The relative importance of local and landscape scale factors to chalk grassland butterflies.
The relative importance of habitat and landscape for chalk grassland butterflies was
investigated among a network of chalk grassland habitat patches. Local scale habitat
selection models were derived from analyses on a single large chalk grassland site
(Porton Down). Landscape scale models were .
then derived from species-specificlo cal
scalev ariables addedt o variablesm easuredin the patch network, which included Porton
Down as a major source site. An investigation was also carried out as to whether
landscapes calep rocessesw ere acting within Porton Down.
Models showedt hat landscapes calep rocessesm ay be acting within the single, large site
for some species. However, lack of patch definition and natural variations in local
population densitiesm ay give rise to apparentl andscapes calep rocessesw ithin this site
which fundamentally differ from true landscape scale processes.
At the true landscape scale across the patch network, patch area was the most commonly
included landscape scale variable in models, although only for five species. For one
species, dark green fritillary Argynnis aglaja, distance from the major source site and
average distance from other sites were of over-riding importance. For most species, local
scale habitat and environment variables were sufficient to describe their presence or
density among the patch network.
Incidence Function Models were used to estimate metapopulation dynamic parameters
for seven species, Both sensitivity to environmental stochasticity and colonisation ability were correlated with species' scores on canonical community analysis axes (related to
local habitat, patch area, distance from source site and inter-patch'distance measures).
This suggests that species conforming to conventional metapopulation models form an
ecological continuum with those responding to local scale habitat and landscape scale
processes in a less prescriptive fashion.
Species groups with either restricted or widespread distributions appear to conform to a
landscape connectivity model. Five ecologically similar species appear to be moving
from the former to latter group across a `connectivity threshold', possibly due to recent
increases in larval food-plant availability in the wider landscape.