Fluctuations in density of an outbreak species drive diversity cascades in food webs

Abstract

Patterns in food-web structure have frequently been examined in static food webs, but few studies have attempted to delineate patterns that materialize in food webs under nonequilibrium conditions. Here, using one of nature's classical nonequilibrium systems as the food-web database, we test the major assumptions of recent advances in food-web theory. We show that a complex web of interactions between insect herbivores and their natural enemies displays significant architectural flexibility over a large fluctuation in the natural abundance of the major herbivore, the spruce budworm ( Choristoneura fumiferana ). Importantly, this flexibility operates precisely in the manner predicted by recent foraging-based food-web theories: higher-order mobile generalists respond rapidly in time and space by converging on areas of increasing prey abundance. This “birdfeeder effect” operates such that increasing budworm densities correspond to a cascade of increasing diversity and food-web complexity. Thus, by integrating foraging theory with food-web ecology and analyzing a long-term, natural data set coupled with manipulative field experiments, we are able to show that food-web structure varies in a predictable manner. Furthermore, both recent food-web theory and longstanding foraging theory suggest that this very same food-web flexibility ought to be a potent stabilizing mechanism. Interestingly, we find that this food-web flexibility tends to be greater in heterogeneous than in homogeneous forest plots. Because our results provide a plausible mechanism for boreal forest effects on populations of forest insect pests, they have implications for forest and pest management practices.

Population studies of tree-defoliating insects in Canada: a century in review

“…Moreover, insights into the temporal and spatial nature of defoliator outbreaks have come to form the basis of pest management strategies, which is perhaps best illustrated in the long-standing debate over whether to manage spruce budworm outbreaks through a "foliage protection" versus "population control" strategy (Royama 1984;Régnière et al 2012). Another important contribution of this work has been confirmation of the importance of forestry practices in affecting the behaviour and structure of defoliator populations (e.g., forest fragmentation, Roland 1993Roland , 2005precommercial thinning, Moreau et al 2006b; forest composition, Eveleigh et al 2007). Understanding the role forest composition and structure play in shaping outbreaks provides a basis for selecting silvicultural practices aimed at reducing forest susceptibility to defoliator damage.…”

Microsporidia Biological Control Agents and Pathogens of Beneficial Insects

“…For example, in the conifer forests of northern North America, Nosema fumiferanae is a microsporidium that is considered a regulating component of outbreaks of the eastern spruce budworm, Choristoneura fumiferana (Régnière 1984;van Frankenhuyzen et al 2007). Extensive defoliating outbreaks of this moth occur in 35-40 year cycles (Royama 1984;Boulanger & Arseneault 2004), in which N. fumiferanae and other entomopathogens and parasites increase and decrease with host density and can dampen the outbreaks (Eveleigh et al 2007). In the extended periods between budworm outbreaks, N. fumiferanae maintains infection across seasonal generations via transovarial transmission (Eveleigh et al 2012).…”

Food-chain length and adaptive foraging

“…Recent reports suggest that consumers may temporally change diet through learning and phenotypic plasticity (Kause et al 1999;Dukas & Bernays 2000;Egas & Sabelis 2001), which may provide a potential driving force for the introduction of temporal variability in food-web structure (Warren 1989;Winemiller 1990;Eveleigh et al 2007). Given this inherent flexibility in trophic interactions (MacArthur & Pianka 1966;Murdock 1969;Stephens & Krebs 1986), an environmental change that may alter a predator's diet selection behaviour (e.g.…”