One of the perceived benefits of biodiversity is resistance to invasion by exotic species. This has relevance for vegetation restoration: according to theory, sowing more species of the desired type would help to exclude the invasion of undesired ones. Oakley & Knox (Applied Vegetation Science, this issue) tested this in a real restoration situation: the revegetation of bare compacted clay after construction or commercial activity. Higher sown diversity did indeed reduce the invasion of non-sown species and, of particular practical relevance, reduced the invasion of exotic species.
Credit for coining the term ‘biodiversity’ is variously given to W.G. Rosen (in person, 1985), L. Tangley (in a table in a scientific paper, 1985) and E.O. Wilson (in a book title, 1986). Within a few years, the term had become an important propaganda tool in persuading the public, and especially politicians, that conservation is important. Tangley (1985) quotes Peter Raven: “Basically, everyone on earth depends on living organisms to sustain the quality of their lives. Thus even from a practical or pragmatic view, it's essential to preserve as many kinds of organisms as we can.” However, more explicit benefits were needed to convince politicians of the importance of biodiversity, and the more cynical would say in order to obtain research money. So, beyond aesthetic considerations and ‘extinction is for ever’ appeals to emotion, ecologists indicated the practical significance of biodiversity: stability, resistance to perturbation, resilience, productivity (overyield), invasion resistance, nutrient cycling, water yield, etc. The trouble was that ecologists, if questioned, had no hard evidence for any of them. In fact, the opposite could sometimes be demonstrated, for example from theory on stability (May 1972) and from the field on invasibility (Stohlgren et al. 1999). However, politicians do not read much scientific literature, and there was enough time-lag for large grants to be awarded, notably the EU-funded BIODEPTH programme. That funding ran from 1996–1999, but most of the experimental sites have continued to be maintained. Similar questions have been answered using David Tilman's Cedar Creek plots, already set up.
There have now been many experiments attempting to validate the supposed benefits of biodiversity. Over yield was probably the first investigated, indeed the concept goes back to the agricultural work of de Wit in the 1950s, followed by work on stability (sensu lato). More recently, effects have been investigated on carbon sequestration, soil microflora and fauna, litter decomposition, fire, herbivory, nutrient cycling (e.g. Dybzinski et al. 2008), water availability (e.g. Jucker & Coomes 2012) and micro-evolution. All these effects are sometimes summarized as ‘ecosystem services’, a term that may have been coined for political as well as ecological purposes.
Diversity and resistance to invasion
However, a continuing theme has been the effect of species diversity on resistance to invasion. Generally, experiments have supported the concept that diversity confers invasion resistance (Fridley et al. 2007), although with the caveat that the functional traits of the species matter too. It seems that the many positive native/exotic diversity correlations reported, the ‘diversity begets diversity’ of Palmer & Maurer (1997), are mainly due to confounding environmental variation: in favourable environments more native species can co-exist and more exotics can persist (actually, this is not the case in Palmer & Maurer's fascinating work, which comprised a randomized experiment). This has theoretical implications: it quantifies community resistance since the invading species can be seen as a perturbation, and it tests whether native communities are saturated (Stohlgren et al. 2008). It also has importance in conservation: perhaps those communities of conservation value that are species-poor are especially liable to invasion, and should be prioritized for protection and monitoring (Souza et al. 2011).
Funk et al. (2008) pointed out that the principles established in BIODEPTH-type experiments could also be applied to vegetation restoration projects. Communities established with more species should, if the BIODEPTH results are followed, be more resistant to invasion. The restoration of tree cover can rely on intensive management, for example with shelter tubes (Padilla et al. 2011). However, with herbaceous vegetation more extensive methods are needed. Oakley & Knox (2013) address the common practical problem of vegetating sites where heavy construction has removed the topsoil and left only compacted clay subsoil. Would the diversity/invasion resistance concept work there? Keeping the total density of seeds constant, three levels of species richness were sown, in a randomized block design. This is like a BIODEPTH experiment in the real world. The only manipulations of the clay subsoil were the spreading of leaves and discing, typical of real restoration projects. The effect of sown diversity was examined not only on the species richness of invaders but also on their cover, and the latter was determined, very commendably, with point quadrats. Subjective guesses of cover may possibly be justifiable in vegetation survey (but see Wilson 2012), but they have no place in experimental protocols.
The results of the above experiment demonstrated that diversity does indeed reduce the invasion of non-sown species in the real world, in terms of the number of invading species and of their cover. In a restoration project, the target species composition might be tightly defined, for example to establish a grassland over limestone that comprised only typical limestone grassland species, or to re-vegetate a cut-over bog with only species of ombrotrophic bogs present. Sometimes the aim may be simply landscape engineering – establishing vegetation cover to prevent erosion – although even then it may be necessary to exclude fast-growing, competitive species that would hinder the establishment of the deep-rooted species more effective in slope stabilization (Guerrero-Campo et al. 2008). However, often the main aim is to establish cover of some native species, excluding exotics, for it will hardly ever be possible to recreate a ‘natural’ community within a few decades. The aim is then to exclude species exotic to the region, and the 24-species richness treatment of Oakley & Knox almost achieved this, with only one exotic species in the ten most abundant, and that with cover considerably reduced compared to the three-species and 12-species plots (Fig. 1). Recent diversity/invasion experiments have used different random selections of species in each replicate plot. Oakley followed an earlier design with species sets nested, a design more relevant to practical restoration when the optimal mixture would be impracticable to determine. It does give the danger that only the species-rich mixture might include a highly competitive species that could suppress invaders (the ‘selection effect’), but in this case the early-dominant, Elymus virginicus, was present even in the three-species seed mixture.
Oakley & Knox's paper is very appropriate for Applied Vegetation Science, which has vegetation restoration as a major speciality, and which very much welcomes work that applies ecological theory to practical problems.