Birds moving due to climate change, but perhaps not fast enough

Via PhysOrg:

Tropical birds are moving to higher elevations because of climate change, but they may not be moving fast enough, according to a new study by Duke University researchers.

The study, published Thursday in the peer-reviewed online journal PLoS ONE, finds that the birds aren't migrating as rapidly as scientists previously anticipated, based on recorded temperature increases.
The animals instead may be tracking changes in vegetation, which can only move slowly via seed dispersal.

This doesn't seem too surprising. If, after all, the birds are tied to specific types of plants due to their life cycle, then moving in such a way as to maximize only their climatic preference won't be useful; they'd be missing a key component of their life cycle. The process of species adapting to the effects of climate change are now being discussed and published in the scientific literature.

Evidence from temperate areas, such as North America and Europe, shows that many animal and plant species are adapting to climate change by migrating northward, breeding earlier or flowering earlier in response to rising temperatures.

Indeed, the USDA relatively recently changed its hardiness zone maps due to already changed climate conditions. But that's what's happening in the northern latitudes (and we can expect that analogous things are happening in southern latitudes. However, what is happening with habitat changes in the tropics?

"However, our understanding of the response of tropical birds to warming is still poor," said German Forero-Medina, a Ph.D. student at Duke's Nicholas School who is lead author of the new study. "Moving to the north doesn't help them, because tropical temperatures do not change very much with latitude. So moving up to higher elevations is the only way to go, but there are few historical data that can serve as baselines for comparison over time."

What is going on with tropical species at higher altitudes is important, Forero-Medina said, because about half of all birds species live 3,500 feet or more above sea level, and of these species, more than 80 percent may live within the tropics.

Oh, shit. What this is saying is that about 40% of all the bird species in the world are known to live in the tropical one at above 3,500 feet (~1,000 meters), which means that understanding how this group of species will adapt (or not) to climate change will be crucial when considering the vast diversity of bird species.

The biologists found that although the ranges of many bird species have shifted uphill since Terborgh's [ornithological observations in mountainous central Peru in the 1970s], the shifts fell short of what scientists had projected based on temperature increases over the four decades.

"This may be bad news," Pimm said. "Species may be damned if they move to higher elevations to keep cool and then simply run out of habitat. But, by staying put, they may have more habitat but they may overheat."

One thing that could also be limiting the species' movement is the physiological demands that change as an organism moves to a higher elevation. In addition, there is a problem of dispersal: how are birds and plants from tropical floodplains to find their way to a climate zone high enough to exist at all (especially if their lifecycles are somehow tied to being floodplain species)? Do we expect (and do we see) similar things happening with aquatic species? (I mean, I like birds, but that's not what I study.) I would expect that fishes might be able to move upstream, but they are going to encounter barriers (such as waterfalls) and changes in hydrology (moving from the 12th stream-order mouth of the Amazon to an 11th order tributary will be a major change in hydrology, and thus habitat).

Finally, the brief doesn't discuss the issue of crowding and crowding out. If, after all, all things are moving from lower elevations to higher elevations (and even if there aren't significant physiological effects to take into account with these movements) in order to maintain their climatic conditions, then one needs to recognize that the available amount of actual space diminishes, too (after all, there isn't going to be more room at 5,000 feet than at 3,500 feet), and the fragmentation of populations will increase (after all, not all members of a species will end up climbing the same mountain), which means that the ultimate survival of species will depend on a variety of conditions, including metapopulation dynamics (i.e., sharing gene flow between scattered populations). Too, some species will diminish to a point where inbreeding becomes deleterious or the amount of available habitat cannot support a minimum viable population. And these are only direct effects on species. Remember that there are going to be system-wide effects, too...