Chapter 52

Population Ecology

Population Ecology• Population ecology is the study of the populations and their interactions with the environment.

• It also explores how the environment influences these populations in terms of size, age structure, and distribution.

Population Ecology

• Ecologists usually begin an investigation of a population by defining appropriate parameters such as density and dispersion.

Density

• How many individuals live within a given area.

• To determine the density of individuals, it is possible to count all of the organisms within a given area, but it is not likely.

Dispersion

• Dispersion is the spacing patterns among individuals within the boundaries of a population.

Sampling Techniques• Usually a wide variety of them are used.

• Scientists can count all individuals in a given area.

• They can do this in a number of different spots.

• Then, average all of the numbers together to make educated estimates about the population density.

Sampling Techniques• Scientists also employ the mark and recapture method.

• Animals are captured, marked, and released.

• The animals can then be tracked or captured at a later date.

• Density and distribution can be studied.

Sampling Techniques

• These methods are okay, but sometimes the data becomes unreliable because the organisms you are studying behave differently during study.

Population Density• The density is always changing.

• Birth, death, immigration, and emigration are ways a population changes.

Dispersal Patterns

• There are varying dispersal patterns of organisms within a population’s geographic range.

• These variations in local populations are extremely important to ecologists.

Three Common Patterns of Dispersal:

• 1. Clumped• 2. Uniform• 3. Random

1. Clumpled

• 1. Organisms are in uniform patches

2. Uniform• Organisms are evenly spaced.

3. Random• Organisms exhibit unpredictable spacing patterns. They could be grouped together, or there could be an uneven distribution pattern.

Demography

• Demography is the study of the vital characteristics of a population.

• For example:– Ecological needs– Spacing of individuals– Interactions of individuals within a population.

Demographers• These are people who study populations.

• They develop life tables to determine the survival pattern of a population.

• The use a cohort--a group of individuals of the same age that are followed from birth to death.

Life Tables

• These are difficult to build and maintain.

• It is easier to graphically depict a life table--a survivorship curve.

Survivorship Curves• These typically involve 1000 individuals from a population.

• The numbers are obtained by multiplying the surviving population by 1000 each year.

• Plotting these numbers vs. age indicates the death rate (or life expectancy).

Survivorship Curves

• There are three types of survivorship curves:

• 1. Type I• 2. Type II• 3. Type III

1. Type I• Type I curves start flat indicating a low death rate for early and middle life.

• They decline sharply as individuals get older indicating a high death rate.

2. Type II

• Type II curves exhibit relatively constant death rates from birth to death.

3. Type III• Type III curves see death rates very high in the beginning but as the animals grow and mature the death rates level off.– Example: animals that produce many young and provide little or no care for them.

Life Histories

• Life histories are products of natural selection.

• The traits that affect an organism’s schedule of reproduction and survival comprise its life history.

Life Histories

• There are three basic variables that life histories entail:

•1. When reproduction begins.•2. How often the organism reproduces.•3. How many offspring are produced during a reproductive cycle.

• For the most part, life histories are the product of evolutionary outcomes because most animals don’t choose when to reproduce.

Reproductive Modes

• In general, there are 2 reproductive modes that are followed:

• 1. Big bang reproduction--semelparity.

• 2. Repeated reproduction--iteroparity.

Reproductive Modes

• The evolutionary events that favor these are determined by the environment.

Semelparity• This is a “one and done” scheme for reproduction.

• The organism takes a big chance.

• Favored when the survival rate of the offspring is low.

• When an organism lives in a highly variable or highly unpredictable environment.

• Examples:– Salmon and agave plants.

Iteroparity

• This is repeated reproduction. Organisms continually give rise to offspring throughout their lives.

• Iteroparity is favored when environments are more stable.

Energy Constraints• Time, energy, and nutrients cannot be used for one thing and something else.

• This is the tradeoff that prevents producing a large number of offspring very frequently.

Population Growth• Unchecked population growth is considered exponential.

• There are mechanisms that prevent exponential population growth.

• This can be estimated using mathematical equations to describe the per capita growth rate.

Population Growth• It essentially boils down to the rate being equal to the number of births minus the number of deaths.

• r = b-m• r>0 the population is increasing• r<0 the population is decreasing• r=0 no change

Population Growth• Because resources are limited populations cannot grow exponentially forever.

• Ecologists try to identify the carrying capacity of an environment, K.

• K is the maximum population size an environment can support.

Population Growth

• To account for changes in the environment, scientists have created a logistic growth model to explain how populations vary in size.

Population Growth• The exponential growth model is used as a starting point.

• We add information about the environment that acts to reduce the per capita rate of increase.

• If K is the maximum, K-N is the number of individuals the environment can accommodate.

• N is the population size.

Population Growth• (K-N)/K is the fraction of the carrying capacity available for population growth.

• Multiplying by the maximum rate of increase of the population, rmax, allows us to modify the growth rate of the population as its size increases.

• rmax N (K-N)/K

Population Growth

• When N = K, the population stops growing.

• The logistic model will produce an S-shaped (sigmoid) growth curve when population size is plotted over time.

Population Growth• New individuals are added at the highest rate at intermediate population sizes.

• This is when the breeding population is of substantial size and space and resources are abundant.

• As N approaches K, the population size slows.

Population Growth

• The logistic model.• This incorporates the idea that every individual added to the population has the same negative effect on population growth.

• This is not true.

Population Growth• Certain populations exhibit the Allee effect.

• Describes a situation where individuals may have a difficult time surviving and reproducing when the population gets too small.

• The logistic model fits few, if any, real populations.

• It serves as a good starting point.

Population Growth• There are two general questions that are asked when studying population growth:

• 1. What environmental factors stop a population from growing?

• 2. Why do some populations show radical size fluctuations while others stay stable?

Population Growth

• To understand the answers to these questions, we have to:

• Examine the birth and death rates

• Immigration and emigration• How these factors affect population density.

Population Growth

• If immigration and emigration are equal, then birth and death rates will affect population size.

Population Growth• A birth rate or death rate that does not change with population density is said to be density independent.

• A death rate that rises when population density rises is said to be density dependent--an example of negative feedback--it halts population growth.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Population Growth

• 1. What environmental factors stop a population from growing?– 1. Competition for resources– 2. Territoriality– 3. Health– 4. Predation– 5. Toxic waste– 6. Intrinsic Factors

1. Competition for Resources

• This occurs when population density increases and organisms compete for resources.

• This results in a reduction in the number of offspring an individual produces.

2. Territoriality

• Territory is a resource and when available space is limited, population density decreases because reproduction is limited.

3. Health

• When population density is high, transmission of disease is easier and more likely.

• If more of the population contracts the disease and dies, the population density will decrease.

4. Predation

• In terms of predation, as the amount of prey increases, the predator eats more and the prey population declines.

5. Toxic Waste

• As levels of toxic wastes increases (waste that is toxic to the organism), the size of the population decreases.

6. Intrinsic Factors

• There are intrinsic factors tied to the behavior of organisms that result in a change in behavior that alters reproduction rates.

• Examples include aggressive behavior and hormonal changes.

Population Growth• The second major question: 2. Why do some populations show radical size fluctuations while others stay stable?

• To understand population stability, researchers often look at population dynamics and how numbers vary from year to year.

Population Growth• Population dynamics focuses on the interactions between biotic and abiotic features that cause population sizes to vary.

Population Growth

• For instance, stability and fluctuation are governed by complex interactions with the environment.– For example: the moose population fluctuates due to harsh winters, lots of snow, predation by wolves.

Population Growth

• All populations go through cyclic fluctuations in size.

• Some cycles are very short, others are longer.

• A famous example is that of the 10 year cycle of the snowshoe hare and the lynx.

Population

Cycles• Lynx are specialist predators, they feed on hares so their populations rise and fall with the hare.

Population Cycles• There are three hypotheses have been proposed to describe the 10-year cycle of these populations:

• 1. The cycles may be caused by a shortage of food in the winter.

• 2. The cycles may be due to predator-prey interactions.

• 3. The cycles may be a combination of the two.

Hypothesis #1• This hypothesis has been discarded because for over 20 years researchers have studied the population dynamics.

• Adding food to the environment increased the numbers of hares, and the population still followed the natural fluctuations.

Hypothesis #2 & #3• These two hypotheses are supported because experiments have revealed that nearly all hares were killed by predators and none died of starvation.

• Also, when predators were eliminated, food was an added factor, especially in the winter.

Another Factor…

• Another factor contributing to the crash of the predators is that when food becomes scarce, lynx often turn on themselves.

• This shows that this cycle is not just a hare-lynx cycle.