Sunday, February 26, 2012

Ecology quotes from Loreau

From Populations to Ecosystems: Theoretical Foundations for a New Ecological Synthesis, Loreau (2010) contains a large number of pithy postulates.

A) Comparative data often indicate a unimodal relationship between diversity and productivity driven by changes in environmental conditions. (B) Experimental variation in species richness under a specific set of environmental conditions produces a pattern of decreasing between-replicate variance and increasing mean response with increasing diversity, as indicated by the thin, curved regression lines through the scatter of response values (shaded areas).

Mutualistic interactions, ecosystem engineering, and trait-mediated indirect interactions (see Schmidz) are not described in simple trophic food webs.

Too much interaction (too many species, too many connections, or too strong connections) destabalize complex ecosystems.

Vertical diversity, in contrast to horizotnal diversity, does not increase productivity but does increase stability. Production (flux) is less affected by top down forces than is biomass or population density (stocks). Horizontal diversity enhances resource exploitation. But note Schmitz: "Weak carnivore indirect effect on plant (biomass, diversity) does not necessarily mean weak effect on ecosystem properties (NPP, N mineralization).

While positive species interactions such as facilitation and mutualism are on eof the biological mechanisms that contribute to generate functional complementarity, and hence positive relationships between species diversity and total biomass and production with single trophic levels (horizontal diversity), their impact in multitrophic systems is more complex because they tend to increase the resource exploitation ability of species at all trophic levels. Consequently, they can enhance the efficiency with which limiting resources are usesd and transferred along the food chain, thereby contributing to enhance ecosystem functioning, but they can also exacerbate the negative effects of trophic interactions when consumers are generalists, including the potential for overexploitation (hyperpredation), intense resource and apparent competition, and reduced functional comiplementarity at consumer trophic levels. Since a higher species diversity provides more opportunities for both trophic and nontrophic interactions, it can also exacerbate these negative effects and result in weaker, or even negative, relationships with total biomass or production.

Overyielding occurs when plant assemblages outperform monocultures. Transgressive Overyielding occurs when when these plant assemblages...

Internal (re)cycling of nutrients (P, N) typically an order of magnitude greater than inputs or outputs (Vitousek and Matson 2009).

Indirect Mutualism Through Nutrient Cycling can occur through Plant-Decomposer Interactions (unless decomposers compete with plants for limiting nutrients?)

Predicts that species traits that improve nutrient cycling efficiency of either plants or decomposers should have a strong positive effect on ecosystem cycling efficiency, primary production, and secondary production. Plants may produce litters of different qualities, thereby controlling patterns of nutrient cycling (Hobbie 1992); they may modify soil structure, which in turn strongly affects nutrient retention (Wood 1984); they may recycle some limiting nutrients internally via biochemical pathways (Switzer and Nelson 1972); or they may directly control nitrification, and hence nitrogen outputs, in the vicinity of their rooting system through inhibition of nitrifying bacteria (Lata et al. 2004).

Grazing Optimization Hypothesis: primary productivity, or even plant fitness, is maximized at an imtermediate rate of herbivory. See McNaughton 1979 for example in Serengeti.

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