Effects of Habitat Patch Size and Isolation on Dispersal by Hesperia comma Butterflies: Implications for Metapopulation Structure

Abstract

1. Metapopulation dynamics of the silver-spotted skipper butterfly Hesperia comma were studied between 1982 and 1991 along a 25-km stretch of chalk hills on the North Downs, Surrey, UK. Sixty-nine patches of suitable habitat were identified, of which 48 were occupied in 1982. Over the 9-year period, 12 patches were colonized, seven went extinct and nine patches remained vacant. Patches were more likely to be colonized if they were relatively large and close to other large, occupied patches. Local populations in small, isolated patches were more likely to go extinct. 2. Within a 70-ha section of a metapopulation, mark-release-recapture techniques were used in 1994 to investigate the effects of local patch area and isolation on movement of individuals among local patches. 988 butterflies were marked of which 133 moved between patches. 67% of between-patch movements were less than 50 m, although the longest recorded distance moved was 1070 m. Butterflies were most likely to move between large patches that were close together. 3. Metapopulation models have assumed that the distribution of distances moved by migrants follows a negative-exponential function. In our study, this distribution fitted an inverse-power function better than a negative-exponential. The under-estimation of long-distance migration by negative exponentials, compared with inverse power functions, may explain why theoretical models have under-estimated the number of occupied patches in metapopulations of H. comma following natural colonization. 4. Per capita emigration and immigration rates were significantly higher in small patches (area < 0.07 ha) compared with medium-sized (0.33-0.78 ha) or large patches (5.66 ha). However, in absolute numbers, more emigrants came from the largest patch where the source population was the largest. 5. The population system studied here shares attributes of several theoretical types of spatially structured population (patchy population, metapopulation, mainland-island system, non-equilibrium system) depending on the temporal and spatial scale examined. The distribution and dynamics of metapopulations should be regarded as being affected by a variety of behavioural and population processes, and real metapopulations can rarely be characterized as a single theoretical type.