Lecture 12.
The Ecological Effects of Invasive Species
Ecological effects can occur at many different levels. Some exotic species, even if
widespread, appear to have little effect on native species (the introduced species
may fill a vacant niche) or on ecosystems (the invader replaces a similar native
species and fills most or all of its function).
The effects of invaders on diversity are often hard to quantify. Few species
extinctions have been directly and solely attributable to an invasive species.
Viewed simply, many invaded ecosystems contain more species than they did prior
to invasion (a diversity increase!!).
We need to be careful though, as we are looking at only a short snapshot of time and
we don’t know what the future outcome of these novel interactions might be.
For example, the three most abundant lady beetles in North America are all
introduced. We know that many native species have decreased as the invasive
species have become common but we don’t know if the invasive generalists are
filling the roles that a larger number of native specialists used to do. We also do not
know the mechanism behind the declines in native species (i.e., is it a direct
relationship, an indirect relationship mediated through food or another resource, an
indirect interaction between the invasive species and other environmental
stressors, etc).