Conservation implications of the dimensions and drivers of rarity
Our framework (Figure 3) provides insight into the particular vulnerabilities of different types of rare species and may help identify effective conservation measures for them. Each of the three rarity dimensions pose different challenges to persistence: species that occur at low abundance may be vulnerable to demographic stochasticity , Allee effects , inbreeding, and drift . Low occupancy is correlated with higher extinction risk , which may be explained in part by metapopulation dynamics: as occupancy declines, populations will become increasingly isolated, which in turn reduces the probability of demographic rescue and increases the risk of local extinctions. Similarly, isolation resulting from low occupancy may also promote drift and lack of gene flow , impacting fitness. Furthermore, species that are characterised by narrow ranges may be particularly vulnerable to correlated population dynamics owing to the spatial correlation in ecological processes and environmental conditions over the relatively small spatial extent of these species’ ranges . For example, factors such as disturbance or habitat loss will affect a larger proportion of the range of narrow endemics as compared to more widespread species. Finally, species that are range restricted by a narrow climatic niche are particularly vulnerable to climate change .
The hypothesised relationships between the three rarity axes and the four underlying processes (Figure 3) may point to measures that could be used to conserve different types of rare species. Note that the measures proposed below are aimed at managing species whose persistence is threatened by their rarity, rather than those that are stable despite being rare.
We hypothesise that species characterised by low abundance are primarily limited by demography and interactions. Demographic challenges to persistence may be mitigated via measures such as assisted breeding and ex-situ conservation, which can increase the probability of survival for species on the brink of extinction, and help to maintain or increase the genetic diversity of very rare species . As for interactions, species may be threatened by new negative interactions (e.g., competition and predation), or, conversely, by the loss of positive interactions (e.g., pollination). Control of predators or invasive species may be necessary for conservation of some species (while avoiding unnecessary and unproductive persecution of predators; ). In the case of facilitative interactions, the conservation of species with obligate symbioses requires the conservation of the symbiont. In some cases, facilitative interactions may be known or suspected to be involved in a species’ rarity, but the identity of critical symbionts unknown; e.g., a rare plant may be threatened by insufficient pollination, but the specific pollinator is not known. In these cases, habitat- or landscape-scale efforts that promote the recovery or maintenance of biodiversity and ecological processes may be the most effective intervention. In short, it is essential to recognize the importance of the trophic network surrounding the target species.
Species characterized by small ranges and/or low occupancy are thought to be mainly limited by environmental filtering and movement. Species limited by environmental filtering may benefit from landscape scale measures to conserve or increase (i.e., restore) high quality habitat. Where movement is a limiting factor, increasing patch connectivity (at a grain size suitable to the target species’ dispersal capacity) and assisted colonisation may be helpful to increase patch occupancy. Assisted migration to climate analogues may also help conserve range-restricted species threatened by climate change.