(Sign available at Animal Den)
One of the central underpinnings of conservation biology is that just about every terrestrial habitat type - from southern New England calcareous sloping fens to the fynbos of South Africa - occurs where it does and in its current form because of three, interrelated factors: Climate, Geology and Disturbance. Even geographically restricted habitats with very specific soil chemistry requirements are subject to these influences. Over time, habitats can shift in species composition, vegetative structure, even physical location on the landscape.
Of these three factors, it is disturbance, particularly that which is caused by patterns of human land use, that has the greatest range of variability over the shortest period of time. Geology (notably bedrock geology and the slope and aspect of terrain) changes very slowly on its own over many millennia. The patterns of regional climate variation fall somewhere in between, though can be augmented by human land use at scales from the very local (deforestation, ground water depletion) to global levels.
This is one of the reasons why conservation biologists advocate conserving large landscapes to account for variability and disturbance and allow for change. Save too small an area, and sooner or later the natural systems that sustained whatever was initially of conservation interest there will become fragmented or degraded by disturbance (human or natural). Over time, changes in climate may cause species and habitat ranges to contract or expand. If the connection between suitable habitat types is broken and species are unable to migrate, occurrences become extirpated. If I were interested in conserving coastal wetlands in southern New England, for example, I'd be planning for their inland migration with projected sea level rise.
Environmental regulators, on the other hand, deal in occurrences here and now, often within jurisdictions that do not encompass the full range of the species or habitat types of concern. What is rare in Massachusetts or Connecticut may be quite abundant elsewhere. It still may be important for genetic reasons to conserve such species at the edge of their ranges (eastern timber rattlesnakes and bog turtles are two examples from our area), but it may also be that declining species such as meadowlarks and bobolinks are falling to levels more appropriate to our presently forested landscape rather than previously agricultural one. There are policy and management implications at work here.
Which brings me to the subject of polar bears. Like all charismatic megafauna, they have an emotional appeal to many of us who do not actually try to coexist with wild populations (though the reality of that association does not necessarily preclude affection). The species has a circumpolar range and population dynamics that appear to be driven by complex associations including prey availability, depth of permanent sea ice, and the ability to range over wide areas. This summer, the polar bear in the United States was listed as Threatened under the Endangered Species Act, due in large measure to changes in habitat linked to projected changes in climate. Losing US polar bear populations because future environmental conditions may no longer allow the bear to hunt seals on the ice could have implications for the entire species through the loss of genetic diversity and range contiguity, even if some populations are less affected by climate change.
Still, it has me thinking.
Most scientists agree that polar bears have existed as a distinct species for at least 100,000 years, and new evidence suggests their origin should be placed perhaps tens of thousands of years earlier. If true, this suggests that when a young species, populations of polar bears survived the Last Interglacial period (127,000 - 110,000 years ago) when global conditions were warmer than today (though cooler than they could become in the coming centuries). Possibly, the modern representatives of the species have evolved with a lower tolerance for climactic variation than their ancestors had nearer to initial speciation. Even more likely, a shift to less suitable prey may bring polar bears into unacceptable competition and ultimate conflict with human populations where none existed during the Last Interglacial. I find myself in agreement with the blog World Climate Report on this point, when it says;
"If the bears fare worse this time around, it will mostly likely be because their natural adaptive response may run up against a human roadblock in the form of habitat disruption or other types of difficulties that an increased human presence may pose to the adapting bears."
Highly specialized species are more vulnerable than generalists to changes in habitat. There were a number of New World vulture species that did not survive the Pleistocene because they lived off the carcases of extinct megafauna. Only the California Condor, which had access to a diet of stranded whales, lingered on. Possibly there are populations of polar bears that have more prey options than others.
If I were a conservation biologist working with polar bears, I would be thinking about the places where the best available climate projections and population dynamics data suggest could be refugia for this species; identifying viable connections between the bear's present range and these areas; and making this the focus of my conservation efforts.
During the last Ice Age, such refugia existed for the temperate broadleaf forests of New England on what is now the Continental shelf off the southeastern United States. It took thousands of years for the trees of our forests to recolonize the land after the glaciers receded, traveling north and west at the rate of seed dispersal. When they did reappear on the New England landscape, the pioneer tree species occupied all available niches, even those considered marginal today.
They were only able to do this in the absence of competition from others. That's the rub, for the species with the greatest habitat requirements and in fiercest competition for its own needs is homo sapiens sapiens.