February 3, 2010
Forest fragmentation and the isolation of the giant panda (a goodbye to Tai Shan and Mei Lan)
Two of the cities I’ve called home in the past 10 years – DC and Atlanta – are each sending a panda home to China tomorrow. Mei Lan and Tai Shan were both born in captivity and both a huge boon for conservation and science education on the east coast. I watched Tai Shan grow up along with other zoo goers in the DC area.
They’re returning to their ancestral home in China, where wild pandas are still endangered. Fossil records show us that giant pandas had a much wider range in Asia, inhabiting subtropical and warm temperate forests. Now, mostly because of human encroachment, they are restricted to 24 isolated populations in China’s fragmented mountain forests where bamboo dominates the understory.
In recent surveys, researchers have shown that the number of individual pandas has increased due to conservation efforts in the country, but the populations remain disparate. A recent study published in the Journal of Biogeography takes a look at how exactly these pandas are distributed in the forests of Southwest China, in relation to the level of fragmentation.
Forest fragmentation is a term we read a lot in newspapers and magazines listing the numerous causes of a population decline or a biological invasion, but it’s rarely fleshed out, so I’m going to take the opportunity to briefly describe its most important aspects.
You’re standing on a rock at the edge of a large stream or small river. A forest stretches from the banks of this stream to the faint peaks of mountains far in the distance. You turn around, looking across the stream to the other bank. There’s a stone like the one you’re standing on, and beyond that an identical forest running seamlessly from river to mountains in the other direction. Where the forest ends, at the bank, it changes from one ecosystem to another. In the river itself, another ecosystem, with microhabitats. On the other bank, a replica, then the forest again.
Now imagine you’re standing in a gravel patch on the side of a highway. There is a forest in front of you with no shrubby transitional area. On the other side, a replica: a gravel patch and a wall of trees extending to the mountains in the distance, or so you assume. You see the difference in the split. One is natural and supports a diversity of dovetailing ecosystems, the other is anthropogenic, effectively splitting one forest “patch” into two patches.
As these forest patches are further split, metapopulations form: smaller, per-patch assemblages of the populations found in the once contiguous forest patch. As land is developed, the patches shrink, becoming more and more isolated until migration and dispersal between them becomes strained due to a lack of food and shelter in the developed land. In the process of development, a higher ratio of forest edge to core is created, a drier, sunnier habitat that supports a different network of organisms. The extension of the forest patch edge also means more access for predators and parasites living outside.
Not surprisingly, the researchers found that dense forest (defined as forests with canopy cover > 30%) is “essential” for giant panda survival in the wild. The highest densities of pandas were found in the Qinling Mountains, which also happened to be an area with low relative fragmentation. Broken down, the most important factors for pandas turned out to be patch area, edge density (distance of edge per unit area) and patch “clumpiness” or how close patches are from one another.
Large mammals like the giant panda are particularly sensitive to fragmentation due to their need for space within a preferred habitat, the dense forest. It’s not just territorial; it has a lot to do with biodiversity. The size of these patches determines the diversity of the forest, which creates these smaller habitats like core or dense forest. In this current situation, where forest has been significantly reduced, pandas are forced to transverse long stretches of alien landscapes, which requires more energy despite the lack of food and exposes them to human influences.
Instead of establishing new reserves for other isolated populations, the authors recommend that future conservation efforts should be focused on creating corridors between the disparate patches. It’s great that the conservation efforts to bolster and protect populations are starting to work and the number of individuals is increasing, but the population needs to be considered as a whole. That means trying to reconnect forest patches and expanding the gene pool.
So as we say goodbye to Tai Shan and Mei Lan, it’s important to recognize just why they’re here in the first place. They’re ambassadors for conservation, for the reestablishment of their species in the wild, not in zoos.
The last time I saw Tai Shan, he was doing this:
It made me smile. The interest he generated, that oblivious little panda cub, just by doing what young mammals do – eating, sleeping, playing, sleeping some more – is remarkable. The crowds that lined up in front of that panda enclosure were enormous; so big, in fact, that they had to expand the area to compensate. Dads of every nationality held their squirming little ones on sweaty shoulders during the summer. In the fall, hundreds of school kids – in uniform and out – would pack in for the keeper’s lecture. And in the winter, after the New Year, Heather and I went to the zoo one weekday afternoon between semesters and had Tai Shan completely to ourselves for almost an hour. You can’t help but vicariously reach out to that little life, stumbling along with him as he paws and climbs and sniffs. It’s our proper place in stewardship. From a distance, we’re touched by the clear, oblivious innocence of nature.
Wang, T., Ye, X., Skidmore, A., & Toxopeus, A. (2010). Characterizing the spatial distribution of giant pandas (
) in fragmented forest landscapes
Journal of Biogeography DOI: 10.1111/j.1365-2699.2009.02259.x