BIOLOGICAL POLLUTION - Marine Pollution: What Everyone Needs to Know - page 188

bacteria once a week, which may not be often enough. Models
can be used to predict how long after a rainfall a particular
beach will be affected by microbial pollution, but if you are
concerned, it might be best not to swim for a couple of days
after a major rainfall. The Waterkeeper Alliance has developed
a swim guide and a website at www.theswimguide.org, which
provides up-to-date information about many beaches.
What are invasive species?
When a species arrives in a new environment, it is unlikely to
have the natural controls that kept its population numbers in
balance in its native area. Without control by predators, par-
asites, or disease, some species increase rapidly, to the point
where they can take over their new environment and harm
native species. Marine organisms have been moved around the
world for thousands of years by ocean currents and attached to
driftwood, and more recently aided by human activities. What
is new is the speed and volume at which marine organisms are
now transported. Recently, marine invasive species have had
major impacts on biodiversity, ecosystems, fisheries, human
health, and economics. Although most introductions fail, a
small percentage of species can thrive and become a problem
in the new area. Certain traits—for example tolerance of envi-
ronmental stress—characterize species that become invasive.
Environments that are stressed (e.g., polluted, eutrophic, low
in native diversity) appear to be more vulnerable to invasions.
As demonstrated by Andrew Cohen and James Carlton, San
Francisco Bay appears to be a hotspot for invasions, containing
many of the invasive marine species in western North America.
How do they get to new locations?
Aquatic species can be transported by various means (vec-
tors), either accidentally or on purpose: in ship ballast water
or by attaching to hulls, as hitchhikers clinging to boots or