Landscape management - Ecological Applications: Toward a Sustainable World - page 280

Fig. 10.5 The southern
emu-wren metapopula-
tion, showing size and
location of patches (1- 6
and new patch 7) and
corridors (dashed, solid
and dotted lines). Five
different strategies were
assessed in the study:
Enlarge patch 2 (E2);
Enlarge patch 5 (E5);
Create new patch 7 plus
corridor (dotted line) to
patch 6 (E7); Create
corridor (solid line)
linking patch 2 with 1
and 3 (C2); Create
corridor (dashed lines)
linking patch 5 with 4
and 6 (C5). (After
Westphal et al., 2003.)

N

Corridor 5 (C5)
Corridor 2 (C2)
Corridor to new patch 7

4

New patch

2

1

3

5

7

1 km

6

to be important for metapopulation persistence. The management strategies evalu-
ated were: (1) enlargement of existing patches; (2) linking patches via newly created
corridors of suitable habitat; and (3) creating a new patch (see Figure 10.5). The
economic 'cost' of the three strategies was standardized to be equivalent to 0.9 ha of
newly vegetated area in each case.
The next step was to compare the effectiveness of the three different management
actions, and also to compare among a variety of multi-step management scenarios
- for example, fi rst build a corridor from the largest patch to its neighbor, then, in
the next time period, enlarge the largest patch, then create a new patch, and so on.
(A process known as stochastic dynamic modeling was used for these analyses.) The
aim was to fi nd the strategy or scenario that reduced the 30-year extinction risk to
the greatest extent.
It turns out that optimal metapopulation management decisions depend on the
current state of the population. For example, if only the two smallest patches (2 and
3) are occupied, the optimal single action would be to enlarge one of them (patch
2; Strategy E2). However, when only the large patch 5 is occupied (which is more
resistant to extinction because of the larger population it contains), connecting it to
neighboring patches is optimal (strategy C5). The best of these fi xed strategies
reduced 30-year extinction probabilities by up to 30%. Even better, when chains of
different actions were taken over successive time periods (e.g. strategy C5 followed
by strategy E2, followed by strategy C2, and so on), the optimal scenario reduced
extinction probabilities by 50-80% compared to no-management models.
These results hold a number of lessons for conservation managers. First, good
decisions rely on knowledge of patch occupancy and understanding of extinction
and recolonization rates. Second, the sequence of actions taken can be critical and
this is where approaches such as stochastic dynamic modeling are of value (Clark
& Mangel, 2000). Most important of all is the point that funds available for conser-
vation will always be limited and tools such as these can help achieve the best
returns from scarce resources.

10.2.2 The wood
thrush - going down
the sink

There is convincing evidence that certain bird species living on the edges of forest
patches are more vulnerable to predation of eggs and young (by predatory mammals
and birds) and nest parasitism (when birds such as the brown-headed cowbird,
Molothrus ater , replace the host's eggs with their own). This pattern occurs because
predators and brood parasites often enter the forest from surrounding agricultural
habitats. Lloyd et al. (2005) estimated for wood thrushes ( Hylocichla mustelina ) how
the rate of mortality (from predation and parasitism), and thus the population's