Karin A. Nilsson

Interaction strength revisited—clarifying the role of energy flux for food web stability

Interaction strength (IS) has been theoretically shown to play a major role in governing the stability and dynamics of food webs. Nonetheless, its definition has been varied and problematic, including a range of recent definitions based on biological rates associated with model parameters (e.g., attack rate). Results from food web theory have been used to argue that IS metrics based on energy flux ought to have a clear relationship with stability. Here, we use simple models to elucidate the actual relationship between local stability and a number of common IS metrics (total flux and per capita fluxes) as well as a more recently suggested metric. We find that the classical IS metrics map to stability in a more complex way than suggested by existing food web theory and that the new IS metric has a much clearer, and biologically interpretable, relationship with local stability. The total energy flux metric falls off existing theoretical predictions when the total resource productivity available to the consumer is reduced despite increased consumer attack rates. The density of a consumer can hence decrease when its attack rate increases. This effect, called the paradox of attack rate, is similar to the well-known hydra effect and can even cascade up a food chain to exclude a predator when consumer attack rate is increased.

Experimental evidence for emergent facilitation: promoting the existence of an invertebrate predator by killing its prey

1. Recent theoretical insights have shown that predator species may help each other to persist by size-selective foraging on a shared prey. By feeding on a certain prey stage, a predator may induce a compensatory response in another stage of the same prey species, thereby favouring other predators; a phenomenon referred to as emergent facilitation. 2. To test whether emergent facilitation may occur in a natural system, we performed an enclosure experiment where we mimicked fish predation by selectively removing large zooplankton and subsequently following the response of the invertebrate predator Bythotrephes longimanus. 3. Positive responses to harvest were observed in the biomass of juvenile individuals of the dominant zooplankton Holopedium gibberum and in Bythotrephes densities. Hence, by removing large prey, we increased the biomass of small prey, i.e. stage-specific biomass overcompensation was present in the juvenile stage of Holopedium. This favoured Bythotrephes, which preferentially feed on small Holopedium. 4. We argue that the stage-specific overcompensation occurred as a result of increased per capita fecundity of adult Holopedium and as a result of competitive release following harvest. If shown to be common, emergent facilitation may be a major mechanism behind observed predator extinctions and patterns of predator invasions.

Guppy populations differ in cannibalistic degree and adaptation to structural environments

There is considerable variation in cannibalism between different species and also between individuals of different species, although relatively little is known about what creates this variation. We investigated the degree of cannibalism in guppy (Poecilia reticulata) populations originating from high and low predation environments in Trinidad, and also how cannibalism was affected by the presence of refuges. Females from two populations were allowed to feed on juveniles from two populations in aquaria trials. The cannibalism was size-dependent and varied depending on both juvenile and female origin. Low predation females were more efficient cannibals and low predation juveniles were better at avoiding cannibalism compared to high predation guppies when no refuges were present. The high predation females were superior cannibals and the high predation juveniles were better at escaping cannibalism than the low predation guppies when refuges were present. We discuss whether the differences in cannibalism and response to refuge addition relate to predation-induced habitat shifts and differences in the guppies’ natural environment.

Refuge availability and within‐species differences in cannibalism determine population variability and dynamics

Theoretical studies show that both cannibalism and intraspecific resource competition can have major effects on population dynamics. Cannibalistic intensity, offspring size, harvesting and refuge a ...

Context-dependent interactions and the regulation of species richness in freshwater fish

Species richness is regulated by a complex network of scale-dependent processes. This complexity can obscure the influence of limiting species interactions, making it difficult to determine if abiotic or biotic drivers are more predominant regulators of richness. Using integrative modeling of freshwater fish richness from 721 lakes along an 11o latitudinal gradient, we find negative interactions to be a relatively minor independent predictor of species richness in lakes despite the widespread presence of predators. Instead, interaction effects, when detectable among major functional groups and 231 species pairs, were strong, often positive, but contextually dependent on environment. These results are consistent with the idea that negative interactions internally structure lake communities but do not consistently ‘scale-up’ to regulate richness independently of the environment. The importance of environment for interaction outcomes and its role in the regulation of species richness highlights the potential sensitivity of fish communities to the environmental changes affecting lakes globally.Species richness patterns are driven by biotic and abiotic factors, the relative strengths of which are unclear. Here, the authors test how species interactions or environmental traits influence fish richness across over 700 Canadian lakes, showing a surprisingly small role of negative interactions.