Density reductions by predatory trout increase adult size and fecundity of surviving caddisfly larvae in a detritus‐based stream food web

Abstract

1. In some situations, individuals surviving in environments where predation is intense can grow faster because the benefits of release from intraspecific competition outweigh costs associated with anti-predator responses. Whether these 'thinning' effects of predation occur in detritus-based food webs where resource renewal occurs independently of consumption by consumers was studied. We investigated how effects of predatory brown trout (Salmo trutta) on the larvae of the detritivorous stream caddisfly, Zelandopsyche ingens, influenced the size and fecundity of the caddisfly adults. 2. Trout substantially reduced the abundance of Z. ingens larvae, but adult male and female Z. ingens were significantly larger in trout streams compared to fishless streams. Females in trout streams had 33% more eggs than fishless stream females, and egg sizes were not significantly different. In mesocosms, Z. ingens larvae in low density treatments reflecting trout stream abundances grew significantly faster than larvae in high density treatments that were characteristic of fishless stream abundances. Non-lethal trout presence did not influence case building behaviour, feeding rates or growth or Z. ingens larvae, indicating non-lethal effects of predators were negligible. 3. Increased adult size and fecundity associated with trout stream individuals were probably a result of predator thinning of larval density indirectly releasing surviving Z. ingens from intraspecific competition. Thus, predator thinning did influence interactions between larvae in this detritus-based food web as larval growth was strongly densitydependent. However, extrapolating the total number of eggs potentially produced indicates the increased fecundity of females in trout streams would not compensate for losses of larvae to trout predation.

Extended larval development compensates for sublethal effects of fish predation in a mayfly population (Rhithrogena semicolorata, Ephemeroptera)

“…Power, 1990;McIntosh and Townsend, 1996;Dahl, 1998a,b;Peckarsky and McIntosh, 1998). There are, however, only few field studies analysing both types of predation effects in natural streams (but see: Peckarsky et al, 2001Peckarsky et al, , 2002Dahl and Peckarsky, 2002;Greig and McIntosh, 2008) and in most of them drift feeding fish species were used. Thus, the clear negative net effect of predation on the abundance and on the reproduction of the grazing mayfly R. semicolorata in this study underlines the importance of benthivorous fish as predators in streams.…”

“…The synthesis of both effect types, lethal and sublethal, reflects the outcome of the mayfly population called average fitness of a population. Similarly to our study, Greig and McIntosh (2008) have also measured the lethal and sublethal effects by trout on a caddisfly population and estimated the net effect by calculating a total egg number potentially produced. This can be understood as the result of the fish effects on both the adult density and the individual fecundity, and represents the theoretically possible offspring of one generation.…”

Invertebrate mesopredators are larger in streams with fish

“…Best models within categories (fish only, fish with riparian, predator abundance, prey abundance or physical/chemical variables) were selected by forward stepwise regression, with sex and year forced as covariates. Model ranks are based on Akaike weights Aquat ) 45:243-253 249 Forrester 1994Grieg and McIntosh 2008). However, there is evidence that when trout do influence benthic communities, the impact falls disproportionately on larger-sized species.…”

“…Reduction of densities by trout has been shown to increase resource availability and lead to higher growth rates for shredding invertebrates (Grieg and McIntosh 2008). In addition, resource limitation or intraguild interactions commonly result in densitydependent growth or mortality of top invertebrate predators in the absence of fish (Lancaster et al 1988;van Buskirk 1989;Hildrew et al 2004).…”

“…Indirect effects of fish on life history traits via effects on food web structure have received less attention, though there are theoretical reasons to expect prey to metamorphose at a larger size if predators indirectly increase resources by lowering prey densities (Abrams and Rowe 1996). Empirical evidence of this effect exists for a shredding caddisfly (McIntosh et al 2005;Grieg and McIntosh 2008). The extent to which drift-feeding fish influence benthic invertebrate densities is highly variable and depends on many factors, including the scale of the study (Englund 2005).…”