Our mark–release–recapture study focused on a subset of the whole population. Nevertheless, two considerations suggest that our estimates of population size represent the main part of the whole population: First, our survey of natural dung pats throughout the region identified the site used for mark–recapture as the population stronghold (Fig. 1). Second, estimated survival rates were high, averaging 74% between trapping periods. Since these estimates actually combine the probability of staying alive and of staying within the study area, we may infer that the population within this stronghold is relatively closed, and that high rates of recruitment evident in Fig. 3 largely reflect individuals hatching from local pupae. Even though specific estimates of peak population size differ by a factor of nearly two among the two methods applied, and while the associated confidence limits were relatively wide, the order of magnitude seems firmly established in the thousands (not in the hundreds or in the tens of thousands; cf. Fig. 3b).
A population size in the thousands will clearly exceed the most simplistic ‘rules of thumb’ for an imminent risk of extinction due to demographic or genetic stochasticity (e.g. Boyce, 1992). Yet, the Finnish population of O. gibbulus has rightfully been classified as critically endagered. It is restricted to a small area, and there is hence a high risk that a single perturbation might affect the full population. One such perturbation would be a break in habitat continuity (cf. Hanski, 1999b; Siitonen and Saaristo, 2000; Fritz et al., 2008). The pasture sustaining the main part of the population has been continuously grazed for hundreds of years (Kontula et al., 2000) – should grazing be discontinued for a single year, this might cause the population to crash.
From a management perspective, dung beetles like O. gibbulus offer interesting challenges: they are dependent on a resource (cattle dung) and habitat (warm, grazed slopes) both related to human activities. While national legislation will often prevent the active destruction of natural habitats inhabited by endangered species, forcing land owners to maintain vital resources for O. gibulus will seem like a complex task. Yet, we stress that the predicament of O. gibbulus is shared by a large number of taxa (Thomas, 1993), posing a challenge which we can hardly neglect. In this context, our results can be used to generate recommendations for the management of O. gibbulus, with ramifications for multiple taxa in changing agricultural habitats. First, since our results show O. gibbulus to be a specialist species confined to specific habitats within a small area, ensuring habitat continuity is a key priority. In this context, we recommend positive economic incentives to encourage local land owners to keep doing what they have done for centuries. Second, given the importance of microclimate revealed by our study, our model of solar radiation might be used as a tool in habitat restoration. By identifying those hot spots in the landscape which will offer the warmest conditions after cutting and grazing, we may maximise the reward of such conservation measures. Finally, our model of solar radiation could be used to scan other regions within Southern Finland for conditions suitable for O. gibbulus. Given dispersal limitation, such sites might offer promising sites for population re-introductions. In this context, we should end with a caveat: clearly, the microclimates of today will not be the microclimates of tomorrow. With globally rising temperatures, there is increasing evidence that several insect species are currently broadening their use of habitat types near their northern distribution limits (Thomas et al., 2001; Roy and Thomas, 2003; Davies et al., 2006). While at their range margins, these species have formerly been compensating for a cool macroclimate by selecting exceptionally warm microhabitats, closer to the centre of their range they have occupied a broader range of microhabitats (Thomas, 1993, Thomas et al., 1999). For such species, climate change might now increase the breadth of habitat use towards high latitudes, hence increasing habitat availability and population sizes, and decreasing dispersal distances between patches of suitable habitat (Thomas et al., 1999; Thomas et al., 2001, Davies et al., 2006). In addition to summer-time effects, warmer winter temperatures may facilitate survival in a wider range of microhabitats (Virtanen et al., 1998; Crozier, 2004). Thus, from the perspective of a thermophile like O. gibbulus, a wider part of the landscape may soon become available, in which case the remnant population of today would become an important source for future expansion (Thomas, 1993). Long-range dispersal from populations beyond Finnish borders will seem both slower and less likely. This underscores the need for conserving the current population of O. gibbulus in Finland – and those of similar species.