The invasion and spread of exotic plant species is a major threat to population persistence and biodiversity in general. A plausible way in which exotic plant species might affect native faunas is via their effects on connectivity among native habitat patches (Cronin & Haynes 2004). Research by Kyle Haynes and me illustrates this point.

The invasive grass, smooth brome (Bromus inermis)

Forrest Dillemuth, PhD Student

Prairie cordgrass (Spartina pectinata) is a dominant plant species in wet prairie fragments of northeastern North Dakota. It is clonal and grows in discrete patches ranging in size from a few stems to several ha monocultures. Smooth brome (Bromus inermis) is an exotic grass that has been introduced into the area for erosion control and as a source of hay for grazing animals such as cattle. Brome has spread into these prairie fragments at an alarming rate and appears to be displacing cordgrass habitat.

My graduate student, Forrest Dillemuth and I have used GPS and GIS technology to map the spread of brome and change in cordgrass patch distribution and abundance in three prairie fragments from 2000-2004. Over the course of 4 years, the structure of the landscape changed dramatically (see representative maps below ). In particular, cordgrass patches declined in size by an average of 61% in each prairie fragment. The change in cordgrass patch size was strongly correlated with the percentage of brome surrounding the patch – patches with a high percentage of neighboring brome had the greatest reduction in size (see Figure 1).

We also determined the number of new cordgrass patches that became established (i.e., colonized) or went extinct between 2000 and 2004 in low (< 25%) and high (> 75%) brome areas. We found that new cordgrass patches were more than 2 times as likely to establish in low, as opposed to high brome habitat (see Figure 2). At the same time it was 2 and 1/2 times more likely that a patch of cordgrass went extinct in areas of high brome versus areas of low brome.

The spatial distribution of cordgrass and brome habitat patches at North Kelly's.

These results suggest that as brome continues to spread in the Great Plains, existing cordgrass patches will disappear with higher frequency, and new patches will appear with less frequency than in prairies free of brome.

Figure 1. The relationship between brome prevalence in the immediate vicinity of a cordgrass patch and the mean ± SE percent change in patch size between 2000 and 2004. For all three prairie fragments, there was a highly significant relationship based on a Kruskall-Wallis test (χ2 > 27.7, df = 4, P < 0.001).

Figure 2. Percentage of cordgrass patches that (A) colonized or (B) went extinct in habitats consisting of a low or high percentage of brome.

Based on our experimental research on the movement of a specialist cordgrass planthopper (Prokelisia crocea) and its primary parasitoid (Anagrus columbi), we can anticipate drastic consequences associated with the increase in prevalence of brome in the matrix and the reductions in cordgrass patch size and number. Relative to native matrix habitats (e.g., mudflat or native grasses), brome makes small cordgrass patches function like sieves for the two insect species. Planthoppers and parasitoids leak out of these patches at such high rates that extinction quickly follows (Cronin & Haynes 2004, Cronin 2007).

As part of his PhD research, Forrest is conducting field experiments in North Dakota to determine whether the invasion of brome is responsible for the drastic changes in the distribution of cordgrass. He is exploring issues regarding the invasibility of brome and its impact on a broader range of native species.