Effects of perceived competition and water temperature on the functional responses of invasive and native crabs

Abstract

As the frequency of species introductions increases globally, the need for predicting the ecological impacts of invaders becomes ever more urgent. Functional responses, i.e. how resource uptake by consumers changes as a function of resource density, describe the species-specific ability to deplete resources under standardized conditions, and their shape has been used as a tool to predict the impact of non-native species. However, functional responses are usually derived from individuals foraging alone, which overlooks the important roles of intra- and interspecific interactions in shaping per capita consumption rates. We tested the extent to which the functional responses of invasive European green crab Carcinus maenas and native graceful rock crab Metacarcinus gracilis are affected by the perceived presence of con- and heterospecific individuals. Overall, crabs did not consume significantly more prey in the perceived presence of either a conspecific or novel heterospecific, but did demonstrate significantly higher attack rates in warmer water in the presence of a conspecific. Regardless of competitor type, green crabs consumed, on average, 16% more prey than the native crabs. This was largely due to their higher attack rate and lower prey handling time in warmer water. Green crab prey consumption increased significantly with increasing water temperature while that of the native crab species was unaffected (12 vs. 3.5% per 1°C increase, respectively). Higher maximum feeding rates are congruent with green crab impacts throughout their invaded range and suggest that green crabs might be undeterred from feeding in the presence of this co-occurring native crab species.

Functional responses of male and female European green crabs suggest potential sex-specific impacts of invasion

“…We conducted functional response experiments in June and July 2022. Prior to the start of a trial, we selected up to 12 crabs haphazardly from the holding sea-tables, isolated them, and withheld food for 48 h to standardise hunger levels ( Howard et al, 2018 ; Howard et al, 2022 ). Trials with female crabs were run first, followed by trials with males to minimise the time female crabs were kept in captivity and thus the risk of accidental introduction of green crabs into Bamfield Inlet.…”

“…It also reveals the potential for predators to destabilise prey populations, particularly at low prey densities when predators can find and consume all prey available ( i.e., a Type II functional response) ( Holling, 1959 ; Hassell, 1978 ). Functional responses can differ between invasive and native species, among invasive species, and within species under different contexts ( Haddaway et al, 2012 ; Dick et al, 2013 ; Alexander et al., 2014 ; Howard et al, 2018 ; Howard et al, 2022 ; Ens et al, 2021 ; Chucholl & Chucholl, 2021 ).…”

“…While the effects of external abiotic factors (such as temperature, salinity, habitat complexity, light regimes) on functional responses have been previously examined ( e.g. , South et al, 2017 ; Howard et al, 2022 ; Cuthbert & Briski, 2022 ), there is usually little consideration of consumer sex. Functional response experiments often pool sexes together ( e.g.…”

“…Those that target invasive predators often rely on the study of males ( e.g. , Howard et al, 2018 ; Howard et al., 2022 ; Ens et al, 2021 ; Chucholl & Chucholl, 2021 ). There is a good reason for this bias: the risk of accidental introductions is eliminated if females, especially gravid ones, are not transported to experimental facilities.…”