Ross N. Cuthbert

Resistance is futile: lack of predator switching and a preference for native prey predict the success of an invasive prey species

Invasive species continue to severely impact biodiversity, yet predicting the success or failure of introduced species has remained elusive. In particular, the relationship between community invasibility and native species diversity remains obscure. Here, we apply two traditional ecological concepts that inform prey population stability and hence invasibility. We first show that the native predatory crustacean Gammarus duebeni celticus exhibited similar type II (destabilizing) functional responses (FRs) towards native mayfly prey and invasive amphipod prey, when these prey species were presented separately. However, when the two prey species were presented simultaneously, the predator did not exhibit prey switching, instead consuming disproportionately more native prey than expected from the relative abundance of native and invasive species. These consumptive propensities foster reductions of native prey, while simultaneously limiting biotic resistance against the invasive species by the native predator. Since our theoretical considerations and laboratory results match known field invasion patterns, we advocate the increased consideration of FR and prey switching studies to understand and predict the success of invasive species.

Calanoid Copepods: An Overlooked Tool in the Control of Disease Vector Mosquitoes

Abstract Biological control can assist in the management of disease vector mosquitoes. However, we urgently require the identification of novel and effective agents to aid population management strategies. Previously, predatory biocontrol of disease vector mosquito species has focused extensively on cyclopoid copepods, but prey size refuge effects have been identified as a hindrance to their predatory efficacy. Calanoid copepods have yet to be examined in the context of mosquito control, despite their high prevalence, diversity, and distribution. Here, we apply functional responses (FRs; resource use as a function of resource density) to examine predation efficiencies of a recently described ephemeral pond specialist species, the freshwater calanoid copepod Lovenula raynerae Suárez-Morales, Wasserman & Dalu 2015 (Calanoida: Diaptomidae), using different size classes of larvae of the disease vector complex Culex pipiens (Diptera: Culicidae) as prey. Lovenula raynerae effectively consumed Cx. pipiens larvae across their ontogeny. A potentially population destabilizing type II FR was exhibited toward both early and late instar mosquitoes, indicative of a lack of prey refuge across ontogenetic stages. Attack rates were greatest and handling times lowest for early instar larvae compared to late instar larvae. These traits contrast to other copepods commonly applied in biocontrol, which are only able to handle early instars, and in much smaller numbers. We thus advocate that calanoid copepods can exert particularly marked predatory impact on lower trophic groups, and that their use in disease vector mosquito control strategies should be further explored.

Examining effects of ontogenic microplastic transference on Culex mosquito mortality and adult weight

Microplastics (MPs) continue to proliferate and pollute aquatic and terrestrial environments globally. The impacts of MP pollution on ecosystems and their functioning remain poorly quantified, with most research hitherto focusing on marine ecosystems. There is a paucity of information on the impacts of MPs in freshwater ecosystems, despite the broad range of pathways through which MPs can proliferate and the extensive range of species which actively ingest MPs in these systems. Of particular interest are organisms that bridge aquatic and terrestrial habitats. The present study thus examines the uptake, ontogenic transference and effect of different concentrations (0, 50, 100 and 200u202fMPsu202fmL-1) and sizes (2 and 15u202fμm) of polystyrene MPs between aquatic and terrestrial life stages of Culex pipiens complex mosquitoes. Both 2 and 15u202fμm MPs transferred from the aquatic larval to terrestrial adult stage of Culex mosquitoes, and uptake correlated tightly with initial exposure concentration. However, neither concentration nor size of MPs significantly influenced mortality rates between the aquatic larval and terrestrial adult stage. There was also no impact of MPs on the weight of emerging mosquito adults. We thus demonstrate that MPs can be transferred ontogenically through organisms with complex life histories, presenting a potential pathway for dispersal of MPs into terrestrial environments. We also show that MPs exposure does not affect mortality rates between life stages of freshwater Culex populations. This suggests that MPs do not impact nutritional uptakes, with unhampered development to adulthood facilitating subsequent dispersal of MPs aerially and between freshwater and terrestrial habitats.

The influence of microplastics on trophic interaction strengths and oviposition preferences of dipterans

Microplastic (MP) pollution continues to proliferate in freshwater, marine and terrestrial environments, but with their biotic implications remaining poorly understood. Biotic interactions such as predation can profoundly influence ecosystem structuring, stability and functioning. However, we currently lack quantitative understandings of how trophic interaction strengths and associated behaviours are influenced by MP pollution, and how transference of MPs between trophic levels relates to consumptive traits. We also lack understanding of key life-history effects of MPs, for example, reproductive strategies such as oviposition. The present study examines the predatory ability of non-biting midge larvae, Chaoborus flavicans, towards larvae of Culex pipiens mosquitoes when the latter are exposed to MPs, using a functional response (FR) approach. Transfer of MPs occurred from larval mosquitoes to larval midges via predation. Microplastics transfer was significantly positively related to predation rates. Predation by C. flavicans followed a Type II FR, with average maximum feeding rates of 6.2 mosquito larvae per hour. These and other FR parameters (attack rates and handling times) were not significantly influenced by the presence of MPs. Further, C. pipiens adults did not avoid ovipositing in habitats with high concentrations of MPs. We thus demonstrate that MPs can move readily through freshwater food webs via biotic processes such as predation, and that uptake correlates strongly with consumption rates. Further, as MPs do not deter adult mosquitoes from ovipositing, our experiments reveal high potential for MP exposure and transference through ecosystems.

Intermediate predator naïveté and sex-skewed vulnerability predict the impact of an invasive higher predator

The spread of invasive species continues to reduce biodiversity across all regions and habitat types globally. However, invader impact prediction can be nebulous, and approaches often fail to integrate coupled direct and indirect invader effects. Here, we examine the ecological impacts of an invasive higher predator on lower trophic groups, further developing methodologies to more holistically quantify invader impact. We employ functional response (FR, resource use under different densities) and prey switching experiments to examine the trait- and density-mediated impacts of the invasive mosquitofish Gambusia affinis on an endemic intermediate predator Lovenula raynerae (Copepoda). Lovenula raynerae effectively consumed larval mosquitoes, but was naïve to mosquitofish cues, with attack rates and handling times of the intermediate predator unaffected by mosquitofish cue-treated water. Mosquitofish did not switch between male and female prey, consistently displaying a strong preference for female copepods. We thus demonstrate a lack of risk-reduction activity in the presence of invasive fish by L. raynerae and, in turn, high susceptibility of such intermediate trophic groups to invader impact. Further, we show that mosquitofish demonstrate sex-skewed predator selectivity towards intermediate predators of mosquito larvae, which may affect predator population demographics and, perversely, increase disease vector proliferations. We advocate the utility of FRs and prey switching combined to holistically quantify invasive species impact potential on native organisms at multiple trophic levels.

Comparative feeding rates of native and invasive ascidians

Ascidians have a recent history of species introductions globally, often with strong ecological impacts. Comparisons of per capita effects of invaders and comparable natives are useful to assess such impacts. Here, we explore ingestion rates (IR) and clearance rates (CR) of Ciona intestinalis and Ciona robusta, co-occurring native and non-native ascidians, respectively, from Brittany, France. IR was positively related to food concentration, with the invader responding more strongly to increasing food concentration. CR also differed by species, with the invader demonstrating higher values. C. robusta exhibited a higher functional response (Type I) than did C. intestinalis (Type II). Relative impact measured using seasonal abundance and IR revealed that C. robusta has a much greater impact than C. intestinalis at all food concentrations tested, though the former has a constrained distribution which limits its regional impact. Nevertheless, when abundant, we expect C. robusta to exert a greater impact on algal foods.

Shifting interaction strength between estuarine mysid species across a temperature gradient

In many coastal regions, mean coastal atmospheric and water temperatures are projected to shift as climate change ensues. Interaction strengths between organisms are likely to change along with environmental changes, given interspecific heterogeneity in responses to physico-chemical variables. Biological interaction outcomes have the potential to alter food web production and trophic level biomass distribution. This is particularly pertinent for key species that are either abundant or play disproportionately large roles in ecosystem processes. Using a functional response approach, we quantified the effects of shifting temperatures on interactions between key mysid species-sympatric in their distribution across a biogeographic transition zone along the east coast of South Africa. The Rhopalophthalmus terranatalis functional response type toward Mesopodopsis wooldridgei prey was independent of temperature, with all treatments producing Type II functional responses. Temperature effects on predator-prey dynamics were, however, evident as interaction strength was greatest at 21u202f°C, as measured by maximum feeding rates. Unlike maximum feeding rate, attack rates increased linearly with increasing temperature across the experimental treatments. Our findings suggest that interaction strength between the mysid shrimp species is likely to vary spatially along the current length of their sympatric distribution and temporally in certain regions where temperatures are projected to change. Such experimental interaction investigations are becoming increasingly important given our relatively poor understanding of the consequences of environmental change for effects on interactions among species and their wider ecosystem implications.

The effect of the alternative prey, Paramecium caudatum (Peniculida: Parameciidae), on the predation of Culex pipiens (Diptera: Culicidae) by the copepods Macrocyclops albidus and Megacyclops viridis (Cyclopoida: Cyclopidae)

Abstract Biological control can be an effective tool to combat public health risks associated with mosquito-borne disease. However, target impacts of biological control agents may be reduced by biotic contexts such as the presence of alternative prey. In turn, this can impede our ability to realistically assess biocontrol agent efficacy. Here, we examine the effects of alternative ciliate prey on the predation potential of two cyclopoid copepods, Macrocyclops albidus Jurine (Cyclopoida: Cyclopidae) and Megacyclops viridis Jurine (Cyclopoida: Cyclopidae), toward larvae of the West Nile virus vector mosquito Culex pipiens Linnaeus (Diptera: Culicidae). Using functional responses (FRs; resource use under different resource densities), we demonstrate that both copepods exhibit potentially destabilizing type II FRs toward mosquito prey. However, where the alternative prey was present, we observed species-specific modulations to FR form and magnitude. For M. albidus, FRs remained type II where ciliate prey were present, however, maximum feeding rates on mosquito larvae were reduced. Conversely, for M. viridis, FRs moved toward more stabilizing type III, while maximum feeding rates on mosquito larvae were not significantly reduced. While both species of cyclopoid copepod were able to effectively target and consume larval mosquitoes in the presence of alternative prey, we demonstrate that overall efficacies may be reduced in aquatic habitats which contain multiple prey types. We thus advocate that biotic contexts such as prey selectivity should be integrated into predatory biocontrol agent examinations for mosquitoes which vector pathogens and parasites, to more holistically assess their efficacy.

Up and away: ontogenic transference as a pathway for aerial dispersal of microplastics

Microplastics (MPs) are ubiquitous pollutants found in marine, freshwater and terrestrial ecosystems. With so many MPs in aquatic systems, it is inevitable that they will be ingested by aquatic organisms and be transferred up through the food chain. However, to date, no study has considered whether MPs can be transmitted by means of ontogenic transference, i.e. between life stages that use different habitats. Here, we determine whether fluorescent polystyrene beads could transfer between Culex mosquito life stages and, particularly, could move into the flying adult stage. We show for the first time that MPs can be transferred ontogenically from a feeding (larva) into a non-feeding (pupa) life stage and subsequently into the adult terrestrial life stage. However, transference is dependent on particle size, with smaller 2 µm MPs transferring readily into pupae and adult stages, while 15 µm MPs transferred at a significantly reduced rate. MPs appear to accumulate in the Malpighian tubule renal excretion system. The transfer of MPs to the adults represents a potential aerial pathway to contamination of new environments. Thus, any organism that feeds on terrestrial life phases of freshwater insects could be impacted by MPs found in aquatic ecosystems.