Duration of propagule pressure affects non-native plant species abundances.

PREMISE OF THE STUDY: Invasions by non-native species are known to be related to present propagule pressure (e.g., the number of non-native seeds arriving in a site each year). However, previous studies have mostly ignored the potential effects of the length of time that a site has experienced propagule pressure. This study is novel in studying past as well as present propagule pressure and in demonstrating the importance of past propagule pressure.

METHODS: We tested the importance of past and present propagule pressure in three plant communities in central Texas to quantify relationships between variables representing past and present propagule pressure, other environmental variables, and the abundance of non-native and native woody plant species.

KEY RESULTS: Duration of propagule pressure predicted non-native species richness. Sites had greater non-native species richness if they were nearer to development (houses, roads) and if nearby development was older. While mesic woodlands had the most native species, streamside woodlands had the most non-native species.

CONCLUSIONS: First, future studies of non-native invasions would benefit from including past as well as present propagule pressure. If past propagule pressure is not considered, its effects may be wrongly ascribed to present propagule pressure. Second, the non-native species in this study are widely used in landscaping, and development age reflects the years that landscaping has been present nearby. As xeriscaping becomes more common, streamside woodlands may someday not have the highest non-native species richness: the new drought-tolerant landscaping plants may be better invaders of drier sites than their predecessors.