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.