Feeding across the food web: The interaction between diet, movement and body size in estuarine crocodiles (Crocodylus porosus)

Abstract

The estuarine crocodile (Crocodylus porosus) is an apex predator across freshwater, estuarine and coastal environments. The impact of a changing C. porosus population upon the ecosystem is unknown, but due to large ontogenetic changes in body mass (>1000-fold) their impact may be wide reaching and substantial. Here we investigated the relationship between diet, movement and body size in a population of C. porosus inhabiting a tidal river in northern Australia. Subcutaneous acoustic transmitters and fixed underwater receivers were used to determine the activity space and movement patterns of 42 individuals (202-451 cm in total length). There was no size-related spatial partitioning among different sized crocodiles. Large individuals (snout-vent length (SVL): 160 cm < SVL < 188.5 cm) did, however, exhibit a much larger activity space than other size classes. Diet and individual specialization was assessed using the composition of stable carbon (δ 13 C) and nitrogen (δ 15 N) isotopes in tissues with different turnover rates. There was a quadratic relationship between body size and δ 15 N, suggesting that medium-sized individuals (110 cm < SVL < 160 cm) incorporated a greater proportion of high trophic prey into their diets than small (SVL < 110 cm) or large individuals (SVL > 160 cm). Tissue δ 13 C composition on the other hand was positively correlated with body size, indicating that different size classes were trophically linked to primary producers in different habitats. Individual-level analyses showed that small crocodiles were generalist feeders while medium and large size classes specialized on particular prey items within the food webs they fed. The findings further our understanding of ontogenetic variation in C. porosus diet, and suggest that change in C. porosus population size or demographics may be influential at various levels across the local food web.

A top predator forages low on species-rich tropical food chains

“…Other predators that feed on eels might exist that could extend the total length of the food chain in the tropics. These species include saltwater crocodiles (Crocodylus porosus) and various piscivorous wading birds, though none of these taxa were common in the relatively small streams examined here, and crocodiles show lower δ 15 N than would be expected of a predator at the top of a long food chain (Hanson et al 2015). The role of piscivorous birds in extending FCL is a key area for further investigation (Steinmetz et al 2003).…”

The global decline of freshwater megafauna

“…Freshwater megafauna species perform essential ecological roles and function as top predators or keystone species in their respective habitats (Bakker, Pages, Arthur, & Alcoverro, 2016;Hammerschlag et al, 2019;He et al, 2017;Moore, 2006). The extirpation of top predators, such as crocodilians and large piscivore fishes, causes the simplification of food webs, which in turn has severe impacts on ecological processes and functioning through trophic cascades (Hanson et al, 2015;Winemiller et al, 2015). This flow of causality ultimately results in the reduced resistance of whole communities and ecosystems to external threats (Brose et al, 2017).…”

Individual diet specialisation in sparrows is driven by phenotypic plasticity in traits related to trade‐offs in animal performance

“…We use a unique combination of isotope-based estimates of dietary variation and enzyme assays (maltase, sucrose and aminopeptidase-N) to quantify digestive plasticity. Stable isotope analysis has become an effective tool to examine IS through the estimation of individual and population isotopic variance using animals' tissues that represent diet over different timescales (Matich et al 2011;Hanson et al 2015, Bond et al 2016Maldonado et al 2017). Nitrogen isotope (d 15 N) values have been widely used to characterise trophic level and food-chain length (Post 2002) and here we measured d 15 N values of whole blood and feathers collected from each individual as a proxy of dietary variation to quantify individual and population trophic niches (Bearhop et al 2004;Vander Zanden et al 2010).…”