Lecture 8: Dispersal & Population Structure. Dispersion, dispersal, and migration. Type of dispersion pattern depends on the scale

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1 Lecture 8: Dispersal & Population Structure Dispersion, dispersal, and migration Types of dispersion patterns Ideal free distribution Types of movement How organisms disperse Why organisms disperse Population consequences of dispersal Dispersion, dispersal, and migration Dispersion: the spatial distribution of individuals in a population e.g. clumped, uniform or random distribution Dispersal: spreading or movement of individuals away from each other, usually during a particular life history stage e.g. pelagic larval stages of marine organisms, seeds of plants carried by wind Migration: mass movement of individuals, often entire populations, from one region to another e.g. seasonal migration of birds Types of dispersion patterns (within 1 location) Type of dispersion pattern depends on the scale Random Uniform; hyperdispersed Clumped; aggregated i.e. nest dispersion in tern colonies Within a colony Across an island 10 m 1 km Results from homogeneous environment; null Competition; territoriality Heterogeneous environment; sociality; seed shadows 10 m 1 km 1

2 Types of dispersion patterns (across locations) Both sites have similar densities Both sites have similar amount of resources, etc. One site has a greater density One site has more resources; One site has more predators, etc. Types of dispersion patterns (across locations) Ideal-free distribution: A form of densitydependence that can lead to a pattern of relatively higher density of a mobile organism at one site compared to another site One site has a greater density One site has more resources; One site has more predators, etc. Ideal-free distribution model Based on two factors that influence an individual s decision on where to settle: 1. Intrinsic quality of habitat/foot patch 2. Behavior of others Assumptions: every individual wants to do the best that they can there is no dominance or territoriality Ideal-free distribution model Problem: Good vs. Bad Habitat Prediction: Once all individuals have decided where to settle (i.e. equilibrium has been reached), everybody should get the same food intake rate How can we test this model?: Create 2 food patches each with different amount of food availability 2

3 Test of the ideal-free distribution model N. American Coots - B. Lyon Test of the ideal-free distribution model Good patch Prediction of experimental outcome: Good patch Actual results of experiment on coots: Bad patch Amount of food per individual Bad patch Ratio of Individuals at Good vs. Bad site N Time Types of movement Migration dispersal with a return to the place of origin (two-way movement) Natal dispersal movement of young away from their birthplace (e.g. in birds, marine organisms) Other types of dispersal Dispersal mechanisms Animals 1) Active: walk, fly or swim 2) Passive: air or water currents e.g. ballooning spiderlings, larvae of some marine organisms 3. Phoresy (hitchhiking) e.g. flower mites on hummingbirds Plants 1. Adhesion to animals e.g. sticky or velcro seeds 2. Floatation e.g. coconut seeds 3. Explosive projection e.g. touch-me-not 4. Bribery of animal dispersers e.g. fruits 5. Wind dispersal e.g. dandelion seeds 3

4 Evolutionary reasons for natal dispersal: 1. Find empty sites - avoid competition with kin 2. Habitat change - quality of habitat may change across generations - get away from relatives to avoid inbreeding costs 4. Other? Avoid predators? 1. Find empty sites (avoid kin competition) Hamilton and May theoretical model: Consider a strategy where parents produce babies that do not disperse Now consider a mutant strategy where parents send half of their kids out into world to seek fortune This new strategy can invade and potentially take over non-disperser populations Disperser: fraction x stays but y disperses Non-disperser: babies only replace parents 1. Find empty sites (avoid kin competition) Hamilton and May theoretical model: Evolutionary Stable Strategy (ESS): 1. Strategy that can invade all others when it is rare 2. Strategy that cannot be invaded by any other strategy, rare or not ESS is dispersal because dispersal is favored even in a stable predictable world However, the fitness of dispersing individuals is only higher initially when dispersal strategy is more rare than non-dispersal strategy 2. Habitat change Conditions now many not be the same later - resources can get used up - environment can change Example: dispersal polymorphisms - 2 types of offspring: 1) stay at home or 2) disperse - e.g. plants that can have both big and little seeds, insects that can have winged or wingless individuals in same species Ex: Dispersal polymorphism in water striders that depends on environmental predictability: winged vs. wingless 4

5 Avoid mating with a relative and avoid inbreeding depression Example: Rocky delphiniums (Wasser + Price) Assumption: - plants growing closer together are more likely to be closely related than those growing further apart - used distance as a proxy for relatedness Experiment on delphiniums in Rockies: - Hand pollinated plants with pollen from: very close, intermediate distance, far - Measured fitness with seed set (#seeds/plant) - Note: would have been better to track survival of offspring Population consequences of dispersal Sources and sinks Sink population: has negative population growth Source population: has positive growth Sink Source Bad habitats (sinks) can still be stable due to immigration from good habitats (sources) Population consequences of dispersal Sources and sinks Example w/ Blue tit (a chickadee) Habitat quality Population growth (based on b + d only) Density Dispersal Deciduous Forest Good (more food) λ = x 100x Evergreen Forest Bad (less food) λ = x 1x Note: not ideal-free because λ s not the same 5