Saturday, October 17, 2015

Metapopulation Dynamics

"When we try to pick out anything by itself, we find it hitched to everything else in the universe." -John Muir

Zion National Park has suffered many more extinctions of mammal species than the much larger Yellowstone National Park. Probably because Zion's area is so much smaller than Yellowstone's.

How can scientists explain the persistence, movement, and dispersal of populations of organisms in a complex and dynamic landscape? How much habitat and how interconnected must habitats be in order to sustain healthy self-sustaining populations of native species? In order to better describe and test the dynamics of populations in complex environments Richard Levins described the idea of a "metapopulation" in 1970, which he described as "a population of populations". In this "population of populations" some habitats are occupied by a species, others are not. Some populations go extinct and are perhaps re-established later, while others persist for long periods of time in spite of the constant flux of individuals.

Why is this idea central to the understanding of desert organisms? The answer lies in the variability of desert environments, even at small scales. Take for instance a desert fish, one which is isolated in only one small stream system. Movement of individuals into other populations is rare, sometimes absent. Perhaps heavy rains and flooding make it possible for this fish to travel long distances and colonize new habitat? The reality of this fishes' world is that there is a small area of suitable habitat(its stream), and large areas of non-existent habitats(the desert), perhaps with small and rare opportunities for dispersal (heavy rains and flash floods connecting drainages).

Think now of a large predator, perhaps a Mountain Lion or bear whose habitat is a high mountain range surrounded by miles of barren desert. The same sort of situation exists here at a larger scale. In order to move into other habitat these animals must traverse miles of unsuitable habitat before finding solace in another desert mountain range.

This is the crux of metapopulation theory. Dynamic populations in complex environments; some populations waning from mortality and emigration while others are increasing from recruitment and immigration. You can now realize why this is so valuable for understanding desert systems; where crucial habitat is often interrupted by areas that are completely unsuitable for the life cycle of certain species. Be this a desert spring surrounded by arid lands, a mountain range surrounded by a sea of cacti, or wild lands interrupted by a large city. Desert species are often composed of many populations which may interact with each other infrequently, but these infrequent events may be crucial to the maintenance of those populations.

In some cases humans have interrupted these important processes and interactions. In many populations of Desert Bighorn Sheep that have been isolated from other populations due to a human improvements (roads, cities, etc) there has been a marked decrease in genetic diversity. This lack of diversity could be a problem for these populations going forward.

Desert Bighorn Sheep have experienced a decrease in genetic diversity where their populations have been isolated by human establishments.

It is often crucial that habitat is of sufficient size in order to maintain healthy population (and metapopulation dynamics). For instance, if conserved habitat is of insufficient size and separated from other important habitat by a large spatial distance local extinctions are likely to occur. The Western National Parks provided an important model to test this idea. Zion National Park, a relatively small protected area has suffered five times as many mammalian extinctions as the much larger Yellowstone National Park. Scientists have found that protected area (or suitable habitat) size is often the best predictor of extinction risk, rather than park heterogeneity or differences in elevation. When it comes to protecting habitat, size does matter. Larger, intact habitats are more likely to enable persistence of populations. Populations in smaller, more
disconnected habitats are less likely to recover from losses due to unforeseen circumstances.

So when you visit your local natural area there is often more at work than meets the eye. Protection of a small area often holds no guarantee that healthy populations will persist there, especially if the area is not connected to other important habitats. Increased variability and habitat changes due to climate change could threaten many desert species and populations. Where can bears and other predators go as the deserts climb up the mountains? What about endemic aquatic species found in desert springs as their habitats dry and change?

References and further reading:

Hanski, Ilkka. "Metapopulation dynamics." (1997).

Epps, Clinton W., et al. "Highways block gene flow and cause a rapid decline in genetic diversity of desert bighorn sheep." Ecology letters 8.10 (2005): 1029-1038.

Newmark, William D. "Extinction of mammal populations in western North American national parks." Conservation Biology (1995): 512-526.

Fagan, William F., et al. "Rarity, fragmentation, and extinction risk in desert fishes." Ecology 83.12 (2002): 3250-3256.

Hellgren, Eric C., David P. Onorato, and J. Raymond Skiles. "Dynamics of a black bear population within a desert metapopulation." Biological conservation122.1 (2005): 131-140.

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