Why species on the edge should be preserved

Often the political boundaries of states and countries do not align with any biologically relevant boundaries, and so species often occur sporadically or rarely in one state, while being extremely abundant in an adjacent state. These species are considered “peripheral species” and are often derided by developers and biologists alike as being unworthy of special regulatory protection.

Peripheral species often occur in habitat conditions that are not ideal for the species. For instance, many of the peripheral fish species in Kansas are surviving in waters that are almost too saline for the species to survive.

In some cases I tend to agree. Species which are not threatened, when their entire range is considered, should generally not be the focus of threatened and endangered species regulations. However, a number of fairly compelling arguments can be made to support the case that peripheral species deserve more protection than we might at first believe. Biologically, I know of three biological reasons for protecting peripheral species:

1. When species collapse, it appears that populations on the edge are more persistent. This is a counter-intuitive effect, and, in fact, I have no good feeling for why it occurs (in general). Some species may have been intentionally targeted (i.e., sea otters) and therefore it wasn’t profitable to visit peripheral ranges. Other peripheral populations may be isolated, and therefore unaffected by disease or invasive species that are decimating the ‘core’ range. Fraser (1999; the paper which got me thinking about this) compiled a number of examples where both vertebrates and invertebrates became extirpated within the “core” of their range, but persist at the edges. Obviously, if an entire species is at risk, preserving the peripheral populations may be extremely important.

2. Populations at the edge of the species’ range may be reservoirs of important genetic diversity. An example of this could be the Broadhead Skink, a peripheral Kansas species that is more common in Missouri. However, the Broadhead Skink in Kansas may experience hotter and generally more prairie-type conditions. As a result, genes that favor those conditions likely occur more frequently in those populations. Each peripheral population may have different gene frequencies than the core population and other peripheral populations. These populations are each adapted to unique stressors that may become more widespread with environmental changes. For instance, increasing global temperatures are driving many species northward. Those species that will be most likely to lead that forced migration are the ones located in peripheral areas, and having genes that allow them to exploit the new conditions most effectively. Alternatively, if conditions change and no migration is possible, then peripheral species may have the genes necessary for the entire species to survive.

3. Peripheral species may also be endangered elsewhere. For the most part, political boundaries determine the scope of influence for any state or government, but those political boundaries are rarely important biologically. A species endangered throughout its range should also be protected on the periphery of its range.

In addition, there are some more pragmatic reasons to list peripheral species. Listing a species automatically attracts scientific attention to it due to the increased availability of funding and ‘practical application.’ Occasionally, we have discovered a peripheral species occurs much more widely than previously thought (rendering its ‘peripheral’ status obsolete), other times we have learned the species is not as threatened as originally believed.

There is also a considerable amount of eco-tourism that is derived from peripheral species. Birds and mammals in particular attract considerable attention that is substantially focused on areas of high diversity (i.e., transitional landscapes).

I’m not saying this means every peripheral species should be protected, but I am arguing that merely designating a species “peripheral” is not justification to avoid protecting it.

Fraser, D.F. 2000. Species at the edge: the case for listing of “peripheral” species. pp 49-53 in L.M. Darling (ed.) Proceedings of a Conference on the Biology and Management of Species and Habitats at Risk. Kamloops, B.C., Canada.