Image: Figure 1 from Schultheis et al. (2015) Ecology.
The enemy release hypothesis (ERH) seeks to explain why some exotic species become invasive. Basically, ERH proposes that a lack of co-evolved natural enemies in the introduced (adventive) range means that there will be less top-down pressure regulating the exotic species, which allows it to reach higher densities than it does in its native range.
Of course, the intensity of top-down pressure is not the only potential difference between the native and adventive range, and any of these differences might explain, at least in part, the invasiveness of a given species. Accordingly, there are several other hypotheses which have been proposed over the years (e.g. Evolution of Increased Competitive Ability hypothesis, Novel Weapons hypothesis, etc.). Scientist test them in various contexts, results vary, meta-analyses and reviews ensue.
The general result: support for these hypotheses depends, but on what factors, we’re not really sure. (See Table 1 & 2 from Roy et al. (2010) Biocontrol for examples of support and refutation of ERH.)
I’m particularly interested in ERH because of its foundational connection to classical biological control. If natural enemies are no more or less important in regulating populations in the native versus the adventive range, then what business do we have moving around biological control agents? Can classical biological control still be justified?
I was excited to see an elegant experimental text of ERH in in Ecology by Schultheis et al. (2015) (title: No release for the wicked: enemy release is dynamic and not associated with invasiveness). As you can guess from the title of their paper, I was a little disappointed with their results. Damn data, always get in the way of my beliefs.
They drew 2 predictions from ERH and 2 additional related predictions about invasion dynamics, and tested them in a common garden of 61 plant species over the course of three years.
Prediction 1: Natives would have more herbivore damage than exotic invasives.
Prediction 2: Non-invasive exotics would have more herbivore damage than exotic invasives.
Prediction 3: Exotic species with a broader geographic adventive range would receive more herbivore damage.
Prediction 4: Exotic species with longer residence time in the introduced range would receive more herbivore damage.
The results for predictions 1 and 2 were nope and nope again. The results for predictions 3 and 4 were a bit more complex, so I won’t rehash the details here, but basically they didn’t provide any real support for ERH.
Hm, well, what does this say for classical biological control? Generally nothing great. One of the major problems facing classical biological control is its relatively low success rate (Hall et al. 1980; Greathead & Greathead 1992; but see Clewley et al. 2012). Perhaps we should do a better job of evaluating whether enemy release is occurring for a given invasive species before attempting a biological control importation, thereby weeding out release efforts doomed to failure. Evidence supporting ERH is sometimes sought ahead of time, but often with rather indirect observations like seeing a greater diversity of natural enemies consuming the species in the native range, which doesn’t necessarily mean that more damage is occurring.
Also, it is possible that adding one more species to the community could have some beneficial impact on invasive species control, even if the top-down pressures are not very important in the native range. In a new system, new rules may apply.