Food webs in nature have multiple, reticulate connections between a diversity of consumers and resources. Such complexity affects web dynamics: it first spreads the direct effects of consumption and productivity throughout the web rather than focusing them at particular "trophic levels." Second, consumer densities are often donor controlled with food from across the trophic spectrum, the herbivore and detrital channels, other habitats, life-history omnivory, and even trophic mutualism. Although consumers usually do not affect these resources, increased numbers often allow consumers to depress other resources to levels lower than if donor-controlled resources were absent. We propose that such donor-controlled and "multichannel" omnivory is a general feature of consumer control and central to food web dynamics. This observation is contrary to the normal practice of inferring dynamics by simplifying webs into a few linear "trophic levels," as per "green world" theories. Such theories do not accommodate commo...

Food Web Complexity and Community Dynamics

http://www3.carleton.ca/fecpl/courses/ESS.pdf

Ecosystem services generated by fish populations

Biologists concerned with the evolutionary consequences of competition seem unaware of a large literature on freshwater fishes that bears directly on the debates over ecological character displacem...

Character release and displacement in fishes: a neglected literature

The recent invasion of San Francisco Bay by the suspension-feeding clam Putamocorbula amurensis has provided an opportunity to document the ecological consequences of a major biological disturbance. Previous work over the last two decades has shown that phytoplankton biomass in the upper estuary is low (2-3 mg Chl a m-3) during seasonal periods of high river flow and short residence time, and it is usually high (peak > 30 mg Chl a m-3) during the summer-autumn seasons of low river flow and long residence time. However since P. amurensis became widespread and abundant in 1987, the summer phytoplankton biomass maximum has disappeared, presumably because of increased grazing pressure by this newly introduced species. For the period 1977-l 990, mean estimated primary production was only 39 g C mm2 yr-’ during years when bivalve suspension feeders were abundant (>2,000 mm2), compared to 106 g C m-2 yr-’ when bivalves were absent or present in low numbers. These observations support the hypothesis that seasonal and interannual fluctuations in estuarine phytoplankton biomass and primary production can be regulated jointly by direct physical effects (e.g. river-driven transport) and trophic interactions (episodes of enhanced grazing pressure by immigrant populations of benthic suspension feeders).

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1992.37.5.0946

Trophic interactions and direct physical effects control phytoplankton biomass and production in an estuary

In lakes, top predators (fish) often influence the abundance of primary producers (phytoplankton) through food web interactions: phytoplankton biomass is often greater when planktivorous fish are abundant than when they are rare, and phytoplankton community structure is often affected by fish. Three mechanisms can account for these “top–down” effects of fish: decreased herbivory by zooplankton when fish biomass is high; modification of nutrient recycling rates by the herbivorous zooplankton assemblage as fish biomass varies; and nutrient recycling by fish. These processes were experimentally separated and their relative importance quantified in Tuesday Lake, Michigan. This was accomplished by manipulating the abundance of planktivorous fish or zooplankton in enclosures containing natural phytoplankton communities, and by incubating phytoplankton in nutrient-permeable chambers (which excluded herbivores) placed inside these enclosures.

In large enclosures with all trophic levels, several phytoplankton taxa and total phytoplankton biomass showed increased abundance in the presence of fish compared to enclosures without fish. Several taxa also showed significantly greater abundance in nutrient-permeable chambers incubated in enclosures with fish than in chambers incubated in enclosures without fish. The latter result indicates that some phytoplankton taxa respond to fish even when separated from direct herbivory but exposed to nutrients recycled by consumers. Thus, consumer-mediated nutrient recycling had strong effects on phytoplankton community dynamics and could partly explain the “top–down” effects of fish. Most phytoplankton taxa responded to consumer-mediated nutrient recycling, especially dinoflagellates and chrysophytes.

In separate enclosures, phytoplankton were exposed to contrasting zooplankton assemblages shaped by fish predation but without fish being present. The response of phytoplankton was not as strong as in the case of fish manipulations. Furthermore, the community-level response of phytoplankton was weaker in nutrient-permeable chambers placed in these enclosures than in chambers incubated in enclosures with fish. Nutrient limitation assays showed that manipulation of fish decreased phosphorus limitation of phytoplankton, while direct manipulation of zooplankton had no effect on phosphorus limitation.

These results provide experimental evidence that food web effects on nutrient recycling are important in controlling phytoplankton community dynamics. Results also suggest that some of these effects are expressed via direct recycling of nutrients by fish, as well as by fish-induced effects on nutrient recycling by herbivores. Nutrient-mediated effects of top predators on primary producers should be incorporated into future models of “top–down” control of food web dynamics.

Nutrient recycling and herbivory as mechanisms in the “top–down” effect of fish on algae in lakes

PREDATION can be a potent force structuring ecological communities and affecting several trophic levels1–7. The cascading trophic interactions hypothesis predicts that lacustrine predators such as fish can have a strong effect on herbivorous zooplankton, which in turn can regulate phytoplankton8,9. Ascertaining the scale, scope and generality of this hypothesis is important for both development of ecological theory and aquatic ecosystem management10. Although small-scale tests of parts of this cascade are common11–15, whole-lake assessments are not, particularly in large, natural lakes9; also the influence of several trophic levels and nutrients, a requisite for unambiguous interpretation of predator effects, has not been considered. Here we present data from a natural experiment in Lake Mendota, Wisconsin, USA, which support the cascading trophic interactions hypothesis. Massive mortality of fish greatly reduced prÉdation on zooplankton, resulting in an increase in the abundance of large Daphnia, and a dramatic decrease in phytoplankton biomass. Physical factors and concentrations of limiting nutrients were unchanged before and after fish mortality, indicating that these factors probably did not cause the observed decrease in phytoplankton. Our results demonstrate strong food web influences on phytoplankton, and support the idea that food web interactions can be managed to reduce phytoplankton abundance.

Effects on lower tropic levels of massive fish mortality

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Effects on lower tropic levels of massive fish mortality