ECOLOGY IMPACTS AND GENETIC VARIABILITY RESEARCH FOR INVASIVE WEEDS. Charles T. Bryson, USDA-ARS, Southern Weed Science Research Unit, Stoneville, MS. (cbryson@ars.usda.gov)

ABSTRACT

The ecological impacts of nonnative invasive weeds are directly related to biological and ecological characteristics. Characteristics of individual invasive species are placed into one or more broad categories including reproduction, dispersal, habitat, inter-specific interactions, phenology, physiology, protection from herbivores, and tolerance to environmental stress (1). All of these factors influence the rate of establishment, spread, and adverse impact on crops and native and acceptable nonnative species. Economic and ecological diversity losses occur from competition, crowding, allelopathy, alteration of environmental factors, reduction in land values, and increased cost of productivity because chemical, biological, and cultural methods are required to control invasive weeds. The overall impact by invasive weeds may be as subtle as slight changes in biodiversity of an area or as devastating as creation of expansive monocultures that eliminate most, if not all, other vegetation and/or altering fire frequency and intensity. Cogongrass [Imperata cylindrica (L.) Beauv.] is an example of an invasive species that forms extensive monocultures and displaces other plants because of direct competition and alleopathy (3, 4), alters the fire regimes (4), and eliminates habitat for wildlife. Unlike cogongrass, Japanese honeysuckle (Lonicera japonica Thunb.), Chinese tallow (Sapium sebiferum L.), and other invasive weeds are able to invade areas and selectively displace species that occupy specific niches within an area (1). Deeprooted sedge (Cyperus entrerianus Boeck.) may form extensive monocultures in open areas or replace the herbaceous layer in forested areas (5). Because each weed's ecological impact differs, research is essential to determine biological processes, ecological range, and most vulnerable growth and reproductive stages to develop effective control strategies. Ecological range studies identify the environmental conditions best suited for survival. For example, ecological range studies indicate that wetland nightshade (Solanum tampacense Dunal) can survive and reproduce much farther north that its current range in the United States (6). The most severe invasive weeds are usually difficult to control and possess reproductive and survival traits that provide advantages over native and nonnative acceptable species. Genetic variability (DNA fingerprinting) research is important to determine the diversity within and among invasive weed populations, source and number of introductions, dispersal rate, occurrence or potential for herbicide resistance, potential areas to search for biological control agents, vectors for dispersal, and basis for differing life events within a species or among species such as genetic basis for flowering and seed production. A recent example of this type research determined that 22 tropical soda apple (Solanum viarum Dunal) populations sampled from six states in the southeastern United States did not significantly differ from one another and from 3 of 4 populations sampled in Brazil (7). This information indicated that the introduction of tropical soda apple was likely a single event or from populations that are genetically similar. The research also identifies an area in Brazil where plant populations are similar and where host-specific biological control agents may be discovered. Bloodscale sedge (Cyperus sanguinolentus Vahl), a native of Asia, was determined to be a nonnative invasive weed that was likely introduced from Japan in rice culture and not a potential threatened and endangered species (Cyperus louisianensis Thieret) (8). The life history of bloodscale sedge is diverse in the world. However, the bloodscale sedge genotype introduced into the southeastern United States flowers and sets fruit from late September until frost unlike other varieties of this species elsewhere in the world and many of the other weedy annual sedges that produce seed most of the growing season.

Much more research is needed to determine the ecological range potential of most nonnative invasive weeds and how they adversely affect native species. Additional research is also needed to determine genetic diversity to provide insight into why some invasive weeds are more competitive, which control measures can be adapted for specific or similar genotypes, and where to look for potential species-specific biological control agents.

LITERATURE CITED

1. Bryson, C.T. and R. Carter. 2004. Biology of pathways for invasive weeds. Weed Technol. 18:1216-1220.
2. Koger, C. H. and C. T. Bryson. 2004. Effects of cogongrass (Imperata cylindrica) extracts on germination and seedling growth of selected grass and broadleaf species. Weed Technol. 18:236-242.
3. Koger, C. H., C. T. Bryson, and J. D. Byrd, Jr. 2004. Response of selected grass and broadleaf species to cogongrass (Imperata cylindrica) residues. Weed Technol. 18:353-357.
4. Byrd, J.D., Jr. and C.T. Bryson. 1999. Biology, ecology, and control of cogongrass [Imperata cylindrica (L.) Beauv.]. Mississippi Dept. Agric. and Commerce, Bureau of Plant Industry, Fact Sheet 1999-01. 2 pp.
5. Bryson, C.T., R. Carter, and D. J. Rosen. 2003. Deeprooted Sedge (Cyperus entrerianus). Proc. South. Weed Sci. Soc. 56:CD-ROM.
6. Bryson, C.T. 2000. Overwintering potential for wetland nightshade (Solanum tampicense) north of its current range. Proc. Weed Sci. Soc. Am. 40:13-14.
7. Kreiser, B., C. T. Bryson, and S. J. Usnick. 2004. Genetic fingerprinting of tropical soda apple (Solanum viarum). Weed Technol. 18:1120-1124.
8. Carter, R. and C. T. Bryson. 2000. Cyperus sanguinolentus (Cyperaceae) new to the southeastern United States, and its relation to the supposed endemic Cyperus louisianensis. Sida. 19:325-343.