Are impacts of an exotic predator on a stream food web influenced by disturbance history?

Abstract

Predatory species have been introduced to habitats spanning a wide range of environmental conditions. To better understand the consequences of predation in natural communities we need to examine how variations in abiotic factors modify the influence of predation. The effects of introduced predators may vary amongst habitats if natural disturbance affects the abundance and taxonomic composition of consumers and their resources, or the predator alters recolonisation after disturbance. We tested whether a bed-moving disturbance altered subsequent interactions involving native and introduced predatory fish, invertebrate grazers and algae in experimental channels within a New Zealand stream. Disturbance reduced the abundance of invertebrates by 84%, and induced mortality of Conoesucidae caddisflies. However, the relative abundance of taxa changed little immediately following the disturbance. Invertebrate communities recovered following disturbance in fishless channels and those with native galaxiids (Galaxias vulgaris), and were almost indistinguishable from undisturbed fishless controls after 2 weeks. Invertebrate abundance declined and algal abundance increased in channels with exotic brown trout (Salmo trutta) and their effect was strongest in previously disturbed channels. However, predators and disturbance only had interactive effects on grazer emigration rates. Trout affected grazers through direct consumption (e.g. Conoesucidae caddisflies), and induced higher emigration rates of grazers from channels via drift (e.g. the mayfly Deleatidium). The effects of predatory trout and galaxiids combined differed in disturbed and stable channels. The observed combined effects of predatory trout and galaxiids on invertebrate grazers were lower than expected in stable channels partly due to low emigration rates of Conoesucidae, whereas emigration of grazers was higher than expected in the disturbed channels. The biomass of algae was higher than expected in disturbed channels with both predators. Collectively, our results indicate that predator substitutability and the non-lethal effects of introduced predators varied depending on disturbance history, but their effects on the biomass of grazers and algae did not.

Integrating stomach content and stable isotope analyses to elucidate the feeding habits of non-native sharptooth catfish Clarias gariepinus

“…Morphologically, as the catfish grows, both its gape size and number of gill rakes increase, conferring more efficiency to filter feeding (Munro 1967). Therefore the trophic impacts of the catfish may be contrary to those of piscivorous invaders, such as different bass and trout species, that cause significant top-down effects within the invaded communities (Flecker and Townsend 1994;Gratwicke and Marshall 2001;Cambray 2003b;Nystrom and McIntosh 2003;Vander Zanden et al 2004;Maezono et al 2005).…”

Linking scales of flow variability to lotic ecosystem structure and function

“…In experimental channels, Nyström and McIntosh (2003) found that trout had a significantly higher effect on invertebrates following sub-catastrophic disturbance compared to undisturbed channels, and this resulted in a progressive decline in invertebrate densities (and commensurate increase in periphyton biomass) during the post-disturbance resilience phase. The mechanism for this was by direct consumption of invertebrates and increased emigration (to avoid predation) by the 'weedy' colonists arriving in the disturbed channels.…”

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

“…Freshwater fishes are among the most widely introduced vertebrate group worldwide, and represent the one of the best-studied indicators of invasion impacts (Rahel, 2002;Leprieur et al, 2008;Strayer, 2010). Non-native fishes have disrupted the biota of their recipient ecosystems directly through predation and competition, and indirectly by altering the behaviour and abundance of prey, and disturbing food-web interrelationships (Flecker & Townsend, 1994;Nyström & McIntosh, 2003;Baxter et al, 2004). The general theory of invasion disturbances posits that impacts are often a consequence of cumulative effects related to the distribution range of invaders, their relative abundances within that range, and their per capita effects on individuals, populations and communities of native species (Parker et al, 1999;Dunham et al, 2002;Young et al, 2010;Kadye & Booth, 2012).…”