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

• Immigration–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

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-

– 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

• Richness–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

– 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