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.