invasions—might be reversed in many landscapes in the Anthropocene.
Although land use can contain invasions,
it can simultaneously increase pressure on
the least-invaded habitats. Harsh environ-
ments that are cold, dry, shaded, or nutrient
poor are often refugia of biodiversity and,
to date, little affected by invasions. Few spe-
cies are able to establish on productive land
suitable for intensive use and spread across
a heterogeneous landscape before growing
under harsh conditions. It is the spatial and
environmental remoteness that protects
some of the last uninvaded places on the
planet, but such protection is increasingly
being lost. Land use is expanding into mar-
ginal land, bringing species introductions
ever closer to the last remnants of natural
areas. Furthermore, preadapted species are
selected for planting in harsh environments,
such as on dry land for biofuel production or
in alpine sites to promote summer tourism
(5). Most alien plants found today at high el-
evations in mountains around the world are
species with a broad climate tolerance that
were introduced through agriculture to low
elevations and that then spread along el-
evational gradients. But increasingly, alpine
plants are directly transferred from one
mountain region to another as ornamentals
(5). By shortcutting the spread phase along
the elevational gradient, humans increase
the vulnerability of alpine ecosystems to in-
vasion by specialized alpine plants.
In other cases, an even more fundamental rethinking is needed. Some alien species
gain a major importance in a local economy;
this is, for instance, the case in plantation
forestry. These socioecological keystone
species form tight interrelationships with
socioeconomic interests and industries.
The social system associated with the species might evolve during the expansion process. For example, over a million hectares
of land have been planted with alien trees,
including Australian acacias, in Vietnam in
large-scale reforestation programs since the
1990s. Today, poor villagers, as well as major wood processing companies, depend on
these alien trees. These socioeconomic dependencies maintain the presence of alien
trees that are known to be invasive in the
landscape (12). Thus, a regime shift of the
socioecological system can fundamentally
change invasion dynamics. In these cases,
it is more appropriate to analyze the spread
and persistence of plant-human systems
than of isolated species.
REFRAMING INVASION THEORY
Ultimately, some alien species reach high
abundances, often resulting in strong ecological effects. Ecologists explain the success of these species as a result of either
better adaptation to abiotic conditions or an
advantage gained through new competitive,
symbiotic, or multitrophic interactions, often modulated by rapid evolution (3). Humans are increasingly involved in all these
interactions, and the main hypotheses in
invasion biology can thus be reformulated
with humankind as a key player.
For instance, the enemy release hypothesis states that some invasive species profit
from a release from specialized natural enemies that did not accompany the species
to the new region. In analogy, the human
release hypothesis proposes that release
from effective human control owing to specialized knowledge and skills that are not
present in a new region can facilitate the
spread of an alien plant (11, 13). Atlan et
al. (13) have shown that gorse (Ulex euro-paeus) might have become invasive partly
because traditional agricultural knowledge
on how to effectively cut the thorny bush
and use it in ground form as fodder was not
transferred from the native range in France
to the alien range in La Réunion Island, a
French overseas region in the Indian Ocean.
Land management can also influence
TOWARD IMPROVED PREDICTION
rapid evolutionary change in invasive
plants and their associated biotic commu-
nities. For instance, control of alien plant
populations can slow the evolutionary ad-
aptation of native insects to the potential
new host (14). Such human effects on inva-
sion dynamics do not stop at boundaries of
protected areas, because landscape-scale
processes reach into what are nowadays
the typically small nature fragments that
are scattered across anthropogenic land-
scapes. For example, humans control the
populations of large herbivores and manip-
ulate the frequency and extent of wildfires,
with important consequences for invasions
across all habitats of a landscape.
A purely biological theory of plant invasions cannot explain or predict invasion
dynamics in the Anthropocene. In particular, descriptive analyses of current invasion
patterns that are fundamental to many generalizations in invasion biology might often
be poor predictors of future invasions in a
fundamentally different socioecological environment (5). Focusing analysis on human
factors can help to improve predictability,
account for context dependences, elucidate
complex socioecological systems, and help
in managing or preventing invasions.
More than 1000 publications on invasive
species are published per year, but less than
10% address social aspects—and most of
the latter deal with human perception and
management issues, not with understanding invasion dynamics (15). The paradigm
change that is needed in the field can be
achieved by collaborating with social scientists and building on fields such as ethnobotany, landscape research, or political
ecology with a long tradition of analyzing
plant-human relationships through an interdisciplinary lens. j
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affect the mechanisms by
which plants become invasive.
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