Conserving Biodiversity on Military Lands: A Guide for Natural Resource Managers 3rd Edition

Sources and sinks and their importance

Upon reflection, it becomes obvious that all sub-populations are not the same. Some occur on tiny patches of acceptable habitat, and never grow to more than a small number of individuals. These, of course, never really escape the consequences of being in a “population bottleneck,” and many have a low probability of persisting in isolation. Others occur on large areas of acceptable habitat and, thus, tend to exist as large healthy populations. These have greater demographic and genetic resilience, and hence a greater probability of persistence. Simply because of their large size, these populations tend to be the source of most of the dispersers that colonize vacant patches and reinvigorate the small sub-populations both by their numbers and by their genetic diversity. These are thought of as “source” subPage 64 of 293 populations, whereas the smaller occurrences which tend to absorb migrants, but do not provide dispersers, are considered “sinks.”

The generalization that small populations tend to be sinks, and large populations sources, is, like all generalizations, only true to a point. The key, which is often difficult to measure, is whether the population produces significant numbers emigrants or not. Source populations do, sinks do not. In general, “sink” subpopulations will not persist without continual immigration from “sources.” Thus, the destruction of a single “source” sub-population can result in the extirpation of many surrounding “sinks” even if they are not directly impacted.

The Sikes Act requires military installations to prevent the loss of threatened, endangered, and listed species found within their boundaries. Understanding the ecology of those species, and how their populations and sub-populations are distributed, is key to meeting this requirement. Conserving a wide-ranging species like the bald eagle might be accomplished simply by protecting a limited number of nesting sites—as only a small piece of a much larger population exists on site. The Karner Blue butterfly, in contrast, exists as a meta-population where sub-populations exist in ephemeral patches of host plants. Conserving this species requires an understanding of the disturbance dynamics creating these patches of host plants, and the dispersal capabilities of the butterfly, so as to manage the entire metapopulation and not just a few occurrences, each with a low probability of persistence in isolation. Understanding and managing a meta-population often requires looking beyond an installation’s borders to sub-populations on neighboring lands.

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