PS 11-145 - The role of propagule pressure, biotic resistance, and nitrogen loading on invasion dynamics in freshwater coastal wetlands

Abstract: Propagule pressure (PP) and nitrogen (N) availability are recognized to enhance the colonization of introduced plants, while biotic resistance is known to deter invasion. However, the interaction of these three contributing factors is less well known, which is of specific interest in areas where all three vary, such as the coastal wetlands of the Great Lakes. Here, we investigate the interaction of PP and biotic resistance along a N gradient for two invasive species, Phragmites australis and Typha x glauca, using MONDRIAN, an individual-based community-ecosystem model that simulates growth and competition among individual ramets. We explored invasion dynamics into a bareground environment (no interspecific competition – simulating conditions shortly after disturbance) or an established 3-species native community at six levels of PP (1 to 30 rhizomes/m²) across a N loading gradient (0.86 to 30 gN m^-2yr^-1). We focus on the 1) failure rate (the number of failed invasions, determined as zero individuals alive at the end of the run, out of 10 replicate runs), 2) NPP and 3) dominance (proportion of the community biomass) of the invader, including how patterns differ between Phragmites and Typha. While Phragmites and Typha had similar failure rates under low biotic resistance – high failure rate (100%) at the lowest N loading and low failure rate (0%) at the highest N loading across all PP levels – Typha’s failure rate was consistently higher under strong biotic resistance regardless of N loading. Typha had a high failure rate at both the low and high N loading (lowest failure rate in the middle of the N gradient), with more PP resulting in a lower failure rate at high N loading. Typha had difficulty invading an established community at high N loading because of competition with the largest native. In contrast, Phragmites’ failure rate was low across biotic resistances. Propagule pressure had much less impact on the success (NPP and dominance) of Phragmites compared to Typha. Neither could successfully invade at either biotic resistance at very low N, but Phragmites was much more successful at medium to high N loading. While PP only somewhat influenced the success of Phragmitesoverall, when under strong biotic resistance, Typha needed high PP to be successful. Our results suggest that invasion is a complex outcome influenced by both external factors (N loading, PP, and competition) and species-specific traits.