- 1.0 Mother Nature's statistical law of density-dependence
Most people have developed and spread the grossly misleading notion that having children is a matter of
personal choice. They tend to focus on the tactical free-will aspects of fertility and overlook the strategic
deterministic constraints. Conversely, when it comes to animals, the same people believe the opposite:
that animals (plural) are mechanical brutes that breed when they are in heat.
Cohen summarizes the worldly wisdom for us. He alleges that women enjoy choices denied to animals:
- “ Human choice is not captured by ecological notions of carrying capacity that are
appropriate for nonhuman populations.” [1]
In the popular culture and in significant segments of the scientific establishment, the notion has
developed that an animal is driven by instinct (whatever that is). The beast copulates in that glorious time
of the year and spits out offspring irresponsibly. If later food happens to go scarce, well then, nature takes
its toll on the weak through competition. This is Mother Nature’s way of controlling runaway sin and
keeping species near their carrying capacities.
Nothing could be further from the truth. It is well-established that intra-specific competition [2] depresses
fecundity when a species approaches its carrying capacity. This phenomenon is known as density-
dependent birth rates. Unfortunately, age-dependent demographic trends have not been universally
recognized as laws by many in the ecology establishment in great measure because some writers
perceive that there are exceptions, or claim that such statements cannot be made until density is
unambiguously defined. [3] [4] Therefore, mainstreamers prudently treat density-dependent birth rates as
an unproven principle or as an interesting curiosity, and have not given this phenomenon the attention it
deserves despite so many examples in the plant and animal kingdoms.
I believe that the failure to see the universality of density-dependent birth rates is a case of too much math
and too little common sense. Apparently, the mathematically minded ecologists and biologists are waiting
for some Einstein to come up with an equation that proves that the rule applies to all species before they
call it a law.
I will be bold and summarily promote density-dependent birth rates to a law. I will do so despite being
unable to produce foolproof evidence. I will proceed with the ‘beyond-a-reasonable-doubt’ standard and
rely on my intuition. This is not a qualitative matter and, therefore, not really a biological or social law.
Qualitative actions of individuals do not result in laws because the participants could have decided
otherwise. I cannot predict whether you’re going to move your left hand right now. The dependency of
birth rates on density is a quantitative issue, a statistical law that can more or less be predicted. In spite of
this, I will not use numbers to prove my case because it is unnecessary. We do not need to collect data or
run any test to conceptualize that more living beings surviving on ever smaller rations will at some point
stop reproducing. This does not make the conclusions unscientific as Murray alleges. [5] He should first
learn the definition of the word science before publishing sweeping statements about the scientific
method. There are sufficient observations to show that density dependent birth rates are universal, if not
widespread, among large mammals. I will mention a few cases here that should convince the reader of the
legitimacy of this rule.
2.0 All species of plants and animals are subject to density-dependent birth rates
However, before we look at specific cases, the reader should ponder again what this entails. If what I say
is true, this phenomenon is as stunning as it sounds. A birth rate that is contingent on density suggests
that plants and animals pro-actively practice some form of family planning! It also means that humans
ultimately have no choice in whether to have babies.
The average person will intuitively reject this as patently absurd. Humans are unlike animals in that they
have the ability to understand trends and to adjust their population as the need arises. If we realize that
the world is getting overcrowded, we implement a series of voluntary or compulsory methods to lower the
birth rate. Animals, on the other hand, do not infer density through a logical thinking process. Certainly,
the lonely tigress does not conduct a survey of the region before she determines the appropriate size of
her next litter or whether to have a little fun. Animals rely on instinct, an innate ability to adapt to their
immediate environment.
In fact, the skeptic’s objection to my argument has changed little since Darwin formulated it 150 years ago:
- “ we may confidently assert, that all plants and animals are tending to increase at a
geometrical ratio, that all would most rapidly stock every station in which they could
any how exist, and that the geometrical tendency to increase must be checked by
destruction at some period of life.” [6]
This notion is so intuitive that just about no one questions it. Of course, animals have no family planning!
Only humans have the ability to reason and make choices and plan the future.
Unfortunately, Darwin and those who opine like him have no idea what they're talking about. If they have
their way, the tigress continues reproducing mechanically according to a preordained estrus schedule,
and when instinct misleads her, the surplus progeny is claimed by nature through positive checks:
- “ Population growth is constantly checked by the Four Riders Of The Apocalypse,
War, Famine, Pestilence and Death… Although Dawkins is correct to say that
animal populations (or, indeed, any populations) do not ‘go on increasing
indefinitely’, it also true that all populations take every opportunity to do so… It
is only the action of Malthusian checks on population which restrain the Lynx
population. As the Lynx population rises so they act as a Malthusian check (Death,
one of the Four Riders of The Apocalypse) upon various prey populations whose
numbers then fall. This results in famine (another of the Four Riders) reducing the
Lynx population, whose numbers then decline. A fall in the Lynx population
reduces the checks on the prey populations, whose numbers then rise, and the
whole cycle starts over again.” [7]
- This is not what we observe! Nature does not work particularly on the death end of the life cycle. In fact, it
works primarily at the front-end fertility point. We have sufficient data to reject Darwin’s unsubstantiated
allegations, which he synthesizes in his dictum:
- “ there can be...no prudential restraint from marriage.”
Plants and animals do in fact restrain from marriage, Mr. Darwin! They make downward adjustments to the
birth rate before starvation or predators have a chance to intervene.
- 3.0 Examples of density-dependent birth rates in plants and animals
A few examples suffice to show that density dependent birth rates apply to different habitats and different
types of living entities. Harper (1961) observes that maize and Watkinson (1978 and 1990) that dune plants
produce less seeds after attaining a certain population density. Other researchers have confirmed their
observations with different types of plants:
- “ Individuals in areas of high density produce fewer seeds than individuals at low
density in the same habitat. Three components of fecundity, the number of flowers
per plant, the number of fruits per plant, and the number of seeds per fruit, were
negatively correlated with density.” [8]
“ the percentage of CH seeds produced per individual was much higher at reduced
density. At natural density total seed production per plant was lower and more
hierarchical than at lower density” [9]
Fowler [10] claims that longhorn cattle reduce their fecundity when they begin to feel density pressures.
Rubenstein (1981) discovers that female fecundity and male reproductive activity in sunfish is inversely
proportional to density. And Southern (1970) reports that owls don't even attempt to breed during
seasons of scarcity (i.e., density with respect to resources). In their study of vicuña, Bonacic and his
group summarize what researchers routinely observe in the field:
- “ Population growth rate declined linearly with population size, which indicates a
degree of density dependence… The principal density dependent effect observed
was that birth rate declined in those family groups with the most breeding females.” [11]
In their study of North American Elk, Stewart and company similarly conclude:
- “ Age-specific pregnancy rates were lower in the high-density area… density-
dependent mechanisms had a much greater effect on physical condition and
fecundity than density-independent factors (e.g., precipitation and temperature).” [12]
In his study of seahorses, Martin-Smith tells us that:
- “ the number of offspring an animal will produce, is dependent on the age and/or
size of an individual and thus the population birth rate will depend on the age
and/or size structure of the population... Populations do not grow indefinitely
and so some form of density-dependent relationship between population size
and birth and/or mortality needs to be measured or assumed.” [13]
- And Klein documents that the population pyramid of reindeer introduced into St. Matthew's Island began
to overturn before the population finally crashed:
“ Reindeer (Rangifer tarandus), introduced to St. Matthew Island in 1944, increased
from 29 animals at that time to 6,000 in the summer of 1963... By 1963, the density
of the reindeer on the island had reached 46.9 per square mile and ratios of fawns
and yearlings to adult cows had dropped from 75 and 45 percent respectively, in
1957 to 60 and 26 percent in 1963... Lichens had been completely eliminated as a
significant component of the winter diet... In the late winter of 1963-64... virtually
the entire population of 6,000 reindeer died of starvation. ” [14]
These observations suffice to show that plants and animals practice some form of contraception.
Demographic crashes do occur, but animals already sense that something is not right when they have
trouble finding food and make adjustments to reproductivity early in the process. The longhorn cattle in
Fowler’s investigation did not die off en masse due to scarce resources or because predators pounced
upon them. The purpose of the study was to study births, not deaths. The cows produced less offspring
per capita as the herd approached a magical food-related density!
Another strategy Mother Nature allows is insular dwarfism. Over a few generations, the members of a
species trapped in a resource-depleted region reduce in size. Perhaps the reindeer in St. Matthew Island
were working on this alternative as well:
- “ Average body weights had decreased from 1957 by 38 percent for adult females
and 43 percent for adult males” [13]
Fowler has generated a list of large animals for which there is strong evidence of density dependent birth
rates. [14] He places Man at the top of the list. Therefore, I wasn’t so bold after all when I converted density-
dependent birth rates into a law. This rule is pretty damned close to a law if not right on the money. If
someone later discovers an exception to this rule, it won’t take away from my argument. Such a finding
would not debunk the theory that density-dependent fecundity is a law for large mammals, specifically for
the species I will be discussing in this site: Man. [15]
I believe that what leads the researchers to be skeptical about density-dependent birth rates is the still
nebulous definition of carrying capacity. Are we talking about living space or food? Density is a word most
people associate with space. Therefore, most ecologists are thinking about elbow room rather than
resources. For example, in a seminal experiment, Calhoun (1962) gave a colony of rats ample food,
allowed them to multiply, but kept the living space constant. The rats eventually changed their behavior
and succumbed to a significantly low level of ‘overcrowding’. It was the scarce living space that finally
drove the rats nuts and which led them to stop procreating. Although space and food may both play a role
in the phenomenon of density-dependent birth rates, in a natural setting migration relieves spatial
pressure and it is food that remains as the ultimate constraint. If we define density as the ratio of the
numbers of individuals of a species to the supply of food, then all life is subject to density-dependence
birth rates.
4.0 How Mother Nature does it
How then does nature trigger these universal responses to population pressures? Are cows and dune
plants that smart? Do they take family planning classes from their mothers? Is there compulsive infertility
imposed by the elders of the herds and gardens?
Researchers have inferred certain conclusions from experiments and observations. Ross and Harper
(1972) demonstrated that plants which establish themselves first by growing faster in a densely sown
area crowd out those that lag behind. The late-comers are compelled to compete for leftover space and
food resources. These plants grow to shorter lengths than warranted by the time interval and dedicate a
smaller proportion of their biomass to seeds. Likewise, Klein concludes:
Does any of this ring a bell?
Darwin’s theory of natural selection predicts a similar phenomenon for animals which are organized in a
hierarchical or pyramidal economic structure, which means all of them. Those that live in unproductive
territories spend more time and efforts searching for food, patrolling their territory, and fighting off rivals.
A hungry tigress is a stressed animal more concerned about her own survival than about gratifying her
lover. Staying alive takes priority over reproduction. The Law of Nature is: always business before
pleasure! Hence, any changes in the density of the population as measured with respect to resources will
have an indirect effect on birth rates. Those that are wealthy enjoy the luxury of breeding. Those that are
poor may not. Territoriality (i.e., density in relation to resources) thus preempts surplus progeny by
limiting the number of parents (Fig. 1). There are ‘poor’ animals that nevertheless breed now and then
despite their precarious situation, mainly because they miscalculated or were caught off guard by
subsequent events. In Southern’s study of owls, death-end mechanisms included failure of eggs to
develop and mortality in the nest. In Harcourt’s (1971) study of potato beetles, the breakdown included
cannibalism and starvation. In all hierarchical structures, the few are wealthy and the many are poor. Serfs
and proletariats tend to be a much larger segment of the population as resources dwindle. Hence, from a
strictly statistical point of view, as density increases and resources become scarcer, all these factors
work together to limit global reproduction.
Humans are not immune to density dependent birth rates; we are, after all, a hierarchical species. An
urbanite living in Bogota does not decide to abstain from having children because Shanghai is
overcrowded. Her decision stems from cultural and economic factors closer to home. The amount of
wealth she possesses is not a measure of space, but a measure of her personal finances. Her relation to
resources, in turn, has an inordinate effect on her decision to have a baby. This problem does not
necessarily require that the woman go to the university to learn about planned-parenthood. Like the
tigress, a proletariat experiences the hardships of life directly, and her intuition directs her to shift
priorities from having a family to watching out for number one. Call it intuition or instinct or whatever. If
she opts to have children in spite of her precarious situation, Mother Nature has the option of claiming the
surplus through disease, starvation, abortion, or outright murder. So far our intelligent human had as
many choices as the ‘dumb’ tigress.
- ________________________________________________________________________________________
- Last modified 03/03/08
- Copyright © by Nila Gaede 2008
| Yesterday |
| Today |
Fig. 1 Balloon Ecology |
The relativists of Biology, Ecology, Economics, and Demography remain skeptical, especially on the
sensitive issue of choice. (How can you possibly say that a woman has no choice, Bill? Women's
associations will burn you at the stake for uttering such heresy!) The scholars claim that a woman has
infinitely more options than an animal:
- “ carrying capacity is determined jointly by human choices and natural constraints…
Human choices about the Earth’s human carrying capacity are constrained by
facts of nature which we understand poorly. So any estimates of human carrying
capacity are only conditional on future human choices and natural events.” [16]
Bailey synthesizes a similar notion in his one-liner:
For example, an individual may decide to forego children despite having access to abundant resources,
or she may think that labor pains are not worth the trouble and use an IUD the next time, or she simply
may not find the appropriate mate. The number of variables is so great that it is impossible to predict what
she is going to do. In other words, the analysts are saying that we are smart, intelligent beings, but we
have no idea what determines our carrying capacity.
Again, this shows that common sense is a very scarce commodity in the establishment. People like
Cohen and Bailey are being penny wise and pound foolish; they can’t see the forest for the trees.
Perhaps we cannot predict what one buffalo will decide to do, but we can certainly predict whether most
of the individuals in the herd will stampede over the cliff. This is what density-dependent birth rates are all
about. They are predictable because they make allowances for behavioral extremes.
| A practical way to understand density-dependent birth rate is through the following analogy. Assume we have a box full of air-filled balloons. Each balloon represents the territory of a unit (individual, clan, pride, colony, etc.). The entire volume of the box represents the local or global carrying capacity in terms of food. The sphere of influence of each unit is inversely proportional to resource availability: the richer the territory, the smaller the average territory of each unit. Therefore, we can fit fewer balloons in a given box when the sphere of influence of each unit expands. The question that you should keep in the back of your mind now is: Why doesn't the box expand? |
| Urban women have no freedom to choose whether to have a baby |
| Adapted for the Internet from: Why God Doesn't Exist |
- Module main page: The mechanism behind background extinctions
Pages in this module:
- 1. The three population curves
2. All species eventually conquer the invisible bugs that keep their numbers back
3. Not education, but urbanization causes infertility
4. This page: Urban women have no freedom to choose whether to have a baby
5. An urban woman doesn't have the privilege or need to have a family
6. Density is what overturns a pyramid
7. Urbanization was inevitable