Risk spreading, connectivity, and optimal reserve spacing
Shane A. Blowes1 and Sean R. Connolly
Ecological applications, Volume 22, Issue 1 (January 2012)
Dispersal and the spatial covariance of demographic fluctuations are the two important processes that determine the dynamics of spatially structured populations. The researchers say Spatially explicit approaches to conservation, such as reserve networks, must consider the tension between these two processes and reach a balance between distances near enough to maintain connectivity, but far enough to benefit from risk spreading.
Here the researchers model this trade-of and show how two measures of metapopulation persistence depend on the shape of the dispersal kernel and the shape of the distance decay in demographic covariance. They then go and have a look at the implications of this trade-off for reserve spacing. They say the relative rates of distance decay in dispersal and demographic covariance determine whether the long-run metapopulation growth rate, and quasi-extinction risk, peak for adjacent patches or intermediately spaced patches; two local maxima in metapopulation persistence are also possible.
When dispersal itself fluctuates over time, the trade-off also changes over time. Temporal variation in mean distance that propagules are dispersed (i.e., propagule advection) decreases metapopulation persistence and decreases the likelihood that persistence will peak for adjacent patches. On the other hand , variation in diffusion (the extent of random spread around mean dispersal) increases metapopulation persistence overall and causes it to peak at shorter inter-patch distances. Thus, failure to consider temporal variation in dispersal processes increases the risk that reserve spacings will fail to meet the objective of ensuring metapopulation persistence.
This study identifies two phenomena that receive relatively little attention in empirical work on reserve spacing, but that can qualitatively change the effectiveness of reserve spacing strategies:
1) The functional form of the distance decay in covariance among patch-specific demographic rates
(2) Temporal variation in the shape of the dispersal kernel.
The researchers conclude that sensitivity of metapopulation recovery and persistence to how covariance of vital rates decreases with distance suggests that estimating the shape of this function is likely to be as important for effective reserve design as estimating connectivity. Similarly, because temporal variation in dispersal dynamics influences the effect of reserve spacing, approaches to reserve design that ignore such variation, and rely instead on long-term average dispersal patterns, are likely to lead to lower metapopulation viability than is actually achievable.