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Population Ecology Chapter 43 Population and Community Ecology • Ecology is concerned with the interaction of organisms with other organisms and the physical environment. – These interactions determine the distribution and abundance of organism over Earth’s surface • Ecologists often study a population of a particular species and examine how this population interacts or is affected by other organisms (the community) and the physical environment • Population - all the organisms within an area belonging to the same species • Community - all the various populations interacting in a particular area Populations • Population can be described by: – Number of organisms – Density – Distribution • Clumped • Random • uniform – Growth rate – Mortality pattern – Age distribution • Age structure diagrams – Life Histories Availability of resources can limit population limiting factors Demographics of a Population • Growth rate = birth rate - death rate • Biotic potential - highest possible rate of growth for a population when resources are unlimited – Life table - follows a cohort, a group of individuals of same age throughout lifetime – Survivorship curves - show the number of individuals that are still living over time from a particular generation • Type I • Type II • Type III • Age structure diagrams - graphically illustrate relative abundance of individuals in population at various ages - immediate patterns illustrate if growth rate increasing, decreasing, or stable Life History • The traits that affect an organism’s schedule of reproduction and survival make up its life history. • The life-histories represent an evolutionary resolution of several conflicting demands. – Sometimes we see trade-offs between survival and reproduction when resources are limited. • Limited resources mandate trade-offs between investments in reproduction and survival The Principle of Allocation The exponential model of population describes an idealized population in an unlimited environment • We define a change in population size based on the following verbal equation. Change in population size during time interval = Births during time interval Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings – Deaths during time interval • Using mathematical notation we can express this relationship as follows: – If N represents population size, and t represents time, then N is the change is population size and t represents the change in time, then: • N/t = B-D • Where B is the number of births and D is the number of deaths Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings – We can simplify the equation and use r to represent the difference in per capita birth and death rates. • N/t = rN OR dN/dt = rN – If B = D then there is zero population growth (ZPG). – Under ideal conditions, a population grows rapidly. • Exponential population growth is said to be happening • Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth dN/dt = rmaxN Population Growth Models • Exponential growth – J shaped curve • Lag phase • Exponential growth phase – Requires unlimited resources rN is sometimes called the biotic potential of the population (R = birth rate - death rate) dN/dt = rmaxN The logistic model of population growth incorporates the concept of carrying capacity • Typically, unlimited resources are rare. – Population growth is therefore regulated by carrying capacity (K), which is the maximum stable population size a particular environment can support. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Population Growth Models • Logistic growth – S shaped curve • • • • Lag phase Exponential growth phase Deceleration phase Stable equilibrium phase – Carrying capacity (K) - maximum number of individuals any environment can support Growth stops when N = K. • The logistic population growth model and life histories. – This model predicts different growth rates for different populations, relative to carrying capacity. • Resource availability depends on the situation. • The life history traits that natural selection favors may vary with population density and environmental conditions. • In K-selection, organisms live and reproduce around K, and are sensitive to population density. • In r-selection, organisms exhibit high rates of reproduction and occur in variable environments in which population densities fluctuate well below K. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Reproduction Strategies and Life History • An organism’s life history strategy describes how it allocates time and energy among the various activities throughout its life. r-strategists—life history strategies that allow for high intrinsic rate of increase. K-strategists—life history strategies allow them to persist at or near the carrying capacity. Exponential Growth Can Not Last • Why do all populations eventually stop growing? • What environmental factors stop a population from growing? • The first step to answering these questions is to examine the effects of increased population density. Regulation of Population Size • Both biotic and abiotic factors play a role in regulation population size – Density-independent factors • Generally abiotic factors like weather, natural disaster that alter population size – Density-dependent factors • increase their affect on a population as population density increases. • This is a type of negative feedback. – Biotic factors like predation, disease, competition, parasitism, limiting resources (food, shelter) – Again, remember, limited resources imply competition within members of population and sometimes between different species Negative feedback prevents unlimited population growth • A variety of factors can cause negative feedback. – Resource limitation in crowded populations can stop population growth by reducing reproduction. • Intraspecific competition for food can also cause density-dependent behavior of populations. – Territoriality. – Predation. – Waste accumulation is another component that can regulate population size. • In wine, as yeast populations increase, they make more alcohol during fermentation. • However, yeast can only withstand an alcohol percentage of approximately 13% before they begin to die. – Disease can also regulate population growth, because it spreads more rapidly in dense populations. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Population dynamics reflect a complex interaction of biotic and abiotic influences • Carrying capacity can vary. • Year-to-year data can be helpful in analyzing population growth. • Some populations fluctuate erratically, based on many factors. Fig. 52.18 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Other populations have regular boom-and-bust cycles. – There are populations that fluctuate greatly. – A good example involves the lynx and snowshoe hare that cycle on a ten year basis. Population Densities are Dynamic and Interconnected