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.
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