Northern Prairie Wildlife Research Center
The invasion of exotic plants is becoming a problem in many ecosystems including some areas in Rocky Mountain National Park (RMNP) (Rocky Mountain National Park Resource Management Reports #1 and #13). Some exotic species, such as leafy spurge and spotted knapweed, are capable of rapidly colonizing areas, altering community composition, and even displacing native species (Belcher and Wilson 1989, Tyser and Key 1988). In many cases, the processes of invasion are poorly documented, and little information is available on an area's past history. However, there is a large amount of information available in the literature which relates to the life history traits of exotic species and the distribution of exotic species. This information can be used to help predict the potential distribution and threat of exotic species to ecosystems.
Exotic plants can be thought of as those plants which did not originally occur in the ecosystem, and have since been introduced to the area. The National Park Service (NPS) defines an exotic species as, "those that occur in a given place as a result of direct or indirect, deliberate, or accidental actions by humans." This somewhat conservative definition of exotic species is necessary to insure that natural resources in national parks are preserved.
NPS policy generally prohibits the introduction of exotic species into natural areas of national parks. Exotic species which threaten park resources or public health are to be managed or eliminated if possible. In addition, the NPS recently signed a memorandum of understanding with 10 other federal and state agencies in the state of Colorado. This agreement states that all paid management agencies will work with private and county entities to manage exotic plants and, in particular, "noxious weeds." RMNP is currently working with Estes Park in exotic plant control as part of this agreement.
The process of invasion by exotic species has been naturally occurring for thousands of years. However modern landscapes present "..unparalleled opportunities for invasive weeds as a result of modern transportation systems and the intensity of modern land-use practices" (Forcella 1992). As a result, exotic species threaten to impact other plant species and communities as they expand their ranges and invade new areas. In western Montana, for example, invasive species such as spotted knapweed have reduced plant community diversity and forage quality Forcella (1992).
The invasion of exotic plants into ecosystems is detrimental on an ecological level because it can potentially alter the balance between the native species. The term "niche" is often used to describe the range of conditions and resource qualities within which the organism or species persists (Ricklefs 1990). Systems that have evolved under natural conditions have niche overlaps which allow different species to exist together. However, the introduction of exotic species can disrupt this balance. As Bedunah (1992) pointed out, "..since the exotic (plant) did not evolve in the community, it has not had time to move toward niche and habitat differentiation there, and it may be a more direct competitor with the dominant and co-dominant plants." There are now many examples where exotic plants have indeed altered this balance and made significant ecological changes to plant communities. A review of the invasion process can provide some understanding about how exotic species become problems in natural areas.
The process of invasion can be thought of as an initial colonization of a system, followed by the establishment of a viable population within the system. There are several important steps to a successful plant invasion, including: seed dispersal, initial seedling establishment, and the establishment and persistence of a viable population (Figure I ). The overall ability of a plant to successfully invade an area is related to its life history traits.
|Figure 1. Diagram illustrating the key components of the "invasion cycle." The ability of a species to invade an ecosystem is directly related to its life history traits.|
Seed dispersal is one of the most important factors that influences the ability of a species to colonize new areas. For an invasion to occur, seeds must first be dispersed to a potential habitat. Seeds have an entire array of morphological and structural adaptations which allow them to be dispersed by natural processes and human activities.
Seeds may be naturally dispersed by wind, water, and animals. For example, Canada thistle and dandelion produce seeds that have a hairy structure called a pappus that allows them to be easily dispersed by wind. Similarly, mature Russian thistle plants break off at the base, allowing the "tumbleweed" to disperse seeds. Many seeds are buoyant and can be dispersed by water. Seeds may also have specialized structures that allow them to cling to animals. Cheatgrass seeds, for example, possess barbs on the caryopses that attach to animal fur. Finally, seeds can remain viable after passing through the digestive tracts of animals. Once eaten, these seeds can be dispersed over potentially large areas by birds, cattle, horses, and other mammals. In RMNP, the use of horses has likely contributed to the spread of a number of exotics including Canada thistle (McLendon 1992).
Humans activities are another important vector for seed dispersal. For example, cultivation has promoted the spread of exotics. A number of exotic plants were intentionally introduced into RMNP area for use as cultivars or as ornamentals before the area became a National Park. More recently, exotic plants have been introduced as part of erosion control programs, or accidentally by park visitors. For example, spotted knapweed was found in one of the RMNP campgrounds and was likely brought in by park visitors. Exotic plants continue to be unintentionally introduced and dispersed in RMNP by clinging to clothing and mud on hiking boots, and by attaching to motor vehicles.
Once seeds reach a new potential habitat, climatic and abiotic factors may affect the establishment of seedlings. For seedlings to successfully become established, the temperature and precipitation regimes of the area must fall within the tolerance ranges of that species. However, many invasive species may be "pre-adapted" to the climatic and abiotic conditions of the new potential habitats(Newsome and Noble 1986). These species may have evolved under similar climatic conditions, or may have broad tolerance ranges that allow them to occupy a variety of habitat types.
The establishment of a single plant in an ecosystem generally does not constitute a successful invasion. Instead, an invasive species must establish a self-sustaining population. Bazaaz (1984) points out that colonizing species are more likely to become established with a large number of repeated introductions of a large number of seeds. The establishment of invasive species is rare with single introductions of a small number of seeds. Thus, species that are capable of producing and distributing a large number of seeds have a higher probability of a successful invasion. Species which have a high number of propagules in close proximity to natural areas such as RMNP also have a high invasion potential.
Species that are good competitors for soil moisture and nutrients have a higher chance of establishment and persistence in ecosystems. Characteristics of good competitors include plants that hold their leaves higher than other plants (in light limited environments), or push roots deeper into the soil (in water limited environments). Good competitors often possess rapid early growth, leading to a rapid development of the root system. The development of a root system early in the spring may allow the plant to access available resources that are unavailable to dormant species. Through the acquisition of these available resources, the exotic plant may then become established in the ecosystem.
Once an exotic plant population becomes established, there are three potential outcomes to a plant invasion: naturalization, facilitation, and species replacement through succession. Naturalization refers to a species that is more or less in equilibrium with the other plants in the community. If a species invades an ecosystem and does not expand its range within that ecosystem, it might be considered 'naturalized." However, naturalization may only be a short term phenomenon.
Facilitation can occur when a disturbance alters the community. Following the disturbance, the exotic species may then begin to invade larger areas. In a sense, the subsequent invasions were 'facilitated" by the disturbance. An exotic species can be facilitated to spread by the introduction of a suitable seed dispersal agent or pollinator, or the provision of disturbance (Cronk and Fuller 1995). Facilitation helps to complete the invasion cycle by allowing the species to disperse seeds and establish plants in new areas. In the absence of any natural controls, such as pathogens or herbivores, these invasive species may continue to expand their range (Bedunah 1992).
Finally, the process of succession can affect the invasion cycle by altering the availability of resources over time. The change in species composition from a simple plant community composed of a few colonizing species to more complex plant communities over time is called succession. Succession is driven by stressors or disturbance, which can affect the availability of resources available to plants. These changes in resource availability influence which plants are able to persist in the ecosystem and may provide an initial opportunity for invasion. Over time, plants that are best adapted to the biotic and abiotic conditions replace the plants that are not well adapted to the conditions.
There are two general types of succession: primary and secondary succession. Primary succession occurs when plants gradually become established in areas not previously vegetated because of the lack of soil development. Examples of primary succession include plants which colonize a gradually filling bog or parent material such as granite (Barbour et al. 1987). In contrast, secondary succession occurs in areas that were previously vegetated, but have had the pre-existing vegetation destroyed. In the case of secondary succession, much of the soil and plant propagules (such as seeds and rhizomes) remain intact (Barbour et al. 1987). Disturbances such as fire, logging, or cultivation can initiate secondary succession.
Secondary succession is tightly linked to the availability of resources and the life history characteristics of the plants. For example, recent research in the Piceance Basin of western Colorado indicates that nitrogen availability in the soil is the key factor driving secondary succession (McLendon and Redente 1992). A commonly observed pattern of plant succession begins with the domination of annual species followed by perennial grasses or shrubs, followed by perennial grasses, and finally either shrubs or trees. The final dominant community of shrubs or trees is sometimes referred to as a climax community.
Changes in nitrogen availability over time will affect the species composition for that ecosystem. Immediately following a disturbance, nitrogen is often highly available. These conditions favor plants which readily exploit the available nitrogen, such as annual plants (sometimes called early seral species). The annual life history involves a relatively rapid rate of growth that requires high levels of nitrogen. Annual plants, such as Russian thistle, generally dominate a disturbed area as long as there is a surplus of available nitrogen.
Over time, the nitrogen from the soil becomes tied up in plant tissue and litter. In addition, the decomposition of litter (which contains organic nitrogen and carbon) is relatively slow during early succession. As a result, the amount of plant available nitrogen decreases over time. These conditions favor slower growing plants with lower nitrogen demands such as perennial fortes and perennial grasses (mid-seral species). During the middle stages of succession, perennial fortes and grasses gradually replace the annual plants. The mid-seral species tend to have slower growth rates, which reduces the amount of litter inputs. Fortunately, the rate of decomposition of litter increases during the midsuccession stages.
In spite of their slow growth rates, shrubs and trees increase in importance during late succession stages. Late seral species are able to tolerate low resource availability because they are good accumulators and competitors for resources. By efficiently exploiting limited resources, or by storing resources (and denying other plants access to resources), late seral species are able to survive in low resource conditions.
Relatively few exotic plants introduced to RMNP can be considered late seral species. However, species which possess traits similar to mid- or late succession species are generally much more persistent. For example, some perennial grasses such as smooth brome store a large amount of resources in below ground tissues. These large food reserves make this plant very difficult to control because these reserves must be depleted before the plant becomes stressed. Many of these species are also capable of slowing down natural succession processes because of their ability to access the limited resources in the ecosystem. Plants which have high reproductive output along with mid to late succession characteristics are among the most threatening and difficult to control.
The invasion potential of an exotic species can be partially predicted by examining its life history characteristics, geographic distribution, and ecological distribution. Table 1 summarizes some of the life history characteristics that are closely related to the overall invasion potential and persistence of plants in ecosystems. The list presents a general "wish list" for the ideal weed adapted from some of the early work on colonizing species (Baker 1965). Fortunately, no single species possesses all these characteristics. However, plants that possess a number of these traits are often "pre-disposed" to being good invaders. In addition to life history characteristics, information on the ecological and geographical range of the species can also be used to help predict the potential distribution of a species in a given area. Species which are found in a wide range of habitat types will likely have a much wider potential range than those species restricted to a small geographic range or few habitats.
Table 1. Characteristics of the "Ideal Weed" (adapted from Baker 1965).
- Has no special requirements for germination.
- Has discontinuous germination (self-controlled) and great longevity of seed.
- Shows rapid seedling growth.
- Spends short time in vegetative condition before beginning to flower.
- Maintains continuous seed production for as long as growing conditions permit.
- Is self-compatible, but not obligatory self-pollinated.
- When cross pollinated, can be achieved by non-specialized flower visitor or by wind.
- Has very high seed output in favorable environmental conditions.
- Can produce seed in a wide variety of environmental circumstances. High tolerance of (and often plasticity in face of) climatic and edaphic variation.
- Has special adaptations to both long and short distance dispersal.
- If perennial, has vigorous vegetative reproduction.
- If perennial, shows ability to regenerate from severed rootstocks.
- Has ability to compete by special means (rosette formation, choking growth, etc.)
Information on the life history and distribution of exotic species is certainly useful in a management context. For example, species that have wide distributions in their native systems and have traits that are characteristic of invasive species (such as adaptations for long distance dispersal, and the ability to compete for resources) should be closely monitored. Other species that have restricted ranges, specialized pollinator relationships, or limited seed dispersal potential may pose less of an immediate threat.
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