Comparing food-web impacts of a native invertebrate and an invasive fish as predators in small floodplain wetlands

Abstract

Gambusia holbrooki is an invasive predatory poeciliid fish in wetlands of south-eastern Australia, where it coexists with the native waterbug Anisops thienemanni (Notonectidae). Gambusia has been shown to produce trophic cascades, leading to increased algal biomass following invasion, whereas these effects relative to the often-dominant invertebrate predator Anisops are unknown. Given its flexible diet, we predicted that Gambusia would feed more broadly than Anisops, thereby reducing the abundance of zooplankton grazers, and increasing chlorophyll a. We tested this hypothesis in experimental 110-L wetland mesocosms, using Gambusia and Anisops alone and in combination, in addition to no-predator treatments. We ran two experiments lasting 91 and 35 days, respectively. Both fish and macroinvertebrates generated weak trophic cascades, resulting in minor increases in chlorophyll a above concentrations in control treatments. Gambusia, in lowering total zooplankton abundances, triggered a larger, although still relatively small, algal response relative to Anisops. Impacts of both predators on dominant invertebrate grazers (e.g. Simocephalus spp., copepod nauplii) were similar, although Anisops was associated with an increase in ostracod (Newnhamia sp.) numbers. The similar trophic role of the two predators on algae was unexpected, given their different effects on planktonic communities and their very different taxonomic positions and zoogeographic origins.

Density‐ and trait‐mediated top–down effects modify bottom–up control of a highly endemic tropical aquatic food web

“…2a–d, 3). Poeciliids are known to drive such cascades in more diverse systems (Hurlbert et al 1972, Nagdali and Gupta 2002, Ho et al 2011), but our study describes a trophic cascade whereby poeciliids alter entire pond ecosystems by reducing the abundance and feeding of a single species, H. rubra . The relative lack of other species of invertebrate grazers in this system, and, specifically, the lack of any species resistant to poeciliid predation, prevents compensation by other members of the community from offsetting the effects of reduced H. rubra grazing.…”

Responses of a wetland ecosystem to the controlled introduction of invasive fish

“…Collectively, our findings suggest that negative effects of mosquitofish on native amphibians can be strong under certain settings, but they may also be influenced by factors such as community structure, habitat characteristics and productivity. Prior research has also shown that hydrological characteristics can mediate the effects of mosquitofish on some native species (Ho, Bond & Lake, ). Repeating our mosquitofish introduction within a less complex or productive habitat, and/or introducing mosquitofish earlier in the season, may have resulted in stronger effects on amphibians.…”

Communities at the extreme: Aquatic food webs in desert landscapes

“…Changes along environmental gradients can alter which taxa are present and their abundance (community composition) and the trophic interactions between taxa (food-web structure). By including all species present and their interactions, we can understand how whole ecosystems respond to stressors such as invasive species (Ho, Bond, & Lake, 2011), drought (Ledger, Brown, Edwards, Milner, & Woodward, 2013), or climate change (Binzer, Guill, Brose, & Rall, 2012). By including all species present and their interactions, we can understand how whole ecosystems respond to stressors such as invasive species (Ho, Bond, & Lake, 2011), drought (Ledger, Brown, Edwards, Milner, & Woodward, 2013), or climate change (Binzer, Guill, Brose, & Rall, 2012).…”