Minimum Viable Metapopulation Size, Extinction Debt, and the Conservation of a Declining Species

Abstract

A key question facing conservation biologists is whether declines in species' distributions are keeping pace with landscape change, or whether current distributions overestimate probabilities of future persistence. We use metapopulations of the marsh fritillary butterfly Euphydryas aurinia in the United Kingdom as a model system to test for extinction debt in a declining species. We derive parameters for a metapopulation model (incidence function model, IFM) using information from a 625-km2 landscape where habitat patch occupancy, colonization, and extinction rates for E. aurinia depend on patch connectivity, area, and quality. We then show that habitat networks in six extant metapopulations in 16-km2 squares were larger, had longer modeled persistence times (using IFM), and higher metapopulation capacity (lambdaM) than six extinct metapopulations. However, there was a > 99% chance that one or more of the six extant metapopulations would go extinct in 100 years in the absence of further habitat loss. For 11 out of 12 networks, minimum areas of habitat needed for 95% persistence of metapopulation simulations after 100 years ranged from 80 to 142 ha (approximately 5-9% of land area), depending on the spatial location of habitat. The area of habitat exceeded the estimated minimum viable metapopulation size (MVM) in only two of the six extant metapopulations, and even then by only 20%. The remaining four extant networks were expected to suffer extinction in 15-126 years. MVM was consistently estimated as approximately 5% of land area based on a sensitivity analysis of IFM parameters and was reduced only marginally (to approximately 4%) by modeling the potential impact of long-distance colonization over wider landscapes. The results suggest a widespread extinction debt among extant metapopulations of a declining species, necessitating conservation management or reserve designation even in apparent strongholds. For threatened species, metapopulation modeling is a potential means to identify landscapes near to extinction thresholds, to which conservation measures can be targeted for the best chance of success.

Dos and Don’ts for butterflies of the Habitats Directive of the European Union

“…If the surrounding landscape is being managed and colonies survive nearby, it will probably recolonize naturally in due course. (Anthes et al 2003a;Anthes et al 2003b;Anthes and Nunner 2006;Barnett and Warren 1995;Betzholtz et al 2007;Botham et al 2011;Bulman et al 2007;Hula et al 2004;Joyce and Pullin 2003;Junker and Schmitt 2010;Konvička et al 2003;Liu et al 2006;Munguira et al 1997;Porter and Ellis 2011;Saarinen et al 2005;Schtickzelle et al 2005;Smee et al 2011;Stefanescu et al 2006;Wang et al 2004;Warren 1994;Warren et al 1994;Zimmermann et al 2011a;Zimmermann et al 2011b)…”

Europe’s other debt crisis caused by the long legacy of future extinctions

“…For a similar taxonomic group, butterflies, the available empirical evidence for delayed population response to habitat loss is mixed with some studies reporting rather fast (several years) (12,27) and others relatively long (several decades or more) relaxation times (17). In addition, theoretical simulations have suggested that the delay in insect metapopulation extinctions can extend well beyond 100 y (28), and that their lag times are particularly long if available habitat networks are reduced to levels close to the extinction threshold (29), as might be the case in many landscapes of Europe where an intensively used agricultural matrix is interspersed by remnants of near-natural and nonintensively used habitats (30).…”

Metapopulation perspective to institutional fit: maintenance of dynamic habitat networks

“…Habitat patch networks may undergo gradual degradation (Mortelliti et al 2010) or loss (Bulman et al 2007) of habitat patches because of human land use activities. Additionally, climate change may cause habitat patch networks to shift gradually toward the poles (Radchuck et al 2014).…”