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Alien Plant Invasion in Mixed-grass Prairie: Effects
of Vegetation Type and Anthropogenic Disturbance

Introduction

Concern over the invasion of non-indigenous plants into natural areas is rapidly escalating to alarm as the number of studies showing the prevalence and effects of such invasions climbs (Pysek 1995). Case studies have focused on individual plant species and their attributes (Young and Evans 1973, Mack 1981, Vitousek and Walker 1989, Thompson 1991, McClaran and Anable 1992, D'Antonio 1993, Qi and Upadhyaya 1993, Edwards et al. 1995) and several authors have attempted to find trends and generalizations that would help predict plant invasiveness (Andersen 1995, Beerling 1995, Pysek et al. 1995, Rejmanek 1995, 1996, Rejmanek and Richardson 1996, Daehler 1998). The role of anthropogenic and other forms of disturbance in promoting invasions of alien plants is well documented (Belcher and Wilson 1989, Hobbs 1989, Hobbs and Huenneke 1992, D'Antonio 1993, Parker et al. 1993, McIntyre and Lavorel 1994, Greenberg et al. 1997, Stapanian et al. 1998, Smith and Knapp 1999). Stochastic events related to the convergence of appropriate abiotic conditions, propagule availability, and phenological timing have also been implicated in the success or failure of plant invasions (Crawley 1989, Mack 1995), as have characteristics of the invaded vegetation or habitat (Mack 1989, Rejmanek 1989, Knops et al. 1995, Kotanen et al. 1998, Stohlgren et al. 1999).

In this study, we examine the influence of three factors, anthropogenic disturbance, native vegetation type, and park unit (a blocking factor reflecting the two geographically separate park areas) on the species composition of alien plants of Theodore Roosevelt National Park (TRNP) in western North Dakota, USA. The park consists of two units separated by roughly 115 km of federally and privately owned rangeland. The south and north units of the park comprise approximately 18 600 and 9600 ha, respectively, of mixed-grass prairie, badlands (an erosional landscape of buttes, tablelands, and valleys), and riparian areas. Twenty-three native vegetation types have been delineated on a geographic information system (GIS; TRNP, unpublished [maps available online])1, many of which occur in both units. Vegetation types are present in a complex mosaic, with high levels of spatial association among the different plant communities such that propagules in one vegetation type are unlikely to be spatially isolated from most other vegetation types. The configuration of TRNP provided an opportunity to examine the frequency of invasion of a suite of alien species into vegetation types occurring in two natural areas managed in similar ways. In addition, the availability of digitized data for roads, trails, and other human-impacted areas allowed us to examine the effects of anthropogenic disturbance on invasibility of each vegetation type. Because each of the alien species we consider here occurred in counties surrounding both units of the park, and thus had the potential to have colonized both units, we assume that differences in alien plant distribution attributed to park unit are associated with stochastic events (e.g., convergence of appropriate conditions for germination, availability of propagules) that influenced colonization.

The objectives of this research were (1) to compare the effects of vegetation type, anthropogenic disturbance, and park unit on the number and frequency (i.e., number of plots occupied per transect) of alien plant species in native vegetation types common to the north and south units of TRNP and (2) to determine which factors and interactions best explain the presence of individual alien plant species on transects. If matters of chance related to availability of propagules and transient establishment opportunities largely determine the success of invasion, we would predict that park unit or anthropogenic disturbance would better explain the variation in alien plant frequency than would vegetation type. However, if invasibility is more strongly related to biotic or physical characteristics of the native plant communities, we would predict that models of alien plant occurrence would more likely include vegetation type as an explanatory variable.

1 http://www.nps.gov/gis/park_gisdata/ndakota/thro.htm

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