Changing disturbance regimes drives riparian plant invasion: evidence from trait-based species responses to flow regulation

Background/Question/Methods

Many factors affect invasion success. This makes it difficult to determine their relative importance, especially when relying on survey data that include confounded variables. Incorporating information about species traits can increase understanding of species-environment relationships and provide additional evidence to test competing hypotheses.

Using variance-components analysis and hierarchical linear models, we examine whether exotic plant invasion in River Murray wetlands, Australia is driven by altered environmental filters, higher colonisation or competitive ability of exotic species or human association. Specifically, we examine how proportional cover of exotic plants relates to flood disturbance, human-mediated dispersal, impacts of flow regulation and livestock grazing. We test whether variation in the occupancy of exotic and native species along environmental gradients can be explained by species traits (life history, specific leaf area, height, seed mass). 

Results/Conclusions

Findings suggest that flow regulation facilitates exotic plant invasion because, compared to the native species group, exotic species possess characteristics (larger seed mass, higher specific leaf area, higher weed status) that were advantageous under altered hydrology.

Proportional exotic cover was unrelated to proximity to towns and disturbances associated with cattle pugging and flood frequency.

Exotic species responses to hydrological change were not based on colonisation or competitive abilities. However, native species with higher competitive ability (perennial, higher SLA) appeared more tolerant of regulation-induced reductions in flood magnitude than less competitive natives. Traits generally modulated occupancy of native and exotic species in a similar way, but there were exceptions, e.g. seed mass.

This study highlights the importance of testing assumptions regarding trait-based responses and illustrates that invasion is not a function of one factor, but many. Modification of the disturbance regime was the precursor to invasion, but human facilitation and increased availability of invader propagules, combined with exotic species trait distributions relative to natives, enabled invasion.