The success of invasion can be determined by landscape structure both indirectly as a response to the way the plant community is modified by landscape characteristics and directly as a response to resource heterogeneity created by edge effects. Understanding how landscape structure alters invasion success is important because fragmentation and non-native species introductions are both expected to continue. We sought to understand how spatial heterogeneity of native species alters invasion success. Taeniatherum caput-medusae is a widespread invasive annual grass in the western United States. Studies have shown that perennial bunchgrasses are resistant to T. caput-medusae invasion, however, it is unclear how configuration of perennial bunchgrasses may alter patch dynamics to influence the success of invasion. In Fall of 2012, we seeded native perennial bunchgrass species in vertical strips of varying widths (2.4 m, 4.8 m, and 7.2 m, N =4) across 44m x 24m fields. We used 0% and 100% seeded fields as non-patchy controls. In Fall of 2015, we seeded T. caput-medusae into 2 m2 plots in the area between native patches. The following spring, we measured the community diversity, abundance and the number of T. caput-medusae seedheads in the 2m2plots and in paired unseeded plots.
We found that heterogeneity, regardless of patch width, reduced the negative impact of T. caput-medusae on the native plant community compared to the non-patchy control treatments. There were fewer T. caput-medusae seedheads in the 2.4 m, 4.8 m, and 7.2 m patch treatments than in the non-patchy treatments. The plots with T. caput-medusae in the non-patchy treatments had a higher cover of non-native species and a nearly two-fold decrease in native cover compared to areas where T. caput-medusae was not seeded. The increase in non-native cover was attributed to both the abundance of T. caput-medusae and an increase in cover of other non-native annual grasses. In the patchy treatments, there was also an increase in non-native cover, however cover of other non-natives was not increased in the presence of T. caput-medusae. Additionally, there was a smaller decrease in native cover when T. caput-medusae was present in patchy treatments compared to non-patchy controls. Overall, spatial heterogeneity in plant communities buffered against the negative effects of the introduction of T. caput-medusae, suggesting that it is important to consider spatial structure when evaluating invasion vulnerability.