Image Credit: Maria Grist, CC BY-SA 4.0, Image Cropped

Platypus predation has differential effects on aquatic invertebrates in contrasting stream and lake ecosystems (2020) McLachlan-Troup, Scientific Reports, https://doi.org/10.1038/s41598-020-69957-1

The Crux

A trophic cascade occurs when a predator’s effects of its prey goes on to affect ‘lower’ levels of that ecosystem. A great example is the effect that sea otters have on kelp: the sea otters prey extensively on sea urchins, which in turn increases the populations of kelp, which the sea urchins prey on. While this is a result of direct predation by otters, often this can occur through a prey species changing its behaviour to avoid the predators.

Yet most ecosystems are more complex than a simple three-level trophic system. Cascades are therefore more likely to occur when the ecosystem is less complex, or when there are well-defined relationships between species, as a result of a predator having preferred prey species or only a few groups of species making up an ecosystem.

This week’s authors investigated how the platypus (our recently-found-to-be-fluorescent friend) influences the abundance and species richness of invertebrates across both rivers and lakes, and whether it’s capable of affecting an ecosystems algae and sediments as well.

What They Did

Two different ecosystems were used, one river in New South Wales, and a shallow lake in Tasmania (thought to be the less complex ecosystem of the two). The researchers set up four cages in the river and six in the lake. These cages allowed small fish and invertebrates into the cages, but prevented platypus from getting in. They also placed an equal number of open cages in both habitats.

These cages were left for six weeks, before the invertebrate, algal and sediment communities were sampled. Further sites among the cages were also sampled, to test whether the presence of the cages themselves changed the immediate invertebrate community.

Did You Know: Platy-Pain

Platypus are not only one of only three monotreme species (a very distinct type of mammal), they are also one of the only venomous mammals in the world. The venom is delivered in male platypus through two spurs located near the back flippers, which can lock into predators (or a human picking up a platypus by anything except the tail), causing immense (bot non-fatal) pain. As an undergrad student I was in a lecture once when Professor Kath Handasyde related the story of a colleague who decided to find out whether the pain was caused by the venom itself or having two massive spurs digging into your arm.

Turns out it was the venom.

What They Found

Across streams, invertebrate abundance and species richness was significantly higher in areas from which platypus were excluded. The same pattern generally was seen across the different groups of invertebrates, though it was less stark in herbivores. Omnivores, detritivores and predators were the invertebrates most affected by platypus predation. There was no effect of platypus on algal biomass or sediment composition.

Problems

As always, getting a high enough number of replicates to ensure reliable results is a matter of time and money, neither or which are often found in massive quantities. However the samples sizes here are very small, and boiling results which compare lakes to rivers down to one of each results in very large assumptions. The authors recognise this, and I hope we can see more experiments like this in the future which build on their work.

So What?

The fact that the lake ecosystem showed little evidence of an effect of platypus predation shows that our assumptions about ecosystem complexity are not always going to be accurate. The lake theoretically should have been the more simplistic of the two ecosystems, yet the stream showed a much stronger effect of platypus predation.

The lack of an effect by platypus on sediments or algae is probably down to the fact that herbivorous invertebrates were the least affected of the groups. Whether this is a product of platypus having specific preferences or a result of the low sampling sizes used is hard to tell.

Experiments like these, which are really simple and yet can tell us so much about predator-prey dynamics, are easily my favourite to read about. I hope the work here is expanded on in future years, as the platypus is a culturally significant and fascinating species, and any information about its importance in freshwater systems is welcome.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who remains fascinated by one of only three monotremes. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.