The quality and isolation of habitat patches both determine where butterflies persist in fragmented landscapes

Abstract

Habitat quality and metapopulation e¡ects are the main hypotheses that currently explain the disproportionate decline of insects in cultivated Holarctic landscapes. The former assumes a degradation in habitat quality for insects within surviving ecosystems, the latter that too few, small or isolated islands of ecosystem remain in landscapes for populations to persist. These hypotheses are often treated as alternatives, and this can lead to serious con£ict in the interpretations of conservationists. We present the ¢rst empirical demonstration that habitat quality and site isolation are both important determinants of where populations persist in modern landscapes. We described the precise habitat requirements of Melitaea cinxia, Polyommatus bellargus and Thymelicus acteon, and quanti¢ed the variation in carrying capacity within each butter£y's niche. We then made detailed surveys to compare the distribution and density of every population of each species with the size, distance apart and quality of their speci¢c habitats in all their potential habitat patches in three UK landscapes. In each case, within-site variation in habitat quality explained which patches supported a species' population two to three times better than site isolation. Site area and occupancy were not correlated in any species. Instead of representing alternative paradigms, habitat quality and spatial e¡ects operate at di¡erent hierarchical levels within the same process: habitat quality is the missing third parameter in metapopulation dynamics, contributing more to species persistence, on the basis of these results, than site area or isolation. A reorientation in conservation priorities is recommended.

Population Dynamics of Lepidoptera Pests in Eucalyptus urophylla Plantations in the Brazilian Amazonia

“…Fragments and corridors of native forest in eucalyptus monocultures can increase the diversity of natural enemies and reduce difficulties with the insect pests [10,23,24]. Furthermore, the distance between the plantations and natural vegetation may be more important than the reserve area, possibly because of the source of natural enemies.…”

Both population size and patch quality affect local extinctions and colonizations

“…Talley 2007). Critics have argued that habitat quality of the patch is as important as patch size per se (Thomas et al 2001). This relates to sink -source dynamics, similar to metapopulation dynamics.…”

Dispersal capacity and diet breadth modify the response of wild bees to habitat loss

“…In addition, the increasing relative amount of edge in small habitat patches may increase the abundance of habitat generalists and matrix species, which can negatively affect habitat specialists (Ewers & Didham 2006). The ability of a patch to support a certain number of species also depends on habitat quality (Thomas et al 2001;Pöyry et al 2009) that often deteriorates as a result of land use change and habitat loss (Kiviniemi & Eriksson 2002). All these mechanisms point to the critical importance of habitat patch size for understanding how communities respond to habitat loss.…”