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Community Structure – Shaking up old paradigms. Bruno, Stachowicz and Bertness (2003) – The inclusion of facilitation into ecological theory. TRENDS in Ecology and Evolution. 18(3) Historically, negative factors have been considered, with the support of experimental data, as the controlling forces in community structure. These include: Competition Predation Abiotic Stress The Role of Predation - Classic study by Robert Paine (1966) Pisaster ochraceus is an efficient predator of the common mussel, Mytilus californicus. Paine’s study is in the intertidal zone of Mukkaw Bay in Washington state. The Role of Predation - Classic study by Robert Paine The food web as described by Paine (1966) 2 spp. 2 spp. 1 spp. 3 spp. The Role of Predation - Classic study by Robert Paine When Pisaster was removed from some of the pools, the number of species dropped from 18 The Role of Predation - Classic study by Robert Paine When Pisaster was removed from some of the pools, the number of species dropped from 18 to 3. What was the limiting resource? Within 3 months, the barnacle Balanus glandula occupied 60%-80% of the space. One year – B. glandula was crowded out by mussels, Benthic algMytus californianus and gooseneck barnacles, Pollicipes polymerus. ae populations declined. Herbivorous chitons and limpets for lack of space and food. Sponges were also crowded out. A nudibranch that feeds on sponges also left. Five years – pools dominated by the mussel M. californianus and gooseneck barnacles, P. polymerus. The Role of Predation - Classic study by Robert Paine Bruno et al. (2003) “Mussel beds are home to hundreds of invertebrates that do poorly in the presence of the mussels competitors.” Does predation really control community structure? Are there other forces at play in structuring communities? Can a community dominated by just a few species really be diverse? Species with a Large Impact • Dominant species have a very large impact on community structure • Such species are highly abundant or play a pivotal role in community dynamics. Species with a Large Impact Keystone species exert strong control on a community by their ecological roles, or niches. • In contrast to dominant species, they are not necessarily abundant in a community. Ecosystem engineers (or “foundation species”) cause physical changes in the environment that affect community structure. E.g. Mussel, Mytilus californianus. Competition – Interference competition Disturbance and diversity in an intertidal algal community Gigartina stellata Ulva sp. (Sousa 1979) Competition – Interference competition Disturbance and diversity in an intertidal algal community Intermediate Disturbance Hypothesis -abiotic stress Intermediate Disturbance Hypothesis – foundation effect? Species richness and abundance of invertebrates increased from early to middle successional stages, then remained similar into the later stage. These changes are paralleled by those in the physical structure (biomass and surface area) of the algae. Increased complexity of the algal physical structure probably influenced the associated invertebrate community through several mechanisms: (1) It decreased mortality caused by predation from fish and crabs. (2) It reduced the severity of physical stresses, primarily wave shock. (3) It increased the accumulation of those individuals and species transported passively by wave action. (4) In mobile species, selection of algal substrates was largely based on physical aspects of algal structure. Any apparent “structure” in this assemblage seems to be the result of the summation of individual responses to changing algal physical structure during succession. No abiotic disturbance.