Agriculture Reference
In-Depth Information
such fine-grain factors are also difficult to quantify for landscape-level models
or habitat-level risk assessments [32]. Climate also drives abiotic and biotic
thresholds for plant growth in particular ecosystems. For example, the retreat
of forests to mountainous areas over the last 10,000 years is a direct result of
changing climate [33]. Climate can also change habitat suitability over short
timescales through drought, seasonal frost, and flooding [34]. Invasive plant
species tend to adapt well to a variety of habitats but usually invade regions
with similar climates to their native range first, then adapt to other climates
later [35]. Edaphic characteristics such as topography and elevation modify
climate, and therefore influence the plant species that can grow at a given loca-
tion. Land use and the resulting condition of the environment is a third driving
component of habitat suitability and invasion. Changes in land use are thought
to be the single most important factor in species extinction [15] and to have
strong influences on invasible sites [36].
Landscapes
Across a region, a species may be detected in the late colonization/naturaliza-
tion or early naturalization/invasive phase [15, 16] and thus be considered a
stable population. However, this stable source population contributes to many
subsequent local infestations through seed dispersal. These small satellite pop-
ulations are sinks from the source and can become additional source popula-
tions themselves, which expand markedly from the original infestation area
[37]. Rates of increase for satellite populations can be extremely high because
satellite introductions have a much higher probability of success than initial
introductions as a result of the constant seed flow that arises from source areas
[38, 39].
Landscape features and connectivity relationships become important for
predicting spread from source populations to new and as yet unoccupied loca-
tions for exotic plant species. Regions are composed of a mosaic of extensive
natural resource areas (forests, rangelands, wetlands, etc.), intensively man-
aged locations (farms, paddocks, holding areas for livestock, etc.), urban areas
of various sizes (intensities), and the corridors (roads, rivers, etc.) that connect
them all [32]. Invasive plants can be present in each landscape feature. The
spread of Bromus tectorum across most of the western United States since
1900 [40], and the replacement of native hardwood forests in Australia and
other regions of the world by Pinus radiata [41] are important examples of
landscape change by exotic plant species. Transportation corridors can
markedly influence the risk associated with species spread as satellite popula-
tions [42].
Interaction with land managers
Scientifically rigorous research is fundamental to a regional research program,
but the gap between scientific knowledge and existing management practices
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