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Patch Disturbance and the Human Niche
by John M. Logan

(copyright held by John Logan <>, all rights reserved)

Some Definitions-

ecology- the study of the interrelationships that occur between organisms and their environment.

environment- the total of circumstances surrounding an organism or a group of organisms.

succession- the orderly change of species in a place over time.

climax- the hypothetical endpoint of succession where species change ceases.

disturbance- any factor which causes succession to halt its progression short of climax or to regress to an earlier successional condition.

central place foraging- way of gathering food which uses a base of operations to which the animal returns regularly.

patch disturbance species- an organism which, usually through central place foraging, disturbs a central point greatly and places further from this point to a lesser extent.

autecology- the study of the environmental influences upon one species.

Part 1: Introduction- We have all been here before!

As someone who is trained to study the environment as a whole system, it has amazed me that, although many persons express a concern for the environment and the problems we seem to have caused, there seems to be so little agreement as to the causes and the cures. I have reached the conclusion that most people are actually well meaning where this issue is concerned. But I do not think that most of us have a real understanding of the problem in terms that will allow us to compare the situation with other similar situations and find analogous solutions.

Some will, in fact, state that never before has the Earth faced such a situation, never before has a species faced such a disastrous fate, or never before has humanity been faced with the choices we face. We all like to think that we are unique and that we live in unique times. It makes our lives seem less futile, perhaps. Uniqueness somehow gives meaning to what might be a pedestrian existence.

And each specialist will find that his specialty will explain the environmental problem and offer "perspective". Ask a sociologist and he will say: "I think that most of the problem is embedded in our cultural ideas and our way of life -- our consumer mentality -- and I'm trying to think of ways to change this." Ask a philosopher and you will hear: "A fundamental gap exists between the personal systems of thought invented by man and the systemic relationships of nature. Our political institutions appear inadequate to the challenge of averting socio-ecological catastrophe." Ask a politician, he will see it as a governmental problem and want laws to limit pollution and to regulate the dismantling of our natural environmental systems. Ask an economist, he will tell you about the "tragedy of the commons" or want market economics to place values on clean air, water, and ancient forests. An ethicist will discuss the lack of ethics extending to other species. Vegetarians will insist that giving up meat will lead to a sustainable future. Non-christians will see the problem as stemming from the Biblical injunction to "be fruitful and multiply, and replenish the earth, and subdue it". Christian ministers will claim that the entire problem stems from a forsaking of the "one true religion" and call for a return to traditional values. Everybody seems to know what the problem is! Everyone has the solution! Unfortunately, everybody's solution is different. Why should ecologists be any different? We also have a perspective. And of course we also have a solution. The difference is that we have been trained to study exactly these problems in other species. How can we as human beings make rational decisions if we do not understand the logic of our own life-support system? How can we understand the consequences of our action or inaction if we do not understand how natural systems function?

There are many different kinds of ecologists. Some study the physical requirements of organisms- how much light they need, how much they use, how much water, heat, etc. Some study animals. Some study plants. Some study communities of plants. Some study communities of animals. Some study communities containing both plants and animals. Avian ecologists study only birds. The list goes on and on. But most ecologists have studied succession- how a the species community of organisms change over time. Most have studied population dynamics- how the numbers of individuals in species change over time. Most have studied the physical requirements for groups of organisms to survive in a community. These skills which are used for other species are very appropriate for studying the human predicament.

But, you may object, the situation in which mankind finds itself is unique in the history of the world. We have no precedents to which we might refer in understanding the human condition. Are we that unique? Once nearly everyone would have agreed that humans, being the only creatures with souls, the only ones made in the image of the Christian God, were unique among all others. Fewer will make that argument today- in those words. But most are still making the same argument. Either because we are technological creatures or because we can communicate or because we are the only rational animal or because we have the capacity to knowingly reflect on our behavior or because we have opposable thumbs or because we can write poetry or because we have the power to separate right and wrong or because we are 'self-conscious' or ....

I've got news for all those Uniqueness Freaks. First off, every species on good old Mother Earth is UNIQUE. That's why each is a different species. Yes, every species on Earth can claim to be unique.

But, you may argue, our uniqueness is so extreme! More extreme than the platypus which looks like a collection of leftover parts? More unique than the orchid which extends its roots into thin air for nourishment? More unique than the one species of insect capable of pollinating that orchid? More unique than the societal honeybee with its division of labor? More unique than the communist ants who keep aphids as farm animals? More unique than the tool creating chimp? More unique than the gorilla which can communicate using American sign language? I had the good fortune to meet one of the researchers who worked on the project teaching chimpanzees to communicate with sign language. It was obvious that his opinion of what we are had been greatly effected by the experience. He did not believe that chimps were little hairy humans. They were unique, with their own unique intelligence. Just as we are unique with ours. The researchers who work with and 'talk' to Koko, the signing gorilla, speak of her emotions, her game playing, her humor, and her intelligence. It is unique. As is ours. But such uniqueness does not make our environmental situation unique.

But we have metallurgy and nuclear energy, you will protest. True, but these technologies don't exempt us from any of the rules that regulate populations of other organisms. In the long run of tens or hundreds of thousands of years they may even make us more susceptible to those rules.

Still, the argument may be put forward that the situation in which we find ourselves is unprecedented at least in human history. I would reply that many civilizations have fallen due to environmental causes. Several authors would argue that all (or almost all) civilizations before ours fell due to environmental reasons. The Greeks are a good example. Once the hills of Greece were covered with lush forest and fertile soil. They produced their own food and lumber. By the time the Greeks reached the period we refer to as Classical Greece their lands were so eroded that they were capable of producing only grapes and olives, crops that produce in a poor soil. The Greeks were forced to develop colonies in North Africa and Italy to supply their grain needs and they developed long trade routes to exchange olive oil and wine for the staples they needed. Of course, with long trade routes came the need for a powerful navy to protect those routes. The final blow to ancient Greece came when it could no longer control such colonies as Rome and Carthage, when it could no longer maintain its supply lines. But its fate had been sealed generations before when it lost its ability to feed itself. This process has been repeated many times in history. A people grow great, destroy their environment, and are forced to seek sustenance far from home. Either they move 'en masse' as nomadic peoples have done or they set up colonies and trading routes. Finally, the center of the powerful society shifts to one of the colonies and the old center fades.

One difference which we are experiencing this time is that we have run out of places of escape. As the whole world is being used up, we no longer have the luxury of moving on. Colonizing space has proved too expensive a task. Even if a few modern nomads should attempt to seek greener pastures, they will find the pickings pretty slim in near space and the cost of maintaining an earthlike environment in space for long periods of time likely more than they can afford. So we have run out of places to run. Surely this is unique. Surely this has no precedents.

Let's examine another example- Easter Island. Noted ecologist Jared Diamond published an excellent article for the general public in the August 1995 issue of "Discovery" titled 'Easter's End' in which he describes the inhabitants of Easter Island, the most remote inhabitable piece of land on Earth. Studies of pollen cores prove that when the Polynesians arrived by boat, these colonists found a lush subtropical forest paradise. Today only one species of native tree remains on the island and it is very rare. Polynesians first settled Easter Island some 1,600 years BP. Destruction of the forests were well under way 400 years later. By 500 years BP the forests were gone. Every species of native land bird became extinct. Shellfish became overexploited. And without trees from which to construct large canoes, the inhabitants could no longer go to sea in search of the porpoises which had been a staple of their diet. Rats became a dietary staple. (Even more ominously, with the felling of the last tree they could no longer built canoes to leave the island for greener -although distant- pastures! Easter became a closed system, a closed experiment in human overpopulation.) To replace the meat supplies which they had lost they began intensive poultry production. (Sound familiar?) Then they turned to the largest remaining source of protein on the island- each other. Without wood for cooking and heating fires they began burning sugarcane scraps, grass, and sedges- a sure way to impoverish the soil. The population crash took place around 300 years BP. When Dutch explorer Jacob Roggeven first sighted this island on Easter Sunday in 1722, the island was a barren grassland. His crew saw not a single tree or shrub. Botanists have identified only 47 species of plants (mostly grasses, sedges, and ferns) on the island. There are two shrub and two tree species. The present carrying capacity of the 64 square mile island is around 2000 persons, about one-tenth of what is considered to be its peak population.

To those who say we are facing a unique situation, I say that you do not know your history. The situation has been faced before and, based upon those previous examples, the odds for us to avoid disaster do not look good. Only by admitting that we are not unique and by studying previous human and nonhuman occurrences of overpopulation will we be able to learn enough to avoid disaster.

Part 2: Some Basic Community Ecology


Succession is the process of change in community composition over time. Some species enter the community. Others are extirpated (ie-become locally extinct). Species which dominated a landscape at one time become minor components of the community while other species are "fruitful and multiply", becoming dominant species. This change is in some way directional and generally predictable. Let's start with the example of a newly deposited expanse of beach sand. It begins as bare sand. Before plants can begin growing on this site, seeds must blow or be carried to it. What species begin to grow will depend upon what seeds arrive. But certain patterns can be predicted. The first plants to arrive will have small, light seeds. They will tend to be short lived, weedy species. These will be followed by grasses. In a few years we will see shrubs and small trees begin to grow amongst the grasses. This is an example of succession.

Here's another example of succession: A few years ago I built a house in the Missouri Ozarks. When the construction of the basement was completed I had an area of bare soil surrounding the house for several feet. Since I was busy with the construction I did not bother to plant grass seed. Seeds which had been in the soil for many years (ie-the seed bank) began to germinate. By the end of the summer I had a crop of pigweeds (Amaranthus sp.), lambsquarter (Chenopodium sp.), smartweeds (Polygonum sp.), and several other species. All of the plants which covered this bare patch had certain things in common- they all grew very rapidly and produced a rank growth, they all completed their life cycles in one growing season, they all produced copious amounts of seeds. They were all what farmers commonly call weeds (although most of them are very useful plants). In the autumn I mowed my rank growth of "weeds", sending most of the tiny seeds flying over the yard. With such seed production I quite expected another crop of these same "critters" the next year. But I was in for a surprise. The next year the yard grew an almost totally different collection of plants- sour dock (Rumex crispus), dandelion (Taraxacum officinale), wild carrot (Daucus carrota) and other species. Very few of them were plants which could finish their lives and produce seed in one year. Several were biennials- plants requiring two years to set seed. Some were short lived perennials- plants living a few years. It was in the third growing season that grass became the dominant life form in my disturbed yard. Grass is a long lived perennial. It quickly replaced all of the annual and biennial species and most of the short lived perennials (dandelions stayed around). The process of succession was taking place on my lawn. Had I not succumbed to the temptation to mow the grass (both to keep down the fire hazard and to keep various wild animals from joining my lawn community), my yard would soon have been invaded by various woody species. As it was, some of my flower and garden beds were invaded by trumpet creeper (Campsis radicans), a woody vine.

Forests also undergo successional change. Had the succession in my yard been allowed to proceed, the first trees would likely have been Chinese elms, persimmons, sumacs, silver maples, and sassafras- trees with light, wind blown seeds and trees with seeds which animals can be induced to transport. (In fact, before persimmon seeds can be induced to sprout, they often need to pass through the gut of a raccoon, possum or cow.) Only later would the heavy seeded oaks and hickories make an appearance. When we start talking about changes involving tree species, it becomes obvious that we are no longer speaking of changes which are noticeable in a year or two. Such changes involve tens and hundreds of years. Some ecologists discuss successional climaxes- that is, a situation where the change in community composition ceases. Others do not. To some extent, these difference are the result from differing definitions of what is and what is not community succession. Changes in climate also enter into these debates. Early ecologist Fredrick Clements assumed that succession progressed towards the development of a stable vegetation type in equilibrium with the regional climate. Today we know much more about prehistoric variability in climate. Such changes undoubtedly resulted in changes in plant communities. Additionally, much of the confusion in the field of succession results from various understandings and misunderstandings of disturbance.


When I decided to mow my yard instead of allowing it to proceed to a successional stage with woody species, I caused a disturbance in the successional sequence. Disturbances can be external to the community involved- such as my lawn mower or the backhoe which dug my basement and created the original bare patch which became my yard. Other disturbances, such as the grazing by herds of bison on the prairies of North America, are internal- caused by a member of the community. Fire is an effective disturbance because it kills most woody species and -like my lawnmower- keeps succession from proceeding beyond the grass stage.

It is my opinion that succession and disturbance should best be understood together as a dialectic in which succession and disturbance work in opposition to produce various communities. A dialectic may be defined as a process of change through the conflict of opposing forces. Through the dialectic process polar opposites work in opposition to resolve into a synthesis. Succession and disturbance can thus be understood to work in opposition to produce the extant community type which is the synthesis of particular succession and disturbance regimes.

Too many ecologists from Clements to modern day view disturbance as a process which acts upon the community from outside the community, causing it "harm". Disturbance is given a bad connotation. Even the choice of the word "disturbance" to identify the process is indicative of this connotation. When we are "disturbed" by someone or something we are kept from doing what we were doing before the disturbance. We view this as bad. It often is!

But in natural communities disturbance may equally be viewed as a process which keeps those communities healthy and resilient. In fact, so important is the process of disturbance in the health of communities that communities actually have mechanisms which produce disturbance built right into them. Why?

Let us consider for a moment an hypothetical community which has undergone no disturbance in thousands of years. Why do I say that such a community is hypothetical? Because, quite plainly, no such community exists! When we begin studying communities, we find that all undergo disturbance in some form. But let us consider one, just the same. What would it look like? First, all of those early successional species such as those which occupied my yard would be gone. Without disturbance they would not be maintained. Furthermore, although their seeds can survive for a considerable length of time (some for well over 100 years) in the seed bank, their viability is limited. No viable seeds for these species would remain- they would be extinct species. In our hypothetical community none of the early successional trees would remain. They would have become extinct in the community for the same reasons which doomed the early successional species from my yard. The undisturbed community would attain that hypothesized steady state called by ecologists "climax". Given enough time the only species remaining would be the climax community species.

Now, just as an experiment, let's smack our hypothetical community with a very large meteorite! The impact crater will be an area with lots of bare ground and mineral soil. Such a place would not be the sort of place in which climax community species could get a foothold, grow and thrive. Early successional species are much more suitable for such habitats. They will grow more quickly, produce lots of seed, cover and protect the remaining soil, rapidly occupy the disturbed area, produce large amounts of organic matter which will decay to built the soil, and modify the environment so that later successional species can reoccupy the site. (This process of early successional species modifying the environment so that later successional species can grow is called "facilitation".)

But wait! Our undisturbed-until-the-meteorite community has no early successional species remaining! It's recovery from the meteorite will take much longer since no facilitation can occur. Perhaps an analogy can be made with the Native American populations of humans which had never been exposed to the disturbances of smallpox and other European diseases. Because the Europeans had known such diseases, they had developed resistance to them. Humans of European lineage could recover from the disturbance of a smallpox bacterium much more quickly- often before disease made them ill. The presence of the disease had produced resistance to disease- a resilience in the human organism which enables it to respond when the disease reappears. Edward Jenner took advantage of that ability to produce resilience (resistence) when he used a weakened form of the smallpox virus (cowpox) to produce immunity in his patients. Analogously, the presence of various types of disturbance in a natural community maintains resilience against major disturbances which might befall those communities (such as our meteorite or perhaps a windstorm).


Ecologists has produced several definitions of "disturbance". Forman and Godron (1986) define disturbance as "an event which causes a significant change from the normal pattern in an ecological system". The "normal pattern" would be the progress of succession without the disturbance. van Andel and van den Bergh (1987) define disturbance as "a change in conditions which interferes with the normal functioning of a given biological system". Such definitions, while useful from a theoretical aspect, are difficult to quantify. (Also, would fire be a disturbance in a prairie since it enhances the functioning of a prairie rather than interfering?)

Grimes (1979) defines disturbances as "the mechanisms which limit plant biomass by causing its partial or total destruction". Biomass is the actual mass of living material in a natural system. Such a definition makes disturbance fairly easy to measure by simply taking representative samples of the biomass and calculating the total biomass. When the total biomass shrinks, a disturbance has occurred. Comparisons of various types of disturbance become possible by comparing relative changes in biomass. However, this definition assumes that biomass always increases throughout succession if disturbance is not present. Ecologists debate whether this assumption is true.

A third set of definitions define disturbance as a set of processes which lead to increased availability of resources. Prairie fire is a disturbance by this definition because fire releases nutrients bound in biomass. These nutrients are then available to the plants which grow after the disturbance. The problem I see with this definition is that, if disturbance and succession are to be seen as dialectic processes, then succession would invariably lead to a decrease in resource availability. Such an assumption is definitely not the case.

The main problem with most approaches to disturbance is that they have not seen disturbance as an integral part of communities but as something outside of and acting upon those communities. For example, Glenn-Lewin and van der Maarel (1992) write that "to the degree that herbivores and plants have evolved together, it may be inappropriate to consider grazing as a disturbance." Such an approach considers disturbance to be an autecological process (ie- a process operating upon individual species) rather than as a community process. Seeing grazing as part of the system prevents some ecologists from seeing it as disturbance. But some disturbances are internal and some are external. Bison which are part of the North American prairie system act as an internal disturbance. An invasion of Asiatic locusts in the same North American prairie could more properly be seen as an external disturbance.

Biological disturbances, often internal, are less likely to be seen as disturbance by some ecologists. A physical disturbance such as a lightning fire can easily be seen as an external disturbance. What of the same fire set by native humans? You can see why the question of disturbance has gotten so murky!

As you can see, modern ecology has not made disturbance an easy concept to understand. Perhaps this is because, rather than being true definitions, the above "definitions" of disturbance actually are descriptions of causes, effects, and mechanisms of disturbance rather than being definitions. I would define disturbance as any activity or process which prevents or retards succession. This definition is obviously in keeping with my concept of the succession-disturbance dialectic. Perhaps it is difficult to quantify. And if disturbance is an integral part of all communities, then it becomes difficult to remove those disturbances and make comparisons of communities with and without them.


Before passing to the more important discussion of the human species in this context, I would like to make one more distinction concerning types of disturbance. Disturbances can be quantified by their extent (size), recurrence rate (timing), and magnitude (intensity). Disturbances such as grazing by a herd of cattle may be of moderate intensity over a large sized area. Other animals such as prairie dogs may create a more intense disturbance over a much smaller area. We call the latter type of disturbance "patch disturbance" and such species "patch disturbance species".

Part 3: How animals effect changes in natural communities


Plants have several functions in ecosystems. By 'function in an ecosystem' I mean how their niche (their way of earning a living) fits into the ecosystem to assist the system in its holistic processes. For example, a specific class of plants, with the aid of symbiotic bacteria, fixes nitrogen- that is, they take nitrogen gas from the air, convert that elemental nitrogen into nitrates, and upon decay deposit those nitrates in the soil for other organisms to utilize. In temperate deciduous forests, a class of plants known as vernal herbs are very important in storing potassium until other plants begin growing and need it. Without the vernal herbs much of the potassium would be lost through leaching before other plants in the system began their period of growth. Deep rooted annuals and grasses bring nutrients up from deep in the soil and, when they rot, deposit these nutrients near the surface for other plants to utilize. It is not the "goal" of any of these plants to assist others by these functions. But the ecosystem has evolved in such a way that these niches perform functions which assist the whole system.

Similarly, animals have functions in ecosystems which also prove useful to the whole system. For example, many plants species would not survive without the symbiotic assistance of one or more animal species to act as pollinators. Additionally, many animals act as vectors to spread the seeds of plants to new locations. Squirrels are somewhat less adept at relocating nuts than they are at hiding them. Due to this difference in efficiency, squirrels are very important in the establishment of tree seedlings. Other animals such as birds spread seeds by eating the fruit and depositing the seeds in their feces. Many types of seeds have adapted so that they need the scarification from an animal's digestive system before they can germinate. Other animals are pruners. When most plants are pruned they will sprout two branches where only one was previously. We do the same thing when we pinch back a "leggy" houseplant in hopes that it will grow back with thicker foliage or when we prune a hedge to thicken it. Some insects chew a ring around tree branches, killing them and causing them to break off cleanly. A pruned tree or shrub is more efficient. It's leaves grow closer together allowing it to collect more light while maintaining fewer limbs. A pruned plant produces more flowers and more flowers mean more seeds and more offspring. Just as squirrels might be said to plant trees, rabbits might be said to thin them by eating the seedlings and chewing the bark off of them. Deer eat seedlings and nibble on branches, sometimes pruning trees of lower branches and also thickening the branching structure of shrubs. Grass which has undergone moderate grazing tillers, producing thicker stands of grass. And as I discussed in the previous section, other animals are disturbance species. These are the only four uses for animals by ecosystems which I have been able to come up with. There may be more and I am open to suggestions. And of course a species of animal can perform more than one of the functions described. For example, grazers can be both pruners of herbaceous plants and generalized disturbance organisms depending upon the intensity of their grazing and the plant species involved.

In general, plants function as pathways to cycle and store nutrients. Animals perform the less basic but no less important function of "tweeking" natural systems. They make the systems run more efficiently. Without pollinators, pruners, planters, and disturbers the ecosystem would most likely still function but it would not be as efficient. Pruning causes plants to produce a more compact photosynthetic area. Planters make more species of plants available to any given location. Disturbers keep early successional plants widely available within the system. And since more efficient ecosystems will replace less efficient ones, ecosystems with animals are the rule. (Lucky for us.)


Lots of folks don't like beavers. Beaver block streams, flood land, and kill trees. Often they will plug up a culvert turning a road into a dam. The water against the road softens the road surface and causes traffic to rut the road. The people who don't like beavers see such activities as bad. The beaver seems to see such activities as its raison d'etre. With the exception of man, beaver manipulate their environment more than any other species. And like human beings, beaver occupy a habitat which is extensively of their own making. When the beaver floods a bottomland forest it begins a cyclic process which takes many years -more than a human lifetime- to complete. We usually only recognize the disturbance which begins the process. Initially the beaver creates an aquatic patch causing the trees, which are not adapted to the newly raised water table, to die. The beaver builds its home safely within this aquatic patch. It further uses the aquatic patch to travel safely to and from its food supply. In relatively flat locations they dig canals for the transport of cut logs. In many instances these canals are hundreds of feet long and equipped with subsidiary dams and "locks". The beaver chooses to eat certain species of trees- not the entire tree but only the living parts, the bark and small branches. (When it gnaws a tree down it is getting at the branches.)

These are the things a beaver (or a family of beaver) do in order to "make a living" in the world. But this activity does things to and for the ecosystem in which it lives which, although byproducts of beaver habits, are very important to and for that ecosystem. At one time beaver were found in nearly all of North America's aquatic habitats from the Arctic tundra to the Mexican deserts. Many attributes of those streams were changed by the beaver removal which preceded and accompanied European settlement. For example, every dam has the potential to retain a substantial amount of sediment. Beaver activities change the absolute and relative amounts of several nutrients in the stream system resulting in big changes to the carbon and nitrogen cycles. They effect the species composition of both the aquatic micro-flora and fauna. And because the beaver prefers certain tree species as food over other species they greatly alter the surrounding forest composition. Beaver have been said to "farm" their environment, producing the conditions which are necessary to cause their environment to produce the species of plants which form the basis of their sustenance. Within the pond, beaver disturbance begins a process of succession known as a hydrarch- ie, a succession beginning in an aquatic environment. Over time, as silt is deposited in the pond, it becomes shallower and shallower. The patch passes from an area of open water, through a period of shallow wetland, and finally forms a wet meadow. By this time the beaver have long since been forced to abandon the site and the filled pond becomes a lasting feature of the valley floor. Most of the bottomland soils of eastern North America are thought to be the product of beaver habitation over thousands of years.


Species such as beaver, which have a much greater influence upon their environment than what would be expected from their metabolic use of energy, are known as "keystone species". They are much more than mere "tweekers" of some portion of their ecosystem. Keystone species play pivotal roles in regulating entire ecosystems. They not only affect nutrient cycling and such processes as ground water recharge and water table levels, but they also act as major factors in determining what plant communities are present and they create niches for many other animals.

The above description obviously describes another species quite well: <Homo sapiens>.


Humans need two basic types of food, proteins and carbohydrates, plus a collection of vitamins and minerals. The archeological record has shown that early humans ate almost any kind of animal which they could catch. By studying the bones left in the refuse piles of early humans, it appears that a considerable amount of their diet was composed of small mammals. Carbohydrates were supplied from tuberous roots and the grains of grasses and other plants.

But the perennial grasses do not produce copious amounts of seed. Early successional species such as annual grasses and forbs are much more productive of seed. But before the discovery of agricultural techniques humans had no way of producing these grains. Or did they?

Consider the family of pre-agricultural humans who travel into a new area. Perhaps there is a stream with fresh water or maybe a spring. There are animals to eat- perhaps beaver or other animals which live along the stream, plus fish, and also small rodents which live in the grasses which grow between the savanna-like trees. The family sets up camp near the water source, constructs shelter, builds a fire, and settles in for a stay which will last as long as the food supply lasts. They probably use grasses in the construction of their shelters, as kindling to start their fires, and for several other purposes. As they move around their camp performing the repetitive tasks of their lives, their foot traffic fairly quickly destroys the carpet of grasses which had covered the ground. After a few weeks or months they find that the game is no longer plentiful and easy to catch. Long before they have extirpated any species from the area, they pack as much of their items as they can carry and they set out to another location. They may move several times in a cycle of the seasons. But being creatures of habit, as we all are, when the next year rolls around they remember the easy living which they had in that place beside the good water source. Hoping that life will be easy there again this year, they return to the site. Perhaps the tree branches which comprised the frames of their shelters are still standing. With somewhat less work than the previous year they restore the shelters and again set up camp.

But some things at the campsite are different this year. Because of their activities the previous year, the perennial grasses are not as thick. The past activities of the humans have made them less vigorous, their ground coverage less complete. Instead other plants are present- plants which are much more palatable to our ancestors. Perhaps there are chenopodiums, amaranths, and other plants which they can use as sources of vitamins and minerals- plants which we call greens. Perhaps there are also annual grasses such as foxtail, spelt, or millet- plants which are not particularly palatable but which produce fairly large seed which the humans can collect easily and consume as a carbohydrate source. When the greens get too mature to eat as greens they also produce seeds which the humans collect and eat. They eat some of these seeds while they stay at the camp. If there are enough, they store some of the seeds to take with them to their next camp. And of course, like the squirrels who can not find all of the nuts they hide, they also accidentally dropped a small percentage of the seeds. These seeds provide a seed bank for another season's crop of disturbance adapted plants. When they move on to their next camp, taking some of the easily stored seeds with them, they inadvertently spread the species which provide their food to another location. Soon these seeds are in most of the locations where the humans make camp. The patch disturbance which they produce anywhere they stay for any length of time allows these food plants to spring magically from the soil and nourish the humans. Just as the manipulations of the beaver result in an increase in the available food, so the manipulations of the humans result in an increase in the available human food.

At least for a while.

But just as the beaver pond finally silts in and the successional state of the forest around them changes over time to species which the beaver do not eat, so it is with the humans. The soil of the campsite slowly gets eroded and compacted. It produces less food after a many years. The family of humans begins to camp elsewhere and Nature begins the successional process of healing the changes brought by human habitation.


The situation described above undoubtedly maintained for tens of thousands of years. It is easy to see how the situation described above would lead with little difficulty to agricultural practices. It may have taken thousands of years to get it right. Perhaps one of the humans discovered that if he fed some of their seed-food to the Earth she would favor the family with a greater amount of food. Planting seed had been discovered. Perhaps another discovered that when the Earth was stirred with a stick and then fed some seed she would grow an even more abundant harvest. It would not take long for planting ceremonies to develop. It would not take long for a complete mythology to develop to explain the process. In pre-agricultural societies where hunting techniques had already developed to a reasonable level, carbohydrates were more difficult to accumulate than were proteins. Families which discovered these secrets of Mother Earth were blessed with better diets and greater numerical increase than others. Their populations would grow.

Still, the soils which were farmed would tire under production, forcing the humans to move to other locations. As long as land was plentiful and people were few, there was little need to develop techniques of soil fertilization. Under patterns of shifting agriculture the environment which included humans was stable. Although agriculture led to a more sedentary life, the villages still needed to be moved from time to time. And since people still died of other causes than malnutrition, the population grew slowly and sporadically. The human patch disturbances remained few in a matrix of undisturbed (or less disturbed) perennial grasses and trees. But as the human population did grow, the number of patches increased. As the family groups developed into tribes, the sizes of the patches grew larger. And as fewer places became available to occupy, because some other group of humans was already there, the ease with which a village could be moved decreased.

Et cetera.

So by doing what we evolved to do- that is by living in small family groups which used a central location as a base of operations from which to scavenge, hunt, and gather foods- we learned to create intentional patch disturbances which supplied us with greater amounts of food. And these greater amounts of food have allowed our populations to grow large over time.

But what, after thousands of years of slow increase or fluctuation of population levels, has caused our exponential increase in numbers which we see today? Most species have factors in their environment which control their numbers. We seem not to have such controls. What happened to them? Perhaps if we study how other animal populations have gotten out of control, we can learn something about our predicament.

Part 4: Population Dynamics


Ecologists once hypothesized that, as ecosystems approached climax conditions their diversity and, therefore, their stability were maximized. But numerous studies of many kinds of communities indicated that this simply is not true. Instead, diversity is maximized before climax is achieved. Beyond that point diversity begins to decline. Diversity is maximized at a point somewhat beyond the middle of the successional pathway but short of the climax endpoint. Recall that disturbance acts to retard or prevent succession. Therefore, some disturbance in an ecosystem will produce a more diverse -and, thus, more stable- system than one without some disturbance. Too little disturbance equals less diversity. But too much disturbance also equals less diversity. Diversity is maximized at a point of intermediate disturbance. Ecologists call this concept of just the right amount of disturbance the "intermediate disturbance hypothesis". In previous parts of this series of posts I have shown how natural systems have disturbance built into them. The intermediate disturbance hypothesis explains why this is so.

For tens of thousands of years human beings have acted as one of the patch disturbance species which keep communities at these intermediate points where diversity and stability are higher. Since we are such an adaptable species, we have been very useful for that task in many kind of communities. We produced patches of early successional grasses in grasslands. We produced open patches in forests. We even set landscape scale fires which keep succession from proceeding and keep prairie communities healthy. Being omnivores, we have acted as controls on the population levels of numerous plant and animal species. Being fairly mobile and capable of adapting to various climates we have acted as distribution vectors for all kinds of plant and animal species. We have the ability to be a very useful species for the living system which is Earth.

And while our patches were few and shifting, occurring in a late successional landscape matrix, we remained very useful. But we have grown great in number. And our patches have multiplied right along with us. When a patch is depleted there is no longer any place to move. Non-agricultural patches are used until they are nearly sterile and have a community of a few very hardy "weed" species. And we use artificial fertilizers to keep agricultural patches productive until the topsoil (or the petroleum which is raw material for the fertilizer) is gone. We expand the size of our patches to increase our production or to simply keep it at the same levels as soils become depleted.

Do you recall my description of the process which the beaver initiate? As their ponds silt up, they abandon their patch. It becomes a wetland; then later a beaver meadow. And finally it returns to forest. The entire process takes a long time. Our patches were meant to undergo a similarly cyclical process of use, abandonment, and finally regrowth. This latter stage of regrowth is the part of the loop which we humans have stopped. We no longer move to another site and let the successional process proceed. Nature can no longer pass through the phases (or seres) of "healing" which take the landscape back to perennial grasses and late successional forests. It is as if the beaver had built bigger and more numerous dams until they flooded their entire world with High Aswan Dams and Boulder Dams. Obviously something would have to give! Such a beaver world would be at the mercy of any huge rain event. As we humans build more and more precipitously, we will be at the mercy of some "event". And, more fundamentally, although we are doing the same thing we always have (creating patch disturbances), we no longer maintain the ecosystems we occupy at that "intermediate" point of maximum diversity.

But how did we get so "out of control"?


Although numerous occurrences of ungulate population irruptions (ie- exponential population increase) have been documented, one of the most famous and most studied occurred on the Kaibab Plateau on the north rim of the Grand Canyon (USA) in the early 1900s. (For the original treatment of this irruption see: Leopold, Aldo. 1943. Deer Irruptions. <Wisconsin Conservation Department Publication> 321: 3-11. In Leopold's classic "Thinking Like a Mountain" found in <A Sand County Almanac>, Leopold discusses the lessons he learned from the Kaibab disaster.)

Mule deer are a native species of the Kaibab Plateau. By the early part of the 20th century people had also brought cattle and sheep to this range. Along with their cattle and sheep they brought the practice of killing wolves and other predators such as mountain lions which they considered to be detrimental to their animals. But Leopold writes of this time and process thusly: "The cow man who cleans his range of wolves does not realize that he is taking over the wolf's job of trimming the herd to fit the range." And he describes the results of this process thusly: "When the wolves are removed from mountains the deer multiply. .... I have watched the face of many a newly wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails. I have seen every available bush and seedling browsed, first to anaemic desuetude, then to death. I have seen every edible tree defoliated to the height of a saddlehorn. Such a mountain looks as if someone had given God a new pruning shears, and forbidden him all other exercise. In the end the starved bones of the hoped-for deer herd, dead of its own too-much, bleach with the bones of the dead sage, or molder under the high-lined junipers."

Leopold's words are very vivid. Here are the facts as well as we can determine them: In 1906 the mule deer population of the Kaibab was around 4,000 head. By 1918, when the first signs of damage were seen, the population had increased to slightly less than 50,000. By the early 1920's (around 1924) the population peaked somewhere near 100,000 and began a drastic decline as starvation and disease decimated the head. By 1926 the population was cut in half. It continued on its downward path until 1940 when the population levels were back to near the 1906 level. The carrying capacity before the irruption is thought to have been between 25,000-30,000 head (ie.- the point just before any noticeable damage was being done). After the irruption, carrying capacity was lowered to around 5,000. This change in carrying capacity indicates that overshooting the carrying capacity caused damage to the system. The reduction in carrying capacity was a long term reduction, a long term injury to the environment.

According to Leopold, the problem began at the turn of the century with the predator control program. But other ecologists have indicated that the real scenario might not be as simple as Leopold indicates. "Revisionist" ecologists such as Graeme Caughley play down the importance of predator removal as the cause. Caughley, studying all known cases of the introduction of large ungulates into new habitats, found that a population irruption invariably occurred whether or not predators were present. Instead, Caughley points to the reduction of the sheep herds from 200,000 head to 5,000 head and the reduction of cattle from 20,000 head to only a few as the culprit. Competitive pressures limiting mule deer population were eased almost to elimination in 1908. The lessening of competition from the livestock had such a great influence that it was as if the mule deer had been introduced into an entirely new range. (For further discussion of this theory read: G. Caughley. 1970. Eruption of Ungulate Populations, with Emphasis on Himalayan Thar in New Zealand. <Ecology> 51:53- 72.)

While we can prove neither cause for the Kaibab Plateau irruption of mule deer, we do know one thing. The irruption coincided with both the removal of competitive pressures and with the elimination of predators. One may argue one cause or the other. However it seems likely that an irruption caused by relaxation of competition was only to be exacerbated by lack of predator controls. Most likely the two factors interacted to produce the irruption.


Okay, so how do we apply this to human beings? We, after all, are not ungulates. But there is no reason to believe that the processes which affect mule deer might not have their counterparts in other species- even humans.

History tells us that the population levels of Europe fluctuated for hundreds and perhaps thousands of years. Each time it rose to near or beyond carrying capacity war, famine, and disease pushed it back down. Then came the Renaissance and the Age of Exploration. The New World was discovered. Europeans moved into the new pastures of North, South and Central America plus Africa, Australia, and much of the Asian subcontinent. Such an expansion of Western Civilization into new areas can be seen as analogous to the introduction of the ungulates into new habitat. Of course, these areas were already occupied by humans. But we know from our history books that the European explorers and pioneers viewed the Americas as a vast untamed and empty wilderness. They saw the "primeval forests" and the great expanses of prairie as empty and unused. And they set about to use them. The Europeans occupied these "new" lands in such a way that the native populations were not able to directly compete with their conquerors. Peoples with non-European backgrounds, religions, and customs were viewed and treated as another, inferior species. Using disease, warfare, and their more advanced technology, plus their concept of private property, the Europeans systematically isolated themselves from competition with the native peoples. Europe imported the wealth of much of the rest of the world and freed itself from the necessity of supplying its own needs. It's population swelled. Much of that population moved across the waters and filled other continents. The irruption had begun.

(As I pointed out in first post in this series, we are not experiencing the first instance of human overpopulation. Not only did it happen in the closed system of Easter Island (where the human species had introduced itself into an unpeopled system), but it also happened in various societies of history. And many times we can probably find a situation where newly colonized lands precipitated the irruptive cycle- or, at least, aggravated it.)

In addition to the human expansion to "new" lands, the new found wealth of the Americans, Africa, and Asia enabled the expansion of science and medicine. Medicine set about conquering the many diseases which had helped keep human populations within limits for most of humanity's existence. Man's "predators" were not eliminated but were at least extremely curtailed. Science also created other "new pastures". Primary among these is the use of petroleum which, among other uses, can -for a while- make the soils more productive, allowing the population irruption to continue. Such technologies act to create still more new "habitats" for the population to fill.

Unfortunately, none of these discoveries actually made the planet any larger. We must still occupy the same physical space and get our food from the patch disturbances in that space. And as the mosaic of our early successional patch disturbances fills the late succession matrix, they spread and flow together until they produce a new matrix of early successional, disturbance dominated communities. What we are witnessing is the creation of a human "browse line"!

And we look out at this strange sight asking, "How did things get this way?" And we blame our leaders, our religions, our philosophies, our technologies. We ask, "Where did we go wrong?" And we blame agriculture, patriarchal society, or even fire. But, folks, we didn't go "wrong". We got here by following the very patterns which make us what we are. We got here by following the patterns which co-evolved with us to make us this nearly hairless, omnivorous species of ground primate which acts as a patch disturbance, keystone species. While these characteristics kept us and our ecosystems in balance in fairly static, limited habitats, the introduction of our species to "new" habitats has caused us to increase our numbers in an irruptive, exponential manner. Yet the long term carrying capacity of the Earth has not increased. Since half of the top soil is already gone, we have probably already decreased Earth's capacity to support humans. And before we get back to sustainable population numbers we will probably decrease carrying capacity still more. How much more depends upon a lot of factors- not the least of which is how quickly we can return to sustainability.


Whatever else can be said about the nature of a sustainable society, it must be stated unequivocally that such a society must have population levels at or below carrying capacity. Considerable ink has been used discussing what CC means for our species. I have not come to terms with much of this argument. Most thinkers on this subject agree that our CC is dependent upon what level of technology we chose. While I do not necessarily disagree, I will withhold final judgement as to whether that is the case. But it seems that a lot of people can't tell the difference between the necessities of species existence (which define carrying capacity) and the wants of present day people (which our society has taught us are necessary for life). Jay Hanson quotes: "In the end," says the Grand Inquisitor, "in the end they will lay their freedom at our feet and say to us, 'Make us your slaves, but feed us.'" But, in the end, to be fed we will give up more than our freedom. We will give up our automobiles, our tv sets, and our electric lights. We will give up our central heating, our indoor plumbing, and every other modern convenience. This is why I think that carrying capacity is very likely only a matter how much food and fuel we can produce in a sustainable manner. It is entirely possible that our species living at carrying capacity will have no time for the pursuits of solar power or sustainable technology. Technology requires time away from the pursuits of basic subsistence. But a species which does not need to spend all of its time pursuing the basics is not at carrying capacity!

If the overshoot phase of population growth which we are now experiencing ends through catastrophic means, humanity will find itself at a new, lower carrying capacity. Plus, the technological level will also fall to a minimum level because, at carrying capacity, all energies will be turned toward survival. As with the Kaibab mule deer, a natural population crash will only proceed to the level of the new carrying capacity. For technological progress to exist in a "sustainable society" population levels must be below carrying capacity. The lower population -less than carrying capacity- will allow resources to be available for non-survival purposes. And this condition will only come about through a planned, controlled reduction in population. Only if the overshoot phase ends with planning (a controlled crash) will it be possible to maintain any "respectable" level of technology.

But what are the odds of getting to those kinds of sub- capacity levels when we are starting so far above capacity?


At this point I admit that I am probably over-reaching my expertise. But I do know that wildlife managers have certain rules which they have sometimes learned the hard way. For example, at one time they fed hungry herds of elk and other game animals in the winter. The idea was that, if they could get the herd through this period of scarcity, then the animals would thrive the next summer and provide more animals to hunt in hunting season. But the animals became very dependent upon the food humans provided. Then a harsh winter would come and deep snow would prevent the wildlife managers from getting either food to the animals or the animals to the food. The wildlife would die by the thousands. Better not to feed the wildlife, was the lesson.

Applying this knowledge to human beings, perhaps we need to rethink food aid to crowded countries. Garrett Hardin has written well on this topic. I recommend an essay titled "An Ecolate View of the Human Predicament" which can be found at . Our food aid allows populations which cannot feed themselves to continue increasing. When the year arrives that we have no surplus food to give them (the equivalent of the harsh winter described above), then they will die in even greater numbers than had they been left to fend for themselves all along. Perhaps we should rethink all manner of aid, including medical assistance, for similar reasons.

But for the affluent nations to influence the non-affluent nations to control and reduce their populations, we will need to practice what we preach. And we can't even reach a consensus on abortion! How could we ever muster the will to adopt the "China Solution" before we get into the population predicament of China. (China had the "good luck" to be a non-christian and non- democratic state! I doubt that they could have mustered the will to limit births had they held to the sanctity -nay, the divineness- of human life as their ultimate precept.) But, as Hardin writes, "Does God give a prize for the maximum number of people?"


Only when we face the fact that we have gotten to this "dead end" position by acting in the very ways which we evolved with and which worked so well throughout our evolutionary history- only then will we be prepared to face the fact that, to escape this trap we have laid for ourselves, we must act in ways which are not natural for us as a species. This will be a much harder task than the one we hold for ourselves when we say that we got here by some mistake or historical error. But acknowledging our "success" will make solving our dilemma possible, while pretending that it is all some tragic mistake will not.

In conclusion, whatever the sustainable society is, we must get there by, first, stopping the increase in our population and, then, by reducing our population to sustainable levels. We must pursue actions which are appropriate to accomplish these goals? And we must not let obsolete religious or humanist precepts adopted when our numbers were limited and our continuance precarious prevent us from achieving them. Any belief or concept which stands in the way of the goal must be expendable. We will undoubtedly view this fact as a tragic loss of much which has made us human. And it undoubtedly is. But is it more tragic than the prospect of an uncontrolled crash, a worldwide Easter Island experience? By observing what has happened previously, both to the human species and to other species, we can see that our future will not be pleasant. Populations have only been reduced by starvation and disease or by colonization of new habitats. We have no new habitats. By most accounts we have already exceeded carrying capacity. I do not think that we will build a consensus to gently and peaceably reduce the population of the planet by more than 50 percent in the next fifty years. We will not go gently. We will very likely go blindly.

Can we reach a consensus that the right to continue as a species is more important that democratic rights or reproductive rights? I am old enough to remember the Cold War slogan "Better dead than Red". Some people actually thought that global nuclear annihilation was preferable to communism! Such an idea is obviously insane. Is it any less insane to say that we must reach a sustainable future only by consensus? Only by democratic means? Or only through free and independent choices to limit our reproduction? After all, most of the rights which we hold as "self-evident" are the result of the frontier mentality. And the frontier is gone.

I take little stock in the rationality of humankind. I do not believe that we can convince our fellow humans with rational arguments (even though I have tried to make one with these posts). Today most Americans (and the majority of persons in many other countries) know that something is wrong. But most of that majority only want changes which don't affect their lifestyles. Most will accept recycling. But fewer try to reduce consumption. Consumption is still their right! And while we on this list discuss reducing population, others are going to extraordinary means to have children- in vitro fertilization, egg implantation, surrogate motherhood.

Have the rich already read the writing on the wall? They are doing all that they can now to maximize short term profits. Their political parties are using religion, corporate power, and money to make certain that they can maximize their hold on what limited resources remain. They have the communication apparatus, the guns, and the money. How can we expect non-democratic methods to do a better job of changing our future? Perhaps the wealthy, having read the future correctly, are preparing to inherit the post-crash planet.

Obviously our society is not built to last. From this point we need to begin discussing what our options are. Perhaps there is no way out of the corner we have painted ourselves into. Bill Devall and George Sessions seem to hint at this in their book <Deep Ecology>. Perhaps, as Jay Hanson has suggested, we should sit on some beach and enjoy what's left while we can. Or perhaps we will have to "dig in" and wait for the paint to dry.