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Healthy Hills

May 31, 2010

If you live here in the Bay Area, or at least have flown in regularly enough, a view like this one might be one you take for granted.

"Healthy Hills" Photo by Stuart Dixon, calibersf.com/

Photographer Stuart Dixon took this from a small airplane near Livermore a few weeks ago, and generously allowed me to use it here.

Beyond being a cool view of the Coast Range, it’s a cool view of what sun exposure and varied topography do for a place. Which is cool, I promise: look, the north-facing slopes are shady and heavily forested; the south-facing slopes are patchier; the tops of the hills, the most exposed, don’t have anything big growing on them.

Varied topography and exposure did that!

It means there’s lots of little niches for different things to grow in. It also means if it’s too hot and dry on top, animals (or, on a longer time-scale, plants) can move over to a north-facing slope. Too cool and wet? Try the top, or the south-facing slope. These little migrations can happen in hours, days, seasons, years, millenia.

Those options are a buffer in el nino years, in droughts, or in a changing climate. But if those options are limited–say a development goes in on one of those slopes, or someone decides to log in the area (these are just hypotheticals), then you end up with pieces of habitat that have fewer options. A wildflower or a butterfly or a lizard or whatever might be able to keep living and breeding in that smaller place for a while, but if anything else changes (precipitation, temperature, availability of food, or presence of predators), that buffer’s not there anymore.

That’s why isolated places, like islands or parks are so susceptible to extinctions. They just have fewer options.

It’s also why healthy hills are cool; they have lots of options.

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