Marine Ecosystems under Toxic Pressure - Vulnerability of Coastal Ecosystems and Adaptation - page 21

dominant grazers (A). The primary producers benefit from the reduction
in pressure from grazing due to the death of (A). Boxes 3 and 4
illustrate two indirect effects able to lead to an increase in the
abundance of grazers (B) (box 4). First of all, the “rare” grazers can
benefit from the death of the common grazers and thus increase their
rate of feeding on the primary producers through a decrease in the
concurrence, which leads to an increase in the population. Box 4 shows
that the increase in the abundance of primary producers generates a
functional response for rare grazers, who become dominant.
The structure of the community has evolved from 1 to 4 following
exposure to a contaminant.
1.4.1.2. Interaction between resources and consumers: “bottom-up”
Contaminants can have indirect effects at the base of the food
chain (vegetables, detritic and/or bacterial biomasses, primary
producers) by modifying, for example, the organic substrate of
detritivores or by affecting the liberation rates of nutritive elements for
primary producers. Hydrocarbons from petrol represent an important
source of organic matter that can lead to a stimulation in productivity
or the bacterial biomass. This stimulation can be sufficiently important
to feed a reaction of the “bottom-up” type on bacteriophagic species
and/or detritivores [PET 96]. In another domain, Podemski and Culp
[POD 01] found evidence for a stimulation in the growth of diatoms
subjected to effluent coming from a paper-whitening factory. This
phenomenon generated an important proliferation of grazers feeding
on the diatoms.
Herbicides can have direct effects on microalgae and generate
indirect effects on communities of zooplankton via variations in
competiveness between species. Through cascade effects, these are the
species at higher trophic levels, such as salmon, which are more
susceptible to being affected [BRO 10].
Fleeger et al . [FLE 03] list 56 cases of indirect effects from
pesticides on the competition between species or predation on the
biota and conclude that, at least in aquatic systems, pesticides exert a
strong selection pressure on invertebrates.