How does trophic skewing affect the functioning of marine communities?

My former PhD student Dr Pamela Reynolds and I just published a paper in PLoS One* that attempts to address this question.

Trophic skew is an ecological term that describes changes in the relative number of species at different tropic levels, i.e., “species richness“.  In most natural food webs, there are more species at the bottom (plants and herbivores) than at the top of the food chain (apex predators). When top predators are removed from ecosystems by over-harvesting or habitat loss, this pyramid shape gets exacerbated, so that the ratio of species at the bottom of the food chain relative to those at the top increases (as in the black pyramid in the figure below).

Pamela and I tested how trophic skew can influence ecosystem functioning by creating tiny estuarine ecosystems in outdoor aquaria. We controlled how many plant and predator species there were in each replicate aquarium, so that we created four experimental skewing scenarios (Fig. 1A) that reflected real or predicted degrees of trophic skewing. We then measured how this affected the productivity and standing biomass of each species and things like the strength of the “trophic cascade” – which is simply how much the predators benefit the plants by eating the herbivores (AKA “grazers” which in this system are mainly tiny crustaceans like amphipods).

My lab has been doing these types of biodiversity manipulations at UNC’s Institute of Marine Sciences for about a decade. So we have a pretty good feel for how these species interact and what their functional roles are. The aquarium’s – which we call “mesocosms” – have buckets that dump fresh seawater into them every minute or so. This keeps them aerated and well-stirred.  The plants and animals in our little ecosystems or mescosms are the critters you’d find inhabiting docks, pier pilings, oyster reefs, seagrass beds, and jetties in temperate estuaries (shrimp, crabs, small fish, etc).

We predicted that top-rich food webs (greater predator to prey richness ratios) would exhibit lower grazer abundances and higher algal biomass compared to bottom-rich food webs (lower predator to prey richness). This is because increasing predator relative to prey richness often increases predation and reduces prey populations (in this case this would benefit the plants via a trophic cascade). However, algal richness is known to promote primary as well as secondary production, and thus bottom-up food webs could have either higher or lower algal biomass depending on the intensity of grazing. If predator (top-down) richness effects drive this system, we expected top-rich food webs to have a stronger positive trophic cascade on algal biomass compared to bottom-rich food webs, and the reverse if algal (bottom-up) richness is more important. Honestly, making reasonable predictions about how different forms  of skewing should affect ecosystems isn’t exactly straightforward. We spend many, many months talking about this before we ran the experiment.

Fig. 1. A) Structure of experimental food webs with varying species richness per trophic level, i.e., trophic skew, and B) experimental species pool (predators: Hypleurochilus geminatus, Monacanthus hispidus, Fundulus heteroclitus, swimming crabs (Callinectes sapidus or C. similis), Lagodon rhomboids, Penaeus aztecus, Palaemonetes vulgaris, mud crabs (Panopeus herbstii, Eurypanopeus depressus orNeopanope sayi); grazers: Gammarus mucronatus, Elasmopus levis, Dulichiella appendiculata, Paracerceis caudata; macroalgae: Dictyota menstrualis, Codium fragile, Padina gymnospora, Sargassum filipendula, Ceramium sp., Gracilaria tikvahiae, G. verrucosa, Hypnea musciformis, Ulva lactuca. Species are not drawn to scale.

What did we discover?  The results of our experiment suggest that trophic skewing of species richness can affect ecosystem functions including primary and secondary production as well as the strength of a trophic cascade.  After 24 days, increasing macroalgal richness promoted both plant biomass and grazer abundance, although the positive effect on plant biomass disappeared in the presence of grazers. The strongest trophic cascade on the experimentally stocked macroalgae emerged in communities with a greater ratio of prey to predator richness (bottom-rich food webs), while stronger cascades on the accumulation of naturally colonizing algae (primarily microalgae with some early successional macroalgae that recruited and grew in the mesocosms) generally emerged in communities with greater predator to prey richness (the more top-rich food webs). These results suggest that trophic skewing of species richness and overall changes in food web topology can influence marine community structure and food web dynamics in complex ways, emphasizing the need for multitrophic approaches to understand the consequences of marine extinctions and invasions.

Take home message:  Removing top predators and skewing food webs changes how they work. Not so surprising, yet very difficult to test scientifically (no other study has ever done this!). Also note, this isn’t just about how removing top predators affects how marine communities function: the study was designed to determine how changes in the number of species might matter.

Reynolds PL, Bruno JF (2012) Effects of Trophic Skewing of Species Richness on Ecosystem Functioning in a Diverse Marine Community. PLoS ONE 7(5): e36196. doi:10.1371/journal.pone.0036196

*This work was funded in part by UNC Chapel Hill and the National Science Foundation.   

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