Evolution on Smallest of Scales Smooths Out Patchwork Patterns of Where Animals and Plants Live

The Douglas fir is a tall iconic pine tree in Western North America forming a forest that winds unbroken from the Western spine of British Columbia all the way to the Mexican cordillera. The environmental conditions of Canada and Mexico are obviously very different, but even on much smaller scales – say, the top of a mountain compared with a valley below it – the rainfall, temperature, soil nutrients and dozens of other factors can vary quite a bit. The Douglas fir grows well in so many of these places that it turns a dramatically varied landscape into one smooth, continuous forest complete with all the species it supports.

I am an ecologist and used to think that the Douglas fir was simply a hardy tree, rarely hemmed in by environmental conditions or other species. But recent research done by my colleagues and me suggests that environmental conditions are not all that determines where plants and animals live in a landscape and the patchwork patterns of those distributions. These spatial patterns are also influenced by evolution.

Over time, species often adapt to local conditions, and these adaptations alter how and where they can live. For example, Douglas fir trees might adapt through evolution to thrive on both a dry mountainside and in a wet valley nearby. But my colleagues and I have taken this idea a step further to explore not just how organisms adapt, but how the process of adaptation itself can have profound effects on the patterns of where organisms live in a landscape.

Without adaptation, you might find a mixed patchwork of where species live – a species of insect lives in the valley, but not on the mountains. When Douglas firs adapt to and grow on a dry mountain as well as in the wet valley, they create one continuous forest habitat where two very different landscapes used to exist. The birds, the insects, the deer, the flowers and all the other organisms that live in the forest can also now occupy both the valley and the mountaintop. Adaptation by the Douglas fir created a smoother distribution of species.

Adaptation, it seems, plays a larger role in determining ecological patterns than scientists previously thought.

A Salamander Mystery

In 1999, when I was a beginning graduate student in Connecticut, I wanted to understand how a predator called the marbled salamander affected the survival of the smaller yellow-spotted salamander in small temporary ponds. Much like the famous wolves in Yellowstone National Park, the marbled salamander is a keystone predator, and just a few individuals in a pond can determine which other species live there.

I spent months watching these ponds, but however much I tried, the patterns I saw just weren’t making sense. In one pond, the yellow-spotted salamanders survived alongside the marbled predator. But in the next pond over, under nearly identical conditions, the spotted salamanders were quickly reduced to predator poop. I couldn’t find an environmental explanation for this.

To figure out what was driving this unevenness of high and low survival, I collected salamander eggs from ponds where the small salamanders survived alongside the predator, as well as eggs from ponds without predators. I then raised these yellow-spotted salamanders in buckets and looked for differences between them.

I found one surprising difference. The salamanders from ponds with the predatory marbled salamander adapted to the predator by becoming gluttonous – eat and get big so you don’t get eaten yourself.

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