A biological community is an association of interacting populations 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 Introduced Norway rats indirectly alter the intertidal community in the Aleutian Islands through direct predation on birds that forage in the intertidal. Dotted arrows indicate indirect effects, whereas solid arrows indicate direct effects. Rats keep Glaucous-winged gull and Black Oystercatcher numbers low, which releases intertidal invertebrates such as barnacles and herbivorous snails and limpets from foraging pressure. Greater numbers of grazing invertebrates leads to a significant decrease in algal cover, which allows more settling space for sessile invertebrates. The marine rocky intertidal is altered from an algae- to an invertebrate-dominated system. 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