LECTURE 24 CH 13-15 SPATIAL STRUCTURE/DYNAMICS OF POPULATIONS
Distribution (range)
Range is seldom continuous or static
Invading species can spread rapidly beyond their initial range
Most species have small ranges
Ranges of mammals increase with increasing latitude
Species with wider distributions have greater abundance
Geographic distributions of populations defined on different spatial scales
Factors limiting geographic distribution
Dispersal, behavior, other species, abiotic factors, chance, history
Over the distribution, much spatial variation in population density
Density (no. / unit area)
Depends on suitable habitat
Declines as body mass increases (log – log) for birds and mammals
At a given biomass, birds have lower density than mammals
Dispersion (spacing of individuals with respect to others in population)
Clumped, random, uniform (even)
Factors affecting dispersion
Habitat heterogeneity, dispersal limitation, reproductive mode, social interactions
Effects of habitat heterogeneity (patches) on populations and species interactions
Landscape ecology: effects of size and arrangement of habitat patches
Fragmentation: creation of habitat patches
Edge effects: organisms have different levels of sensitivity to edge vs. interior
Smaller fragments have greater proportion of area as edge habitat
How large a fragment is necessary to maintain a population?
Species interactions (e.g. nest parasitism) affected by fragmentation + edge effects
Spatial dynamics of populations
Habitat patchiness  3 models of populations
Metapopulation: matrix is unsuitable for species; migration between habitats
Source-sink: source (B>D  emigration); source (D>B  immigration)
Landscape: patches offer different resources; species uses multiple patches
suitability of landscape influences migration between patches
Metapopulation: subpopulations in different patches with some migration between
What determines if matrix is a barrier to movement?
Distance between patches, nature of matrix, mobility of species
Small, isolated populations with no migration have different genetic structure
Patches differ in size, habitat quality, degree of isolation (corridors)
Possible patterns of migration between patches
Metapopulation dynamics depends on
Birth and death of subpopulations
Migration between patches
Colonization to form new subpopulations
Extinction of existing subpopulations
Connectivity (corridors) to allow migration
Corridors enhance migration and maintain population cohesion
Factors determining patch occupancy
Patch size
Patch isolation
Rescue effect: immigration from large subpopulation keeps a declining population
from going extinct (source provides emigrants to sink)
Small populations: greater risk of extinction due to
Stochastic events
Subpopulations more isolated
Subpopulations with more synchronized fluctuations
Deterministic models
Based on large size; no variation in average birth and death rates
Stochastic (random) models
Randomness affects populations
Catastrophe
Temporal variation in environment
Stochastic (random sampling) processes
Chance events may cause small populations to go extinct
Probability of extinction
increases over time
decreases with larger initial population size
Small populations can go extinct due to random fluctuations in population size