Invasive Species - Ecological Systems - page 164

even highly diverse native communities are often readily invaded by nonnative
species, but the reduction of local species richness may accelerate invasion [ 35 ].
Most recent studies of invasion mechanisms focus on two popular hypotheses:
fluctuating resource availability and enemy release . The former hypothesis
proposes that a system's susceptibility to plant invasions varies with fluctuations
in unused resources (e.g., light, water, space, nutrients). Where propagule pressure
exists, invasion will be promoted by a sudden increase in resource supply (such as
through nutrient pollution) or reduced uptake by resident species (following
a disturbance such as clearcutting or fire) [ 39 , 40 ]. Nutrient-rich habitats do
experience more plant invasions, but native plants may not always outperform
nonnatives in low-resource conditions [ 41 ]. Highly disturbed environments are
also believed to be more invasible [ 1 ]. Nonnative species may dominate a habitat
following a disturbance event that is outside the evolutionary experience of the
natives; otherwise, natural disturbance may contribute to a system's resistance to
invasion [ 42 ].
The enemy release hypothesis attributes the success of nonnative species to their
escape from specialized natural enemies upon arrival to a new region, and their
inherent advantage over resident competitors that are burdened by their own
enemies [ 43 ]. One reason why plants that are subject to strong herbivory in their
native range can thrive in novel regions is that, in the absence of specialized
enemies, they may reallocate the energetic costs of defense toward reproduc-
tion and growth, and thus become more competitive [ 44 ]. It follows that fast-
growing species adapted to resource-rich environments may benefit most from
the absence of specialized enemies; thus, multiple mechanisms (enemy release,
disturbance, resource addition) may act synergistically to drive such invasions [ 45 ].
Modern Invasions as Unprecedented Global Change
The spread of species into regions beyond their native range has accelerated
exponentially during the past millennium because of human activities such as
agriculture, international travel, and global trade. There is a strong link between
trade activity and the global distribution of nonnative species [ 46 , 47 ]. International
trade often involves cargo moved by transoceanic ships, which can carry an
enormous number of organisms on their hulls and especially in their ballast tanks.
Tens of thousands of ships are estimated to be collectively transporting several
thousand species around the planet on any given day [ 48 ].
Most countries have recorded the establishment of several hundred nonnative
species, including invertebrates, vertebrates, plants, bacteria, and fungi ( Fig. 10.1 ).
Human influence is reflected in the improbable composition of modern species
assemblages worldwide: African grasses dominate large tracts of the Neotropical
region [ 30 ], European mammals and birds are abundant in Australia and New Zealand
[ 29 , 32 ], Eurasian invertebrates and fishes dominate food webs in the North American
Great Lakes [ 34 ], and over 25% of the nonnative species in the Baltic Sea originate
from the Pacific and Indian Oceans [ 50 ]. Over a decade ago, it was estimated that