In an effort to make order out of the chaos of existence, scientists often resort to classifying stuff.  To make order of the natural world, ecologists classify different regions of the world into distinct biomes – large geographical areas with characteristic groups of organisms adapted to that particular environment.  Familiar examples of terrestrial biomes are tropical forests, temperate grasslands and desert, and in the aquatic world examples include open ocean, coral reefs and rivers. But what happens at ecotones, where two or more biomes come together? Research has shown that ecotones can be biodiversity hotspots, as the diverse habitats attract many different species, and may also attract edge specialists – species that are particularly adapted to conditions on the border between the two biomes.

Sara Weinstein collects data at the ocean to land ecotone. Credit: Anand Varma.

Sara Weinstein’s graduate research explored the ecology and transmission of raccoon roundworm, Baylisascaris procyonis, a widespread raccoon parasite that causes severe disease in other animals (including humans).  She was dissecting raccoons to study infection patterns and as she describes “it would have been a waste of perfectly good raccoon guts to not also examine the rest of the parasite community.”  This examination would allow her to determine whether the generalization that ecotones are biodiversity hotspots for terrestrial and aquatic organisms also applies to the much more murky world of gut parasites.

A raccoon poses next to a culvert. Credit: SB Weinstein.

Working with four other researchers, Weinstein compiled a database of published accounts of gastrointestinal parasites from surveys of 256 raccoon populations.  They then used this database to classify parasites as either core or satellite.  Core parasites are locally abundant, common over a large region and can occupy a broad ecological niche.  Satellite parasites are rare, restricted to a small portion of a region and have narrow ecological niches.

Microphallus sp. – a group of relatively rare satellite trematodes collected from a raccoon gut. Credit: SB Weinstein.

Weinstein and her colleagues found that the data divided raccoon gut parasites into two distinct groups.

Top graph. Parasite frequency across raccoon populations. Most parasite genera were found in less than 10% of the raccoon populations.  Dashed line indicates 30% cutoff between satellite and core genera.  Bottom graph. Proportion of raccoons infected with each parasite  in relation to range-wide prevalence.  Larger data points indicate more populations surveyed for a given parasite.

There were eight taxa (genera) that were found in more than 40% of raccoon populations. In contrast there were 51 genera that were found in fewer than 30% of raccoon populations, with the vast majority of these found in fewer than 10% of raccoon populations in the survey (top graph on left).  The eight common taxa – core parasites – also tended to be present in more individuals within each population than did the 51 less common genera of satellite parasites (bottom graph on left).

Having defined core and satellite parasites, the researchers then did a thorough analysis of the gut contents of 180 raccoon collected by trappers and animal control agents in Santa Barbara County between 2012 – 2015. They hypothesized that the prevalence of core parasites should not be overly affected by ecotones.  In contrast, satellite parasites should increase in ecotones, because ecotones provide unique environmental conditions that would be suitable to some of the less common species in the parasite community.

In Santa Barbara County, Weinstein and her colleagues identified four core parasites and nine satellite parasites within the population, with a mean of 2.24 parasite species per raccoon. Racoons nearer to the marine ecotone harbored more parasite species than did raccoons more distant from the marine ecotone, a result of much greater richness of satellite species (left graph below). The story was very different for the freshwater ecotone.  Overall, parasite richness was relatively constant in relation to distance from the freshwater ecotone.  There were actually fewer core parasites but more satellite parasites near the freshwater ecotone (right graph below).

Left graph. Total parasite richness (orange line) in relation to distance from shore.  Satellites (orange fill) increased in abundance near the shore, while core parasites (maroon line) were steady. Right graph. Total parasite richness in relation to distance from freshwater.

Why did core parasite richness decline near the freshwater ecotone?  Weinstein and her colleagues believe that diet may play an important role.  For example, the core parasites Atriotaenia procyonis and Physoloptera rara were more common in raccoons far from freshwater, probably because racoons are infected by these two parasites as a result of eating terrestrial (but not aquatic) insect species that are intermediate hosts for these two parasite species.  As it turns out, these intermediate insect hosts prefer upland habitats that tend to be located relatively distant from the freshwater ecotone.

Increased abundance of rare parasites at ecotones has important implications for human health.  Several emerging infectious diseases, such as lyme disease, yellow fever and Nipoh virus are associated with ecotones. Habitat development by the expanding human population is causing increased habitat fragmentation, creating more ecotones, and potentially increasing the prevalence of these and other, equally unfriendly, parasites.

note: the paper that describes this research is from the journal Ecology. The reference is Weinstein, S. B., J. C. Van Wert, M. Kinsella, V. V. Tkach, and K. D. Lafferty. 2019. Infection at an ecotone: cross-system foraging increases satellite parasites but decreases core parasites in raccoons. Ecology 100(9):e02808. 10.1002/ecy.2808.  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.