Comparative functional responses of introduced and native ladybird beetles track ecological impact through predation and competition

Predicting invasive consumer impact via the comparative functional response approach: linking application to ecological theory

The Comparative Functional Response Approach (CFRA) was developed to provide a practical methodology by which short-term experiments can be used to forecast the longer-term impacts of a potential invading consumer. The CFRA makes inferences about potential invader impact based on comparisons of the functional responses of invader and native consumers on native resources in a common experimental venue. Application of the CFRA and derivative approaches have proliferated since it was introduced in 2014. Here we examine the conceptual foundations of the CFRA within the context of basic Lotka–Volterra consumer-resource theory. Our goals are to assess whether core predictions of the CFRA hold within this framework, to consider the relative importance of background mortality and consumer assimilation efficiency in determining predator impact, and to leverage this conceptual framework to expand the discussion regarding stability and long term consumer and resource dynamics. The CFRA assertion that consumers with a higher functional response will have larger impacts on resources only holds as long as all other parameters are equal, but basic theory indicates that predator impacts on prey abundance and stability will depend more on variation in conversion efficiency and background mortality. While examination of the CFRA within this framework highlights limitations about its current application, it also points to potential strengths that are only revealed when a theoretical context is identified, in this case the implications for stability and conceptual links to competition theory.

Predatory ability and abundance forecast the ecological impacts of two aquatic invasive species

Characterising interspecific interaction strengths, combined with population abundances of prey and their novel predators, is critical to develop predictive invasion ecology. This is especially true of aquatic invasive species, which can pose a significant threat to the structure and stability of the ecosystems to which they are introduced. Here, we investigated consumer-resource dynamics of two globally-established aquatic invasive species, European green crab (Carcinus maenas) and brown trout (Salmo trutta). We explored the mediating effect of prey density on predatory impact in these invaders relative to functionally analogous native rock crab (Cancer irroratus) and Atlantic salmon (Salmo salar), respectively, feeding on shared prey (Mytilus sp. and Tenebrio molitor, respectively). We subsequently combined feeding rates with each predator’s regional abundance to forecast relative ecological impacts. All predators demonstrated potentially destabilising Type II functional responses towards prey, with native rock crab and invasive brown trout exhibiting greater per capita impacts relative to their trophic analogues. Functional Response Ratios (attack rates divided by handling times) were higher for both invasive species, reflecting greater overall per capita effects compared to natives. Impact projections that incorporated predator abundances with per capita effects predicted severe impacts by European green crabs. However, brown trout, despite possessing higher per capita effects than Atlantic salmon, are projected to have low impact owing to currently low abundances in the sampled watershed. Should brown trout density increase sixfold, we predict it would exert higher impact than Atlantic salmon. Such impact-forecasting metrics and methods are thus vital tools to assist in the determination of current and future adverse impacts associated with aquatic invasive species.

Predator selection and predator-prey interactions for the biological control of mosquito dengue vectors in northern Vietnam

Predators and their interactions with target prey influence the efficiency of control strategies. In the present study, we demonstrate the implementation of natural predator selection for controlling dengue vectors in northern Vietnam through field-based observation of aquatic insect predators in natural habitats and lab-based assessment of predatorial capacities for several aquatic insect predators. The selected species was then used to evaluate the predatory-prey interaction using functional responses (FRs) toward 3^rd- and 4^th-instar larvae of four major medical mosquito species (Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and Anopheles minimus). The preference of selected predators for Ae. aegypti larvae over other mosquito larvae was also investigated. Both field observation and lab experiments indicated that the giant water bug Diplonychus rusticus was abundant and exhibited the highest predatory capacity for mosquito larvae. The predator exhibited type II FRs when offered each of the four prey species, and the greatest attack rates were observed for Ae. aegypti and Ae. albopictus, with only negligible differences observed in the handling times of the prey species. Further, Manly's selectivity (α) values calculated from the prey choice experiments showed that Ae. aegypti was preferred over both Cx. quinquefasciatus and An. minimus. Together, these findings indicate that D. rusticus could be successfully used to facilitate the biological control of both Ae. aegypti and Ae. albopictus within the species' distributional overlap in Southeast Asia.

Exotic predators are not better biocontrol agents: the harlequin ladybird is not the most voracious in Mexico

The use of exotic species for pest biocontrol has been a common pathway for introduction and dispersal of invasive species that may have undesired outcomes. Biocontrol agents are believed to be a less damaging alternative than pesticides, but some species may also prey on or parasitize native species or outcompete them for resources. The harlequin ladybird (Harmonia axyridis) is a well-known biocontrol agent originally from Asia that has established invasive populations in 59 countries around the globe. Harlequin ladybirds are generalist predators that in addition to pests prey on an array of different species including other coccinelids’ eggs and larvae. In Mexico, native ladybirds that share ecological requirements with harlequin ladybirds are at risk of being outcompeted and predated upon. The aim of our study was to compare the foraging efficiency of harlequin ladybirds against three species of native coccinelids when preying on aphids. We investigated the foraging behaviour of ladybirds alone and in pairs with a conspecific, a native heterospecific or an exotic heterospecific. We found that the native Cycloneda sanguinea was the species that consumed the most aphids, while Hippodamia convergens was the fastest to find and consume each aphid. Harlequin ladybirds and H. convergens consumed the same number of aphids while P. vittigera consumed less. Conspecific competition was stronger than heterospecific competition. We discuss the suitability of using the exotic invasive harlequin ladybird for aphid biocontrol in comparison with native coccinelids.

Comparison of native and non-native predator consumption rates and prey avoidance behavior in North America and Europe

Novel predator-prey interactions can contribute to the invasion success of non-native predators. For example, native prey can fail to recognize and avoid non-native predators due to a lack of co-evolutionary history and cue dissimilarity with native predators. This might result in a competitive advantage for non-native predators. Numerous lady beetle species were globally redistributed as biological control agents against aphids, resulting in novel predator-prey interactions. Here, we investigated the strength of avoidance behavior of the pea aphid (Acyrthosiphon pisum) toward chemical cues of native lady beetles and non-native Asian Harmonia axyridis and European Coccinella septempunctata and Hippodamia variegata in North America, hypothesizing that cues of non-native lady beetles induce weaker avoidance behavior than cues of co-evolved native lady beetles. Additionally, we compared aphid consumption of lady beetles, examining potential predation advantages of non-native lady beetles. Finally, we compared cue avoidance behavior between North American and European pea aphid populations and aphid consumption of native and non-native lady beetles in North America and Europe. In North America, pea aphids avoided chemical cues of all ladybeetle species tested, regardless of their origin. In contrast to pea aphids in North America, European pea aphids did not avoid cues of the non-native H. axyridis. The non-native H. axyridis and C. septempunctata were among the largest and most voracious lady beetle species tested, on both continents. Consequently, in North America non-native lady beetle species might have a competitive advantage on shared food resources due to their relatively large body size, compared to several native American lady beetle species. In Europe, however, non-native H. axyridis might benefit from missing aphid cue avoidance as well as a large body size. The co-evolutionary time gap between the European and North American invasion of H. axyridis likely explains the intercontinental differences in cue avoidance behavior and might indicate evolution in aphids toward non-native predators.

Interactions between different predator-prey states: a method for the derivation of the functional and numerical response

In this paper we introduce a formal method for the derivation of a predator's functional response from a system of fast state transitions of the prey or predator on a time scale during which the total prey and predator densities remain constant. Such derivation permits an explicit interpretation of the structure and parameters of the functional response in terms of individual behaviour. The same method is also used here to derive the corresponding numerical response of the predator as well as of the prey.

On the RIP: using Relative Impact Potential to assess the ecological impacts of invasive alien species

Invasive alien species continue to arrive in new locations with no abatement in rate, and thus greater predictive powers surrounding their ecological impacts are required. In particular, we need improved means of quantifying the ecological impacts of new invasive species under different contexts. Here, we develop a suite of metrics based upon the novel Relative Impact Potential (RIP) metric, combining the functional response (consumer per capita effect), with proxies for the numerical response (consumer population response), providing quantification of invasive species ecological impact. These metrics are comparative in relation to the eco-evolutionary baseline of trophically analogous natives, as well as other invasive species and across multiple populations. Crucially, the metrics also reveal how impacts of invasive species change under abiotic and biotic contexts. While studies focused solely on functional responses have been successful in predictive invasion ecology, RIP retains these advantages while adding vital other predictive elements, principally consumer abundance. RIP can also be combined with propagule pressure to quantify overall invasion risk. By highlighting functional response and numerical response proxies, we outline a user-friendly method for assessing the impacts of invaders of all trophic levels and taxonomic groups. We apply the metric to impact assessment in the face of climate change by taking account of both changing predator consumption rates and prey reproduction rates. We proceed to outline the application of RIP to assess biotic resistance against incoming invasive species, the effect of evolution on invasive species impacts, application to interspecific competition, changing spatio-temporal patterns of invasion, and how RIP can inform biological control. We propose that RIP provides scientists and practitioners with a user-friendly, customisable and, crucially, powerful technique to inform invasive species policy and management.