Geographic range and extinction

It's almost a truism to say that species with broad geographic ranges are less susceptible to extinction than those &narrow ranges. I say “almost”, because it's not hard to imagine exceptions.

Consider the prospects of a narrow serpentine endemic like Calochortus tiburonensis, for example. It is restricted to a single serpentine outcrop on the Tiburon Peninsula in Marin County, California (source), but the U.S. Fish & Wildlife Service identifies the primary threats to its persistence as “catastrophic events such as fire, disease or pest outbreak, severe drought, or other natural or human-caused disasters” (source). In other words, as long as developers don't build on this part of the Tiburon peninsula, its future is reasonably secure.

In contrast, Isotria medeoloides is currently known to occur in 15 states from Maine to Georgia. In spite of its broad geographic range, all populations are relatively small, and some have declined for unknown reasons (source). In other words, its future may be secure, but we really don't know, because we don't know why some of its populations are declining.

But notice that qualifier on my assessment of Calochortus tiburonensis: “as long as developers don't build on this part of the Tiburon Peninsula” (and there aren't other catastrophes). Species with narrow geographic ranges are inherently more susceptible to catastrophic events, because those with large ranges are likely to have at least some populations that escape the catastrophe. There's even a paper in the most recent Proceedings of the National Academy of Sciences providing data to back this assertion up.

Jonathan Payne and Seth Finnegan use the extensive database fossil invertebrates from the Cambrian to the present (ca. 500 million years of earth history) to examine the relationship between geographic range and extinction (source). They report that:

• Wide geographic range is associated with resistance to extinction during normal “background” periods of extinction.

• Geographic range is less strongly associated with resistance to extinction during mass extinction events

Mass extinction events are presumably associated with massive environmental disturbances, so even those species with wide geographic range may have populations that escape. In short, when looked at over a geological time scale¹, species with broad geographical ranges are less susceptible to extinction than those with narrow ranges.

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¹Or a human one, if human-caused catastrophes can't be prevented or natural ones can be expected to occur.