Environmental Engineering Reference
In-Depth Information
individual invasive species have positive impacts from the perspectives of one or
more interest groups and negative impacts from the perspectives of others (see
section 5.4). h ese issues will infl uence decisions about how much eff ort is put
into control, where that eff ort is expended, and how the costs and benefi ts are dis-
tributed amongst stakeholders. Because impacts may be ecological, economic, or
social, they cannot be readily quantifi ed in a single currency though some attempts
have been made to quantify environmental impacts using economic concepts (e.g.
Sinden
et al
.
2003). Decisions about what, if any, action to take will inevitably
involve a degree of subjectivity taking various societal values into account.
h e relationship between the abundance and impact of an invasive species is
important. h e best documented cases of this relationship are probably those
involving weeds of agriculture or pastoralism (Cousens 1985). h e relationship
between percentage yield loss (of a crop) and the abundance (density) of a weed
has been described by a rectangular hyperbolic curve (Fig. 5.2). Applying this rela-
tionship to invasive species in general, impacts increase with higher abundance but
at a decreasing rate. At low abundance, an invasive species will have negligible or
undetectable impacts, while above some threshold, further increases in abundance
result in no greater impacts.
h e abundance of an invasive species depends on the carrying capacity of the
invaded environment and the stage to which the invasion has progressed. h is
means that in the early stages of an invasion (Fig. 5.1), impacts are likely to be low.
A period of low impact may extend for some time given the time-lags that have
been documented to occur during many invasion processes (Mack
et al
.
2000;
Grice and Ainsworth 2002; Groves 2006). h e costs of control during an early
Weed density
Fig. 5.2
A rectangular hyperbolic curve describes a relationship between
percentage yield loss of a crop and the abundance (density) of a weed
(Cousens 1985). A similar relationship could be expected to describe the
relationships between density and other impacts of invasive species.
