Spatial Dynamics in a Metapopulation Network: Recovery of a Rare Grasshopper Stauroderus Scalaris from Population Refuges

ECOGRAPHY 24: 452–460. Copenhagen 2001 Spatial dynamics in a metapopulation network: recovery of a rare grasshopper Stauroderus scalaris from population refuges Allan Carlsson and Oskar Kindvall Carlsson, A. and Kindvall, O. 2001. Spatial dynamics in a metapopulation network: recovery of a rare grasshopper Stauroderus scalaris from population refuges. – Ecography 24: 452–460. A characteristic feature of the spatial distribution of many species is patchiness. This spatial patchiness may be generated by very different processes, e.g. fragmentation, succession and extinction-colonisation dynamics. In this study, we apply a spatial realistic metapopulation model to analyse the occupancy pattern of a rare and endangered grasshopper, Stauroderus scalaris, found in an extensive network of 158 patches. When the study was initiated in 1985 the regional occupancy was 9.3% declining down to 7.1% in 1989. Then there was a spatial expansion of the population and in 1993 as many as 27.3% of the patches were occupied and 32.9% in 1995. During this expansion phase, the dynamics obeyed metapopulation principles; large patches and less isolated ones were more likely to be colonised. In the beginning, local extinction risks were negatively related to patch size and positively influenced by isolation. However, later on neither area nor isolation affected extinction probabilities. Altogether, 20 extinctions and 56 colonisations were observed. The shift in regional occupancy, with a growth of ca 20%, coincides with perturbations to the patch network and the warmest summer in 140 yr. Our results suggest that S. scalaris persists on a dynamic habitat mosaic, where refuges are crucial during adverse periods, and stochastic environmental factors (disturbances and climate), that are correlated over large areas, are generating population dynamic patterns that are hard to predict using current modelling techniques. A. Carlsson, Dept of Conser6ation Biology, Swedish Uni6. of Agricultural Sciences, Box 7044, SE-750 07 Uppsala, Sweden.–O. Kind6all (correspondence: os- [email protected]), Dept of Entomology, Swedish Uni6. of Agricultural Sci- ences, Box 7044, SE-750 07 Uppsala, Sweden. As a consequence of natural distribution of suitable There is empirical evidence that the spatial dynamics habitats and of secondary fragmentation, many species of several species appears to be driven by a dynamic are found as spatially structured populations. Species habitat mosaic rather than stochastic factors. Thus, for may persist in such landscapes as metapopulations in a instance, extinctions are frequently due to habitat dete- balance between local extinction and recolonisation. rioration, i.e. successional changes in vegetation, habi- There is a growing literature reporting the effects of tat loss and habitat management (Harrison 1991, landscape heterogeneity on metapopulation dynamics. Warren 1993, Thomas 1994, Singer and Thomas 1996), Empirical studies have found differences in extinction while colonisations are facilitated when environmental and colonisation probabilities of local populations due conditions are improved for some reasons (Thomas to differences in local population size (Hanski 1999), 1994). Based on such findings, it has been suggested patch size (Kindvall and Ahleń 1991, Soule´ et al. 1992), that habitat dynamics rather than stochastic factors are degree of isolation (Harrison et al. 1988, Sjo¨gren 1991), a key to the persistence of many metapopulations and population variability (Pimm et al. 1988, Bengtsson (Warren 1991, Warren and Thomas 1992, Thomas and Milbrink 1995). 1994). One strength with metapopulation models as an Accepted 1 December 2000 Copyright © ECOGRAPHY 2001 ISSN 0906-7590 Printed in Ireland – all rights reserved 452 ECOGRAPHY 24:4 (2001) analytical tool is that they can be used to predict the We digitised all potentially suitable habitat patches consequences of possible future management options, based on aerial photographs (IR-film; 1982, 1987 and i.e. explore how size and persistence of a particular 1994) and economic maps (1:10 000) onto which habitat metapopulation is affected by patch removal or the patches were manually drawn during each inventory. potential to recover if conditions improve and new We also carefully described the vegetation and other patches of suitable habitat are created (Thomas and habitat structures in 1995 on each of the identified Jones 1993, Thomas 1994, Hanski and Thomas 1994, patches in order to assess the correspondence between Sjo¨gren-Gulve and Ray 1996). the remote sensing technique and field observations. In this study, we analyse the pattern of presence-ab- Based on the digitised map, it was possible to calculate sence of the grasshopper Stauroderus scalaris in a re- patch sizes (m2) and central co-ordinates (m) on a gional network of habitat patches differing in size and 10×10 m resolution. isolation. For this purpose we apply a spatially realistic metapopulation model, the incidence function model (Hanski 1994), to analyse whether the recorded occu- The incidence function model pancy pattern may be understood by extinction-recolo- nization dynamics. With the model we try to predict The incidence function model of metapopulation dy- observed changes in the number of occupied patches. namics (Hanski 1994) has been shown to give an appro- We also conduct a sensitivity analysis based on the priate description of extinction-recolonisation processes parameterised incidence function model attempting to in vertebrates (Hanski 1991, Cook and Hanski 1995, identify which mechanisms are responsible for the ob- Moilanen et al. 1998) as well as in insects (Hanski et al. served dynamic patterns. 1995, 1996, Kindvall 2000) living in fragmented populations. Detailed descriptions of the model can be found in Hanski (1994), ter Braak et al. (1998) and Moilanen Material and methods (1999). Here we only summarise the model concept and key assumptions. The species Each patch in a patch network is assumed to have two possible states, occupied or empty. Furthermore, Stauroderus scalaris is a large (18–27 mm) grasshopper assuming constant and patch specific colonisation (Ci) (Orthoptera: Acrididae, Gomphocerinae) with discrete and extinction (E ) probabilities, and a possible rescue annual generations. It is widely distributed in the i effect, then the long-term probability (Ji) of a patch i mountains of central and southern Europe but there is being occupied, the so-called incidence, is given by: an isolated relict population on the island of O8land off the south-eastern coast of Sweden. Within the main Ci distribution range of the species it prefers to live in dry, J = . (1) i C +E −C · E stony mountain meadows (Bellmann 1985). The i i i i Swedish fringe population occupies meadows located The extinction probability is assumed to depend on the on poor, dry sandy soils (Kindvall and de Jong 1991). local population size, which is assumed to be correlated These dry meadows are very distinct features in the to patch area (A ), to a certain degree (x): landscape, which has a very simple structure, being i composed of pine forest, agricultural fields and dry e meadows. E =min 1, , (2) i Ax The O8land population is isolated from the main i European distribution, and restricted to an area of 150 where e is a parameter that sets the overall extinction km2 at the northernmost tip of the island. About 500 risk. The probability that a local patch will become ha (3.3% of the landscape) of suitable habitat are found colonised (C ), is assumed to depend on the spatial patchily distributed in the area. i locations of surrounding occupied patches, and their areas (Aj). This probability is calculated as Censuses 1 C = , (3) We censused all dry meadows for presence or absence i y2 of the species in the entire range during four years: 1+ Si 1985, 1989, 1993 and 1995. The species stridulates in late July–August during warm sunny days. Each year, where y is a parameter that inversely relates to the all patches were acoustically and visually monitored in migrant’s ability to establish on a patch once reaching appropriate weather. If no specimens were detected on it. The number of migrants arriving at patch i is a particular patch, another visit was made some days determined by an index (Si) describing the later. connectedness: ECOGRAPHY 24:4 (2001) 453 % a 8 Si = (pj · exp(− · Dij) · Aj). (4) island of Oland. Here, the species is found in dry j"i meadows on sandy soils, a resource that is patchily With this index, it is assumed that the number of distributed in the landscape, as is evident from Fig. 1. migrants declines exponentially with increasing inter- There were 140 suitable patches in the first year. Due to a changes in agricultural subsidies, farming practice in patch distance (Dij). The parameter sets the overall migration ability and assumes that migrants move the region changed and the number of suitable patches equally to the inverse of the average dispersal distance. increased to 158 in 1995. The metapopulation was Incidence function model parameters were estimated censused four times, during which the landscape re- from three sets of occupancy data, using two different mained very stable between the first two censuses and methods (MC and NLRB, see Moilanen 1999) depend- between the last two censuses (Fig. 2). However, being on whether the data from a single inventory or two tween 1989 and 1993 as many as 46.4% of the patches inventories were included in an analysis. Estimates were were subjected to disturbances

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