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Lecture 11

BIOL 3170 Lecture 11: Lecture 11 - Community structure

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BIOL 3170
Mark Vicari

Community structure Trophic structure: • The feeding relationships of a community (i.e. who eats who) • Have 3 main trophic levels: o Primary producers (plants) o Herbivores o Carnivores • Predation occurs between and within the trophic levels • Competition occurs within the trophic levels • This leads to complex interactions and food webs o Effects on one species may have difficult-to-predict effects on other species ▪ Especially because of indirect interactions Indirect interactions in communities: 1. Apparent competition o When two species share the same predator ▪ One prey species present --> predator population small or nonexistent ▪ Two prey species present --> predator population larger (more food) --> both prey populations smaller ▪ Prey populations appear to be competing o E.g. decline of woodland caribou in northern Ontario ▪ Result of moose expanding into caribou habitat which has also drawn in wolves ▪ Increased predation by wolves has diminished caribou numbers 2. Indirect commensalism o When two competing species have different predators The salamander benefits D. rosea and its predator, but is unaffected by them. Keystone species: • Species who have a profound effect on the density of other species (positive or negative) • They themselves do not have to be abundant • E.g. sea star which controls mussel population and allows other species to exist o w/o sea star, mussels dominate and outcompete the other species • E.g. lesser snow goose which have over-grazed their wintering habitat leading to loss of vegetation and ultimately increased salinity and erosion of the area o Plants cannot re-establish in those soil conditions o Animals cannot live in the area because there are no plants • Thus, the mussels in the above example are also considered keystone species Ecosystem engineers/bioengineers • Species which greatly modify their habitat • Often keystone species as well o E.g. the lesser snow goose from above example o E.g. Beavers Dominant species • Have very high abundance or biomass in a community • Exert powerful control over the distribution of other species • Why? o One hypothesis: dominant species are most competitive in exploiting resources o Another hypothesis: they are most successful at avoiding predators Top-down regulation • When diversity and abundance of species in a community is regulated by predators o i.e. regulated by predation Bottom-up regulation • When diversity and abundance of species in a community is regula
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