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L2A Determinants of Distribution and Abundance (SS) • Habitat is a broad term, niche is more defined • Realised niches are affected by competition and interaction • Species are limited in their distribution by the set of biotic and abiotic conditions that define their niches. • Transplant experiments can help identify the potential range of a species.

Transplant Experiments • Means of determining if species absence is due to being outside its fundamental niche • Moves individuals to unoccupied area Þ see if they can survive and reproduce

successfully (for generations) • Gives info on WHY species aren’t occupying an area (e.g. competition) • Transplant Success = the fundamental niche (potential range) must be larger than the

realised niche (actual range).

• Barrier to range change: If species doesn’t occupy potential range, can it move there or not? If not = dispersal

barrier • Some species can move, but don’t = habitat selection barrier • If the transplant was unsuccessful = other species or abiotic factors are the barrier • Need for a control (within species distribution) Þ To determine the effects of handling and transplanting

(monitor through multiple generations)

Dispersal • Dispersal: movement of individuals away from their place of birth #Migration: mass directional movements of large numbers of a species back and forth between habitats • Dispersal = simplest explanation of species absence from an area, occurs on different spatial and temporal scales • Adaptations for dispersal = common Þ few species are limited on a local scale by their ability to disperse • Human impacts on dispersal are a major conservation issue

Who disperses? • Most species can disperse • Many species have a dispersive stage in their life cycle → large scale dispersal

• E.g. sessile (adult in one spot) but have a moving phase (e.g. larva of sea urchins) Who has limited dispersal? • Seeds dropping directly from trees – good micro conditions (will wait for parental death then grow up) • Flightless birds and insects from New Zealand

2 Dispersal is a trade-off • Provides a way for individuals to leave a crowded site and colonise an empty one • Most dispersing organisms die (unfavourable conditions) • \ Trade-off between staying at home (suitable but competitive) or dispersing (and taking a risk on survival)

Types of dispersal • Diffusive dispersal: Movement across hospitable terrain over generations

• Rapid expansion if few individuals can disperse much farther than the average (e.g. Starlings) • E.g. Cane toad spread (at the forefront are the bigger/ stronger individuals)

• Jump dispersal: Movement within an individual’s lifetime across unsuitable terrain (e.g. Ballast water from ships) • Secular dispersal: Movement over geological time + species undergoes evolutionary change (e.g. canid (dog)

expansion)

Spatial Scales of Colonisation • Dispersal plays a key role in colonisation! Local scale • Dispersal rarely limits local distributions as almost all organisms can

disperse • E.g. recolonization of Krakatau Islands after volcanic eruption – many

early colonising species were wind and sea-dispersed. Later colonisers were dispersed by animals (e.g. birds and bats)

Global scale • Barriers to dispersal = key role in explaining global distribution

patterns of species • Dispersal limitation: the absence of a population from a suitable habitat due to barriers to dispersal (e.g.

mountain range, ocean) (e.g. Australian desert separating bird species from East to West coast)

Habitat Selection • Can restrict the distribution of organisms • Operates via behavioural decisions at individual level and several spatial scales • Evolved because individuals in the better habitats have more offspring • Hierarchical structure of decision making when dispersing = destination→ habitat → micro habitat • Species may be able to disperse to an area, however may not survive/remain there • Related to body size → smaller species prefer cover (closed habitat) (e.g. desert mouse) • E.g. Mosquitoes absent in rice fields where plants > 30cm (can’t do buzz dance) – seen in transplant experiment Evolution of Habitat Selection Preference • Natural selection favours individuals that select habitats in which the

most offspring can be successful raised • Individuals choosing poor habitats will fledge fewer offspring and be

selected against • Selection can be directional (e.g. on right)

Habitat Selection Models 1. Ideal Free Distribution Model

• Individuals are free to move into any habitat without constraint

• Predicts that fitness at high densities in good habitats = low density in poor habitats

3 2. Ideal Despotic Distribution Model

• Individuals constrained to lesser habitats by the aggressive behaviour of others • Density will not be lower in marginal habitats • Breeding success is lower in marginal habitats than ideal ones #Despot = a tyrant or oppressor

Example: Decision rules for habitat selection in birds • Ideal Free Model:

• Later arrivals choose pre-occupied sites as this signifies suitability • Higher densities provide protection from predators which is traded off against lower fitness

• Despotic Model: • Early arrivals pre-empt breeding places and displace later arrivals • Later arrivals get sub-optimal breeding places and so have lower fitness