Fortune favours the bold: a higher predator reduces the impact of a native but not an invasive intermediate predator

Abstract

Summary1. Emergent multiple predator effects (MPEs) might radically alter predictions of predatory impact that are based solely on the impact of individuals. In the context of biological invasions, determining if and how the individual-level impacts of invasive predators relates to their impacts in multiple-individual situations will inform understanding of how such impacts might propagate through recipient communities. 2. Here, we use functional responses (the relationship between prey consumption rate and prey density) to compare the impacts of the invasive freshwater mysid crustacean Hemimysis anomala with a native counterpart Mysis salemaai when feeding on basal cladoceran prey (i) as individuals, (ii) in conspecific groups and (iii) in conspecific groups in the presence of a higher fish predator, Gasterosteus aculeatus. 3. In the absence of the higher predator, the invader consumed significantly more basal prey than the native, and consumption was additive for both mysid species -that is, group consumption was predictable from individual-level consumption. 4. Invaders and natives were themselves equally susceptible to predation when feeding with the higher fish predator, but an MPE occurred only between the natives and higher predator, where consumption of basal prey was significantly reduced. In contrast, consumption by the invaders and higher predator remained additive. 5. The presence of a higher predator serves to exacerbate the existing difference in individuallevel consumption between invasive and native mysids. We attribute the mechanism responsible for the MPE associated with the native to a trait-mediated indirect interaction, and further suggest that the relative indifference to predator threat on the part of the invader contributes to its success and impacts within invaded communities.

Predicting the predatory impacts of the “demon shrimp” Dikerogammarus haemobaphes, on native and previously introduced species

“…That being said, there are other factors to be considered when examining impacts in natural ecosystems, such as habitat complexity, competition and prey switching. Nevertheless, laboratory derived comparative functional responses can be used to detect inherent predatory differences between species, and have accurately predicted impacts in other aquatic systems (Bollache et al 2008;Dick et al 2013;Barrios-O'Neill et al 2014;Alexander et al 2014;Dodd et al 2014). Without such work, it can be difficult to tease out effects that are observed on the field, notably when dealing with information poor species such as D. haemobaphes.…”

“…One recently proposed framework suggests the use of comparative functional responses as a means to assess invader impacts, since damaging invasive species may be characterized by higher magnitude functional responses than native analogues and less damaging invaders (Dick et al 2013Alexander et al 2014;Barrios-O'Neill et al 2014;Dodd et al 2014). Pioneered in the late 1940s by Solomon, functional responses were further developed by Holling (1959a, b) in order to understand the relationship between resource density and consumer utilization rate.…”

Differential ecological impacts of invader and native predatory freshwater amphipods under environmental change are revealed by comparative functional responses

“…While empirical studies are rare, the impacts of predators may be strongly context-dependent (MacNeil et al 2009;Ricciardi et al 2013;Dick et al 2014;Paterson et al 2014), with abiotic and biotic factors potentially influencing the degree of impact both spatially and temporally (Barrios-O'Neill et al 2014;Iacarella et al in press;Paterson et al 2014). This is especially true when considering the impacts of invasive species (Thomsen et al 2011) and we thus require methods to both understand and predict the impacts of such invaders under changing environmental conditions (Walther et al 2009;Dick et al 2014).…”

“…The parameter values estimated from our functional response curves can provide insight into the impact of the predators on prey species and assist in resolving the reasons for damaging invader impacts. Particular attention must be given to low prey densities where it becomes evident if a prey species is in danger of local extinction, however, field functional responses may be different due to the multitude of abiotic and biotic factors that are present in the environment (Barrios-O'Neill et al 2014;Paterson et al 2014;Iacarella et al in press). Further investigation of context-dependent invader impacts using comparative functional responses is needed, including the impact of switching and mutual interference and facilitation as these have been shown to modify functional responses and therefore may modify community-level impacts (Murdoch 1969;Médoc et al 2013;Smout et al 2013;Dick et al 2014).…”

“…For example, large and small differences in the functional responses of the invasive shrimp Hemimysis anomala and native shrimp Mysis salemaai were found to reflect large and small impacts of these mysids on native zooplankton species in the field ). In addition, this predictive methodology has shown promise when considering context-dependencies in invasive species impact, as relevant abiotic (Iacarella et al in press) and biotic (Barrios-O'Neill et al 2014) factors can be incorporated into experiments. This allows the impact of an invader to be predicted under conditions that are likely to change or fluctuate, providing more realistic and robust impact forecasting.…”