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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