untitled

A similar result was observed in southern Sweden with forest fragments embed-

ded in an agricultural landscape. Andren (1992) recorded the impact of various species

of the crow family as predators of artificial nests placed in the forest. Two species,

the European jay (Garrulus glandarius) and the raven (Corvus corax), were confined

to forest and were absent from small fragments, so their impact declined with

fragmentation. Jackdaws (C.monedula) and black-billed magpies (Pica pica) were largely

in agriculture. The hooded crow (C.corone) lived in agriculture but invaded forest

patches, causing increased predation along the forest edge and within small fragments.

In Kruger National Park, the expansion of zebra (Equus burchelli) populations into

dry habitats when water holes were constructed in the middle of last century allowed

lions to move into those areas. Consequently, rare secondary prey, such as roan

antelope (Hippotragus equinus) and tsessebe (Damaliscus lunatus), have been driven

towards extinction (Harrington et al. 1999).

PREDATION 175

0.10

0.05

0

• 0.05


• 0.10


• 0.15


• 0.20
  0 100 200 300 400 500 600
  Population size

Rate of increase,

r

(a)

100

80

60

40

20

0

1 10 100 1000 10 000 100 000 1 000 000

Forest size (ha)

% Predation

(b)

Fig. 10.10Depensatory
total responses. (a) Wolf
predation on different
woodland caribou herds
in British Columbia.
Predation rate increases
as caribou density
declines, causing the
populations to decline
even faster. (After
Wittmer et al. 2005.)
(b) Various mammal
and bird predators on
passerine bird nests as a
function of forest patch
size. These patches are
an index of prey
population size. (After
Wilcove 1985.)