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Lecture 5

Biology Lecture 5 Article 3.docx

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University of Toronto St. George
Scott Browning

BIOLOGY LECTURE #5 Allee Effects Aka positive density dependence Allee effects are small pop phenomenon where PGR is reduced by undercrowding History + Def - Occur in small populations - At low density, population growth of populations subject to Allee is reduced - Darwin observed that large pop size important to stop extinctions when there are predators, BUT, it Maltusian printiple + logistic model for PG was accepted - Malthusian principle: intra-specific comp doesn’t decrease with pop size - PG = limited by UNDERCROWDING, not competition Ex) Aggregation had (+) effects on survival of land isopods that decreased fast when isolated - These benefits contradict Malth + log model ALLEE EFFECTS HAVE 2 MANIFESTATIONS: 1) COMPONENT ALLEE EFFECTS: occur in populations where (+) relationship between fitness + population size (competition can limit size) 2) DEMOGRAPHIC ALLEE EFFECTS: occur when CAE make a (+) relationship between per capita PG + population size (per capita GR of small pop = fitness - An Allee effect it (+) relationship between AAI fitness + pop size over some infinite interval - (+) Relationship might give rise to a critical pop size - Below this population can’t persist - STRONG Allee effect results in this - WEAK don’t Mechs that Cause It 1) Mechs that affect R + S Ex) mate limitation = under crowding in species that reproduce sexually, it reduces R when plants of animals can’t release gametes/find each other (plants + animals) - COOPERATIVE BEHAVIORS: better in social groups, and result in increased R success + S 2) Mechs with presence of conspecific individuals - Conspecific: belonging to the same species Ex) PC risk of predation is smaller in large prey pop than small prey pop - presence of multiple individuals effect biotic in favorable ways (niche construction) 3) Demographic + Genetic Mech - active dispersal away from low-density pops = decreased rates of PG - pop size is small, inbreeding depr can cause Allee by reducing fitness as pop declines GENERAL PATTERN FOR ALL: SMALL POPS SUFFER FROM REDUCED FITNESS EVIDENCE FOR ALLEE: Evidence for CAE: - Most commonly observed mech is mate limitation - Allee found in S found in taxonomic groups - Predator satiation can affect any pop fed upon by a predator that feeds on many species - Mate limitation, cooperative (feeding, defense, breeding), predator satiation, dispersal, habitat amelioration Evidence for DAE: - Harder to demonstrate for 2 reasons 1) Allee effects in fitness may be offset at low density by increases in other fitness 2) Natural pops at low density hard to find + high variance from small size obscures statistics - SO… unusual cases of PG important: invasive species - Likely effects of (+) density dependence = EXTINCTION (can’t say if Allee to blame) Dynamics of Populations with Allee - Most dramatic consequences with strong Allee - Diff between Allee vs Malth logistic theory seen in the relationship between PCGR + pop size Dividing both sides by pop size = PCGR POPULATION GR - Strong Allee = critical size occurs: intersects the H line at 0 = unstable eq - Pops greater than critical size will increase to carrying capacity
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