Chap 52 Population Ecology

oThe study of populations in their natural environment. oA populations environment affects the density, distribution, age structure and size.

Figure 52.2 Population dynamics Births and immigration add individuals to a population. Births Immigration PopuIation size Emigration Deaths Deaths and emigration remove individuals from a population.

I. Density and Dispersion oAll population sizes are limited by resources, space and mate availability. oDensity is not a static property. Birth, death, immigration and emigration greatly affect a population at any given moment. oHow the individuals within the population are spaced out indicates the dispersion of the population. oEnvironment and social interactions also contribute to variation in density

Figure 52.3 Patterns of dispersion within a population ’ s geographic range (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. (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. (c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

II. Life History results from Natural Selection The traits that affect an organism’s reproduction and survival. Life history patterns are very diverse. Semelparity = “one shot” pattern of reproduction. Iteroparity = repeated reproduction

Figure 52.6 An agave, or century plant

Population Growth and exponential model Life histories provide a quantitative model for the understanding of population growth. Population growth does not occur indefinitely for any species. Factors such as resources, mates and space limit population growth.

EQUATION TIME!!!!!!!!!!!

Exponential Growth Considers ideal conditions and unlimited resources. The per capita rate of increase may assume the maximum rate for the species  r max. The size of a population growing exponentially, increases at a constant rate, resulting in a J-shaped curve.

Figure 52.9 Population growth predicted by the exponential model ,000 1,500 2,000 Number of generations Population size (N) dN dt  1.0N dN dt  0.5N

Exponential growth assumes: 1.Continuous reproduction. 2.Environment is constant in space and time (unlimited resources). 3.All organisms are identical.

Logistic Growth Model As population density increases, each individual has access to fewer resources. There is a limit to the number of individuals that can occupy a habitat  carrying capacity (K). K is not fixed but fluxuates based on available resources.

If individuals cannot obtain sufficient resources to reproduce, (b) will decline. If individuals cannot not find and consume enough energy to maintain themselves (m) may increase. A decrease in (b) or increase in (m) results in a lower per capita rate of increase (r). Therefore the per capita rate of increase declines as carrying capacity is reached.

Figure Influence of population size (N) on per capita rate of increase (r) Maximum Positive Negative 0 N  KN  K Population size (N) Per capita rate of increase (r)

Logistic growth assumes: 1.Population growth rate declines when population #’s increase. 2.Population growth rate stops when N=K. 3.When N<K, population growth rates reach maximum  intrinsic rate of increase. 4.When N>K, resources are limiting and growth rates become negative.

Table 52.3 A Hypothetical Example of Logistic Population Growth, Where K  1,000 and r max  0.05 per Individual per Year

In the logistic model of growth, population increases the most during periods of intermediate size, when there is a substantial breeding population but also available space and resources available. Logistic models produce a sigmoid (s- shaped) growth curve.

Figure Population growth predicted by the logistic model dN dt  1.0N Exponential growth Logistic growth dN dt  1.0N 1,500  N 1,500 K  1, ,000 1,500 2,000 Number of generations Population size (N)

K/R Selection By connecting the favored life traits to the logistic model of population density, scientists have developed these terms. K selection (density dependent)  selection of life traits that are sensitive to population densities. R selection (density independent)  favors life traits that maximize reproductive success in low density environments.

K selection tends to maximize population size and operates in populations living at/near carrying capacity (K). R selection tends to maximize the rate of increase (r) and operates in populations that fluxuate well below K.

r - unstable environment, density independent K - stable environment, density dependent interactions small size of organismlarge size of organism energy used to make each individual is lowenergy used to make each individual is high many offspring are producedfew offspring are produced early maturity late maturity, often after a prolonged period of parental care short life expectancylong life expectancy each individual reproduces only once individuals can reproduce more than once in their lifetime type III survivorship pattern in which most of the individuals die within a short time but a few live much longer type I or II survivorship pattern in which most individuals live to near the maximum life s

Abiotic/Biotic Influence Populations How do we regulate a population? What environmental factors stop populations from growing? Why do some populations show change over time while others remain stable? Population dynamics focus on the abiotic/biotic factors that cause variation in population size.

To regulate population growth a negative feedback mechanism must exist between density and the vital rates of birth and death. Many factors can cause a change in birth and death rates: 1.Competition for resources 5. Predation 2.Territoriality 6. Intrinsic factors 3.Health 4.Toxic waste

Age Structure Diagram that shows present growth trends is a country’s age structure. Commonly represented in “pyramids”. Age structures not only predict a populations growth trends but also give a look into social conditions.

Figure Age-structure pyramids for the human population of three countries (data as of 2003) Rapid growth Afghanistan Slow growth United States Decrease Italy Male Female Male FemaleMale Female Age Percent of population 80–84 85  75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19 80–84 85  75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19