Resistance of the Prey-to-Predator Ratio to Environmental Gradients and to Biomanipulations

Abstract

Species and their abundances change along environmental gradients, and populations of some species are decimated by introductions of foreign species. We show that the prey‐to‐predator species ratio is resistant to change. We determined that ratios were about 3:1 for 50 lakes with diversity ranging from 5 to 58 taxa, and with a total combined fauna of 140 insects, crustaceans, and fish. The prey‐to‐predator ratio range (1.25:1 to 5.20:1) was only 9% of the potential range. Ratios were not related to chemical, thermal, or physical characteristics of lakes. Ratios were generally the same for fishless lakes, for lakes with one or two introduced predators (salmonids), and for lakes with more complex communities with up to 14 predators including five fish species. For the simplest model, this suggests that when a fish species is introduced and becomes established in a lake there is a fundamental ecological compensatory adjustment with either a three species increase in the diversity of prey or the loss of a predator (but no change in diversity). However, we show more complex prey–predator ratio changes occur because certain keystone species regulated community structure and prey diversity. When the keystone fish (a predatory salmonid) was removed from five lakes, the penultimate littoral predator (lake chub) occupied both the pelagic and littoral zones. Consequently, amphipod density decreased, chironomid density increased, and total zooplankton and benthic diversity increased by 31% or 7.8 taxa per lake on average. The mean prey–predator ratio for these five lakes increased significantly from 2.32:1 to 4.16:1. These community based prey–predator ratios were established by single keystone species from their influence on prey, other predators, and community structure.

Higher temperatures enhance the effects of invasive sportfish on mountain zooplankton communities

“…Planktivorous fish can suppress competitively superior, larger zooplankton (Amundsen et al, 2009) and predatory invertebrates, resulting in increased abundance and coexistence of several less readily detectable smaller zooplankton (Shurin & Allen, 2001;Donald & Anderson, 2003;Gliwicz et al, 2010). Planktivorous fish can suppress competitively superior, larger zooplankton (Amundsen et al, 2009) and predatory invertebrates, resulting in increased abundance and coexistence of several less readily detectable smaller zooplankton (Shurin & Allen, 2001;Donald & Anderson, 2003;Gliwicz et al, 2010).…”

“…Large-bodied organisms such as calanoid copepods (e.g. Daphnia middendorffiana), thereby releasing smaller zooplankton from competition and invertebrate predation pressure (Anderson, 1980;Paul et al, 1995;Donald & Anderson, 2003). amphipods and dipteran larvae) are conspicuous in fishless alpine lakes where they often function as the stenothermic coldwater top predator (Anderson, 1971;Paul et al, 1995;Weidman, Schindler & Vinebrooke, 2011).…”

The impact of non-native rainbow trout within Afro-montane streams in eastern Zimbabwe

“…Since invasive predators, such as trout, are opportunistic feeders, they tend to target the most conspicuous and accessible prey that usually result in a corresponding decline in their abundance and biomass (Englund & Polhemus, 2001;Meissner & Moutka, 2006;Miller & Crowl, 2006;Johnson et al, 2009). Odonates are considered to be keystone macro-invertebrate predators (Fincke et al, 1997) that regulate abundance of their prey thereby reducing dominance of certain species and maintaining diversity (Donald & Anderson, 2003). In this study, the high abundance of some taxa such as Baetis spp., Chironominae and Simulium sp., which are considered the common prey, may suggest a response to the low abundance of macro-invertebrate predators in habitats with trout.…”

Increasing isolation reduces predator:prey species richness ratios in aquatic food webs

“…However, just like the more general studies discussed above, studies in freshwater ponds have found mixed results with regards to predator:prey richness ratios. In some cases, predator:prey ratios in ponds appear to be constant (Jeffries and Lawton 1985, Warren and Gaston 1992, Jeffries 2002, Donald and Anderson 2003, Urban 2004). For example, in an important experimental study, Jeffries (2002) manipulated the ratio of predator:prey richness in experimental ponds and found that, although the identity of species often changed after perturbation, the predator:prey ratio of species richness remained constant.…”