Populations and communities
Direct and indirect effects of predatory trout in detritus-based stream food webs
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While the role of predators in autochthonous food webs have been well documented, predator impacts on detritivore populations and organic matter dynamics in detritus-based food webs remain poorly understood. Hamish Greig's BSc Hons research investigated the effects of predatory trout on detritivore communities dominated by the oeconesid caddisfly, Zelandopsyche ingens (left) in beech forest streams at Cass, Canterbury, New Zealand. Field surveys indicated Z. ingens density was significantly lower in trout streams than fishless streams (McIntosh et al. 2005, pdf, 300kb). |
Subsequent mesocosm experiments showed trout predation reduced densities of Z. ingens by 80-100%, resulting in slower breakdown of coarse particulate organic matter (CPOM) and reduced production of fine particulate organic matter (FPOM). A second experiment that prevented trout access to Z. ingens, but allowed the transmission of trout cues, resulted in no change in litter processing rates in the presence of trout. Thus trout effects on litter processing were due to reduced Z. ingens densities, not trout-induced modifications to Z. ingens feeding behaviour. Field assays of litter processing rates in natural streams (Fig. 1) indicated that the indirect effects of trout on litter processing observed in mesocosms scaled up to complex, natural food webs (Greig and McIntosh 2006, pdf, 120kb).
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Figure 1. Artificial leaf packs used in a field assay of litter breakdown rates in trout and fishless streams. |
Additional surveys indicated that although trout substantially reduced the abundance of Z. ingens larvae, adult Z. ingens were significantly larger and more fecund in trout streams than fishless streams (Greig & McIntosh 2008, pdf, 220kb). Mesocosms indicated Z. ingens larvae reared at densities observed in trout streams grew significantly faster than larvae in high density treatments that were characteristic of fishless stream abundances. However I did not observe non-consumptive effects of trout cues on Z. ingens behaviour or morphology (Greig & McIntosh 2008, pdf, 220kb). This suggests increased adult size and fecundity associated with trout stream individuals was probably a result of predator thinning of larval density indirectly releasing surviving Z. ingens from intraspecific competition. 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 (Fig. 2). Overall Hamish's honours work provided evidence that predators can indirectly influence both resource dynamics and correlates of prey fitness in donor-controlled, detritus based food webs.
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Figure 2. Summary of interactions between brown trout (Salmo trutta) and larval Zelandopsyche ingens in beech forest streams and the subsequent influence on adult Z. ingens in the Cass region, South Island, New Zealand. Arrows represent hypothesised effect strengths, with + or – indicating positive and negative effects, respectively, and dashed arrows indicating inferred or unknown effects. |
People involved in this project
University of Canterbury
Hamish Greig (Completed PhD student)
Angus McIntosh
Publications
Greig & McIntosh 2006. Indirect effects of predatory trout on organic matter processing in detritus-based stream food webs. Oikos 112: 31-40 (pdf, 120kb).
Greig & McIntosh 2008. Density reductions by predatory trout increase adult size and fecundity of surviving caddisfly larvae in a detritus-based stream food web. Freshwater Biology 53: 1579-1591 (pdf, 220kb).
McIntosh, Greig, McMurtrie, Nyström & Winterbourn 2005. Top-down and bottom-up influences on populations of a stream detritivore. Freshwater Biology 50: 1206-1218 (pdf, 300kb).