Environmental Engineering Reference
In-Depth Information
diversity will remain low. However, it is possible that, with more moderate
pollution, tolerant species may replace those that have been lost. This is
possible if these more tolerant species are poor competitors that are normally
excluded from the site by competitively superior but less stress-tolerant species
(this would also be consistent with the observation that pollution can increase
the success of invasive species, c.f. Piola & Johnston
2008
). If there are sufficient
tolerant species available in the local species pool to colonise, this may produce
a community with similar diversity but a different composition to that present
before the pollution occurred. A similar change in community composition
may occur even if the pollutant is not acutely toxic. Chronic toxic effects in
which a pollutant reduces survival, growth or reproduction of sensitive species,
may reduce the population size of these sensitive species, relaxing the intensity
of competition on less sensitive species and allowing them to become estab-
lished or increase.
Does this pattern occur in reality?
If this conceptual model is correct, we can classify sites into three categories:
1. Pristine sites which may be contaminated, but where there are negligible
effects of the contaminants on the species that are present;
2. Severely polluted sites at which diversity shows a significant reduction
relative to that at pristine sites, as a result of adverse effects of contaminants
3. Moderately polluted sites where diversity is comparable to that at pristine
sites, but where adverse effects of contaminants on more sensitive species
have altered community composition to favour pollution-tolerant species.
What evidence is there that this corresponds to what actually does occur in
the field? At the pristine end of the continuum, we hope that there are still
sites where effects of contamination are negligible, although proving this
would be difficult and there is evidence that virtually none of the world's coral
reefs are unaffected by human activity (Pandolfi et al.
2003
). At the other end of
the spectrum, there are many examples where pollution has reduced the
diversity of communities. Detecting pollution that is sufficiently severe to
cause considerable reduction in diversity is straightforward, and I do not
intend to review examples of this here. Detecting subtle changes in community
composition that are not accompanied by a reduction in diversity presents a
much greater challenge. Differences in community composition between sites
can be caused by changes in environmental characteristics other than pollu-
tion. So detecting changes due to pollution requires the availability of good
control sites, which are identical to the impacted sites in every respect apart
from the level of contamination (Grant & Millward
1997
). In studies where this
condition is satisfied, subtle ecological changes associated with impacts of
pollution may be present over quite extensive areas. In a classic study of the
ecological effects of the discharge of oil-based drilling muds from the Ekofisk
