Abstract
Understanding determinants of the invasiveness and ecological impacts of alien species is amongst the most sought-after and urgent research questions in ecology. Several studies have shown the value of comparing the functional responses (FRs) of alien and native predators towards native prey, however, the technique is under-explored with herbivorous alien species and as a predictor of invasiveness as distinct from ecological impact. Here, in China, we conducted a mesocosm experiment to compare the FRs among three herbivorous snail species: the golden apple snail, Pomacea canaliculata, a highly invasive and high impact alien listed in “100 of the World's Worst Invasive Alien Species”; Planorbarius corneus, a non-invasive, low impact alien; and the Chinese native snail, Bellamya aeruginosa, when feeding on four locally occurring plant species. Further, by using a numerical response equation, we modelled the population dynamics of the snail consumers. For standard FR parameters, we found that the invasive and damaging alien snail had the highest “attack rates” a, shortest “handling times” h and also the highest estimated maximum feeding rates, 1/hT, whereas the native species had the lowest attack rates, longest handling times and lowest maximum feeding rates. The non-invasive, low impact alien species had consistently intermediate FR parameters. The invasive alien species had higher population growth potential than the native snail species, whilst that of the non-invasive alien species was intermediate. Thus, while the comparative FR approach has been proposed as a reliable method for predicting the ecological impacts of invasive predators, our results further suggest that comparative FRs could extend to predict the invasiveness and ecological impacts of alien herbivores and should be explored in other taxa and trophic groups to determine the general utility of the approach.
Highlights
Attempts to understand determinants of the invasiveness and ecological impacts of alien species have led to various comparisons of species traits amongst invaders and natives [1]
Consumer-resource interactions have long been the focus of ecology, and the “functional response” (FR), describing the relationship between resource density and resource consumption rate, has been recognized as a crucial process influencing the dynamics of populations and communities and the stability of food webs [9,10,11,12]
Attack rates a of Po. canaliculata feeding on the four plant species were all significantly higher than those of both other snails (Tukey’s test, P < 0.001; Fig 2A), and those of Pl. corneus feeding on I. aquatica and C. caroliniana were intermediate, and when feeding on the other two plants those of Pl. corneus were not significantly different from those of B. aeruginosa
Summary
Attempts to understand determinants of the invasiveness and ecological impacts of alien species have led to various comparisons of species traits amongst invaders and natives [1]. In the Type II FR, the amount of resource consumed approaches an asymptote hyperbolically as resource density/availability increases; this corresponds to declining proportional consumption. This represents inverse density dependence, where resources at low density are more likely to be consumed, potentially leading to de-stabilization of the resource population. The Type III FR is characterized by an asymptotical sigmoidal curve as resource density/availability increases; this corresponds to an increase and decrease in proportional consumption and as such may impart a refuge when resource density falls below a threshold level, potentially stabilizing consumer-resource dynamics [3]
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