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Transcript
Strong Interactions and Food Web Structure • Paine suggested feeding activities of a few species may have a dominant influence on community structure. – One approach would be energy flow – Which species would be the most important given this criterion? – Alternative criterion is degree of influence on community structure. – What happens when we remove a species? Strong Interactions and Food Web Structure (Fig. 17.5) • • • • • • Remove the blue tit, what happens? Remove A. geminipuncta, what happens? Remove A. gratus, what happens? Keystone Species If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities. Food Web Structure, Species Diversity and Intertidal Organisms (Fig. 17.6) As species richness increases, proportion of the web represented by predators also increased. – Do all predators use similar resources? Increased predator diversity produces increased predation pressure. – Are all prey types equally affected? Promotes higher diversity. Paine and Intertidal Predators (Fig. 17.7) Consumers’ Effects on Local Diversity To understand the effect herbivores have on plant diversity, you need to know: – Herbivore food preference. – Competitive relationships between plant species in the local community. – Variance in feeding preferences and competitive relationships across environments. Consumers’ Effects on Local Diversity (Fig. 17.8) Lubchenko studied influence of intertidal snail (Littorina littorea) on structure of an algal community. – Snails fed on green (Enteromorpha spp.) and red (Chondrus crispus) algae. – Under normal conditions, Enteromorpha out-competes Chondrus in tide pools, and Littornia prefers Enteromorpha. – In the absence of snails, Chondrus is competitively displaced. • Consumers’ Effects on Local Diversity When snails are present in high densities, Littorina grazes down Enteromorpha, releasing Chondrus from competition. – Green crabs (Carcinus maenus) prey on young snails, preventing juveniles from colonizing tide pools. – Populations of Carcinus are controlled by seagulls. Consumers’ Effects on Local Diversity – Low snail density - Enteromorpha dominates tide pool. – Medium snail density - Competitive exclusion eliminated, and algal diversity increased. – High snail density - Feeding requirements are high enough that snails eat preferred algae and less-preferred algae. • Algal diversity decreased. • • • • Keystone Species: Summation Power : Keystone species exert strong effects on their community structure, despite low biomass. Exotic Predators (Fig. 17.15) Exotic species have dramatic impacts on communities because they were outside the evolutionary experience of local prey populations. Nile Perch (Lates nilotica) exotic fish predator in Lake Victoria. Fish fauna dramatically reduced.