GIRAE: a generalised approach for linking the total impact of invasion to species' range, abundance and per-unit effects

The total impact of an alien species was conceptualised as the product of its range size, local abundance and per-unit effect in a seminal paper by Parker et al. (Biol Invasions 1:3-19, 1999). However, a practical approach for estimating the three components has been lacking. Here, we generalise the impact formula and, through use of regression models, estimate the relationship between the three components of impact, an approach we term GIRAE (Generalised Impact = Range size x Abundance x per-unit Effect). We discuss how GIRAE can be applied to multiple types of impact, including environmental impacts, damage and management costs. We propose two methods for applying GIRAE. The species-specific method computes the relationship between impact, range size, abundance and per-unit effect for a given species across multiple invaded sites or regions of different sizes. The multi-species method combines data from multiple species across multiple sites or regions to calculate a per-unit effect for each species and is computed using a single regression model. The species-specific method is more accurate, but it requires a large amount of data for each species and assumes a constant perunit effect for a species across the invaded area. The multi-species method is more easily applicable and data-parsimonious, but assumes the same relationship between impact, range size and abundance for all considered species. We illustrate these methods using data about money spent managing plant invasions in different biomes of South Africa. We found clear differences between species in terms of money spent per unit area invaded, with per-unit expenditure varying substantially between biomes for some species-insights that are useful for monitoring and evaluating management. GIRAE offers a versatile and practical method that can be applied to many different types of data to better understand and manage the impacts of biological invasions.

Automated summary

The total impact of an alien species can be estimated by a formula called GIRAE, which considers the range size, local abundance, and per-unit effect. This formula can be applied to different types of impact, such as environmental impacts and management costs. The paper proposes two methods for using GIRAE: the species-specific method and the multi-species method. While the species-specific method is more accurate, it requires a large amount of data for each species, while the multi-species method is more easily applicable but assumes the same relationship between impact and other factors for all species. The methods are illustrated using data on managing plant invasions in South Africa, showing clear differences in expenditure between species and biomes. Overall, the GIRAE method offers a versatile and practical approach for understanding and managing the impacts of biological invasions.