The evolutionary invention of food-webs: a palaeobiological and macroecological approach

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• Nick Butterfield, Department of Earth Sciences, University of Cambridge
• Wednesday 12 June 2013, 14:00-15:00
• British Antarctic Survey, Room 330B.

If you have a question about this talk, please contact Christian Franzke.

If external to BAS, please email Anje Neutel (anjute@bas.ac.uk) in advance to gain access to the building.

Food webs are patterns of who eats whom, and the associated cycling of nutrients and biomass. At the base of all food webs are primary producers, which are consumed and redistributed through a variety of heterotrophic organisms, often arranged sequentially via trophic tiering. The local expression of any particular food web is determined by the interplay of ‘bottom-up’ resources (competition) and ‘top-down’ consumers (predation). On a larger scale, however, they are dependent on the overall diversity, body sizes and connectivity of the constituent nodes, which in turn controls ecosystem function, biogeochemical feedbacks and evolutionary potential.

Prior to the evolution of ingestive multicellular heterotrophs (animals), food-webs were presumably limited to few tiers and microscopic size, comparable to the bacteria-flagellate-ciliate microbial loop that dominates modern oligotrophic oceans. The small size and billion-year stasis exhibited by early (pre-Cryogenian) fossil assemblages attests to the absence of animal-based ecology and limited trophic innovation.

The Cryogenian-Ediacaran appearance of sponge-grade multicellularity allowed flagellate organisms to overcome the tyranny of viscous flow, introducing a fundamentally new process to food-web dynamics. Macroscopic, turbulent-flow suspension feeding would have had major effects on water column ventilation and nutrient cycling in shallow shelf settings. Even so, the limited ability of sponges to consume larger particles or to move, combined with their low palatability, suggests that they contributed relatively little to the expansion of trophic diversity.

The Ediacaran appearance of organ/cnidarian-grade multicellularity introduced both muscular motility and a potential for macrophagous ingestion into the trophic mix. Like sponges, however, jellies are largely a trophic dead-end, and their blind gut and limited brain-power would have had only modest, primarily indirect, effects on early food-web dynamics.

By contrast, the late Ediacaran evolution of cephalized bilaterians with a through gut revolutionized the structure, function and evolutionary potential of food webs. Bilaterians readily eat other bilaterians, and have the neural, locomotory and developmental capacity to derive sequentially higher-order trophic tiers, opening up fundamentally new regions of ecospace based on large body size and complex multi-trophic interactions. Compounding counter-responses in underlying tiers further multiplied diversity and ecological repertoires, including escape into sediments and plankton. These in turn had profound feedback effects on ecosystem function and evolutionary dynamics, ultimately giving rise to the modern marine. The early exponential phase of this planetary regime change is known as the Cambrian explosion.

Butterfield, N.J. 2011. Animals and the invention of the Phanerozoic Earth system. Trends in Ecology & Evolution 26, 81–87.

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