Environmental Engineering Reference
In-Depth Information
100
Beta
2
between
ecoregions
Indiana
Ohio
75
Western
Allegheny
Plateau
North-central
Till Plain
Beta
1
between sites
50
1
2
3
4
25
Alpha
among sites
6
5
0
Fig.
10.4
In the study region as a whole, which encompasses six forest sites (1-6) in the
states of Indiana and Ohio (boundaries shown as solid lines), a total of 467 species of beetles
were identifi ed in early summer. The histogram shows how this overall regional beetle
species richness (gamma richness) is partitioned within habitats (alpha richness - average
species richness per site) and between habitats in the region as a whole (beta
1
- the extra
species added as extra forest sites are included, beta
2
- the further species added when two
ecoregions are included (North-central Till Plain and Western Allegheny Plateau - bounda-
ries shown as dotted lines)). (After Gering et al., 2003.)
10.2
Conservation
of metapopulations
It is not unusual for endangered populations to exist as a series of subpopulations
in a metapopulation. And even in cases where numbers have become so small that
only a single 'subpopulation' remains, a successful fi rst management step might well
be to increase the size of the population so that it can be subdivided into a meta-
population, thus moving away from the all-eggs-in-one-basket syndrome. When it
comes to predicting the persistence of metapopulations, the manager has to focus
on three things - the size (and therefore vulnerability to extinction) of each sub-
population, the extent to which subpopulations are connected by emigration and
immigration, and the relative favorability of the landscape within which the sub-
populations are embedded. To illustrate the nature of the problem and the approaches
managers may take, I present three case studies. The fi rst concerns an endangered
Australian bird and makes explicit the relationship between economics and popula-
tion viability. Various management options are compared to determine not only the
likely outcome for the bird but also to minimize the economic cost of achieving a
viable population (Section 10.2.1). Then I turn to a North American bird in decline,
and show how at a broad geographical scale managers can identify areas that deserve
conservation effort and others that are lost causes. Finally, I compare the conserva-
tion gain of providing greater connectivity among existing grizzly bear reserves as
opposed to simply enlarging the reserves.
10.2.1
The emu-wren
- making the most of
the conservation
dollar
When a species exists as a metapopulation it is necessary to take into account the
probabilities of local extinctions of subpopulations and of their recolonization by
individuals from surviving subpopulations. With this in mind, Westphal et al.
(2003) built a metapopulation simulation model for the critically endangered south-
ern emu-wren (
Stipiturus
malachurus
intermedius
), follow ing a procedure similar to
those described in Section 5.4. The metapopulation in a South Australian upland
area occurs in six remaining patches of dense swamp habitat (Figure 10.5). Emu-
wrens are poor fl yers and interpatch corridors of appropriate vegetation are likely
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