Discussion
Understanding the dynamics of small populations is critical for the
effective conservation of at-risk species. Previous work has
demonstrated an increase in extinction proneness in declining
populations (Fagan & Holmes 2006). Here, we corroborate preexisting
theoretical and empirical studies on the extinction dynamics of
populations and, additionally, show for the first time that the rate of
the extinction vortex can be altered by body size.
Reinforcing previous findings, our results show that the proximity of a
population to extinction is dependent on the logarithm of population
size (Lande 1993; Fagan & Holmes 2006). This suggests that the
proximity to extinction decreases at an increasing rate as a population
declines, indicative of an extinction vortex. Accordingly, care should
be taken to maintain populations at high densities to avoid
self-reinforcing spirals to extinction and to maximize the probability
of long-term persistence (Fagan & Holmes 2006). We cannot rule out the
influence of body size; the best fitting models did not provide a
significantly improved fit compared to those that included an
interaction term between body size and population size (Table 2). The
positive coefficients for the interaction between populations size and
body size (Table 2), suggests population size becomes increasingly
important in determining the distance from extinction as body size
increases. As such, smaller-bodied species appear to be more vulnerable
to imminent extinction across a greater range of population sizes, in
agreement with previous studies reporting greater population persistence
among species with slower life history traits (Newmark 1995; Saether et
al. 2005). Our relatively small dataset may account for the fact that
this is not the clear best performing model in this analysis.
According to the extinction vortex, genetic deterioration and Allee
effects are expected to result in proportionally larger declines as
population size diminishes (Brook et al. 2008). Indeed, we found an
increase in the year-to-year per capita rate of decline as population
shrinks (Table 2; Fig. 1). The implication of this is that even with
conservation intervention, species that fall into the extinction vortex
may struggle to be saved and require a non-linear increase in the
magnitude of the change required to save a population as it moves
towards extinction. Well-studied populations on the verge of extirpation
support this; the decline of the Florida panther population (Puma
concolor coryi ) was only reversed after the introduction of several
individuals translocated from healthy populations leading to the
restoration of genetic diversity (Johnson et al. 2010). In practical
terms, this emphasizes the need for early conservation intervention,
with a strong focus on ensuring species do not fall into the extinction
vortex.
Our results suggest that the key question of when a species is at risk
of rapidly collapsing to extinction is not only a function of population
size, but is also affected by the body size of the species; we found
evidence that small body size exacerbates the rate of decline in
geometric growth rate as population size declines (Fig. 1). This was
true for all groups (Table 2), suggesting a similar trend across the
vertebrate phylum. Though it is acknowledged that extinction risk is an
emergent property of the interaction between biological traits and the
type of threatening process (Owens & Bennett 2000; Isaac & Cowlishaw
2004; Price & Gittleman 2007; Brook et al. 2008; Davidson et al. 2009;
Ripple et al. 2017), our findings may seem at odds with the frequently
reported positive association between body size and extinction threat
level (e.g. IUCN threat status) (Gaston & Blackburn 1995; Bennett &
Owens 1997; Cardillo et al. 2005; Liow et al. 2008; Dirzo et al. 2014).
However, the extinction risk of highly fecund species is tempered by
naturally larger populations (Tracy & George 1992; Newmark 1995);
species at the fast end of the life history speed continuum seem to be
more vulnerable after controlling for the confounding effect of
population size (Cook & Hanski 1995; Johst & Brandl 1997; Saether et
al. 2005; Hilbers et al. 2016). A possible explanation is that
smaller-bodied species have faster life histories and are more
susceptible to stochastic elements (Peltonen & Hanski 2001; Sinclair
2003; Saether & Engen 2002; Saether et al. 2004; Wilson & Martin
2012), therefore they are predisposed to respond faster to the
deleterious demographic impacts of genetic decay, Allee effects and
other stressors.
The results of our third analysis, investigating population variability
through time, supports the idea that stochastic processes are involved
in causing the extirpation of these populations (Fagan & Holmes 2006;
Brook et al . 2008). That the magnitude of annual population
variability is higher in smaller-bodied species is indicative of lower
population stability in smaller-bodied species, as has been noted
elsewhere (Sinclair 2003). This may also help to explain the significant
negative interaction between body size and years to extinction in the
non-avian subset (Table 2); given that the population dynamics of
smaller-bodied species is inherently more stochastic, any increase in
year-to-year variability due to stochastic elements could be less
detectable. We suspect that a similar pattern is not observed in our
avian subset because of the small variation in body size; the range of
body masses in our avian subset (~7kg) is two orders of
magnitude smaller than that of our non-avian subset
(~350kg).
In conclusion, despite the large disparity in ecological and
environmental contexts among populations constituting this study, we
find evidence that small body size exacerbates the rate of the
extinction vortex, providing one of the first studies to investigate
differential vulnerability to the extinction vortex in relation to
intrinsic biological traits and, to our knowledge, the first to
specifically investigate this in real-life populations. The practical
relevance of our findings is highlighted by the fact that
species-specific data on body size is arguably the most widely available
across all taxa, and our results demonstrate the need for a generally
conservative approach to population targets especially in small-bodied
taxa with fast life history speeds and a high susceptibility to
stochastic processes.