Divided and Conquered

Image credit: Alex Proimos, CC BY-NC 2.0, Image Cropped

Experimental habitat fragmentation disrupts nematode infections in Australian skinks (2019), Resasco et al., Ecology. https://doi.org/10.1002/ecy.2547

The Crux

Habitat destruction is an all-too-familiar side effect of human development and expansion. But another prevalent issue is habitat fragmentation, whereby habitat isn’t completely destroyed, but instead broken up into fragments and separated by developed areas. While some may think this is good, because there is still habitat available for wildlife to inhabit, the disconnected nature of what is left makes it very difficult for most wildlife to thrive, as they require much more connected landscapes.

Though fragmentation has been well studied in the past, less is known about how it affects parasites. Because they depend on other organisms for their own survival, parasites in particular are at risk of local or even extinction due to the cascading effects of species loss (i.e., coextinction, see Did You Know?). The complex nature of many parasite life cycles, in addition to a scarcity of experimental studies, makes it difficult to predict what effects that fragmentation will have on parasites. Today’s authors used a long-running, large-scale fragmentation experiment (The Wog Wog Habitat Fragmentation Experiment) to determine how fragmentation affects host-parasite interactions.

Did You Know: Coextinction

Extinction is when one species dies out, whether due to environmental change or a cataclysmic event like a meteor (or more commonly these days, through human intervention). Coextinction, on the other hand, is when one (or more) species goes extinct when an organism that they depend on goes extinct. Think parasites that rely on a specific host species to complete their life cycle. If that host goes extinct, the parasite species will soon follow.

What They Did

The authors used the pale-flecked garden sunskink (Lampropholis guichenoti) as their host species and a nematode (Hedruris wogwogensis) as the parasite. This nematode is a trophically-transmitted parasite, meaning that the skink has to eat infected prey items to become infected itself. The authors checked pitfall traps to count how many prey items and skinks they were catching in the study area, in addition to checking both the prey and the skinks for parasites. Relating the degree of fragmentation to parasite prevalence (i.e., the proportion of the population with parasites) in both the prey and skinks allowed them to understand how fragmentation affected parasitism in this host-parasite system.

The skinks and nematodes in today’s study can be found in the South East Forests National Park in New South Wales, Australia. (Image credit: Andrew Purdam, CC BY-NC-ND 2.0)

What They Found

The authors found that as habitat became more fragmented, the number of prey declined, yet the number of skinks increased. They also found that the prevalence of nematodes in both the prey and skinks declined as fragmentation increased. Specifically, one finding was that fragmentation reduced the amount of nematodes in prey, which reduced the amount of nematodes that could then infect skinks.

Problems?

My only major comment is that I would have liked to see this study include the entire life cycle of the parasite. That is, the skink and prey (an amphipod) were only two of the three (or more) host species involved in the development of this parasite. The amphipod is what’s known as an “intermediate host”, which means that it is neither the first nor final host. I’m curious as the know how fragmentation affects the entire life cycle. If, for example, fragmentation actually increased the abundance of the first host, then we might expect it to negate or “balance out” the loss seen at later stages in the parasite’s life.

So What?

Fragmentation is only going to get worse in the future as humans continue to develop and change the natural world. Studies like this one allow scientists and policy makers to make informed predictions about how such fragmentation will affect native organisms. My hope is that seeing results like these, where fragmentation is directly linked to species loss, will improve protections for wild spaces.


Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who hopes that he isn’t secretly harboring a nematode or five. 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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