Population and Community Ecology

Population Characteristics Density- # of individuals per unit of area Determined by… counts sample size estimate indirect indicators mark-recapture Dispersion- pattern of spacing Types: Random- unpredictable, patternless spacing (c) Clumped- patchy aggregation (a) Uniform- even spacing (b)

Immigration vs. Emigration Movement into an area Emigration Movement out of an area

Demography: factors that affect growth & decline of populations Birthrate (natality, fecundity)- # of offspring produced Death rate (mortality) Age structure- relative number of individuals of each age Survivorship curve- plot of numbers still alive at each age

Types of Survivorship Curves Type I Relatively low death rates until later in life Ex: humans Type II Constant death rate throughout lifespan Ex: lizards Type III More death of young individuals Ex: Sea turtles

Population Growth Models Exponential model (blue)   idealized population in an unlimited environment (J-curve) r-selected species (r=per capita growth rate) Logistic model (red) carrying capacity (K): maximum population size that a particular environment can support (S-curve) K-selected species

Mathematical Models N = population size t = time Exponential Growth d = discrete/fixed time interval r = per capita rate of increase r = births – deaths If r > 0, population is growing If r < 0, population is declining K= carrying capacity Exponential Growth Logistic Growth

Life History “Strategies” r-selected (opportunistic) Short maturation & lifespan Many (small) offspring; usually 1 (early) reproduction; no parental care High death rate K-selected (equilibrial) Long maturation & lifespan Few (large) offspring; usually several (late) reproductions; extensive parental care Low death rate

Population Limiting Factors Density-dependent factors competition predation stress/crowding waste accumulation Density-independent factors weather/climate periodic disturbances

Community Ecology Community an assemblage of populations living close enough together for potential interaction

Community Structure Richness (number of species) & abundance Species diversity Hypotheses: Individualistic- chance assemblage with similar abiotic requirements Interactive- assemblage locked into association by mandatory biotic interactions

Interactions Interspecific- interactions between populations of different species within a community: Predation Includes herbivory and parasitism Competition Commensalism Mutualism

Predation Defense Cryptic (camouflage) coloration Aposematic (warning) coloration Mimicry- superficial resemblance to another species Batesian- palatable/ harmless species mimics an unpalatable/ harmful model Mullerian- 2 or more unpalatable, aposematically colored species resemble each other

Competition: a closer look Interference- actual fighting over resources Exploitative- consumption or use of similar resources Competitive Exclusion Principle- 2 species with similar needs for the same limiting resources cannot coexist in the same place Gause experiment

The Niche Ecological niche- the sum total of an organism’s use of biotic and abiotic resources in its environment; its “ecological role” Fundamental- the set of resources a population is theoretically capable of using under ideal conditions Realized- the resources a population actually uses 2 species cannot coexist in a community if their niches are identical

Competition Evidence Resource partitioning- Character displacement- sympatric species consume slightly different foods or use other resources in slightly different ways Character displacement- Allopatric species are similar Sympatric species show morphological differences

Species Richness and Diversity Total number of different species Relative Abundance Proportion each species represents of the total individuals

Trophic Structure Transfer of food energy through a community About 10% of the energy can be transferred from one level to the next Food Chain- linear feeding relationship Food Web- shows all the possible feeding relationships

Arctic Food Web

Special Species Dominant Species Keystone Species Foundation Species Most abundant Keystone Species Strong control on community structure Not necessarily most abundant Foundation Species Cause physiological changes to community

Succession Ecological succession- transition in species composition over ecological time Primary begun in lifeless area; no soil, perhaps volcanic activity or retreating glacier Secondary an existing community has been cleared by some disturbance that leaves the soil intact