Sexual dimorphism in a top predator (Notophthalmus viridescens) drives aquatic prey community assembly (2018) Start & De Lisle, Proceedings B, doi:10.1098/rspb.2018.1717 Proceedings B

The Crux

Ecology is a scientific discipline focused on the interactions between the biotic (living) and abiotic (non-living) parts of the environment, and within ecology the subdiscipline of community ecology focuses on how these biotic and abiotic parts interact to determine what species live where. When researchers investigate these relationships, they tend to only consider differences between species, instead of differences within a single species. This means that we are missing a big part of the picture, as differences within a single species can outnumber those between multiple species.

One of the most common differences within a species are those between males and females. Depending on the species in question, one sex can be bigger, eat more, live longer, or eat different things, and this can have an effect on the community that the species in question lives in. Despite these many differences between the sexes, there weren’t any direct empirical examples in the scientific literature of these differences affecting community dynamics. The authors of this paper were the first to use an experiment to investigate this phenomenon, using the red-spotted newt (Notophthalmus viridescens), which is an important predator in aquatic communities.

Did You Know: Sexual Dimorphism How do you tell of a deer is male or female? Look for antlers, of course. This difference between the sexes is known as sexual dimorphism, and there are plenty of examples in nature of this phenomena. Female spiders are larger than male spiders, male elephant seals are MUCH bigger than their female counterparts, and human males tend to be larger than human females.

What They Did

The researchers set up 10 artificial ponds, stocking them with leaf litter, plankton, and large invertebrates. Each pond received the same amounts and kinds of leaf litter, plankton, and large invertebrates, but 5 of the ponds were female-biased (16 females and 9 males, while the other 5 received the opposite sex-ratio. After two months, the researchers ended the experiment and collected the different prey for counting.

Because male and female newts of this species differ in the type of habitat that they tend to hunt in, the experimental design used in this study allows for the detection of differences in community structure due to differences in the sexes. Females hunt along the bottom of the pond, while males tend to swim more in the water column to hunt. As such, male-biased ponds could be expected to have less swimming prey, while female-biased ponds could have less prey that “walk” along the bottom.

What They Found Out

The sex ratio of the ponds affected the abundance of the different prey groups (plankton and large invertebrates), and also affected the abundance of the different prey types (swimming vs. “walking”).

There were more swimming prey in the female-biased ponds, while male-biased ponds had more “walking” prey. This is a reflection of the different hunting tactics employed by the two sexes, as having more of a given sex meant that they would have a larger effect on (eat more of) the prey in their normal hunting habitat.

Problems

This experiment, while well-designed and executed, did not exhaustively sample the invertebrates from the cattle tanks. It was not logistically possible to filter ~27,000L of water to count out and identify every single prey item, so the researchers used a pipe to sample the water column.

While some reviewers had issues with this method, and pointed out that they may have missed the more fast-swimming prey, or the prey that lives UNDER the surface itself, the authors point out why this is unlikely in the paper itself.

So What?

This study is the first to experimentally demonstrate that sexual dimorphism can have community-level effects. This is not a new idea in ecology, but all of the previous work has been correlational and as such cannot determine the cause and effect relationship between sexual dimorphism and community structure.

Though this study used newts, these results can be applied to make predictions in other communities. Any predator species that exhibits sexual dimorphism in the form of hunting behavior, prey preference, or the amount of prey consumed can theoretically have different effects on prey species and alter the structure of the communities that they inhabit.