45.3 Environmental Limits to Population Growth

Experimental animal models have been used in research into how animals allocate their energy resources.

The common fruit fly, Drosophila melanogaster, has been used in some of the work.
Studies have shown that the cost of reproduction is more than how long a male flies live, and that fruit flies that have already had multiple sexual partners have less sperm left for reproduction.
Fruit flies use optimal mates to maximize their chances of reproduction.

In 1981 male fruit flies were placed in enclosures with either virgin or inseminated females.

The life spans of males who were unable to mate with virgin females were shorter than those who were able to.
The effect was the same regardless of how large the males were.
Men who did not mate lived longer, allowing them more chances to find mates in the future.

The 2006 studies show how males select the female with which they will mate and how this is affected by previous matings.

The findings showed that larger females produced more offspring than smaller females.
The selection of partners was more pronounced in resource-depleted males than in non-resource-depleted males.
In order to maximize their chances for offspring, males with low sperm supplies were limited in the number of times they could mate before they were able to replenish their supply.
One of the first studies to show that the male's behavior affects its use of reproductive resources is this one.

Male fruit flies that had previously had sex picked larger, more fecund females than those that had not.

In the context of natural selection, the early death of an animal is not important because they have already reproduced.
When resources such as sperm are low, the behavior of the organisms can change to give them the best chance of passing their genes on to the next generation.
Behavioral biology, or ethology, is a discipline that studies the changes in behavior that are important to evolution.

Population ecologists use a variety of methods to model population dynamics, despite the fact that life histories describe the way many characteristics of a population change over time.

Due to their lack of predictive ability, certain long-accepted models are being modified or even abandoned, and scholars strive to create effective new models.

The English clergyman Thomas Malthus influenced Charles Darwin in his theory of natural selection.

Malthus wrote a book in 1798 that stated that populations with unlimited natural resources grow rapidly and then their population growth decreases as their natural resources are exhausted.

The division takes about an hour.
After an hour, if 1000bacteria are placed in a large flask with an unlimited supply of nutrients, there is one round of division and each organisms divides, resulting in 2000 organisms--an increase of 1000.
Each of the 2000 organisms will double in an hour.
The number of organisms in the flask should increase after the third hour.
The number of organisms added in each reproductive generation is increasing at a greater and greater rate.
The population would have increased from 1000 to 16 billion after 1 day and 24 cycles.

The growth rate will be lowered because somebacteria will die during the experiment and not reproduce.

The per capita birth rate "b" is the number of individuals "N" and the per capita death rate "d" is the number of individuals "N".
Ecologists are interested in the population at a particular point in time.
The "instantaneous" growth rate is obtained by replacing the change in number and time with an instant-specific measurement of number and time.

Different species have different rates of increase, even under ideal conditions, which is recognized by a further refinement of the formula.

Populations grow when resources are limited.
When the carrying capacity of the environment is reached, population expansion decreases as resources become scarce, leading to an S-shaped curve.

In the real world, exponential growth is not possible unless infinite natural resources are available.

In his description of the struggle for existence, Charles Darwin states that individuals will compete for limited resources.
Natural selection will allow the successful ones to pass on their own characteristics and traits to the next generation at a greater rate.

In environments where there are few individuals and plentiful resources, exponential growth may occur, but when the number of individuals gets large enough, resources will be exhausted, slowing the growth rate.

The expression "K - N" indicates how many individuals may be added to a population at a given stage, and "K - N" is the fraction of the carrying capacity available for further growth.

When N is small, the right side of the equation reduces to rmaxN, which means the population is growing and not influenced by carrying capacity.

When N is large, population growth will be slowed or even stopped if K comes close to zero.
The carrying capacity K slows population growth in large populations.
The carrying capacity is affected by the value of (K-N)/K.

Growth is exponential because there are few individuals.
The growth rate decreases as resources become limited.
Growth levels off at the carrying capacity of the environment, with little change in population size over time.

The model assumes that every person in the population will have the same chance for survival.

In animals, important resources include food, water, shelter, and mates, whereas in plants, important resources include water, sunlight, and space to grow.

Some people will be better adapted to their environment than others in the real world.

Resources are plentiful and all individuals can get what they need.
The competition increases as the population grows.
The carrying capacity of an environment can be reduced by the accumulated waste products.

As the population depletes the nutrients, its growth levels off.
There are variations to the idealized curve in the real world.
Animals in wild populations include sheep and harbor seals.
The population size exceeds the carrying capacity for a short time and then falls below it after a while.
As the population fluctuates around its carrying capacity, the population's size continues to change.
The model is confirmed even with this oscillation.