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Community Structure A biological community is an association of interacting populations Community Diversity & Community Structure • Community diversity relates the number of different species in a community • A community’s structure is based on the interactions between all of its species Individualistic holistic Community Structure • Consumer-resource interactions are a significant component of community structure • Feeding relationships are often used to describe community structure Community Structure • Consumer-resource (feeding) interactions are critical to community structure, because they represent the travel of energy through the ecosystem Community Structure – Trophic Levels • Primary producers - autotrophs (plants and other ‘self-feeders’) • Primary consumers – herbivores • Secondary consumers – carnivores that eat herbivores • Tertiary consumers – carnivores that eat secondary consumers… Community Structure – Food Web • Food web = a summary of all the feeding relationships within a community Effects of species richness on food web structure Is there a relationship between the species richness of a community and the complexity of its feeding interactions? Complexity of a food web can be characterized based on: • number of feeding interactions • number of trophic levels Effect of species richness on food web complexity • The number of feeding interactions that each species has with other species is independent of the overall diversity of the community a b Effect of species richness on food web complexity • However, the number of trophic levels does tend to increase with species richness, so higher community diversity is usually associated with increased food web complexity Effect of species richness on food web complexity Effect of food web structure on species diversity • feeding relationships (and other species interactions) can affect species diversity within a community • for example: when a predator controls the population of an otherwise dominant competitor, it may allow other less competitive species to persist Effect of food web structure on species diversity • The number of species at one trophic level may depend on the presence of consumer species at a higher trophic level There are different ways to visualize community structure • Robert Paine – noted that some feeding relationships are more important than others in terms of shaping community diversity • “strong interactions” • defining criterion not necessarily the quantity of energy flow, but the degree of influence on population sizes and community structure There are different ways to visualize community structure Effect of food web structure on species diversity: Keystone species • Keystone species are those species whose interactions (usually feeding) have a disproportionately large influence on the structure of their community • influence is disproportionate relative to the keystone species’ actual biomass • distinct from dominant species (whose influence is due to high biomass, e.g., redwoods) Community structure - alternative stable states • Similar to population sizes in predator-prey cycles, community structure can exist at different stable equilibria • e.g., kelp forest / urchin barren Community structure & trophic cascades • Trophic levels are influenced from above by predation and from below by production • Top-down control (predator-mediated) • Bottom-up control (producer-mediated) • Trophic Cascade – the activity of one trophic level has far-reaching effects on the biomass of distant trophic levels Community structure & trophic cascades Community structure & trophic cascades Community structure & trophic cascades Community structure & trophic cascades • Trophic cascades can have far-reaching and unpredictable effects Introduced Norway rats indirectly alter the intertidal community in the Aleutian Islands through direct predation on birds that forage in the intertidal Kurle C. M. et.al. PNAS 2008;105:3800-3804 ©2008 by National Academy of Sciences Introduced species & community structure • Introduced species tend to reduce the complexity of community structure by simplifying trophic structure / eliminating trophic levels Energy flow constrains community structure • Trophic dynamics: as energy is transferred from one trophic level to the next, some of it is lost due to: • limited assimilation • respiration • heat production Energy flow constrains community structure - Demo • paper clips = energy units • must have at least 2 energy units to survive • front row = primary producers (3 clips) • second row = primary consumers (forage 3 clips from primary consumers, pay 2 clips for respiration) • third row = secondary consumers (get 3 clips worth of energy from prey, pay 2 clips for respiration) Energy flow constrains community structure • Energy transfer between trophic levels is inefficient enough to make numerous trophic levels an impossibility • The most productive ecosystems usually max out at six trophic levels Energy Pyramids