Inter‐specific differences in invader and native fish functional responses illustrate neutral effects on prey but superior invader competitive ability

Abstract

Inter‐specific interactions are key factors in the structuring and functioning of ecological communities. Therefore, it is necessary to assess species interactions, such as predator–prey dynamics and competitive exclusion, within the context of continuing global species introductions. The aim of the present study was to assess ecological impacts and competitive exclusion dynamics involving co‐occurring introduced and native fish species, using the multiple predator functional response (FR) approach. We use comparative FR analysis (resource uptake as a function of resource density) to assess inter‐specific interactions between the invasive western mosquitofish Gambusia affinis and the native freshwater river goby Glossogobius callidus towards chironomid larvae. The FR was derived for each fish species as individuals and when in heterospecific pairings. Data from single individuals were used to predict the expected FR of individuals when in heterospecific multiple predator combinations. Expected FRs were then compared to the observed FRs of each predator in combination trials, enumerated using prey items in their gut contents. Both fish species displayed Type II FRs, however, in single fish trials, invasive mosquitofish had significantly higher FRs (curve asymptotes; i.e. shorter handling times and higher maximum feeding rates) than native river gobies. Heterospecific mosquitofish‐river goby combinations revealed that the FR of the river goby was reduced (i.e. longer handling times and lower maximum feeding rates) by the presence of mosquitofish, whereas this combination greatly enhanced mosquitofish FR magnitudes (i.e. shorter handling times and higher maximum feeding rates). The heterospecific treatments resulted in neutral impacts on prey and there were clear inter‐species interference in favour of the non‐native mosquitofish. This study demonstrates how multiple predators can alter the shape of individual FRs, with neutral effects on prey but superior invader competitive ability. This has implications for predicting the naturalisation success of invasive predators, as predatory and competitive interactions with native predators and prey may facilitate rather than impede establishment.

Breathing space: deoxygenation of aquatic environments can drive differential ecological impacts across biological invasion stages

“…Across all three oxygen treatments, consumption was significantly greater in the Arrival and Proliferation stages, versus the Pre-invasion and Replacement stages, although this is to be expected considering the presence of four predators present in the Arrival and Proliferation stages versus two in the Pre-invasion and Replacement stages. However, it is feasible that multiple predators are so antagonistic as to actually lower per capita and even overall predation effects (Mofu et al 2019). Arrival, Replacement and Proliferation stages had RTIP scores [ 1 (i.e.…”

“…1). We assessed the differing impacts of the two fish species within each invasion stage in two ways: (1) using consumption data from our FR experiment with individual predators and combining those data with actual recorded field density data of the two fish species (from areas of one river at different stages of N. melanostomus invasion, and an uninvaded river for a ''baseline'' native species density), and; (2) a mesocosm experiment to simulate the four invasion stages, with multiple predators of both fish species, which is important to capture multiple predator effects (MPEs) that may be additive, synergistic or antagonistic (Mofu et al 2019).…”

Invasive crayfish outperform Potamonautid crabs at higher temperatures

“…Species interactions shape community dynamics through both consumptive and non‐consumptive manners such as intra‐ and interspecific competition for resources such as food, shelter, and reproduction (Lopez et al., 2019; Mofu et al., 2019; Sih et al., 2010; Zeng et al., 2019). Evidence of effects of crayfish introductions upon freshwater crabs are unequivocal across regions due to context dependencies largely relating to co‐evolutionary history (Gherardi & Cioni, 2004; Mazza et al., 2017; Savvides et al., 2015; Zeng et al., 2019).…”

“…Functional responses can be linear (Type I), hyperbolic (Type II), or sigmoidal (Type III) (Holling, 1966) and Type II responses in particular may potentially destabilise prey populations and lead to localised extirpations (Dick et al., 2013; Dickey et al., 2020). Invasion impacts are highly context dependent with regards to abiotic and biotic characteristics of the recipient environment; thus, FR provides a standardised method to compare resource consumption across contexts (Mofu et al., 2019; Strayer, 2020; Uiterwaal & DeLong, 2020). The parameter FRR [ a/h ] combines variables to elucidate potential intermediate impact (where previously distinguishing differences between FR curves was problematic) and may give a further mechanistic insight into the ultimate driver of impact (Cuthbert et al., 2019; Dickey et al., 2020; South et al., 2019).…”

In a Pinch: Mechanisms Behind Potential Biotic Resistance Toward Two Invasive Crayfish by Native African Freshwater Crabs

“…Species interactions shape community dynamics through consumptive effects, but also in non-consumptive manners such as intraspecific and interspecific competition for resources such as food, shelter, and reproduction (Sih et al, 2010;Lopez et al, 2019;Mofu et al, 2019;Zeng et al, 2019). Biotic resistance can thus be exhibited in both consumptive and competitive manners, however, in freshwater systems biotic resistance is driven overwhelmingly by consumption (Alofs and Jackson, 2014).…”