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Ecology & Evolutionary Biology

EEB365: Final Notes Module 2 and 3 Lecture 1: Topic 1. From Population to Metapopulation How can we tell that a species/population is at risk? What processes increase or decrease the risk? Population growth Environmental variation Harvesting species sustainable Genetics Population size Key Conservation Strategies Captive breeding with reintroduction Habitat restoration Habitat protection Elimination of harmful exotic predators or competitors Reduction or elimination of harvesting Removing disease agents Restoration of natural environmental conditions A population is a group of coexisting, interacting individuals of the same species at a given location and time Population: All coexisting individuals of the same species living in the same area at the same time Spatial disjunction Genetic disjunction Demography disjunction N = Birth + Immigration Death Emigration Historically, most population models assumed panmixus: all animals within a population equally likely to mate There are barriers to panmixus, like Spatial structure, road/traffic, clumped distributions of animals, and sub-populations located in isolated patches Fragmentaion: N (population size) is a function of Demographic (random fluctuations in age structure and birth death rates), Environment. For a population to persist ,its colonization rate must be equal or larger to its extinction rate. Dispersal of the animal is important. The extinction vortex: Things acting to lower the effective population (Ne) size: Environmental variation and catastrophic events or: 1. Habitat destruction 2. Environmental degradation 3. Habitat fragmentation 4. Over harvesting 5. Effects of exotic species These create changes cause: 1. Demographic variation 2. Population which is more subdivided by fragmentation, 3. Inbreeding depression, 4. Genetic drift and less of an ability to adapt 5. ALL lowering effective population size. What can be done to counteract the risk factors? Increase birth and immigration Decrease death and emigration Several populations act as metapopulations. A metapopulation is a population made up of discrete local (sub)-populations, each with its own probability of extinction (i.e., unique set of fecundity and survival values), which are connected by migration. Subpopulations that go extinct, can become re-established through migration from existing subpopulations. When should metapopulations exist? 1. Suitable habitat occurs in discrete patches 2. Demes have substantial risk of extinction 3. Dispersal among patches sufficient for recolonization 4. Dynamics of demes asynchronous, environmental and demographic variation Demes contained in discrete patches 1 Patches undefined: Shape, content, size Distance between patches undefined Extinction and immigration rates constant across all patches Different Metapopulation Models Remember however that a model can be at most 2 of 3 things: General Realistic (accurate) Precise Levins (1970) Metatpopulation Model: Levins was the first to model dynamics of local populations vs. overall population. He coined the term metapopulation. He looked at the distribution of sub-populations (demes) among patches and patch occupancy. Patch occupancy depends on rates of extinction and recolonization (dispersal). Levins (Classic) Metapopulation Model the patch areas are undefined and are assumed equal. The distance between patches is irrelevant. In visual model dashed circles: subpopulations gone extinct due to lack of migration; Filled circles: occupied patches; Arrows indicate migration. Assumptions of Levins model A metapopulation is made up of discrete local (sub)populations Habitat patches are equal in area, isolation and quality Local populations have independent (uncorrelated) population dynamics (demographic independence) Migration occurs among local populations and is so low that it does not affect local dynamics (except to rescue local populations that have gone extinct) Math for Levins Model dP / dt = cP(1 - P) eP ^P= 1-e/c 1. P = fraction of currently occupied patches 2. ^P = equilibrium fraction of occupied patches 3. e = probability of extant local population going extinct 4. c = colonization rate per empty patch and extant local population Key Predictions Metapopulation persists if e/c<1 because probability of the population going extinct is smaller then the colonization rate. P (occupied patches) increases with increasing patch area, due to decreasing extinction P(occupied patches) increases with decreasing distance among patches due to increasing colonization Refinements to Levins Model 1. Populations are structured into local (breeding) populations (i.e., subpopulations) 2. Migration occurs among local populations 3. Subpopulation reestablishment in an area occurs following extinction because of migration. Island Biogeography vs. Metapopulation: both emphasize balance between extinction and immigration rates. The differences in Island Biogeography: the patches are defined; there is a mainland and island structure. The area, and distance between the main land and the island is taken into account. In metapopulation the Patch areas undefined (i.e., assumed equal) , and the area, and distance between populations is irrelevant There are modifications made to Metapopulation Theory which are take into account: 1. The Effects of patch size and density 2. Rescue effect recolonization 3. Size of demes (sup-populations) 4. Location of the patches occur and also the territory it is in 5. Stochasticity (environmental changes and variation) The new models therefore do not assume, equal area, quality, or isolation of habitat patches. The relaxation of these assumptions have led to the identification of three other types of metapopulations: 1. Mainland-island 2. Source-sink 3. Non-equilibrium metapopulations Most naturally fragmented populations 2 When Patch Area Differs: Mainland-Island Metapopulation Model When Patch Quality Differs: Source-Sink Metapopulation Model the green circle is high quality habitat (source), while the white are poor quality habitat sinks Fragmentation reduces Metapopulation Viability because it reduces patch and population sizes, thereby increasing extinction rates. It also increases inter-patch distance, reduces migration rates between patches, reducing the likelihood of local populations sustaining one another When Isolation Changes: Nonequilibrium Metapopulation Model: White, dashed circles: declining subpopulations; unfilled, dashed circles: extinct subpopulations; arrows indicate migration and are thin to indicate very low migration Scale Matters: Dispersal abilities of animals determine metapopulation boundaries, and point out key connections in the landscape Importance of Dispersal between Populations 1.In non-equilibrium dispersal distance is low as well as the variance in patch size (a determinant of population persistence). 2. In the Classic Levins dispersal distance is at a medium level and patch size variance is low 3. For mainland the variance in patch size is high and the dispersal distance is at a medium level 4. In Patchy populations the dispersal distance is high and the variance in patch size is low. Metapopulations and dispersal: The probability of dispersal between habitats is high when dispersal distance is low. It is species dependant however, because some species can disperse better then others. Barriers and cut off source sink patch and cause extirpation of a species After fragmentation of patches dispersal distance is increased. How Fragmentation and Area Effect Reduce Metapopulation Viability 1.Reduces patch and population sizes, thereby increasing extinction rates of subpopulations 2.Increases inter-patch distance, reduces migration rates between patches, reducing the likelihood of local populations sustaining one another What a metapopulation is NOT: migration is so high that populations exhibit demographic panmixia! Synchrony What is spatial synchrony? It refers to synchronized changes in abundance (or other time-varying characteristics) of 3 geographically disjunctive populations. Generally, populations located near each other tend to be more synchronous than those located farther apart, and the patterns of variation in spatial synchrony with distance differ between species. Synchrony among spatially separated populations can be caused by: 1) Migration, or dispersal, of individuals among population is liable to cause population synchrony 2) Synchronous stochastic effects; correlated environmental disturbances (Moran effect) 3) Trophic (eg. predation) interactions with other species that are either themselves synchronized or mobile Metapopulation Theory and Owl Conservation Use spatially explicit models and explored habitat geometry on population viability They Incorporated: Distance between patches related to juvenile dispersal distances, and Patch sizes in terms of nu
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