Marine Ecosystems under Toxic Pressure - Vulnerability of Coastal Ecosystems and Adaptation - page 32

A system's “recuperation” time (resilience) depends on the
capacity of species to recolonize the system (generation time)
[BRO 00] and the half-life of the contaminant, which can vary from a
few days to several years. The resilience can occur due to an increase
in the abundance of species still in the system or by the migration of
species coming from the exterior of the system. For example, studies
in lotic systems have shown that the diversity of macroinvertebrates
can fluctuate following the application of insecticides, because
individuals can colonize sites upstream that are not affected by the
insecticide [WAL 96]. In the same way, insects can recolonize lentic
systems after exposure to pesticides, which contributes to the system's
resilience [WAN 96]. This suggests that the geographic distribution of
species and the structuration of metapopulations can play an important
role in the recuperation of ecosystems.
The contaminants can have long-term effects on the ecosystems;
Woin [WOI 98] has demonstrated that two years after the application
of fenvalerate insecticide (a pyrethrinoid, organochlorinated
insecticide), a community of invertebrates in a pond was significantly
different in the diversity of the species and in abundance compared to
control sites. In the same way, the application of herbicides can lead to
a reduction in the biomass of producers, which persists because of
recuperation periods specific to the species [SPA 97]. Overall, these
studies show that the resilience of ecosystems to exposure to
pesticides will depend on a certain number of parameters (beyond
lethal and sublethal effects on each member of the community) such
as the type of habitat, rates of migration, the dynamics of extinction
and recolonization, the specific sensitivity of the species and the
species' specific rates of recuperation.
The resilience stricto sensu of an ecosystem, following an
improvement in the quality of the water and/or sediment, can be
retarded by the installation of tolerant species that check
recolonization by sensitive species, such as has been reported in
polluted lakes [FRO 06]. The ecosystem has, therefore, evolved in a
lasting manner, eventually returning to its initial functions, supported
by other species.