Application of Island Biogeography to the design of nature reserves

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The theory of island biogeography has been termed the 'First Law of Conservation Biology.'  Why is this so?

A.  Because of human actions, natural habitats are becoming increasingly isolated and island-like.  As such, the number of species on these remaining areas may change as immigration decreases and extinction increases.

B.  By identifying potential mechanisms underlying the loss of species diversity, Island Biogeography Theory may help suggest ways in which we can design nature reserves to maximize their ability to maintain diversity.

C.  Specific recommendations:

  1)  A large reserve is better than a small reserve
 
  2)  A single undivided reserves is better than a number of small reserves

  3)  If divided, reserves is better than a number of small reserves

  4)  If divided, reserves should be spaced equally from another, not linearly

  5)  If reserves are linear, they should be connected with corridors

  6)  If reserve is small and isolated, it should be circular and not linear

II.  This application of Island Biogeography has been very controversial.

The two recommendations which have generated the most argument have been B and E.

A.  Will single, large reserves protect more biodiversity than several smaller reserves of equal size?  This has been termed the Single Large or Several Small (SLOSS) debate.

Island biogeography theory only predicts species richness for single sites.  It makes no predictions about diversity across multiple sites, or how that diversity will compare to other regions.

In fact, much research has shown that many islands will almost always have more diversity than single areas of the same size.  There are many factors responsible for this, including greater isolation between areas which allows for trophic equivalency.

However, others have argued that while this may be true, most species of conservation importance will not survive on small reserves due to high extinction rates.  Only 'weeds' will survive on small islands.

Yet, many plant and invertebrate populations (almost 90% of global biodiversity) need very small amounts of habitat to survive (a few square feet), so what is 'small' for these species will be quite different from what is 'small' for a cougar.

B.  Are corridors essential for protection of biodiversity?  Are they a good investment of conservation resources?

Supporters of corridors argues that unless reserves are connected in some way, many species with large home ranges (wolves, bear, cougar) will face increased extinction rates, as current reserves may not be large enough to support them.

They also argue that migration between reserves is vital to maintain the genetic health of populations.

However, critics argue that:

  1)  Little data exists to show that corridors are used by target species.  The data which is known suggests that many species will not use them.  For instance, Minnesota research has demonstrated that wolves will only use roadless corridors more than 6 miles wide!  Few such corridors exist in the eastern U.S.

  2)  Corridors may be used by non-target species with better dispersal abilities.  By linking reserves, corridors may decrease the isolation and increase the chance of exotic pests, weeds, or diseases to invade sites.

  3)  Corridors are often very expensive to purchase.  Given the lack of data supporting the benefits of corridors, would money be better spent protecting other, more important pieces of land?

C.  It is also important to note that many small, 'non-optimally' designed reserves may protect much of their biodiversity.  Remnant prairies in Wisconsin and Iowa along rail roads are small and linear.  Yet, they support more diverse butterfly communities than many larger reserves.

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Created 2 September 2011, Last Update 02 September 2011

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