Limiting Spatial Dispersal

Invasive plant propagules (most often seeds) create new infestations by being spatially dispersed to new areas. Limiting the spatial dispersal of invasive plants can greatly improve prevention success (Davies and Sheley 2007a). If invasive plants do not have propagules present at a site or are unable to disperse propagules to the site, invasion will not occur. Furthermore, the likelihood of invasion is significantly less with decreasing propagule pressure (D'Antonio et al. 2001; Davies 2008; Davies et al. 2008).

National and regional regulations to prevent the introduction of exotic species play an important role in limiting the spatial dispersal of invasive plants into new geographic regions. However, once an invasive species establishes in a geographic region, management becomes the primary means to preventing its spread within that region. Thus, our discussion is focused on research and management to prevent the spread of regionally-established invasive plant species.

One strategy to limit the spatial dispersal of invasive plant species is to reduce the amount of propagules they produce. Depending on the invasive plant species and location of infestations, various methods can be deployed to accomplish this task. Grazing or mowing prior to propagule development, introduction of biological control agents, and decreasing resource availability can potentially limit spatial dispersal by reducing propagule production. Defoliation by mowing or grazing of invasive plant species prior to propagule development causes a shift in resource allocation away from reproduction to growth (DiTomaso 2000; Sheley et al. 1998). The closer defoliation occurs to the end of the growing season, the less likely the invasive plant will have enough time and resources to grow new foliage and reproduce. Biological control agents can also be a defoliator that reduces the general production of the invader or they can be propagule predators. For example, two biological control insects reduced Centaurea solstitialis L. (yellow star-thistle) seed production by 4376% (Pitcairn and DiTomaso 2000). A potential method to reduce resource availability to invasive plants is to practice management that increases competition for resources by other plants growing in the infestations. These tools to reduce propagule production of invasive plant species are only efficient under specific circumstances and often other management actions are needed.

Additional general management, such as applying herbicides around the edge of established invasive plant infestations can limit their expansion (Sheley et al. 1999) or defoliating invasive plants at the edge of infestations prior to seed development can reduce the spatial dispersal of invasive plant species without expending resources to control the entire infestation. However, specific management focused on restricting dispersal mechanisms can further improve the effectiveness of a prevention program. Limiting the spatial dispersal of invasive plant species can be accomplished by identifying and developing management that obstructs dispersal vectors. Most invasive species have special adaptations that facilitate their spatial dispersal by specific vectors. Davies and Sheley (2007a) identified seed characteristics (potential adaptations for dispersal) and location of the infestations relative to vector pathways as critical elements to consider in determining which vectors are major contributors to the spread of the invasive plant species (Fig. 2). Specific strategies could then be focused on these vectors to improve the effectiveness and efficiency of prevention efforts. Factors that promote specific dispersal are attachment, attractant, aerodynamic properties, buoyancy, and self-propelled adaptations, and infestation location (Davies and Sheley 2007a).

Invasive plants may have adaptations that facilitate attachment to vectors to increase spatial dispersal. Plants that are dispersed by animals and humans often have barbs, hooks, or viscid outgrowths on their seeds (Sorensen 1986; Cousens and Mortimer 1995). These outgrowths allow them to attach to clothing, hair, and fur and thus, can be dispersed great distances (Cousens and Mortimer 1995). Some invasive plants are dispersed by attracting potential vectors. Seeds that are edible or surrounded by edible flesh would likely be dispersed by animals and humans. These seeds may be consumed and then dispersed in animal feces. For example, horses can be major disperses of invasive plants species by transporting seeds in their digestive tract (Wells and Lauenroth 2007). Seeds surrounded by edible flesh may also be transported by humans and animals to a new location for consumption, the edible flesh is consumed, and then the remaining seed is discarded. Limiting dispersal of invasive plants that are commonly transported by animals and humans should focus on limiting their contact with potential vectors when propagules can be dispersed. Physical barriers to animal and humans can decrease their contact with the invasive plant species and thus reduce the spread of these plants. Feeding animals invasive plant-free feed in containment prior to moving them from locations with infestations to non-infested areas would also reduce the spread of invasive plants.

Many invasive plant species have seeds with aerodynamic properties that enhance their ability to be dispersed by wind (Fig. 3). Wind dispersed seeds have plumes or wing appendages (Burrows 1986); alternatively, whole plants or their panicles may disarticulate before seed shatter and be wind dispersed. For example, entire above ground structure of tumbleweeds breaks off and can be dispersed by wind (Howe and Smallwood 1982, Cousens and Mortimer 1995). Invasive plants may also have mechanisms for self-propelled dispersal. Euphorbia esula L. (leafy spurge) self-propels its seeds away from the parent plants with capsules that erupt during periods of low humidity and hot temperatures (Selleck et al. 1962). Self-propelled seeds may be projected several meters (Riley 1930; Beattie and Lyons 1975).

Other Seeds

More research needed to identify major vectors

Location (near vector pathway)

More research needed to identify major vectors

Seeds viable when surrounding foliage acts as an

Adaptations for dispersal

Fleshy,

Buoyant

edible

Plant or panicle detaches

Location (near vector pathway)

Seeds viable when surrounding foliage acts as an

Adaptations for dispersal

Very small

Plumed, winged

Ballistic seed shatter

Plant or panicle detaches

Very small

Plumed, winged

Ballistic seed shatter

Insects

Wind

Self

- close trails through weed infestations

- clean seeds off clothing and equipment

- maintain weed free zones along trails

- educate public about weeds

- create barriers around weed infestations

- pen livestock for 48 hrs after using an infested pasture before moving them

- deter or prevent animal use or movement through weed infestations when seeds are viable limit travel through infestations

- wash/clean vehicles maintain weed free zone around roads and boat launching areas harvest weed infested hay fields before weed seeds are viable maintain weed free zone along waterways

- create barriers between weed infestations and waterways screen seeds out of waterways deter or prevent insect use of or movement through weed infestations

- prevent insects from traveling outside of weed infested areas

Figure 2. A framework for identifying potential major spatial dispersal vectors of an invasive plant species and management strategies to limit specific dispersal. (Copyright © 2007 Weed Science Society of America: Reprinted from Davies, K.W., and R.L. Sheley. 2007. A conceptual framework for preventing the spatial dispersal of invasive plants. Weed Science 55:178-184 by permission of Allen Press Publishing Services).

Figure 3. A seed with aerodynamic properties that are an adaptation for wind-dispersal. Note the high surface area of the plumes compared to the seed size.

Invasive plants with adaptations for wind- and self-propelled dispersal can be managed with similar strategies. The focus should be to intercept the aerial projected seeds or structures containing seeds. Davies and Sheley (2007b) demonstrated that wind-dispersal of invasive plants could be significantly reduced by maintaining taller neighboring vegetation around invasive plants.

Buoyant invasive plant propagules have the potential to be water dispersed. Some seeds can float for months (Hocking and Liddle 1986; Schneider and Sharitz 1988) increasing their dispersal potential and becoming a greater challenge to management. Small seeds can also be water dispersed because the amount of energy required to transport them is small. Strategies for limiting water-dispersal of invasive plants should strive to keep the propagules from reaching the water, removing them from the water, and/or stopping them from traveling from the water to a safe site while still viable (Davies and Sheley 2007a).

Locations of invasive plant infestations relative to potential vector pathways are also important factors to consider when managing invasive plant species. Infestations near roads and animal trails can accentuate dispersal by vehicles (Plummer and Keever 1963) and animals (Darwin 1859) even if plants lack specific adaptations for those methods of dispersal. To prevent dispersal along vector pathways, management needs to limit contact between vectors and invasive plants when dispersal can occur. Intensive management of infestations along roads, trails, and waterways to create an invasive plant-free zone next to the vector pathway would be justified. Closing or rerouting remote hiking trails and dirt roads that intersect invasive plant infestations may also be necessary.

Another major factor contributing to the spread of invasive plants is the dispersal of their propagules with desirable plant materials. Dispersal of invasive plant propagules with hay, other feeds, and seed mixes is a serious management concern. Seeds of Centaurea L. (knapweeds) species are often dispersed with hay and seed mixes (Sheley et al. 1998). Thus, recreational activities that are seemingly innocuous, such as using hay or grain on backcountry horse-packing trips could easily spread invasive plants to remote wildlands. Requirements for certified weed-free hay, other feeds, and seed on wildlands are probably the only effective strategy for curtailing this threat.

To limit the spatial dispersal of invasive plant species propagules, combinations of strategies will probably need to be applied because many invaders are dispersed by more than one mechanism or more than one invader is of concern. For a more detailed description of spatial dispersal of invasive plant species and suggested management options refer to Davies and Sheley (2007a). Preventing the spatial dispersal of invasive plants species in wildlands will also require educating the general public and, specifically, users of wildlands about invasive plants and implementing regulations to prevent invasive species introduction. Limiting the spatial dispersal of invasive plant propagules could be improved with additional research developing new methods and strategies to reduce propagule production, intercept dispersing propagules, and prevent vector contact with invasive plants.

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Responses

  • RICHARD ROBINSON
    How are invasive species selflimiting?
    8 years ago
  • Elena Donaldson
    How can invasive species be self limiting?
    8 years ago

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