Monday, October 23, 2006

The Good, the Bad, and the Confounding: Biological Response to Climate Change

Environmentalists are concerned that current and future climate change could adversely affect life on earth, causing many species to go extinct and dramatically altering the composition of ecosystems worldwide. In response to climate change, organisms may undergo ecological and/or evolutionary changes to avoid extinction. Ecologically, populations may shift their ranges to stay within an optimal climate envelope. For example, populations in Virginia might expand their range north to New England. Populations may also, simultaneously, adapt to changing climate by altering their optimal climate envelope. Rather than expanding into New England, organisms may stay in Virginia and become accustomed to South Carolina weather. In this post, I will discuss three recent scientific findings that weigh in on this matter.

First, the good news, sort of. A recent paper in Science examined a well-characterized species of fruit fly, Drosophila subobscura. D. subobscura is native to Europe, but invaded North and South America within the last 30 years. The genome of D. subobscura, like many other species, contains chromosomal inversions. The details are complex, but essentially, chromosomal inversions allow for gene complexes to be passed on intact from parent to offspring. Recall that, normally, the process of recombination breaks genes apart, such that every gamete (sperm and egg) is a unique combination of the maternal and paternal genomes. Researchers have known for many years that chromosomal inversions vary in frequency with latitude, a strong indication that they are associated with adaptation to local climate. In this paper, researchers took historical data on chromosomal inversion frequency and climate on all three continents where D. subobscura now lives. They found that, in less than three decades, chromosomal inversion frequencies shifted accordingly with changes in climate. For example, chromosomal inversions once found in Spain might now be found in France. The importance of this study is that 1) as we already knew, climate really is changing; 2) organisms have the ability to adapt; and 3) they can adapt fast and on a human time-scale.

Second, the bad news. If every organism could perfectly compensate for changing climate, then perhaps climate change wouldn’t have much impact on biology. Unfortunately, this simply isn’t the case. D. subobscura, like many other small organisms, is special in that generation times are short and population sizes are often large, two factors that are known to facilitate rapid evolution. In contrast, other organisms of have long generation times (e.g. all trees) and small population sizes, and may not perfectly adapt to changing climate. However, a study published in this month’s American Naturalist suggests that even small insects like D. subobscura may not be safe. Using physiological data from 65 insect species occupying many climates, researchers found that cold-adapted species grow more slowly in their optimal climate envelope than warm-adapated species in their optimal climate envelope. Even though natural selection allows some species to inhabit cold climate, it cannot fully compensate for thermodynamics. Extrapolating a bit, this study suggests that as warm-adapted species expand their range north (or to higher elevation) they will likely displace cold-adapted species that do not grow as quickly. This study only examined insects (which, along with flowering plants, make up the vast majority of earth’s biodiversity), so it remains to be seen whether similar patterns emerge in other taxa with differing behavior and physiology.

And finally, the confounding. Organisms are not merely adapted to abiotic factors such as climate, but to other organisms as well. A paper published Nature a couple years ago, reported on the results from experimental ecosystems containing 3 species of Drosophila and a parasitic wasp. The enclosed ecosystems had varying temperatures, thus mimicking climatic changes associated with latitude. The details are not important, but the take home message from the study was that optimal climate envelope did not accurately predict changes in species composition at different temperatures. They conclude, therefore, that biological communities may respond idiosyncratically to climate change, causing many species to be extirpated from large parts of their range, while others expand their range rapidly.

Together with other studies, these three papers confirm that natural selection is indeed rapid and potent, but not perfect. Also, ecological factors may confound our ability to predict and therefore mitigate adverse affects of climate change. Taken together, such studies demonstrate how basic biological research can inform (and complicate) decisions on important environmental issues, if politicians care to take notice.

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