McCard, Nathan, Coughlan, Neil, Britton, Robert, Dick, Jamie and McCard, Monica (2026) Herbivore or Hunter? Ontogenetic shifts in the feeding impact of the invasive turtle Trachemys scripta. [Data Collection]
Abstract
Invasive alien species (IAS) are a major driver of global biodiversity loss and ecosystem change, yet predicting their ecological impacts remains a key challenge in invasion ecology. Functional response (FR) approaches provide a mechanistic framework for quantifying consumer-resource interactions and forecasting invader impacts across trophic groups and ecological contexts. Here, we assessed the ecological impact of juvenile and adult life stages of the invasive turtle Trachemys scripta by quantifying their FRs and functional response ratios (FRRs) toward four aquatic prey species: two macrophytes (Potamogeton crispus and Elodea canadensis) and two bivalves (Dreissena polymorpha and Corbicula fluminea). Through controlled laboratory experiments, we examined prey consumption across a range of prey densities to determine whether ontogenetic dietary shifts altered predicted ecological impacts. Both juvenile and adult turtles exhibited Type II FRs toward all prey species, indicating the potential for strong ecological impacts, particularly at low prey densities. FRR analyses revealed substantially higher values for macrophytes relative to bivalves, especially for E. canadensis, suggesting that aquatic vegetation may be particularly vulnerable to T. scripta consumption. Contrary to expectations of a pronounced ontogenetic shift from carnivory in juveniles to herbivory in adults, both life stages maintained strong impacts toward macrophyte prey, although adults showed comparatively greater consumption of bivalves than juveniles. Lower FRRs for bivalves likely reflected increased handling constraints associated with shelled prey. Our findings demonstrate that the ecological impacts of invasive consumers can vary considerably across prey groups and trophic resources, with potential implications for both native and non-native species within invaded ecosystems. Strong impacts on macrophytes may alter freshwater ecosystem functioning and associated ecosystem services, while relatively weak impacts on invasive bivalves suggest limited biotic resistance toward these co-occurring invaders. This study highlights the importance of incorporating ontogenetic variation into invasion risk assessment and reinforces the utility of FR and FRR frameworks for predicting the ecological impacts of aquatic IAS. Given the widespread global distribution of T. scripta, continued management and ecological risk assessment remain important for mitigating freshwater ecosystem disruption.
| Additional Information: | Depositing user's licence comment: | ||||
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| Creators: | McCard, Nathan, Coughlan, Neil, Britton, Robert, Dick, Jamie and McCard, Monica | ||||
| Uncontrolled Keywords: | 410202 Biosecurity science and invasive species ecology; 3103 Ecology; 310301 Behavioural ecology | ||||
| DOI: | https://doi.org/10.24377/LJMU.d.00000294 | ||||
| Division: | Biological & Environmental Sciences (new Sep 19) | ||||
| Field of Research: | Biological sciences > Ecology Biological sciences > Ecology > Behavioural ecology Environmental sciences Environmental sciences > Ecological applications > Biosecurity science and invasive species ecology |
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| Date Deposited: | 10 Nov 2026 11:41 | ||||
| Last Modified: | 10 Nov 2026 11:41 | ||||
| URI: | https://opendata.ljmu.ac.uk/id/eprint/294 | ||||
| Data collection method: | Eight small (122.0 ± 3.6 mm: mean total carapace length ± SE) and eight large turtles (208.0 ± 6.4 mm) of mixed sexes were provided by Jungle Pets, Cloughmills, Ballymena, Northern Ireland, UK. All individuals were housed indoors within a single round open-top holding tank (215 cm dia.× 74 cm h., with a water depth of ~33 cm, i.e., ~1200 L) containing de-chlorinated UV (Filter pump eco 3300) filtered tap-water. Water was changed weekly and maintained at 22.0 (± 1.0 °C) using an aquarium heater (Hagen, Marina heater 50Hz) under a natural light regime. Two basking platforms were also provided, along with basking lights to create a hot spot of 23 °C. Turtles were maintained on a diet of dead shrimp and commercial floating turtle food (JBL, Germany). Prey species used for the experiment consisted of four problematic invaders; two macrophyte species: Potamogetum crispus and Elodea canadensis and two bivalves: Dreissena polymorph and Corbicula fluminea. All prey species were collected in April-May 2019 (see Table 1) and transported in source water to Queen’s University Marine Laboratory, Northern Ireland, UK. In the laboratory, bivalve specimens were separately maintained in 200 L aerated aquaria using locally sourced lake water (Lough Cowey, Northern Ireland: 54°24′41.79″N, 5°32′25.96″W) within a controlled temperature (CT) room at 13 °C, on a 12:12 h light-to-dark regime. Only living and feeding bivalves were selected for experimental work, i.e., selected specimens that were observed opening to feed, and reclosed when disturbed. Further, intraspecific prey size of bivalve species was standardised throughout trials (both bivalves = 20.0 ± 0.7 mm: C. fluminea = shell height; D. polymorpha = shell length: see Coughlan et al. 2021). Following collection, macrophytes were separately maintained in 100 L aerated aquaria containing locally sourced lake water (Lough Cowey, as above), without substrate. Aquaria were outdoors under ambient conditions, where macrophytes showed sustained growth over a three-month cultivation period prior to experimentation. All prey species were allowed to acclimatise to the laboratory conditions for at least one month prior to experimentation. Functional responses (FRs) Feeding experiments were undertaken at Queen’s Marine Laboratory, Portaferry, Northern Ireland, between July and September 2019. Experiments were conducted within a round open-top tank (Diameter: 151cm × Height: 45cm), using 90 L of de-chlorinated tap water for small turtles and 150 L for the larger turtles. The water was maintained at 22.0 (± 1.0 °C). Predators were allowed to acclimate to experimental arenas for 30 minutes, while prey species were separately acclimated to the temperature regime for 60 minutes prior to experimentation. Each prey species was separately supplied at four or five different densities in a randomised pattern temporally to eliminate time as a confounding factor (bivalves: 2, 4, 8, 16, 20 individuals; macrophytes: 5, 10, 15 and 20 g). Treatment n = 8 per prey species per density. Functional response experiments were initiated through the addition of the allotted prey treatment to experimental tanks containing an individual turtle. Turtles were allowed to feed for a one-hour period before being removed from the experimental arena for enumeration of prey consumed. All 16 turtles were used, eight small and eight large, while no turtle was used more than once at each prey density and prey species to avoid pseudoreplication. Due to the limited numbers of turtles available, individuals were reused systematically in experiments following a designated recovery time (≥ 48 hours) under standard diet and housing conditions (e.g. Dickey et al., 2021; McCard et al., 2021). Controls consisted of one replicate of each prey type across all densities in the absence of predators. | ||||
| Resource language: | English | ||||
| Metadata language: | English | ||||
| Collection period: |
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