A phast-er framework for integrating invasions: The state factor model

Background/Question/Methods

Invasive species have been a topic of ecological interest since Darwin’s Beagle voyage when he noted “introduced plants which have become common throughout whole islands in a period of less than ten years”, which now cause tremendous environmental and economic damage worldwide. Charles Elton is credited with formalizing the field of invasion ecology in 1958 with the publication of The Ecology of Invasions by Animals and Plants, though we remain mired in complexity without common direction even today. In the past two decades multiple hypotheses have been proposed to explain why the vast majority of introduced species remain benign components of the landscape, while a minority become aggressive invaders. Despite these advances in understanding, no single thesis has universal explanatory power across the multitude of invaders or systems, much less lead to rational control strategies. In response to the increasingly bloated “marketplace of theories,” we have adapted a framework that organized soil science and plant ecology in the previous century to unify biological invasions. The state factor framework integrates phenomenological variables into a quantifiable system based on the theory that specific components of invasions exist and together determine the characteristics of the invasive species.

Results/Conclusions

Just as Hans Jenny identified that soils are a result of climate, organisms, topography, parent material, and time, we have identified five state factors that together comprise the variation associated with any quantifiable characteristic of a biological invasion (i): propagule pressure (p), the introduced habitat (h), the invasive species autecology (a), the source environment (s), and time since introduction (t). The “phast” factor-function allows variance accounting and hypothesis testing via partial differential equations for factors of interest. This accounting bypasses the need to suggest that any component (eg, allelopathy, lack of natural enemies) is singly causal in a particular invasion. Knowledge gaps are quickly elucidated with using a phast-factor matrix, which can drive research priorities. The complexity of biological invasions can only be fully realized with an appropriate framework, which phast attempts to do using the elegant framework of state factors.