Local population dynamics are important to the conservation of metapopulations in highly fragmented landscapes

Abstract

Summary 1. Population viability analyses (PVA) are extremely useful tools for the management of endangered species at the landscape scale. Two main families of spatially explicit models are available to perform PVA: (i) presence–absence models, in which local populations are either existing or extinct at each generation, and (ii) structured population models, in which the dynamics of each local population are modelled. In this study we compared the usefulness of both approaches for the prediction of the persistence of a species living in a highly fragmented landscape. 2. The cranberry fritillary Boloria aquilonaris is an arctic–alpine relict species in western Europe. It lives in landscapes where altitude or the proximity of oceans provide the high humidity conditions required for the formation of peat bogs, the only habitat of the butterfly. In such landscapes, the distribution of the butterfly is fragmented, following the natural distribution of peat bogs; this fragmentation is increased by human‐induced peat bog destruction. 3. The study of the dynamics of a highly fragmented metapopulation of the cranberry fritillary in a network of 14 habitat patches totalling 26·23 ha revealed that: (i) the density at the equilibrium was c. 700 butterflies ha−1; (ii) local population dynamics in small populations were negative; and (iii) six estimates of the growth rate Rt showed large variations even in a large population. Both local extinctions and recolonization events are likely to occur, and two sites (one of 7 ha) unoccupied during two generations were recolonized in the third year. 4. We conclude that the future of this metapopulation is not guaranteed, given its large spatial scale and unbalanced, erratic local population dynamics. We hypothesize that global warming could explain the instability of local population dynamics detected here in addition to the decline of this arctic–alpine relict species, which is currently reported in all its refuge localities in the Netherlands. 5. Presence–absence models based on metapopulation structure and habitat characteristics should provide relatively safe predictions, as the population network (14 patches) is just below the minimum amount of suitable habitat (MASH) for the long‐term persistence of a viable metapopulation. However, careful investigation of local population dynamics showed a high instability in the network, which is confirmed by population extinction at a large site. 6. Synthesis and applications. We urge conservation biologists to consider the local population dynamics of endangered species for the management of metapopulations in fragmented landscapes. In the case of the cranberry fritillary, continuing site protection is essential.