(Continued from previous post)
Density of population, in the birdie world, means density of territories. But the territories minimum sizes. In a food-rich environment, the robin territory may be as small as a half acre, which contains more food than the robin family needs. And no matter how much food is there, the territory will not be smaller than a half acre, even if the breeding pair could survive on a smaller territory. Nor will the robins start increasing the number of birds within their territories simply because more food is available. Only the breeding pair and their young live in the territory, no more.
Every bird has a minimum territory size, during breeding season. For colony nesters, the territory may be as small as a few square feet, but will be defended as fiercely against others of the same species as larger territories. For colony nesters (such as purple martins, rock doves, and herring gulls) the food range is shared.
The breeding territories of colony nesters are small and not used by the breeding pair as the food source, so nesting colonies can be and usually are densely populated. Why then do colony nesters not cover the land with their dense breed themselves into starvation?
Of course, many colony breeders are seabirds, and others like purple martins are limited by lack of suitable nesting sites, but even where this is not the case, colony nesters – or colony animals like prairie dogs, who also live in dense populations – do not saturate the land till they starve themselves. This is because the colonies themselves have territories, whose boundaries they defend against other colonies. Rock doves (also known as common city pigeons), who normally nest in colonies on cliffs, find perfect sites for nesting colonies on urban skyscrapers, and each colony has its own territory within the city that balances out the pigeon population with the food supply. This is why efforts to exterminate pigeons are fruitless, because the removal of some birds makes it possible for other birds to gain nest sites and keep the population the same. (This is also why pigeons can be a steady source of protein for urban dwellers.)
Like colony breeders, grazing herd animals share territories as a community and keep members of other herds out. The territory of the herd is somewhat mobile within the shared range, which allows them not to overgraze any one spot, but other herds must keep their distance, which keeps the population density below the level that would stress the range as a whole.
So now back to the bobcat question (how do bobcat populations fall when rabbit populations fall?) Bobcat populations regulate themselves much in the same way as bird populations do, except that wild felines do not form breeding pairs. Rather (with most felines other than lions), among felines, females will establish territories, and a male’s territory will encompass the territories of several females. Each territory establishes exclusive hunting rights for bobcats of that sex; females defend their territory against strange females, males against strange males, but both of them will tolerate intrusions by strange felines of the opposite sex.
Young bobcats are, at a certain age, driven out of their parents’ territory by the same-sex parent, and must search for their own territories. If the population of rabbits is low, then the existing territories will expand in size. The lower the rabbit population, the less chance a young bobcat will have of finding a vacant territory, especially one with enough food resources to support her and her kits. If there are no available territories, the young bobcat simply will not breed at all. So the answer to the bobcat question is that the decline in rabbits causes a decline in bobcats because many bobcats will be unable to breed. And some of them will starve, because without their own hunting territories they will be unable to hunt.
However, for the bobcats who have territories, the territory is always large enough to have more rabbits than the bobcat needs, so that in every territory, some rabbits will survive to populate the next rabbit generation, and the rabbit population will continue to be spread out throughout the various territories.
Wolves, as we all know, are pack animals, and packs have fixed territories whose boundaries are strictly defended. A pack is an extended family, with the unmated grown young from previous seasons helping to raise the next generation of cubs. Eventually, when they are several years old, the young will go off and look for territories of their own. But if food is not abundant and vacant territories unlikely to be found, the grown young may remain with their pack of origin for a long time. Then the territory must be large enough to support a larger pack, and there are more pack members to help defend a larger territory. And like all pack members, they help to feed and support the newest cubs. But these grown young wolves, as long as they remain with their pack of origin, do not reproduce. In each territory, there is only one breeding pair of wolves. So at times of less food, territory sizes are larger, while fewer wolves are reproducing. Thus, territory helps wolves to keep their populations in balance.
Territory size is an adaptation of all animal species to ensure that they do not just multiply and eat up all the food this year and starve the next. Under conditions of abundant food, there will be plenty of vacant territories – animals do not “quickly reproduce to fill their carrying capacity,” even if you define “carrying capacity” as number of potential breeding territories rather than as potential food supply. During periods of abundance, the population does not become so dense as to overexploit its food resources. Indeed, unoccupied territories could be considered to represent a reserve. During periods of of food stress, all territories may become occupied, but a food-gathering territory is always of a size that contains MORE food than is necessary, so that no matter how much the The carrying capacity of the land – in the sense of the amount of food energy available – IS the main determiner of territory size, but territory size is always large enough that food resources are not exploited up to their limits – that animals do NOT “quickly reproduce to fill up their carrying capacity.”
If overbreeding, overconsuming food, and then crashing through mass starvation were the normal way that animal populations were regulated, in nature, that would be very maladaptive for the ecosystems. A food source that is overconsumed may not recover. The disappearance of species because of overconsumption by the species that eat them would have serious repercussions for the whole ecosystem. If the predators have to wipe out the prey population and exhaust their food supply in order for their population to decline, if the land is full of desperate, starving, and overpopulated predators roaming and searching out the last remnants of the declining prey population, prey would often suffer local extinction, and neither prey nor predator would recover. If grasses and other edible plants are constantly overgrazed by overpopulated ungulates, it would accelerate erosion and open the door for other, less edible plant species to take over (as sagebrush has taken over many overgrazed grasslands). And eventually, through natural selection, only inedible plants would remain.
Moreover, since many different animal species eat the same foods – seeds, insects, grass, rodents, etc – if one animal species overpopulates, consumes all its available food, and crashes because of starvation, a lot of other species who have not overpopulated are going to starve to death as well. If the antelope multiplied to the limit of the available grass, to the point where they starved for lack of food, other grazing animals who depend upon that grass would have no food either. But overgrazing is a problem only where humans’ domestic herds graze. Domestic pastoralists overgraze; buffalo, antelope and wildebeest do not.
Now to Daniel Quinn and his mice. (The mouse story, from The Story of B, can be read at http://www.awok.org/boiling-frog/ , starting about halfway down under the heading “Food availability.” Basically, he postulates an expandable mouse cage and the scenario of increasing or stopping mouse population growth by increasing or limiting their food supply.) I have a lot of experience with wild mice, and was going to write about them in detail. But suffice to say that mice are territorial as well. Mouse reproduction rates are directly linked to food supply; the more food, the larger and more frequent their litters. Like other rodents they are dependent on predation to keep their numbers in check, but their main defense against predation is their high reproductive rate. (Coyotes, like mice, can adjust their litter size from one generation to the next, so that if a number of coyotes are poisoned in a territory, the population can be quickly replenished.)
But mice are territorial, and when their territories are filled to capacity, they have smaller litters, less frequent litters, and will even eat the babies they have. Daniel Quinn casually mentions that his mouse cage can be expanded to any size, but other than that, the issue of space for the mice is not mentioned. He doesn’t mention that if you stop expanding the mouse cage, the mouse population increase will stop when the territories are saturated. The mice will stop multiplying at that point no matter how many tons of food you give them. And, if you severely overcrowd them, they will go insane and start killing each other.
An ecosystem based on the principle every animal is supposed to compulsively reproduce until it fills its carrying capacity would lose biodiversity, would be continually degraded in its ability to support life, would have species dying out through no fault of their own, but rather because of another species’ destructiveness.
But wait! That is what is happening, isn’t it! Not because every animal has been following this principle, but because one species has been doing this.
This is a description of modern civilization’s effect upon the world, as though by believing that this is the principle by which the living world operates, we have created a self-fulfilling prophecy for our species
Since the beginning of the industrial / capitalist era, the view has developed that humans are essentially selfish, and that all people are in competition with one another. Adam Smith in the eighteenth century articulated a moral foundation for the emerging capitalist system, that even if individuals were driven by selfishness and greed, the competition in the free market would tend to benefit society as a whole. Society ran on selfishness and competitiveness, and society was benefited if everyone fanatically pursued their own personal interests to the absolute limits possible.
Enter Darwin into this cultural milieu, and interpreted through the filter of capitalism and industrialism, natural selection becomes all about competition. “Survival of the fittest” becomes survival of the most ruthlessly competitive, those who pursue their own individual/ family/ species interest most relentlessly. If species help each other, it is inadvertent, through pursuing their own interests (like rabbits and bobcats). In ecosystems or in human society, the system runs by everyone pursuing their own selfish greed to the max. (That is what the statement that “every animal reproduces very quickly to fill up their carrying capacity” translates to. And it is simply not true. Kind of like the lie that is pounded into us, that many people believe, that human beings have an intrinsic need to accumulate material stuff.)
As a person from a non-“civ” cultural background, I feel uncomfortable with some of the subtle “civ” influences I see in this movement.
The idea every species is competing to increase its own population to the absolute maximum possible is simply the projection of the capitalist principle of greed onto the natural world. (This movement lacks a clear understanding of the differences between precapitalist and capitalist civilizations. Precapitalist civilizations had more rigid hierarchies and less freedom than capitalist civilization, but capitalist civilization is far more destructive than precapitalist civilization. And understanding the difference between capitalist and precapitalist civilizations is important to understanding where we are.)
Quinnian population dynamics is very much an idea born of “civ,” and of capitalist civilization specifically. It has subtle but deep ramifications for this movement, some of which I feel are harmful. So that is why I feel this is worth discussing.
Next I will talk about human hunter-gatherers, then horticultural tribes, then the relationship of agriculture, population growth, and civilization, and then the modern population explosion.
