Island Biogeography - Conservation Biology - Lecture Slides, Slides of Biology

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Landscape Ecology Lecture 3: Theories & Models Island biogeography, metapopulations & the source-sink theory Docsity.com Island biogeography theory • Developed originally in 1963 by MacArthur & Wilson, & further developed by these & others • Influenced understanding of spatial influences on organisms • For a while, it was the principle design paradigm for conservation reserves • “The number of species on an island will reach an equilibrium that is positively related to island size & negatively related to distance from mainland” • Hence, large islands have more species • Islands distant from the mainland have fewer species (far from the source of new colonists) Docsity.com Island biogeography: criticisms • Criticisms: – assumption of equilibrium (can take a long time) – Other factors may affect diversity on a fragment: • resistance to invasion (eg: heathland remnants: Webb & Vermaat, 1990) • habitat quality/ interspecific competition (Hanski, 1981) • catastrophes (eg: hurricanes) may dominate extinction rates, independent of size (Ehrlich et al., 1980) • trophic dynamics. (eg): Bahamian spider distributions follow IB predictions unless predatory lizards are present. Otherwise predation drives extinction rates (Toft & Schoener, 1983) • Despite this, IB was the primary concept in reserve design until the evolution of metapopulation models in the 1980s Docsity.com Metapopulation model • Most populations have a finite probability of extinction m which is greater than 0 • This implies that all populations will go extinct on a large enough time frame • Fragmentation can therefore benefit a species, allowing recolonization from neighbouring populations • This creates a locally dynamic, but regionally stable population • This regional population, or collection of local populations, was termed a metapopulation by Levins (1969) • This depends on the ability to maintain an exchange of species Docsity.com Metapopulation model • p = proportion of locations colonized at time t • c = probability of colonization • m = probability of extinction • Populations persist regionally only if m < c • This model allows assessment of damage to regional populations by habitat destruction mpc p d t d p −−= )1( Docsity.com Metapopulation model • Example: in Rana lessonae populations (Gulve, 1994) the rate of extinction depends of deterministic & stochastic effects. • Deterministic extinction is through drainage of ponds or natural succession. • Permanent ponds experience extinction through population stochastic effects (random dry periods, over predation by migrant species, low seasonal birth success) • However, extinction in permanent ponds is low (<=8.5%), indicating migration between ponds and consequent reduction in local extinctions. Docsity.com Source-sink model • The metapopulation model assumes all patches are of the same quality, & hence birth/death rates are the same across the landscape • A special-case model was proposed (Pulliam, 1988) in which local populations have unique demographics in response to local variation in habitat quality • This naturally gives rise to the source-sink concept (Dias, 1996) • Areas with greater reproductive success than death rates must have a net excess of individuals, making the areas sources • Other areas, where local mortality is greater than birth rates, have a net deficit in individuals, making them a sink Docsity.com Source-sink model • Individuals will tend to move from sources to sinks to avoid overpopulation of their areas, despite the poorer quality of sinks • Patch quality is often related to size – the source effect is greater for large patches with increased per capita production. • Long-term studies needed to determine whether a patch is source or sink: – Stochastic events (high rainfall) in a generally unfavourable site (desert) may give a false impression that it is a source • There are a number of observable special cases of the source-sink model that can lead to erroneous assumptions of carrying capacity of the area Docsity.com Source-sink: Stable maladaptation • Exemplified by bluetit (Parus caerulus) populations breeding in deciduous and evergreen oak (Blondel et al, 1992) • Birds synchronise laying dates with food availability in deciduous forest • In evergreen forest, the food availability is 3 weeks later, giving lower bird fertility • Birds adapted to deciduous forest, but emigrate to evergreen forest in a patchy landscape • In Corsica (all evergreen), the same species of bird is adapted to the altered timing, because it is an island population (gradual speciation through evolutionary adaptation) Docsity.com Summary • Island biogeography: The number of species on an island is a function of island size and proximity to the main population body • Metapopulation: locally dynamic but regionally stable population. Migration between fragments may allow species to repopulate areas after local extinctions • Source: Area with a net surplus of individuals, from which migration occurs • Sink: Area with net deficit in the growth rate that receives immigrants. • Pseudo-sink: optimal area with lower carrying capacity that receives too many immigrants, lowering overall species fitness locally • Traps: an area appears beneficial but is unable to sustain a full species life cycle • Stable maladaptation: occurs where migration into suboptimal patches from an optimal matrix is common Docsity.com