Monday, March 12, 2012

Galapagos 2012 - Day 4-5 (Blood Sucking Maggots)


Day 4 was a diving day, with manta rays, garden eels, and hammerhead sharks to distract me from eco-evolutionary musings. But on day 5 it was back to work; this time in the wet highlands (picture below) rather than the dry(ish) lowlands. Our goal in sampling these multiple sites is to examine morphological variation in finches across different habitats in order to gain further insight into the ecological forces driving evolutionary diversification. The highlands are simultaneously invigorating and depressing. Invigorating because they are lush and wet – like a miniature rainforest – depressing because so little habitat is left and much of it is over run by invasive species. The most dramatic of these is the yummy blackberry, which carpets the ground below the trees. It is so stressful to the park service that their solution for control is simply spray to roundup. This kills the blackberries plenty dead but it also kills all the native plants. Go figure.

Invasive species are known to be the drivers of contemporary (rapid) evolution the world over, so why should it be any different in Galapagos? This famous crucible of evolution is presumably still crucibiling - perhaps more so now that invasive species are becoming so widespread. For instance, Darwin’s finches are famous for evolving rapidly in response to changing environmental conditions: larger beaks evolve when small seeds are depleted during droughts – but larger beaks evolve during droughts when a larger beaked competitor depletes the larger seeds. Remarkably, it seems that a major player in this evolution might be an invasive plant. By “might” I mean that the major player is known – a very large/hard seed of the plant Tribulus cistoides – but it might or might not be invasive depending on who you talk to. Perhaps one of best examples of evolution in action in nature is actually driven by an invasive plant species.

Now it seems that Darwin’s finches are experienced a dramatic new selective pressure – parasitism. Sometime in the past few decades the fly Philornis downsi invaded Galapagos and began to parasite the nestlings of native Galapagos birds, including at least 11 finch species. The adult flies are not parasitic but their spawn are – they creep out of the nesting material at night to suck the blood and juices of the nestlings. The earliest stages (instars) of this parasite even live inside the nostrils of the poor little finch babies. Now this sounds nasty enough but it looks even nastier. If you pick apart the nests of finches, you find large numbers of big fat juicy grubs in the nesting material (picture below) and the nestlings themselves show rather large and hideous holes in them from the nocturnal activities of their unwelcome nest mates.

Adding injury to insult, these blood sucking maggots seem to have a huge impact on nestling survival and therefore the reproductive success of finches – as shown by a number of researchers (see citations below). For instance, some studies show very high failure rates of infected nests – and infected nests are very common. If infections levels are experimentally reduced, fledging success goes up. Given that these parasites seem to be wide spread on the islands, great concern has been raised regarding the possible negative impacts on finch populations. So why, then, do finches seem to be as abundant as ever. And why, in some years, do we catch lots of fledglings? How can we reconcile very high nest failure with demonstrable evidence of nesting success? One hypothesis is that the timing of parasitism is tied to the first clutch of finches, which then fails, but not the second clutch, which then succeeds. Another hypothesis is that some (unknown) areas of low parasitism are providing a pool of finch immigrants that are keeping populations high even in areas of severe nesting failure. Or maybe we have yet to see major impacts simply because finches are very long lived and, when the adults produced before the current major infestations die of old age or other insults, the populations will start to decline. My own hypothesis is that fledging success may not be the limiting factor in Darwin’s finch population size – perhaps it is instead food resources for adults during dry periods. If so, a decline in the number of fledglings owing to parasitism won’t cause the population to decline.

What remains indisputable is that these darn maggots do kill lots of fledglings, so selection would presumably favor the evolution of adaptive responses by the finches. Some work has shown that finches do show acquired immunity, meaning that exposure to parasites increases subsequent resistance to (or tolerance of) parasites, but whether or not this or other forms of resistance/tolerance are evolving is not certain. This seems to me a great place to look for eco-evolutionary dynamics. That is, one could test whether resistance or tolerance were evolving and then assess the impact of this evolution of finch population dynamics. This is not for me, however, as it would require studying nocturnal maggots that suck the blood of cute little nestings.

Sadly, this is the end of my short but productive trip to Galapagos. I am now sitting in the Guayaquil Airport waiting for thundershowers to clear in hopes of catching the red eye to Miami.

Ah, it seems we are finally boarding ...


References
1.Sarah K. Huber et al. 2010. Ecoimmunity in Darwin’s Finches: Invasive Parasites Trigger Acquired Immunity in the Medium Ground Finch (Geospiza fortis). PLoS ONE 5:e8605.

2. Jennifer A. H. Koop et al. 2011. Experimental Demonstration of the Fitness Consequences of an Introduced Parasite of Darwin’s Finches. PLoS ONE 6:e19706.

3. Sarah K. Huber. 2008. Effects of the introduced parasite Philornis downsi on nestling growth and mortality in the medium ground finch (Geospiza fortis). Biological Conservation 141:601-609.

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