Piscivores, trophic cascades, and lake management.

Ray W Drenner,Ray K David Hambright

doi:10.1100/tsw.2002.138

Abstract

The concept of cascading trophic interactions predicts that an increase in piscivore biomass in lakes will result in decreased planktivorous fish biomass, increased herbivorous zooplankton biomass, and decreased phytoplankton biomass. Though often accepted as a paradigm in the ecological literature and adopted by lake managers as a basis for lake management strategies, the trophic cascading interactions hypothesis has not received the unequivocal support (in the form of rigorous experimental testing) that might be expected of a paradigm. Here we review field experiments and surveys, testing the hypothesis that effects of increasing piscivore biomass will cascade down through the food web yielding a decline in phytoplankton biomass. We found 39 studies in the scientific literature examining piscivore effects on phytoplankton biomass. Of the studies, 22 were confounded by supplemental manipulations (e.g., simultaneous reduction of nutrients or removal of planktivores) and could not be used to assess piscivore effects. Of the 17 nonconfounded studies, most did not find piscivore effects on phytoplankton biomass and therefore did not support the trophic cascading interactions hypothesis. However, the trophic cascading interactions hypothesis also predicts that lake systems containing piscivores will have lower phytoplankton biomass for any given phosphorus concentration. Based on regression analyses of chlorophyll�total phosphorus relationships in the 17 nonconfounded piscivore studies, this aspect of the trophic cascading interactions hypothesis was supported. The slope of the chlorophyll vs. total phosphorus regression was lower in lakes with planktivores and piscivores compared with lakes containing only planktivores but no piscivores. We hypothesize that this slope can be used as an indicator of “functional piscivory” and that communities with extremes of functional piscivory (zero and very high) represent classical 3- and 4-trophic level food webs.

Highlights

Food Webs and Trophic LevelsUnderstanding food webs is a formidable challenge because food webs in nature may contain hundreds of species, each connected by multiple links of various strengths[1,2,3]
Rather than an absolute reduction in phytoplankton biomass, our analysis suggests that the amount of phytoplankton biomass relative to the amount of phosphorus should be lower in systems containing piscivores and that the difference between chlorophyll–total phosphorus ratios in 3-link and 4-link systems is expected to be greatest in high-phosphorus, eutrophic lakes
While it is true that great support exists for cascading effects of planktivorous fish to phytoplankton, via strong effects on the zooplankton[44,45,47], there have been remarkably few tests of piscivores and their cascading effects on phytoplankton

Summary

Introduction

Food Webs and Trophic LevelsUnderstanding food webs is a formidable challenge because food webs in nature may contain hundreds of species, each connected by multiple links of various strengths[1,2,3]. One of the common approaches used to generalize food webs into tractable concepts has been to group organisms into trophic levels, i.e., according to their primary energy source[4,5,6]. In 1960 this view was challenged by the seminal paper of Hairston, Smith, and Slobodkin (hereafter HSS)[11], in which they hypothesized that in a 3-level trophic system, herbivore populations were regulated not by available resources, but by consumption by carnivores, while producers and carnivores were resource regulated. According to the EEH model, even numbers of trophic levels (2 or 4) produce low-standing crops of plants because the herbivore populations flourish. Odd numbers of trophic levels (1 or 3) result in plants controlled by the availability of their resources because herbivores are either absent or suppressed by carnivores

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