Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 248

Fig. 9.4 The Lyme
bacterium Borrelia
burgdorferi , transmitted
by ticks and carried by
hosts such as the white-
footed mouse, causes
disease in humans.
Density of nymphs of
the tick Ixodes
scapularis that are
infected with the
bacterium responsible
for human Lyme
disease increases
dramatically in small
forest fragments where
white-footed mice are
common, but other less
effi cient disease
transmitters are absent.
(After Allan et al.,
2003.)

0.12

0.10

0.08

0.06

0.04

0.02

0 0

1 2 3 4 5 6 7 8

Area (ha)

invading mosquito might conceivably reduce human disease risk if it is a less effi -
cient disease vector than native species.
Whether or not a new mosquito becomes invasive depends on where it fi ts into
the food web of the small water bodies that mosquitos inhabit - and whether it is
more or less competitive for food, or vulnerable to predation, than the native species
(Juliano & Lounibos, 2005). Thus, although the invading Aedes albopictus can out-
compete the native North American mosquito Ochlerotata triseriatus in the absence
of predators, the invader is more vulnerable than the native to insects that feed on
mosquito larvae in their tree hole habitat. In fact, because the native mosquito
hatches earlier, it achieves a size at which it can itself prey upon the smaller larvae
of the invader. For these reasons A. albopictus fails to displace O. triseriatus . Biose-
curity managers can glean clues about the likely harm (or good) of a potential
mosquito invader through knowledge of its disease vector effi ciency and food web
relations.

9.3 Food webs and
harvest management

9.3 Food webs and
harvest management

Harvests of abalone (gastropods in the family Haliotidae) are prone to collapse
through overfi shing. Because adult abalones are relatively sedentary, protection of
broodstock in no-take marine areas may promote export of planktonic larvae and
enhance the harvested populations outside the reserves (see Section 7.2.4). But note
that the function of marine protected areas is most often to conserve biodiversity.
The question arises whether protected areas can serve both harvest management
and biodiversity objectives. The answer is yes in some cases, but not apparently
where abalone and sea otters are concerned.
A keystone species in coastal habitats along the Pacifi c coast of North America is
the sea otter ( Enhydra lutris ), hunted almost to extinction in the eighteenth and
nineteenth centuries but increasingly widespread as a result of protected status. Sea
otters eat abalones and while otters were rare, valuable fi sheries for red abalone
( Haliotus rufescens ) developed. Now there is concern that the fi shery will be unsus-
tainable in the presence of the resurgent sea otter.
Fanshawe et al. (2003) compared the population characteristics of abalone in sites
that varied in harvest intensity and sea otter presence: two sites had no sea otters
and had been 'no-take' abalone zones for 20 years or more; two sites lacked sea otters
but allowed recreational fi shing; and four sites were 'no-take' zones that contained
sea otters. The aim was to determine whether marine protected areas can help make

9.3.1 Who gets top
spot in the abalone
food web - otters or
humans?

9.3.1 Who gets top
spot in the abalone
food web - otters or
humans?