Change you can believe in: A novel empirical model to integrate invasive species impacts

Background/Question/Methods

Invasive plants have become synonymous with displacement of native species and negative impacts on our ecosystem processes and services. Despite this dogmatic understanding of invasions, the overwhelming majority of studies focus on single consequences in specific locations, with few attempts to scale or integrate impact metrics. In fact there has only been a single attempt to scale ecological impacts, but it suffers from defining impact on a per capita basis and has ironically only been calculated for a single species (Heracleum mantegazzium). Therefore, we are incapable of knowing what the total impact of an invasion is on a system (i.e, impacts to diversity, structure, functions, etc.). To overcome the limitations of current models, we have created a novel functional relationship that is capable of integrating all metrics of interest, and allows a more holistic accounting of ecological impact.

Results/Conclusions

To achieve an integrated impact value, we designed each metric (eg, richness, nitrogen pool, liter production) to be calculated in the common currency of percent difference between the invaded and uninvaded (reference) sites. Metrics that were once examined independently can now be combined into a common unit of measure. An integrating function combines an infinite possible number of metrics, which approaches a theoretical true impact value. Achieving the actual ecological impact value is therefore non-linearly proportional to the number of metrics measured, which suggests that simply measuring a single metric grossly oversimplifies the “actual” impact. Interestingly, our model also suggests that it may not matter what metrics are quantified from a site as long as enough metrics are sampled.

Measuring impact on a per capita basis precludes interspecies comparisons due to the tremendous differences in size among invaders (annual grasses to large trees). By using percentage cover, interspecies comparisons to other invaders and natives with dramatically different size is achievable, which also allows measurements to be taken across the range of possible densities to create an impact curve. This curve is then scaled over invaded area size to create a species-specific impact surface.  With this surface it will be possible to make interspecies impact comparisons, and to estimate population level impacts. Testing this conceptual model with empirical evidence may lead to an understanding of the true impact an invasive species has on its surrounding habitat previously unavailable.