Defining a regional approach for invasive plant research and management - Invasive Plants: Ecological and Agricultural Aspects - page 124

biological characteristics play an important role in the invasion process as well
as site factors and disturbance.
Species invasions can generally be depicted by a logistic growth curve [22].
Population growth curves may then be generated for plant species given their
initial population size, intrinsic growth rate, and time. Small populations are
often undetected during the early introduction phase of invasion because indi-
vidual plants may remain unnoticed for long periods of time. Dispersed seed
must compete with the established flora that is well adapted to the site in the
absence of disturbance; thus few introductions proceed to the next phase of
invasion. For example, Williamson [23, 24] estimates that only about one per-
cent of all introduced species into the British flora become established and are
invasive enough to be considered as pests [25].
Colonization/naturalization [15, 16] is characterized by exponential popula-
tion growth of the introduced species. During this explosive growth phase, the
species often becomes apparent to land managers and control efforts begin.
This phase of invasion is closely related to the intrinsic rate of increase for the
plant species. Hence, predictions of colonization/naturalization rates and man-
agement options should focus on the intrinsic biology of the species. For
example, Maxwell et al. [26] divided the life history of Euphorbia esula L. into
five stages: seed, buds, seedlings, vegetative shoots and flowering shoots. By
identifying these stages, the process of population development was deter-
mined using a stage-based matrix model. In this way the process of population
development and expansion was determined.
Theoretically, at some carrying capacity, K , the species' population
approaches a quasi-threshold density where its population growth stabilizes or
does not expand quickly. At this point colonization/naturalization is complete.
The K density occurs when niche occupancy and available resources limit the
rate of species spread, which is controlled by environmental or habitat-level
factors like competition or herbivory. Thus predictions of risk for species
approaching K also should focus on habitats or the disturbance of them.
Table 2 delineates disturbance types important for habitat susceptibility to
non-native invasive plants in one of our study areas in northeast Oregon.
Descriptions of biological and environmental characteristics for most inva-
sive species are often lacking or only general in nature. Thus, it is difficult to
determine which plant species are most likely to be invasive [1, 10, 27].
Formation of a landscape-level model augmented by empirical study of habi-
tat (environmental) and species (biological) factors can inform the overall pro-
gram for non-native invasive plant management in extensive production sys-
tems (Fig. 2).
Relevant scales and disturbances
Sauer [28] and Forman [29] indicate that ecological processes that determine
the function and stability of plant communities during plant invasions are