Small Creatures, Large Effects

Arthropod predation of vertebrates structures trophic dynamics in island ecosystems (2021) Halpin et al., The American Naturalist, https://doi.org/10.1086/715702

Image credit: Bernard Dupont, CC BY-SA 2.0, via Wikimedia Commons

The Crux

Predator-prey dynamics are (I think) the most well-known species interaction out there. Not everyone is a scientist, but almost everyone has seen an image of a cheetah running down a gazelle, a great white shark exploding out of the water as it hammers a seal from below, or wolves teaming up on a much larger herbivore.

These interactions are not only fascinating and captivating, they are also key to structuring communities. For example, the damselflies that I worked with during my PhD occur in two different kinds of lakes: fish lakes and dragonfly lakes. The type of predator alters the lake significantly: damselflies that live in fish lakes are adapted to “hide” from their fish predators by not moving. Not moving in a dragonfly lake means that a dragonfly will eat you.

Though these interactions have been (justifiably) studied to an extreme extent, there are still knowledge gaps out there. Of interest for today’s study is the effect of invertebrate predators on vertebrate prey. While these invertebrate predator/vertebrate prey interactions have been studied in marine and freshwater environments, little work has been conducted in terrestrial systems. This is especially hard to do with invertebrate predators of vertebrate prey, because such predators tend to be hard to find, nocturnal, and they also hunt in more “concealed” environments like leaf litter. To overcome these challenges, today’s authors utilized the Phillip Island centipede (Cormocephalus coynei, which is NOT the centipede featured in this post’s image) and stable isotope analyses (see Did You Know) to understand how invertebrate predators structure food web dynamics.

What They Did

Did You Know: Stable Isotope Analysis

Stable isotope analysis is a technique used by many ecologists when it is either difficult or impossible to actually observe a given animal or species interaction. Isotopes are types of atoms that have very similar chemical properties to a given element (like carbon, oxygen, or nitrogen), but they do differ in their mass and physical properties. Stable isotope analysis investigates these isotopes and how they occur in various organic materials to understand where they came from/what they are. Think about the stable isotope analysis like reading a recipe. If you look at a recipe and it says something along the lines of “crust, pizza sauce, cheese, and veggies” then you know you’re probably analyzing a pizza.

The authors first conducted several surveys of Phillip Island to identify the range of species that the centipedes prey on. They used survey plots and transects. The survey plots were 100 sq. meters, and were surveyed for 30 minutes at a time each night. The transects were 300 meters long, and the authors searched along them for the presence of the centipedes. They noted the body length of the centipedes, as well as the prey species if the centipedes were hunting (stalking or striking at a prey item) or consuming a specific prey item. In addition to these observations, the authors also used stable isotope analyses to further investigate the many species that the centipedes prey on.

What They Found

Through their observations, the authors found that the centipedes consumed 11 different types of prey, including birds, spiders, insects, lizards, and even fish (though these fish had previously been regurgitated by another predator, not hunted down by the centipede itself, but I wouldn’t put it past a centipede to do that….). Crickets, however, were the most targeted prey item by far. Of interest were the predation events on the nestlings of the black-winged petrols, as these birds lay a single egg and this predation completely negates the reproductive effort of those birds.

From their analyses of stable isotopes, the authors confirmed their observations that crickets make up the majority (~52%) of the diet of these centipedes. However, the other ~48% was made up of vertebrates like geckos, fish, skinks, and birds. The below figure summarizes how energy flows through this system, with centipedes acting to drive the majority of the nutrient exchange.

Fig. 5 from today’s paper. Blue arrows denote the predicted direction of the links between the various animal species, with the thickness of the arrow denoting the proportion of the diet that by a given prey item. Black arrows are the hypothesized links between the various animals, and the grey arrows show how all of these animals contribute to the nutrient flow in this system (Image credit: Halpin et al. 2021).

Problems?

Sampling organisms in the field is super tough, and it involves an absolute boatload of work. For this study, the authors did not invasively sample as they only observed the forest floor. Because of this, they likely missed underground predation events or those occuring deeper in the leaf litter. This means that some underground species are likely underrepresented in the modeling framework.

So What?

Today’s authors were able to demonstrate that invertebrate predators only facilitate energy flow from marine systems to terrestrial ones via predation on seabirds and fish, but they can also exert strong pressure on vertebrates by consuming their offspring. Because gigantic predatory arthropods like these are common on islands, the results from this study indicate that there may be a role for invertebrate predators in structuring island food webs.


Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who saw a centipede this week, but is wasn’t nearly as cool as these are. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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