Summary
1. Understanding the trophic status of consumers in freshwater habitats is central to understanding their ecological roles and significance. Tadpoles are a diverse and abundant component of many freshwater habitats, yet we know relatively little about their feeding ecology and true trophic status compared with many other consumer groups. While many tadpole species are labelled herbivores or detritivores, there is surprisingly little evidence to support these trophic assignments.

2. Here we discuss shortcomings in our knowledge of the feeding ecology and trophic status of tadpoles and provide suggestions and examples of how we can more accurately quantify their trophic status and ecological significance.

3. Given the catastrophic amphibian declines that are ongoing in many regions of the planet, there is a sense of urgency regarding this information. Understanding the varied ecological roles of tadpoles will allow for more effective conservation of remaining populations, benefit captive breeding programmes, and allow for more accurate predictions of the ecological consequences of their losses.

http://fwf.ag.utk.edu/mgray/wfs493/Altigetal2007.pdf

What do tadpoles really eat? Assessing the trophic status of an understudied and imperiled group of consumers in freshwater habitats

Hunting live prey is risky and thought to require specialized adaptations. Therefore, observations of predatory cannibalism in otherwise non-carnivorous animals raise questions about its function, adaptive significance and evolutionary potential. Here we document predatory cannibalism on larger conspecifics in Drosophila melanogaster larvae and address its evolutionary significance. We found that under crowded laboratory conditions younger larvae regularly attack and consume 'wandering-stage' conspecifics, forming aggregations mediated by chemical cues from the attacked victim. Nutrition gained this way can be significant: an exclusively cannibalistic diet was sufficient for normal development from eggs to fertile adults. Cannibalistic diet also induced plasticity of larval mouth parts. Finally, during 118 generations of experimental evolution, replicated populations maintained under larval malnutrition evolved enhanced propensity towards cannibalism. These results suggest that, at least under laboratory conditions, predation on conspecifics in Drosophila is a functional, adaptive behaviour, which can rapidly evolve in response to nutritional conditions.

https://serval.unil.ch/resource/serval:BIB_90BFBB93A6C2.P001/REF.pdf

Predatory cannibalism in Drosophila melanogaster larvae

Summary

1. Understanding the trophic relationships of consumers is central to ecology, but constructing meaningful food webs is often difficult because of a lack of detailed information on consumption versus assimilation and high degrees of omnivory.



2. We used fatty acid analyses to examine the trophic relationships of three common larval anurans (Pseudacris crucifer, Lithobates catesbeianus and Lithobates clamitans) that are often classified as grazers or detritivores. Tadpoles and potential food sources were sampled in four ponds in southern Illinois and analysed for fatty acid composition. Single linkage cluster analysis was then used to compare fatty acid profiles among tadpole gut contents, tadpole muscle tissues and available food resources.



3. Diets varied among species and within species among ponds, but organic sediments consistently contributed most to the fatty acid composition of the gut contents of all species. Fatty acid profiles also indicated that larval insects and phytoplankton were consumed by both L. catesbeianus and L. clamitans in one pond, while L. clamitans and P. crucifer consumed mainly periphyton along with sediments in another pond, and these diet differences appeared linked to physical differences among ponds, with periphyton and/or phytoplankton contributing more to tadpole diets in less shaded ponds.



4. The fatty acid composition of muscle tissues of L. clamitans, the dominant tadpole in these systems, indicated that plant detritus and bacteria, which were the dominant components of organic sediments in the ponds, were common components of the assimilatory diet.



5. Results demonstrate the utility of fatty acid analyses for assessing both consumption and assimilation. The tadpole assemblages we examined derive much of their energy from heterotrophic and allochthonous sources and exhibit high dietary plasticity. This information will allow for more accurate and comprehensive assessments of trophic interactions in freshwater habitats, as well as aid in amphibian conservation, management and captive propagation efforts.

Fatty acid analyses reveal high degrees of omnivory and dietary plasticity in pond‐dwelling tadpoles

Although animals of many species kill and consume conspecifics, most such cases probably involve serendipitous encounters between the individuals concerned. In some taxa, however, cannibalism is an active process, with predatory individuals searching out and consuming specific types of conspecific prey items. Although anuran tadpoles often have been reported to consume conspecific eggs, this behaviour has been interpreted as a by-product of usual foraging behaviours rather than a result of targeted searching. Our field and laboratory studies in tropical Australia show that the tadpoles of invasive cane toads Bufo marinus are strongly attracted to chemical cues from conspecific eggs; the effective cues are released late in embryonic development, as the jelly coat breaks down. Tadpoles of native Australian frog species were attracted to the eggs of toads only rarely. If deployed as bait in traps, chemical cues from toad eggs could provide a way to selectively remove toad larvae from waterbodies.

Cues for cannibalism: cane toad tadpoles use chemical signals to locate and consume conspecific eggs.

Why be a cannibal? The benefits to cane toad, Rhinella marina [=Bufo marinus], tadpoles of consuming conspecific eggs

We performed two experiments to address the questions of whether toad tadpoles (Bufo a. americanus): (1) gain an advantage from consuming conspecifics and (2) engage in active cannibalism as opposed to scavenging. Our results show that tadpoles fed algal mats (and associated debris found within them) from their natal pond and supplemented on the bodies of dead tadpoles for 28 d attained an average of 5 more stages of development than tadpoles fed only on algal mats, which suggests a strong advantage to eating conspecifics. No tadpoles fed algal mats died, which suggests that this is an adequate food source. In a series of trials used to address the second question, we found no indication that tadpoles engage in active cannibalism. We tested this under conditions of hunger and using injured tadpoles as potential prey and progressively older tadpoles as potential cannibals. In no cases did we observe any tadpole attacking living tadpoles (even if injured) within 1-h after being placed in experimental tanks. Although some tadpoles were missing from tanks after 3-d periods, our results suggest that some (e.g., injured) tadpoles may succumb and are eaten thereafter. Test tadpoles began consuming dead crushed tadpoles significantly more quickly than dead intact tadpoles, suggesting that cues used to induce feeding are chemical and perhaps a result of some bacterial decomposition. We suggest that toad tadpoles have not evolved active cannibalism because of energetic and other costs and because mortality rates of tadpoles are naturally high in shallow ephemeral breeding ponds. Living tadpoles thus have access to dead tadpoles as a food source in such circumstances without incurring some costs of cannibalism.

Jennifer A. Abdella

Papers

On the Advantages of Putative Cannibalism in American Toad Tadpoles (Bufo a. americanus): Is it Active or Passive and Why?