Food abundance and diet variation in freshwater turtles from the mid-Murray River, Australia

Abstract

Consumers usually respond to variations in prey availability by altering their foraging strategies. Generalist consumers forage on a diversity of resources and have greater potential to ‘switch’ their diet in response to fluctuations in prey availability, in comparison to specialist consumers. We aimed to determine how the diets of two specialist species (the eastern long-necked turtle (Chelodina longicollis) and the broad-shelled turtle (Chelodina expansa) and the more generalist Murray River short-necked turtle (Emydura macquarii) respond to variation in habitat and prey availability. We trapped and stomach-flushed turtles, and compared their diets along with environmental variables (turbidity, macrophyte and filamentous green algae cover, and aquatic invertebrate diversity and abundance) at four wetlands in north-central Victoria. Diets of E. macquarii differed from those of both Chelodina species, which overlapped, across all four sites. However, samples sizes for the two Chelodina species were too small to compare among-wetland variation in diet. Dietary composition of E. macquarii was variable but did not differ statistically among sites. Emydura macquarii preferentially selected filamentous green algae at three of the four sites. Where filamentous green algae were rare, total food bolus volume was reduced and E. macquarii only partially replaced it with other food items, including other vegetation, wood, and animal prey. Many turtles at these sites also had empty stomachs. Thus, filamentous green algae may be a limiting food for E. macquarii. Although E. macquarii has previously been described as a generalist, it appears to have limited ability to replace filamentous green algae with other food items when filamentous green algae are rare.

Prey-switching does not protect a generalist turtle from bioenergetic consequences when its preferred food is scarce

“…Chelodina expansa, C. longicollis, and E. macquarii are sympatric in these wetlands, and stomach contents data indicate that they broadly overlap in the food they consume [29]. The stomach contents of both Chelodina species do not differ across sites, whereas E. macquarii stomach contents vary considerably with location, and largely reflect the availability of prey within a given wetland, which differed in association with turbidity and macrophyte diversity [29]. These results were consistent with optimal foraging theory since the generalist (E. macquarii) should be expected to be more capable of changing its diet in response to changes in food abundance than the more specialized Chelodina species.…”

“…We have previously compared the stomach contents of C. expansa, C. longicollis, and E. macquarii at four sites in north-central Victoria [29]. These wetlands differ broadly in turbidity, and in plant and invertebrate communities [29].…”

“…We have previously compared the stomach contents of C. expansa, C. longicollis, and E. macquarii at four sites in north-central Victoria [29]. These wetlands differ broadly in turbidity, and in plant and invertebrate communities [29]. Chelodina expansa, C. longicollis, and E. macquarii are sympatric in these wetlands, and stomach contents data indicate that they broadly overlap in the food they consume [29].…”

“…These wetlands differ broadly in turbidity, and in plant and invertebrate communities [29]. Chelodina expansa, C. longicollis, and E. macquarii are sympatric in these wetlands, and stomach contents data indicate that they broadly overlap in the food they consume [29]. The stomach contents of both Chelodina species do not differ across sites, whereas E. macquarii stomach contents vary considerably with location, and largely reflect the availability of prey within a given wetland, which differed in association with turbidity and macrophyte diversity [29].…”

“…Here, we used stable isotope analysis to determine whether turtle diets vary consistently over time across site and among species, based on the environmental and stomach-contents differences identified previously in Petrov et al [29]. We then compared turtle body conditions to test whether long-term trends in diet impacted on turtle life histories.…”