Landscape management - Ecological Applications: Toward a Sustainable World - page 281

; see Box 5.1), vary in forest patches of different sizes
and in landscapes containing more or less agricultural land. Recall that a population
will increase in size when

fundamental rate of increase ( λ

λ > 1.0, and decrease when λ < 1.0 (Box 5.1). In this

analysis of 30 landscapes across the USA, was often reduced below the 1.0 thresh-

old by the combined effects of enemies, and this reduction is partly due to the degree
of forest fragmentation (smaller fragments more likely to have

λ

1.0) and partly
to the extent of agricultural land development in the surrounding landscape
(

λ <

1.0 when percentage forest cover in a 10 km radius is less than 80%). Taking
all this into account, Lloyd's team produced a map of the USA showing areas where
the populations are likely to decline to extinction and, more importantly, areas
where wood thrush populations are currently safe (Figure 10.6). The populations in
decline (

λ <

1.0) can be thought of as 'sink' populations (dependent on recruits from
other populations) while the safe populations (

λ <

1.0) can be thought of as 'source'
populations whose excess recruits help sink populations to persist. Areas where
source populations predominate should be the focus of conservation efforts, because
further declines here would be particularly damaging for the species as a whole. On
the other hand, areas where sink populations predominate must either be targeted
for remedial action or given up as lost causes.

λ >

10.2.3 The problem
with large carnivores
- connecting with
grizzly bears

Wildlife reserves are islands in an ocean of more or less favorable habitat that
becomes increasingly inhospitable with time because of the relentless encroach-
ment of human-dominated landscapes. You saw in Box 10.1 how Island Biogeo-
graphy Theory predicts smaller and more isolated reserves will lose species faster
than larger and well-connected reserves. It is important to remember that large and

Fig. 10.6 Predicted
population growth rate
( ) of wood thrush

populations in relation
to forest cover (colored
cells) in the eastern
USA. The extent of
their breeding range is
shown by dotted lines.
Blue areas depict source
populations
that are currently safe
(

λ

Limits of breeding range
100th Meridian
State borders
Lambda 1.0), while pink

areas show sink
populations likely to go
extinct (

λ >

0.9
0.9 - 1
1 - 1.1

1.0). Safe
populations are
generally those in larger
forest fragments and/or
in landscapes contain-
ing only a small
proportion of nonforest
habitat. (After Lloyd et
al., 2005.) (This fi gure
also reproduced as
color plate 10.6.)

λ < 1.1

Non-forest habitat

Wood Thrush