| |
|
Introduced
species have serious effects on ecosystems. Depending on where the
species lie in the food chain, they may alter existing predator-prey
relationships, take over ecological niches, and out-compete native
species. Additionally, they may cause changes in the physical and
chemical characteristics
of the habitat, reducing its suitability for native species. Introduced
bacteria and viruses may cause disease and parasites may attack native
species. The changes in the food web alter nutrient cycling and energy
flow through the ecosystem. (2) These
effects combine to reduce ecosystem complexity and cause a loss of
biodiversity. On a global
scale,
introduced species are the second leading cause for the loss in biodiversity (2). |
Alteration of the base of the food web
The recent invasion of the Great Lakes by small, prolific invertebrates has
upset the balance within the lower trophic levels of the ecosystem. An
example of this is Diporeia, a small shrimp. In the past, nearly 80%
of the living biomass in the lakebed has been made up of Diporeia.
This native
invertebrate is a very important source of food for many fish species
in the Great Lakes. Recently, Diporeia populations have declined drastically,
from over 10,000 shrimp per square meter to zero in some areas of the
Lakes.
It is thought that the dense zebra mussel colonies on the lake floor
have reduced the ability of Diporeia to survive and reproduce. (1)
As a cascade effect, the zebra mussels are efficient filter-feeders
and have removed much of Diporeia’s food source from the water
column, further stressing the shrimp’s populations.
|
 |
|
Changes in
species distribution
The influx of nonnative and invasive species, and the subsequent extinctions
and population depletions of native species has changes the distribution
of fish and other species within the Great Lakes. White et al depict this
spatial change in the diagram to the left.
“Invasional
Meltdown”
Although it is commonly believed that increased biodiversity makes an ecosystem
more resistant to disturbance, it now appears that invasive species are
facilitating the introduction of new invasive species. Some invasive species
alter the
environment in ways that make it easier for others to become established
(12). The rate of Great Lakes invasions continues
to increase. In fact, a new species invades every eight months (13). |
