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Allee Effects Notes.docx

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University of Toronto St. George
Paul Thompson

Allee Effects: History & Definition:  occur in small or sparse populations, but are rarely detected, and are widely common in nature  population growth of populations subject to Allee effects is reduced at low density  Darwin: large pop size important hedge against extinction in presence of predators or other natural enemies  Malthusian Principle: intra-specific competition doesn’t decrease with population size  Allee: noticed that in many species it was under-crowding, not competition, that limited pop growth o Observed that aggregation had positive effects on the survival of land isopods, which were subject to rapid withering when isolated  Allee effects have 2 manifestations: o Generally, allee affect is a positive association b/w absolute average individual fitness and pop size over some finite interval o Such association may give rise to critical pop size below which the pop can’t persist  Allee effects with critical pop sizes are called strong, while allee effects w/o critical sizes are called weak  Positive Density Dependance: (in contrast to classical negative density dependence) and depensatory dynamics (in contrast to classical compensatory dynamics) Mechanisms That Cause Allee Effects: Mate Limitation, Cooperative Feeding, Cooperative Defense, & Habitat Alteration  Mate Limitation – results in undercrowding in species that reproduce sexually cuz it requires contact b/w male and female gametes. o Reduces reproduction when plants or animals release gametes into environment or when males/females have difficulty locating each other  When breeding, feeding ,and defense are cooperative, they become more efficient/successful in larger social groups – results in increased reproductive success or survivorship o Most obvious in social vertebrates (prairie dogs, birds), but can also arise in insects (bark beetles), aquatic organisms (fish) if Allee effects results from group feeding or defense  Other mechanisms only require conspecific individuals (belonging to the same species) o Ex. the risk of predation per individual is smaller in large prey pops than small prey pops.  Presence of multiple individuals can alter enviro or biotic conditions in favorable ways  Such niche (particular range of conditions that species. can tolerate, and how their physiological responses impact species. geographic distributions.) constructions include reducing physical damages in intertidal zones or exclusion of competitors via allelopathy (Biochemical production by a plant which alters growth and survival of other plants or itself.)  Demographic and genetic mechanisms can cause Allee effects – in animals active dispersal away from low-density pops can result in decreased rates of pop growth o When pop size is small, inbreeding depression (Decreased fitness arising from inbreeding; often due to the increased expression of lethal and deleterious recessive traits), can cause an Allee effect by reducing average fitness as pop size declines  Generally, small pops suffer from reduced average individual fitness Evidence of Allee Effects:  Evidence found from naturals populations due to all of the above mechanisms  Most observed is Mate Limitation – causes Allee effects in both animals and plants (pollen limitation)  Positive Density dependence in survivorship due to coop defense/predator satiation is found in taxonomic groups too cuz the tendency of predators to become satisfied and stop increasing consumption depends on predator, not prey, and thus they have potential to affect any pop Evidence for Demographic Allee Effects:  Hard to demonstrate cuz ① Allee effects in one component of fitness may be offset at low density by increases in other components of fitness, such as decreased competition for resources;  ② Natural pops at low density are often difficult to detect and high variance th
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