Chapter 53 ~ Population Ecology

Dispersal and Distribution Several factors influence where species occur and why species are found in certain places Biotic factors – do other species limit the distribution of a species? Abiotic factors – do nonliving factors limit the distribution of a species? Temperature, water availability, oxygen levels, salinity, sunlight, rocks and soil (pH, mineral composition, ect)

Population Characteristics Density - number of individuals per unit of area Individual counts Sample size estimate Indirect indicators Mark-recapture Dispersion - pattern of spacing Random: unpredictable, patternless spacing Clumped: patchy aggregation Uniform: even spacing

Animals often exhibit uniform dispersion due to territoriality behavior

Demography: factors that affect growth & decline of populations Birth rate - number of offspring produced Death rate - number of individuals that die in a population Growth or decline in populations can be calculated by using the rate of change when comparing births and deaths Change in population over time is birth rate minus death rate

Demography Age structure diagram: relative number of individuals of each age and sex in a population at a given point in time

Age Structure Diagrams Analysis of age structure diagrams can determine if the population is experiencing an increase, decrease, or no population change A growing population would have higher numbers of younger ages indicating a high birth rate (true pyramid shape) A declining population would have lower numbers in the pre- reproductive age groups compared to the reproductive ages A stable population would have age groups generally the same lengths

Examples of Changes to Populations

Demography Survivorship curve: plot of the fraction of a population still alive at each age or from one age to the next

Survivorship Curves Type I = low death rates during early and midlife, then death rate increases sharply in older age Ex: humans and most mammals Type II = constant death rate over the organism’s life span Ex: Belding’s ground squirrel, other rodents, some lizards, and some annual plants Type III = very high early death rates, then a flat rate for the few surviving to older age Ex: most marine invertebrates, some plants, many fishes

Population Growth Models Models are used to describe the growth of populations Per capita rate of increase (r) Population ecologist are interested in the difference between the per capita (individual) birth and death rates and the effect on the population size Basically, ecologist want to know how many new individuals will be added to the population each year and what the new population numbers will be So, change in N (number in the population) over change in time = rN

Population Growth Models Exponential model (blue) Equation: dN/dt = rN Idealized population in an unlimited environment Utilized by r-selected species (r= per capita growth rate) Populations accumulate more new individuals per unit of time when it is large than when it is small giving the graph a J-curve shape

Population Growth Models Logistic model (red) Equation: dN/dt = rN (K-N/K) (K-N/K) represents the fraction of K that is still available for population growth Carrying capacity (K): maximum population size that a particular environment can support which creates the S- curve shape K-selected species utilize this type of model

Population Change Population dynamics are influenced by population density and life history traits Population limiting factors are either affected by or not affected by the density of a given population Density-dependent and density-independent factors can limit population size in different ways

Population limiting factor examples Density-dependent factors Competition Predation Intrinsic factors – physiological responses within individuals to change reproductive rates Territoriality Waste accumulation Disease Density-independent factors Weather/climate Periodic disturbances

Life History Life history includes traits that affect an organism’s schedule of reproduction and survival 3 Variables: When reproduction begins and age of sexual maturity How often the organism reproduces Some organisms save their resources for one big reproductive event Coral Spawning and Coral Spawning 2 The number of offspring during each reproductive event

Population life history strategies r-selected Maximize reproductive success in uncrowded environments (density- independent selection) Short maturation & lifespan Many (small) offspring; usually one (early) reproduction; no parental care High death rate K-selected Sensitive to population density changes (density-dependent selection) Long maturation & lifespan Few (large) offspring; usually several (late) reproductions; extensive parental care Low death rate