Biology Reference
In-Depth Information
such as roads and trails, vectors such as livestock, land use practices such as
seeding burned areas, and the extent of area open to invasion (Forman et al. 2003;
Brooks 2007). The species pool is the number of nonnative species in a region, and
the larger that pool the greater the likelihood that at least one or several species will
invade other areas within the region (Lockwood et al. 2005). Propagules can origi-
nate from populations outside of, or within, a management unit.
Plant resource availability is a function of the supply of light, water, and mineral
nutrients, and the proportion of these resources that are unused by existing vegetation
(Davis et al. 2000; Brooks 2007). Resource availability can increase due to direct
additions (e.g., atmospheric nitrogen deposition), increased rates of production (e.g.,
nutrient cycling rates), or by reduced rates of uptake following declines in plant abun-
dance after they are thinned or removed. Feedback processes from established popu-
lations of nonnative plants can also affect resource supply. This can occur by direct
increases in nutrient supply (e.g., nitrifying plants) or indirect increases brought about
by limiting the growth of other species through competition or inhibition. Areas of
high resource availability are often disturbed sites. Fire, landslides, floods, and graz-
ing not only increase the pool of available resources but may also reduce abundance
of native species that would otherwise compete with invading species or, conversely,
reduce invasion rates by consuming potential colonizers (Marty 2005).
2.3.2 Generalized Monitoring Plan
The role of disturbance in facilitating invasions is well established (Lonsdale
1999; Mack and D'Antonio 1998; Mack et al. 2000), probably because they often
lead to increases in both propagule pressure and resource availability. Accordingly,
disturbed areas are often high or very high priorities for early detection monitor-
ing. However, disturbances are typically pulsed events that often cannot be pre-
dicted. Early detection monitoring plans must, therefore, include two parts: (1) a
strategic baseline plan that should be updated periodically (e.g., 5 year intervals)
on the basis of an assessment of propagule pressure and resource availability
across the entire management unit; and (2) tactical incident plans for each major
event that results in major landscape-scale pulses of propagules and/or resources
(e.g., a large fire or construction project). Part 1 should be supported by a consist-
ent and predictable source of funding, whereas part 2 should be supported as part
of monitoring efforts associated with each major landscape-scale event.
Very high priority
areas for early detection monitoring occur where both prop-
agule pressure and resource availability are high (Fig. 2.3). If significant sources of
invading species are present, and resources are readily available, then plant inva-
sions have the greatest probability of occurring.
High priority
areas for monitoring occur where propagule pressure is high,
but resource availability is low (Fig. 2.3). Any time when propagule pressure is
high there is a chance that invasive plants can establish following unanticipated
