recent papers on invasive species and birds

In the fine tradition of Snail's Tales and Afarensis, a recap of some interesting new papers I recently read.

• Jones, Z.F. and C.E. Bock. 2005.  The Botteri's Sparrow and exotic Arizona grasslands: an ecological trap or habitat regained?  Condor 107:731-741.

Botteri's Sparrows (Aimophila botterii) are non-descript birds of tall grasslands in the southwestern U.S. (So non-descript, I chose to illustrate the grass instead!)  Due to heavy grazing, they pretty much disappeared from Arizona grasslands at the turn of the 20th century. 
As the landscape changed over the next 70 years, the were once again nesting in Arizona, in local patches of a tall native grass, sacaton (Sporobolus wrightii).  Meanwhile, a couple of species of lovegrass (Eragrostis spp.) were introduced from Africa around the 1930s. 

In this study, spanning three years, 18 plots, and 314 nests, the authors found that flightless young Botteri's Sparrows required dense cover to avoid predation, just the kind of cover provided by Eragrostis. They concluded that rather than being an ecological trap (e.g., superficially similar to the sparrow's preferred habitat, but lacking some critical component), the introduced Eragrostis was providing an essential service.  While sparrow abundance was intermediate between native upland grasslands and native sacaton, the sacaton is less common, so the exotic lovegrasses are providing increased opportunities for Botteri's Sparrows, and may help them regain their prior abundance. It's not often you hear of a "good" introduced species story, although I'm sure there are more downsides to lovegrass invasion than benefits.  Invasive Species Weblog mentioned Eragrostis here.

• Freed, L.A., R.L. Cann, M. L. Goff, W.A. Kuntz, and G. R. Bodner.  2005.  Increase in avian malaria at upper elevations in Hawaii.  Condor 107:753-764.

Hawaii is full of endemic bird species, but 40% of them are extinct, and half of those that remain are endangered.  Hawaii's birds evolved in the absence of avian malaria (Plasmodium relictum); there were no native mosquito vectors in the islands.  A major mosquito vector was introduced in the early 1800s, followed by numerous species of non-native birds, some of which carried avian malaria.  The stage was set for disaster.

Many of Hawaii's birds succumbed to avian malaria.  Some survived by retreating to higher elevations (about 1500 meters), above which the mosquitoes become rare. This study documents the spread of avian malaria above the 1500 meter mark, up to the limit of the appropriate forest habitats at 1900 meters.  This was first detected a dozen years ago, and the instances of infected birds at higher elevations are accelerating.  The authors attribute the spread of avian malaria to increasing temperatures, which facilitate the survival of the mosquito vector, and discuss the weaknesses of alternate hypotheses. 

A few species are showing some evidence of evolving a tolerance to avian malaria.  This would be encouraging, except that as some more common species evolve tolerance, they become reservoirs of the disease, increasing risk to rarer species that are vulnerable to avian malaria. Clearly, this is another example of the complicated impact of climate change.

Comments

[Wisecrow]

On a positive note, some evidence suggests that weaker and less virulent strains of malaria and pox are being selected for (which makes sense - killing all of your hosts is not a good strategy). Too bad so many wonderful species of honeycreepers had to be lost before this happened (Damn Evolution - never does exactly what you want it to :-)). The Hawaiian strain of malaria is one of the least virulent strains known (which is why introduced birds are rarely affected), according to Doug Pratt. I would highly recommend his recent book "The Hawaiian Honeycreepers", which is worth having for the illustrations alone. Unfortunately, Oxford Press has decided to charge a staggering $250 for the book. Go figure.

[Aydin]

I finally had a chance to read this post. I didn't know birds could get malaria. I've always wondered whose blood mosquitos suck at remote places where there are no humans. I guess they go to birds.

[stacey proudlock]

i woundered if it would be possible to have the email addresse of M.L Goff as i require a number of his journals for my 3rd year dissertation.

[Nuthatch]

Stacey, you'll have to look up the cited paper and contact the corresponding author, find a paper in which Goff is the corresponding author, or contact his organization/institution. I don't know him.