Population Ecology

Population Ecology Chapter 52

Overview: Earth’s Fluctuating Populations • Human population growth is a current hot topic • Population ecology – the study of populations and how they relate to the environment

Concept 52.1: Dynamic biological processes influence population density, dispersion, and demography • Population – group of individuals of a single species that live in the same general area • Rely on same resources • Influenced by similar environment • High level of interacting • High likelihood of breeding with one another

Every population has a specific size and geographical boundaries • Density - # of individuals per unit area • Dispersion – pattern of spacing among individuals in the same geographical boundary • These factors are very difficult to measure

We can count individuals, but we usually estimate populations • Use sampling techniques • Count individuals in randomly selected plots  calculate average density  estimate population size for entire area • Usually pretty accurate when many samples are taken

Mark-recapture method • Individuals captured, marked with tag, recorded, and released • Wait a long time  capture and identify again • Capture both marked and unmarked • Estimate pop by assuming both marked and unmarked have same probability of being captured

Density is a result from many processes that interact • Additions: birth and immigration • Removal: death and emigration • Density is influenced by environmental and social factors

(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory. Figure 52.3a • Clumped dispersion • Individuals aggregate in patches • Influenced by resource availability and behavior

(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors. Figure 52.3b • Uniform dispersion • Individuals evenly distributed • Influenced by social interactions like territoriality

Random dispersion • Position of each individual is NOT influenced by others (c) Random. Dandelions grow from windblown seeds that land at random and later germinate. Figure 52.3c

Demography – the study of the vital statistics of a population and how they change over time • Death and birth rates most important

Life table – age-specific summary of the survival pattern of a population • Built by determining the number of individuals that die in each age group • Calculate the surviving proportion

Represented by a survivorship curve • A plot of the numbers of individuals in a study of 1000 still alive at each age • Natural populations show 3 different patterns

Type I – relatively flat at start • Low death rate in early and middle life • Death rates increase in older age groups • Ex) humans and other large mammals

Type II • Intermediate • Constant mortality over life • Ex) rodents, invertebrates, some plants

Type III • Drops fast at start • Reflects very high death rates in early life • Flattens out at ‘critical age’ • Ex) fish • Have large numbers of offspring, but provide little/no parental care

1,000 I 100 Number of survivors (log scale) II 10 III 1 0 50 100 Percentage of maximum life span

Table 52.2 • Reproductive table • Also called fertility schedule • Age-specific summary of the reproductive rates in a population • Describes reproductive patterns of a population

Concept 52.2: Life history traits are products of natural selection • Natural selection favors traits that improve an organism’s chances of survival and reproductive success • There are trade-offs between survival traits • Frequency of reproduction • # of offspring • Investment in parental care

Life histories have 3 basic variables: • When reproduction begins • How often the organism reproduces • How many offspring are produced during each reproductive episode

“Big-bang” reproduction • Reproduce a single time and die • Aka semelparity • Ex) agave plant

Some organisms produce only a few offspring during repeated reproductive episodes • Aka iteroparity

Organisms have finite resources • Leads to trade-offs between survival and reproduction • Ex) dandelions • Produce large number of small seeds • Ensures at least some of them will grow and eventually reproduce

Other types of plants produce a moderate number of seeds • Provide a large store of energy that help seedling become established

Brood size also influences trade-offs • Primates = extended period of parental care, few offspring • Rodents = little parental care, lots of offspring

Concept 52.3: The exponential model describes population growth in an idealized, unlimited environment • All populations have a tremendous capacity for growth • Always limited by environmental constraints • Studying populations need to happen in an idealized, unlimited environment • Reveals capacity of a species to increase

dN  rN dt • Must ignore immigration and emigration • So, a population’s growth rate equals birth rate – death rate • Zero population growth occurs when the birth rate equals the death rate • Equation:

N = population size • t = time • r = per capita rate of increase • Growing: r > 0 • Declining: r < 0 • Zero population growth: r = 0 • d = change

In other words, the amount of change in the total number of organisms in the population (dN) during a given period of time (dt = change in time) will be equal to the population growth rate (r) times the original number of organisms (N)

Population growth under ideal conditions is called exponential population growth • The rate of reproduction is at its maximum • Called the intrinsic rate or increase

Equation of exponential population growth is: dN rmaxN  dt

Exponential growth increases at a constant rate • Results in a J-shaped curve

The J-shaped curve of exponential growth is characteristic of some populations that are rebounding 8,000 6,000 Elephant population 4,000 2,000 0 1900 1920 1940 1960 1980 Year

Concept 52.4: The logistic growth model includes the concept of carrying capacity • Exponential growth cannot be sustained for long in any population • A more realistic population model limits growth by incorporating carrying capacity

Carrying capacity (K) is the maximum stable population size that an environment can support • Varies with resources • Shelters • Soil nutrients • Water • Nesting sites • Most important: energy limitation

In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached

We construct the logistic model by starting with the exponential model • Then add an expression that reduces the rate of increase as N increases Maximum Per capita rate of increase (r) Positive N  K 0 Negative Population size (N)

The logistic growth equation includes the carrying capacity (K):

K – N is the number of additional individuals the environment can accommodate • (K – N) / K is the fraction of K that is still available for population growth

Table 52.3

Population growth is greatest when the population is approximately half of the carrying capacity • Many reproducing individuals • Per capita rate of increase remains high

The logistic model produces a sigmoid (S-shaped) growth curve • New individuals added rapidly at intermediate sizes • Rate slows dramatically as N approaches K

Fairly accurate • Logistic model assumes that populations adjust to and approach the carrying capacity smoothly • In natural populations, there is a lag time before negative things happen • Usually overshoot their K before settling down to a stable density

Some populations fluctuate greatly • Makes it difficult to define carrying capacity

The logistic model fits few real populations • Useful for estimating possible growth • Conservation biologists use to manage populations • Also use to establish sustainable harvest rates

Life history traits favored by natural selection vary with population density and environmental conditions • High population density: traits favored that help organisms survive and reproduce with few resources • Iteroparity • Low population density: Traits favored that promote rapid reproduction • semelparity

K-selection, or density-dependent selection • Selects for life history traits that are sensitive to population density • Maximizes population size • Operates in populations living near K

r-selection, or density-independent selection • Selects for life history traits that maximize reproduction • Maximizes the rate of increase • Occurs in environments well below K or when there is little competition

Different populations of the same species may show both selection characteristics • Depends on conditions • Both are criticized by ecologists as being oversimplified

Concept 52.5: Populations are regulated by a complex interaction of biotic and abiotic influences • There are two general questions we can ask about regulation of population growth • What environmental factors stop a population from growing? • Why do some populations show radical fluctuations in size over time, while others remain stable?

Population Ecology