As human populations expand, we are converting ecosystems from one state to another. In the case of tropical forests, conversion of forest to cropland may leave behind fragments of relatively undisturbed forest surrounded by a matrix of cropland or other forms of development. Conservation ecologists are exploring whether ecological processes and ecosystem structure in these fragments work pretty much like normal forested regions, or whether fragments behave differently. To do this, in a few locations around the world such as the Wog Wog Fragmentation Experiment in New South Wales, Australia, researchers have systematically created forest fragments of various sizes. They can then ask a variety of questions comparing fragments vs. intact forest. For example, how does species diversity, or how do processes such as competition, predation and mutualism differ in the two landscapes?
Julian Resasco was working as a postdoctoral associate in Kendi Davies’ lab at the University of Colorado on a study that looked at changes in invertebrate communities in response to fragmentation at Wog Wog. Beginning in 1985, researchers had set up a network of pitfall traps, which are cups that are buried with their tops level to the ground, so that any careless organism that wanders in will be trapped in the cup. Some pale-flecked garden skinks, Lampropholis guichenoti, also had the misfortune to become entrapped and became subjects for the study. The invertebrates, and the 186 unfortunate skinks were preserved in alcohol and stored as part of the Australian National Wildlife Collection.
Much later, Resasco arrived and began dissecting skink guts to analyze the prey items for a study that looked at how the skinks shifted prey consumption (their feeding niche) in response to fragmentation. While dissecting the skink guts, he noticed that some of the skinks had worms (nematodes) inside their guts. These nematodes were relatively common among skinks from continuous eucalypt forests, rare among skinks from eucalypt fragments, and absent from skinks in the cleared, pine plantation matrix.
As it turned out, the nematode was a new species, which Resasco and a colleague (Hugh Jones) named Hedruris wogwogensis. Nematodes in the genus Hedruris use crustaceans as intermediate hosts, which alerted Resasco and his colleagues that the terrestrial amphipod Arcitalitrus sylvaticus, which was very common in the pitfall traps, was probably an important intermediate host. When amphipods from pitfall traps were examined microscopically, a small portion of them were infected with Hedruris wogwogensis. The researchers concluded that amphipods became infected when they ate plants that harbored nematode eggs or young nematodes, which then developed in amphipod guts, and were passed on to skinks that ate the amphipods. Thus somewhat inadvertently, one aspect of the study transitioned into the question of how fragmentation can influence the transmission of parasites.
After concluding their skink dissections, Resasco and his colleagues discovered that skinks in continuous forest had five times the infection rate as did skinks in fragmented forest. In addition, no skinks collected in the matrix were infected. Infected skinks harbored a similar number of nematodes, whether they lived in continuous forest or fragments (see the Table below). Lastly, amphipods were considerably more common in skink guts and pitfall traps from continuous forest, less so in fragments, and least in the matrix.
The researchers put these findings together in a structural equation model. The boxes in the model below represent the variables, while the numbers in smaller boxes over the arrows are the regression coefficients, with larger positive numbers (in black) indicating stronger positive effects, and larger negative numbers (in red) indicating stronger negative effects. The model revealed three important findings. First, habitat fragmentation strongly reduced amphipod abundance. High amphipod abundance was associated with high nematode abundance (that is the +0.20 in the model), so lower amphipod abundance from fragmentation reduced nematode abundance. Second, habitat fragmentation positively affected skink abundance – more skinks were captured in fragments than in intact forest, but this increase had no effect on nematode abundance in skinks. Finally, note the direct arrows connecting “Fragmentation” to “Log nematode abundance in skinks”. This indicates that other variables (beside amphipod abundance) are reducing infection rates in skinks that live in fragments and the matrix.
At this point, we still have an incomplete understanding of the system. We know that fragmentation reduces amphipods, which require a moist and shaded environment to thrive. Reduced amphipod abundance leads to lower nematode infection rates in skinks. But we know that other variables are important as well; perhaps nematodes survive more poorly in fragment and matrix soils. Interestingly, pine trees were planted in the matrix and are beginning to mature and shade out the matrix environment. Amphipod abundances are on the rise, so the researchers predict that nematode infection rates will begin to increase accordingly. Those studies have begun.
Looking at the bigger picture, it is clear that fragmentation may decrease (as in this study) or increase the abundance of an intermediate host. As an example of fragmentation increasing intermediate host abundance, the researchers describe a study in which fragmentation increased the abundance of the white footed mouse, an intermediate host for black-legged ticks (that host the bacteria that causes Lyme disease). We need to unravel the connections between landscape factors and the various species they influence, so we can begin to understand how human changes to the landscape can influence the transmission of diseases.
note: the paper that describes this research is from the journal Ecology. The reference is Resasco, J., Bitters, M. E., Cunningham, S. A., Jones, H. I., McKenzie, V. J., and Davies, K. F.. 2019. Experimental habitat fragmentation disrupts nematode infections in Australian skinks. Ecology 100( 1):e02547. 10.1002/ecy.2547. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.