Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 251

Fig. 9.7 Analysis of
trophic level averaged
for all fi sh species in
the fi sheries catch
statistics for the
western central
Atlantic. Open circles
are for the USA, and
closed circles are for all
other areas. Note in
both cases how mean
trophic level has been
declining since 1950 as
a result of overfi shing
of the larger, most
preferred species. These
patterns are examples
of a worldwide trend of
'fi shing down'. (From
Pauly & Palomares,
2005.)

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

the same food web. Simulation models, involving reductions in one fi shing method
at a time, indicate that restrictions on shallow-set longline gear would be the step
most likely to allow recovery of top predators in this food web.
A worldwide pattern, mirroring what has happened in the north Pacifi c, is a
decline in the mean trophic level of fi sh in fi sheries catches (Figure 9.7). In other
words, by overfi shing the largest and most desirable species, which in itself allows
an increase in biomass of smaller species, fi sheries are increasingly coming to rely
on species from much lower down the food chain (i.e. the prey of the previously
targeted fi sh). This general pattern is known as 'fi shing down marine food webs'
(Pauly & Palomares, 2005). Sustainability in fi sheries can be expected to require
management of the use of gears to allow recovery of the larger species and stability
in the composition of the catch.

9.4 Food webs and
conservation
management

Knowledge of the intricacies of food webs can also guide managers who seek to
conserve species at risk. The kokako ( Callaeas cinerea ), an endangered bird, is now
restricted to about 15 populations in forest fragments in the North Island of New
Zealand. Nesting success is generally poor because of predation of eggs and chicks
by introduced mammals - ship rats ( Rattus rattus ), which arrived on the fi rst sailing
ships, brushtail possums ( Tr i ch os ur u s vulpecula ) brought from Australia to start a
fur trade, and stoats ( Mustela erminea ) introduced in a vain attempt to control
rabbits. The diet of kokako includes fruits and foliage, items also consumed by
possums and rats. The causal links in this food web are illustrated in Figure 9.8.
Population modeling has shown, fi rst, that reduction of any one mammalian preda-
tor is not suffi cient to improve the fl edging success of kokako. At the other extreme,
reducing all three predators provides the strongest benefi t for kokako fl edging
success - something that is hardly surprising. But of most note is the fi nding that
reduction of just ship rats and possums produces very nearly as positive an outcome
as reducing all three predators. This is because stoats prey on ship rats so that
reduction of rats reduced stoat success, and the reduction in both predators was
benefi cial to the birds.
The advice to managers, then, is to reduce all three predators or, if funds are
limited (which they invariably are), target the rats and possums. But don't waste
money trying to reduce just one of the predators.