The effect of prey identity and substrate type on the functional response of a globally invasive crayfish

Elements in Invasive Redclaw Crayfish Cherax quadricarinatus Pose Human Health Risks in the Largest Floodplain System of South Africa

The Australian redclaw crayfish, Cherax quadricarinatus, has been introduced globally for aquacultural purposes, and invasive wild populations have established in several countries. An increase in population growth, has caused several low-income and rural communities to utilise it as an inexpensive protein source. Despite this introduction, limited research has been done on element accumulation, and the risks for human consumption. This study focused on elemental accumulation in C. quadricarinatus from two rivers in the Phongolo River Floodplain (PRF), South Africa. The non-carcinogenic and carcinogenic human health risks associated with its consumption were calculated. The accumulation order in the PRF was Zn > Cu > Pb > Cr > As > Ni > Hg > Cd. Elevated concentrations of As and Hg, as well as As, Cr, and Ni posed non-carcinogenic and carcinogenic risks, respectively. These results highlight that wild populations of C. quadricarinatus not only pose a threat to aquatic ecosystems but also potentially cause human health risks when consumed.

When worlds collide: Invader-driven benthic habitat complexity alters predatory impacts of invasive and native predatory fishes

Interactions between multiple invasive alien species (IAS) might increase their ecological impacts, yet relatively few studies have attempted to quantify the effects of facilitative interactions on the success and impact of aquatic IAS. Further, the effect of abiotic factors, such as habitat structure, have lacked consideration in ecological impact prediction for many high-profile IAS, with most data acquired through simplified assessments that do not account for real environmental complexities. In the present study, we assessed a potential facilitative interaction between a predatory invasive fish, the Ponto-Caspian round goby (Neogobius melanostomus), and an invasive bivalve, the Asian clam (Corbicula fluminea). We compared N. melanostomus functional responses (feeding-rates under different prey densities) to a co-occurring endangered European native analogue fish, the bullhead (Cottus gobio), in the presence of increased levels of habitat complexity driven by the accumulation of dead C. fluminea biomass that persists within the environment (i.e. 0, 10, 20 empty bivalve shells). Habitat complexity significantly influenced predation, with consumption in the absence of shells being greater than where 10 or 20 shells were present. However, at the highest shell density, invasive N. melanostomus maximum feeding-rates and functional response ratios were substantially higher than those of native C. gobio. Further, the Relative Impact Potential metric, by combining per capita effects and population abundances, indicated that higher shell densities exacerbate the relative impact of the invader. It therefore appears that N. melanostomus can better tolerate higher IAS shell abundances when foraging at high prey densities, suggesting the occurrence of an important facilitative interaction. Our data are thus fully congruent with field data that link establishment success of N. melanostomus with the presence of C. fluminea. Overall, we show that invader-driven benthic habitat complexity can alter the feeding-rates and thus impacts of predatory fishes, and highlight the importance of inclusion of abiotic factors in impact prediction assessments for IAS.

Identifying economic costs and knowledge gaps of invasive aquatic crustaceans

Despite voluminous literature identifying the impacts of invasive species, summaries of monetary costs for some taxonomic groups remain limited. Invasive alien crustaceans often have profound impacts on recipient ecosystems, but there may be great unknowns related to their economic costs. Using the InvaCost database, we quantify and analyse reported costs associated with invasive crustaceans globally across taxonomic, spatial, and temporal descriptors. Specifically, we quantify the costs of prominent aquatic crustaceans - crayfish, crabs, amphipods, and lobsters. Between 2000 and 2020, crayfish caused US$ 120.5 million in reported costs; the vast majority (99%) being attributed to representatives of Astacidae and Cambaridae. Crayfish-related costs were unevenly distributed across countries, with a strong bias towards European economies (US$ 116.4 million; mainly due to the signal crayfish in Sweden), followed by costs reported from North America and Asia. The costs were also largely predicted or extrapolated, and thus not based on empirical observations. Despite these limitations, the costs of invasive crayfish have increased considerably over the past two decades, averaging US$ 5.7 million per year. Invasive crabs have caused costs of US$ 150.2 million since 1960 and the ratios were again uneven (57% in North America and 42% in Europe). Damage-related costs dominated for both crayfish (80%) and crabs (99%), with management costs lacking or even more under-reported. Reported costs for invasive amphipods (US$ 178.8 thousand) and lobsters (US$ 44.6 thousand) were considerably lower, suggesting a lack of effort in reporting costs for these groups or effects that are largely non-monetised. Despite the well-known damage caused by invasive crustaceans, we identify data limitations that prevent a full accounting of the economic costs of these invasive groups, while highlighting the increasing costs at several scales based on the available literature. Further cost reports are needed to better assess the true magnitude of monetary costs caused by invasive aquatic crustaceans.

The Invasive Round Goby Neogobius melanostomus as a Potential Threat to Native Crayfish Populations

Simple Summary

Neogobius melanostomus is a highly invasive fish that has colonized most major European rivers and is dispersing into their tributaries. Its foraging behaviour does not show particular prey preferences, which makes predicting its interactions with endangered members of the macrozoobenthic community in tributaries a challenge. We observed the interaction of N. melanostomus and crayfish juvenile or A. aquaticus in single- and multiple-prey systems to better predict its ecological impact. The results suggest an impact of N. melanostomus on crayfish similar to that on A. aquaticus, potentially making it a threat to crayfish population stability. Destabilization of a keystone species such as crayfish in river tributaries may lead to a trophic cascade in the ecosystem with irreversible consequences.

Abstract

Despite the spread of round goby Neogobius melanostomus into freshwater streams, there is a lack of information with respect to its effect on macroinvertebrate communities, especially crustaceans. We studied foraging efficiency of N. melanostomus on Procambarus virginalis and Asellus aquaticus, using a functional response (FR) approach. Stocking density of the prey species was manipulated to determine its effect on consumer utilization, with prey offered separately or combined at 1:1, 3:1, and 1:3 at each tested density. For both prey species, N. melanostomus exhibited type II FR, occasionally with a high proportion of non-consumptive mortality. Procambarus virginalis suffered a significantly higher attack rate compared to A. aquaticus. Neogobius melanostomus killed significantly more of the most prevalent prey, regardless of species. In trials with prey species of equal proportions, a difference in the number of each species killed was observed only at the highest density, at which P. virginalis was preferred. Neogobius melanostomus may be an important driver of population dynamics of prey species in the wild. The non-selective prey consumption makes N. melanostomus a potential threat to macrozoobenthic communities of river tributaries.

Pushing the switch: functional responses and prey switching by invasive lionfish may mediate their ecological impact

Biodiversity is declining on a global scale and the spread of invasive alien species (IAS) is a major driver, particularly through predatory impacts. Thus, effective means of assessing and predicting the consequences of IAS predation on native prey population stability remains a vital goal for conservation. Here, we applied two classic ecological concepts, consumer functional response (FR) and prey switching, to predict and understand the ecological impacts of juveniles of the lionfish (Pterois volitans), a notorious and widespread marine invader. Functional responses and prey switching propensities were quantified towards three representative prey species: Artemia salina, Palaemonetes varians, and Gammarus oceanicus. Lionfish exhibited potentially destabilising Type II FRs towards individual prey species, owing to high consumption rates at low prey densities, whilst FR magnitudes differed among prey species. Functional response attack rates (a) were highest, and handling times (h) lowest, towards A. salina, followed by P. varians and then G. oceanicus. Maximum feeding rates (1/h) and functional response ratios (FRR; a/h) also followed this impact gradient for the three prey species. Lionfish, however, displayed a potentially population stabilising prey switching propensity (i.e. frequency-dependent predation) when multiple prey species were presented simultaneously, where disproportionately less of rare prey, and more of abundant prey, were consumed. Whilst FR and FRR magnitudes indicate marked per capita lionfish predatory impacts towards prey species, a strong prey switching propensity may reduce in-field impacts by offering low density prey refuge in biodiverse communities. Our results thus corroborate field patterns documenting variable impacts of lionfish, with prey extirpations less likely in diverse communities owing to frequency-dependent predation.

Intraguild Predation between Chrysoperla carnea (Neuroptera: Chrysopidae) and Hippodamia variegata (Coleoptera: Coccinellidae) at Various Extraguild Prey Densities and Arena Complexities

Intraguild predation (IGP) is a ubiquitous, important and common interaction that occurs in aphidophagous guilds. The effects of extraguild prey (EGP, i.e., aphids) density, predator life stage combinations and duration of the interaction on the level, asymmetry and direction of intraguild predation between lacewing Chrysoperla carnea and ladybird Hippodamia variegata were examined in simple laboratory arena and more complex microcosm environment. Three initial densities of 50, 150 and 400 Aphis fabae third instar nymphs and a control without aphids were provided to six combinations of predator life stages (2nd and 3rd larval instars of lacewing and 3rd and 4th instars and adult females of ladybird). The remaining aphid density and occurrence of IGP were checked after 24, 48 and 72 h. The IGP intensity (IGP level, IL) was similar in the simple arena (reaching 0.6 between larvae in absence of EGP and 0.3 between lacewing larvae and ladybird females) and microcosm environment (0.3 without EGP). In both environments, increasing EGP density lowered IL according to negative exponential relationship. IGP was asymmetric (general average asymmetry was 0.82 in simple arena and 0.93 in microcosm, the difference was not significant) and mostly in favour of larvae of C. carnea, except in the combination of 2nd larvae of C. carnea with the 4th larvae and adults of H. variegata. The direction of IGP, but not other characteristics, partially changed during the duration of the experiment. The incidence of IGP interactions among aphid predators under real conditions and its consequences on aphid biological control are discussed.