species and communities - nc state university...species diversity and abundance a species is the...
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Species and CommunitiesSpecies
What is a species?
Population structure
Evolution of geographic variation
Gene flow and clines
Assortative mating
Local variation
Geographic isolation
Secondary contact and hybridization
Ecology of speciation
Geographic replacement
Behavior and speciation
Communities
Factors shaping patterns of
species diversity and abundance
A species is the smallest aggregation of populations diagnosable by a unique combination of character states in comparable individuals. The goal of the phylogenetic species concept is to define indivisible taxa that can be used in cladistic analysis.
BIOLOGICAL SPECIES CONCEPT (MAYR 1970, POPULATIONS SPECIES AND EVOLUTION)
PHYLOGENETIC SPECIES CONCEPT (CRACRAFT 1989, SPECIATION AND ITS ONTOLOGY)
“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”
Generalized model of speciation
Genetic Divergence
A A’ B
Isolation
Colonization
Fragmentation
Isolating Mechanisms
Reproductive
Ecological
Behavioral
Hybrids
Speciation is driven by changes in the spatial structure of populations
• Dispersal– Where young birds settle
to reproduce
• Philopatry– Tendency of birds to
return to their birth place
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Other factors that decrease effective population size
Fragmentation of populations into small isolated populations with limited dispersal
Founding of new populations by a small number of colonists
Existence of non-monogamous breeding systems
Dispersal processes determine effective population size
Deme - a reproductively cohesive population
Effective Population Size – number of individuals in a deme
The more genetically distinct individuals interbreeding the higher the effective population size
Therefore, the effective population size decreases with shorter juvenile dispersal distances
Small demes evolve faster (often by chance)
Geographic variation drives speciation
• Geographic variation in color and size is characteristic of about 1/3 of American birds
Opposing forces explain geographic differences
• Natural Selection– Changes in gene
frequencies caused by differential fecundity and survival
• Gene Flow– Genetic blending caused by
interbreeding
Proportion of red-phase Screech Owls
Assortative mating
• Assortative mating produces geographic variation– Preferred pairing of like
types
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Geographic isolation is an important component of speciation
• Large Scale (allopatric)– Colonization of islands
• Galapagos finches• Hawaiian Honeycreepers
– Habitat fragmentation• Toucans
• Small Scale (sympatric)• Reunion Gray White-eye
Secondary contact tests species
• Reproductive isolation limits effects of hybrids
• Stable hybrid zones can persist for centuries
• Ecological isolation is less resistant to hybridization
Hybrids occur when ecological isolation breaks down
• When ecologically similar species become sympatric one will usually replace the other
Oriole hybrids
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Blue-winged, Golden-winged Warblers
Lawrence’s Brewster’s
Behavioral isolating mechanisms promote speciation
• Speciation promoted by– Complex behaviors
– Enhanced brains
– Cultural transmission of information
What is a species?• Exploding Species (TREE
1996 11:314-315, 11:509)– Typical estimate for birds is
10,000 species– Molecular techniques and
better field studies may increase this to 20,000
– Authors argue that biological species concept needs revision “Failure to observe inbreeding does not mean lineages are completely independent; neither does interbreeding necessarily prohibit the continued divergence of two lineages in contact”
Communities• Factors shaping
patterns of bird species diversity and abundance are:– Time
– Energy
– Space
Robert H. MacArthur. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39:599-619.
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Bird diversity reflects habitat and prey diversity
• Vegetation structure correlated with bird species diversity
(Karr and Roth 1971)
• Prey size diversity associated with bird diversity
(Schoener 1971) Tropical Temperate
Character displacement indicates competition
• Character displacement refers to morphological or behavioral adaptations caused by competition.
• Gant’s (1986) study of Galapagos Finches is a classic example of character displacement driven by competitive exclusion. – When three species occur on
one island (Santa Cruz) they have distinct bill sizes. Populations on single species islands (Daphne Major) have intermediate bill sizes.
Other evidence of competition• Ecological displacement in
Caribbean hummingbirds
• Altitudinal displacement in Appalachian thrushes
• Ecological equivalents in European and North American tits
Patterns of species diversity
• Temperate – tropical gradients in diversity reflect:– More food
– Benign climate
– Stable environment
(MacArthur 1969)
Number of landbird species
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Island biogeography
• The number of species on an island reflects a balance between rates of immigration (colonization) and extinction.
• (A) Extinction rates increase and immigration rate decrease as the number of species present on an island increases.
• (B) The intersection of the two curves for any particular island size defines the expected equilibrium number of species (S).– Immigration rates on islands that are distant from source areas are lower
than rates on islands close to source areas
– Extinction rates on large islands are lower than rates on small islands
(MacArthur and Wilson 1967
Evidence of equilibrium dynamics
Sources and sinks
• Habitat patches on a landscape function as islands of varying habitat suitability
• Source habitats generate surplus productivity (R0>1), sink habitats cannot sustain stable populations (R0<1)
• Populations in source habitats show less variability than populations in sink habitats
• Fragmentation effects: Landscapes with larger and less isolated patches show less variability than landscapes with smaller or more isolated patches
Pulliam, H.R. 1988. Sources, sinks, and population regulation. The American Naturalist 132: 652-661.