travismulthaupt.com chapter 52 population ecology

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Chapter 52

Population Ecology

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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.

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Population Ecology

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

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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.

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Dispersion

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

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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.

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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.

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Sampling Techniques

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

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Population Density

• The density is always changing.

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

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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.

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Three Common Patterns of Dispersal:

• 1. Clumped

• 2. Uniform

• 3. Random

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1. Clumpled

• 1. Organisms are in uniform patches

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2. Uniform• Organisms are evenly spaced.

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3. Random• Organisms exhibit

unpredictable spacing patterns. They could be grouped together, or there could be an uneven distribution pattern.

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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.

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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.

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Life Tables

• These are difficult to build and maintain.

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

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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).

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Survivorship Curves

• There are three types of survivorship curves:– 1. Type I– 2. Type II– 3. Type III

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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.

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2. Type II

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

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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.

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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.

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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.

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Reproductive Modes

• In general, there are 2 reproductive modes that are followed:– 1. Big bang reproduction--semelparity.– 2. Repeated reproduction--iteroparity.

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Reproductive Modes

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

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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.– Occurs when an organism

lives in a highly variable or highly unpredictable environment.

• Examples:– Salmon and agave plants.

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Iteroparity

• This is repeated reproduction. Organisms continually give rise to offspring throughout their lives.– Iteroparity is favored when environments

are more stable.

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Energy Constraints• Time, energy, and

nutrients cannot be used for one thing as well as something else.

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

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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.

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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

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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.

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Population Growth

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

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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.

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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

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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.

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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.

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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.

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Population Growth• Certain populations exhibit the Allee effect.• This 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.

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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?

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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.

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Population Growth

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

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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.

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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

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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.

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2. Territoriality

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

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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.

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4. Predation

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

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5. Toxic Waste

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

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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.

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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.

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Population Growth• Population dynamics focuses on the

interactions between biotic and abiotic features that cause population sizes to vary.

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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, disease, and parasites.

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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.

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Population

Cycles

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

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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.

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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.

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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.

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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.