In this post Associate Editor Steven Vamosi discusses a paper he recently handled by Gareth D. Davies and J. Robert Britton ‘Assessing the efficacy and ecology of biocontrol and biomanipulation for managing invasive pest fish’
The threats to native biodiversity are manifold, with the “big three” generally agreed to be, in order of decreasing importance, habitat destruction (i.e. loss, degradation, fragmentation), introduced species, and overharvesting. Habitat destruction and overharvesting have in common that they are, at least in principle, easy to address – in both cases reductions in these activities will tend to minimize extinction risk. Introduced species present a more complicated and difficult challenge, especially after they have become established. As an example, a non-native species that I study, Scotch broom (Cytisus scoparius), is now a prominent member of roadside and open field communities in much of the Pacific Northwest of North America. The two most prevalent methods for its management are mechanical removal and the application of herbicides – the latter is undesirable for potential negative effects on native flora and the former is highly labour-intensive. Furthermore, even removal is nuanced, with pulling being effective with young (small) individuals but not recommended for large/mature ones. Neither method has been successful beyond very local scales, such as along main roads running through small urban centres on Vancouver Island, and they require monitoring and periodic action at least in part because Scotch broom seeds may stay viable for three decades in the seed bank.
Even if complete eradication is unlikely, management strategies that may reduce the abundance or spread of non-native species are important, which is what Davies and Britton (2015) investigated for a pest cyprinid fish, the topmouth gudgeon (Pseudorasbora parva). Pseudorasbora parva is native to Asia, having first appeared in Europe in the 1960s and spreading to the study area in the UK by the middle of the last decade. Davies and Britton refer to studies that show that, once it has become established, it can significantly alter food web structure and pathogen transmission, both of which can have devastating effects on native species. The most effective management tool is, unfortunately, the non-specific biocide rotenone, which is (for good reason) prohibited in parts of its new range. Davies and Britton used a two-pronged approach – a naturally affected field site and an experimental field trial – to assess the individual and combined effects of (i) biomanipulation, or the removal of P. parva by nets or traps, and (ii) introduction of a biocontrol, the native facultatively piscivorous perch Perca fluviatilis.
The field site, a shallow pond with P. parva, was exposed to removal of P. parva with nets every six months for two years and the introduction of 10 P. fluviatilis (Photo 1). Collectively, these treatments reduced P. parva densities by an astounding two orders of magnitude. Stable isotope data revealed that P. fluviatilis included P. parva in their diet, although the contribution dropped over the course of the experiment (from 49% to 21%).
The field trial, which was conducted in replicate ponds at an inactive aquaculture site, contrasted the removal (N = 4 ponds) and biocontrol (N = 4) treatments with unmanipulated controls (N = 4). The results from this experiment stressed the importance of the biocontrol in dampening compensatory responses in response to the initially large drops in abundance that may be achieved with removals (Photo 2). Davies and Britton (2015; DOI: 10.1111/1365-2664.12488) stressed the complementary effects of these targets with “removals of mature P. parva prior to their spawning season reduced their reproductive effort, biocontrol minimized their compensatory responses through increased predation pressure, and removals at the end of the reproductive season reduced their recruitment.” In addition to summarizing the main effects of the two treatments, this quote reveals the importance of knowing the natural history of pest species: it is likely that for many, as with P. parva, getting the most bang for the buck (or, perhaps, fish per sweep) will require effective timing of removals.
There are, of course, caveats to these approaches. First, efforts associated with successful removal treatments are nontrivial, with Davies and Britton visiting a single shallow pond several times over a two-year period. Second, there are ethics to be considered when introducing a biocontrol agent to a pond or lake, even if they are native to the region, if they are not obligate consumers of the target species, which applies to most control agents that are not specialized parasites or parasitoids. Practitioners considering applying a double-pronged approach are encouraged to assess the condition of other species that may have been negatively impacted by the pest species prior to introducing a biocontrol species that may parasitize, compete with, or prey upon them. Finally, and as Davies and Britton (2015) were careful to point out, even when applied together these treatments were not successful in totally eradicating P. parva from small ponds. The decision to pursue this target will require case-by-case analyses of perceived benefits relative to potential ecological costs.
In conclusion, this study highlights the promise of managing non-native species with well-researched multi-pronged control methods, which will hopefully lead to other success stories in a variety of species and locales. Perhaps the right combination will lead to serious reductions in other pest species previously considered unmanageable.
Davies, G.D. & Britton, J.R. 2015. Assessing the efficacy and ecology of biocontrol and biomanipulation for managing invasive pest fish. Journal of Applied Ecology DOI: 10.1111/1365-2664.12488.