Sunday, December 4, 2011

Different worms for different gulls

Old wives may say that the early bird gets the worm, but there are many worms that birds may not want to get.  These are the helminths (a.k.a parasitic worms), and whether they are early to the table or late, birds end up with a lot of them.  Helminths come in many shapes and sizes; nematodes are round and generally look like a worm, digeneans have two suckers and are solid all the way through, and cestodes (tapeworms) are basically a miniature toothy q-tip head trailing a toilet paper tail.  Birds and other vertebrate animals have such a diverse collection of these gut parasites that scientists have started to think about the insides of an animal as a mini-ecosystem and are asking whether ecological theory developed for larger animals also works in the same way for parasites.

A study published online this month in Ecography asked whether ring-billed gulls have more similar helminthes in their guts if they live closer to each other or if they are closer together in age.  The idea that as places become farther apart they have more different plants and animals is called ‘distance decay’ and biogeographers have gotten pretty excited about measuring it in the last ten years, even though for geographers it has been the ‘first law of geography’ since 1970.

For gulls living along 300 km of the St. Lawrence River in Quebec, Canada, the differences between the parasites living in different gulls was more related to their age than how far apart they lived from each other.  Older gulls had a wider variety of parasites than younger gulls because they forage for food in many places and ingest different types of parasites, whereas chicks and juveniles don’t venture as far.  However, when the researchers compared the parasites living inside Quebec gulls to those living 3000 km away in Alberta, their differences in parasites were much more related to how far apart gulls lived- age barely had any effect.

This distance decay in parasite communities has also been looked at in the parasites of fish, mammals, and molluscs.  The gull researchers wanted to know whether different characteristics of the host animal, such as its size and mobility, or whether it is homeothermic (warm-blooded) affects how quickly the difference between the parasites of two hosts increases as the distance between them increases.  One might think that animals that migrate across wide areas would have similar parasites even across large distances because each animal is likely to pick up most of the same parasites, whereas mussels (that are affixed to rocks on the ocean shore) could have very different types of parasites living inside them even across short distances.

Intriguingly, this doesn’t actually seem to be the case.  In a preliminary analysis of 25 studies of different host animals, the researchers found that only two factors affected the rate of distance decay in parasites; latitude and how widely across space animals were sampled in the study- host mobility didn’t matter at all.  Studies that analyzed parasites in animals across a large extent (1,000 -10,000 km) tended to find slower rates of distance decay.  Parasites in animals living farther north at higher latitudes had faster rates of distance decay.  That is, there is greater variation in the parasites that animals have over shorter distances. To me, it’s not exactly clear why this may be, but the researchers suggest that it could be because animals living up north have to deal with a wider range of climates.  As more scientists study the ecology of parasites it will be interesting to see whether host characteristics affect other aspects of parasite communities.

You can find this article at:

ResearchBlogging.orgLocke, S., Levy, M., Marcogliese, D., Ackerman, S., & McLaughlin, J. (2011). The decay of parasite community similarity in ring-billed gulls Larus delawarensis and other hosts Ecography DOI: 10.1111/j.1600-0587.2011.07244.x

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