Tuesday, September 7, 2010

Alewife feeding leads to feedbacks

The concept of eco-evolutionary feedbacks unites two well-established theories, which have traditionally existed in isolation. The theory of adaptive radiation, which comes from evolutionary biology, posits that the biotic and abiotic environment determines niche availability, and that lineages diversify until available niches are filled. In contrast, theories of environmental modification, which come from ecology, posit that the activities of organisms modify important aspects of the biotic and abiotic environment. The concept of eco-evolutionary feedbacks integrates these perspectives by recognizing that both processes can occur simultaneously. According to this integrated theoretical framework, adaptive diversification can simultaneously shape and be shaped by the niche-modifying abilities of organisms.

Compelling evidence for the importance of eco-evolutionary feedbacks in the wild comes from our work on alewife-zooplankton interactions in lakes. Anadromous alewives (Alosa pseudoharengus) migrate annually from the Atlantic Ocean to inland lakes where they spawn. Juveniles rear in these lakes, usually for one summer, before departing for the ocean to mature. Our research shows that the isolation of lakes from the ocean by human-constructed dams has led to the evolution of landlocked alewife populations. The ecological effects of landlocked alewife populations are well known from classic work in Connecticut lakes by John Brooks and Stanley Dodson and from studies of the alewife invasion of the Great Lakes. Predation by landlocked alewives drives dramatic shifts in mean body size, biomass, and species composition of crustacean zooplankton. But what about anadromous alewives? Our work shows that the ecological effects of anadromous alewives differ markedly from those of landlocked alewives. In lakes harboring anadromous populations, zooplankton effects are largely seasonal – when the juveniles depart each fall, large-bodied zooplankton recover. However, in landlocked lakes predation pressure is year-round – zooplankton communities are continuously dominated by small-bodied species. A key question is: how do landlocked alewives persist under such unfavorable (small-bodied) prey conditions? Our research shows that the answer lies in contemporary evolution. After eliminating large zooplankton from the lake, landlocked alewife populations rapidly evolve morphological traits (gape width and gill raker spacing) and behavioral traits (prey selectivity) that enable them to be efficient foragers on small-bodied zooplankton. Altered foraging traits then further modify zooplankton community structure and influenced the strength of trophic cascades.

The above results point to an eco-evolutionary feedback. The contemporary evolution of landlocked alewife populations from anadromous ancestors led to changes in zooplankton communities, which in turn shaped the evolution of alewife foraging traits, with then further impacts the structure of prey communities and cascading trophic interactions. Thus, rather than adapting to an existing ecological template (niche), which would be the assumption based on traditional theory, alewives participate in creating the niche to which they are adapted. And the resulting phenotypic divergence then drives further ecological effects. Not a bad days work for a hungry little fish.

A 25-year quest for the Holy Grail of evolutionary biology

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