Biology Reference
In-Depth Information
(relatively) the same rate, making total rate of new range expansion proportional to
the number of populations. This has tremendous practical implications for control-
ling invasive species. When tackling an invasion, managers often deem it best to
attack the largest population(s) first. Instead, modelling indicates that limiting the
number of new localities infested and eliminating small satellite populations should
be higher priorities (Moody and Mack, 1988).
Impacts
It would probably be fair to say that greatest research progress in the past 20 years
has been had in a broader elucidation of the numerous impacts that invasive alien
species can impose. These impacts are remarkably variable and include extinctions
of species, biotic homogenization, disruptions to food-webs, changes to primary
productivity of ecosystems, changes in soil formation, alterations of community
structure, wholesale conversion or replacement of ecosystems, changes in nutrient-
cycling dynamics, collapse of fisheries, degradation of watersheds, promotion of
increased fire frequency and extent, increases in erosion and flooding rates, losses
to agriculture, damage to human structures, disease epidemics, and degradation of
human quality of life (Greenway, 1967; Ebenhard, 1988; van Wilgen et al., 1996;
Wilcove et al., 1998; Mack et al., 2000; Pimentel et al., 2000, 2005; Mooney and
Cleland, 2001; Pimentel, 2002; Mooney, 2005; Towns et al., 2006; Binimelis et al.,
2007; Charles and Dukes, 2007; Reaser et al., 2007). Examples of these impacts are
too many to enumerate but can be found by the score in the articles just cited or in
the scientific and popular topics cited at the beginning of this chapter. Hence, I will
not discuss this issue in detail but will merely give one brief example from the non-
herpetological literature to illustrate both the novelty, unpredictability, and damage
that are so frequently wedded in invasion biology.
The comb jelly,
Mnemiopsis leidyi
, a zooplankton feeder native to western
Atlantic estuaries, was introduced to the Black Sea around 1982. It quickly formed
extremely dense (1.5-2kg/m
2
) biomass, and zooplankton communities declined
15-40 fold (Kideys, 1994). As a result of jelly predation on their food and fry,
anchovies (
Engraulis encrasicolus
) and other planktivorous fish species declined
dramatically, with fisheries collapsing by 4-40 fold, depending on the fish species
and country (Kideys, 1994, 2002; M. Williamson, 1996). Anchovies and other
fisheries had been an important source of human protein for communities around
the Black Sea, so it is not difficult to imagine the economic hardship and decline in
quality of life occasioned by this introduction. It is estimated that fisheries profits
declined from US$17 million/year before the invasion to US$0.3 million afterwards
(Knowler and Barbier, 2000). This cost does not include the estimated several thou-
sand lost jobs as well as secondary effects on economically linked enterprises
(Knowler and Barbier, 2000). The jelly population happened to be brought under
control a few years later by the inadvertent but fortuitous introduction of a second
comb jelly,
Beroe
, which feeds on
Mnemiopsis
. This led to recovery of some
