(Coleoptera: Carabidae) and Habitat Fragmentation

REVIEW Eur. J.Entomol. 98: 127-132, 2001 ISSN 1210-5759 Carabid beetles (Coleóptera: Carabidae) and habitat fragmentation: a review Ja r i NIEMELÁ Department ofEcology and Systematics, PO Box 17, FIN-00014 University ofHelsinki, Finland e-mail:[email protected] Key words. Carabids, conservation, dispersal, forests, habitat fragmentation, habitat heterogeneity, metapopulations, species richness, generalists, specialists Abstract. I review the effects of habitat fragmentation on carabid beetles (Coleoptera, Carabidae) and examine whether the taxon could be used as an indicator of fragmentation. Related to this, I study the conservation needs of carabids. The reviewed studies showed that habitat fragmentation affects carabid assemblages. Many species that require habitat types found in interiors of frag ments are threatened by fragmentation. On the other hand, the species composition of small fragments of habitat (up to a few hec tares) is often altered by species invading from the surroundings. Recommendations for mitigating these adverse effects include maintenance of large habitat patches and connections between them. Furthermore, landscape homogenisation should be avoided by maintaining heterogeneity ofhabitat types. It appears that at least in the Northern Hemisphere there is enough data about carabids for them to be fruitfully used to signal changes in land use practices. Many carabid species have been classified as threatened. Mainte nance of the red-listed carabids in the landscape requires species-specific or assemblage-specific measures. INTRODUCTION HABITAT FRAGMENTATION AND CARABID ASSEMBLAGES Destruction and fragmentation of habitats, overkill, introduction of alien species, and cascading effects of Effects of fragmentation on carabid assemblages species extinctions have been labelled the “evil quartet”, Habitat fragmentation is the partitioning of a con i.e. the four most detrimental, human-caused environ tinuous habitat into many small remnants (Saunders et al., mental changes affecting biodiversity (Pimm & Gilpin, 1991; Haila, 1999; Debinski & Holt, 2000). Sometimes 1989). It is known that many species are negatively the associated habitat loss is included in the concept of affected by habitat fragmentation, but lack of ecological fragmentation. Here, I attempt to separate these two knowledge of many taxonomic groups precludes detailed aspects and focus on the effects of fragmentation^er se, assessments of the magnitude and ecological meaning of i.e. isolation, size and configuration of the habitat rem these changes (McGeoch, 1998). nants. Assessment of the effects of fragmentation on well- Studies on carabids have shown that habitat fragmenta known taxa provides a useful insight that may guide fur tion influences assemblages by altering abundances and ther research and management recommendations. Such a species richness. In a study comparing different sized well-known taxon could be the carabid beetles. This fragments of coniferous forest surrounded by clearcut family is among the best known insect taxa in the areas in Finland, Niemela et al. (1988) found that Northern Hemisphere (Lindroth, 1961-69; Thiele, 1977; although there were no differences in species richness, Desender et al., 1994; Lovei & Sunderland, 1996; Nieabundance of many species was higher in small (< 5 ha) mela, 1996; Niemela et al., 2000), although not neces than in large (> 30 ha) forest remnants. This was most sarily in the Southern Hemisphere (New, 1998). In likely caused by changed environmental conditions in the addition to being abundant and speciose, these beetles are small remnants making them more favourable for taxonomically and ecologically well-studied providing a carabids. good basis for the assessment of the effects of various In another study in Finland, species richness increased human actions on their assemblages (Luff, 1996). Thus, with decreasing size of forest patches surrounded by agri carabids can provide the basis for taking the next step cultural land (Halme & Niemela, 1993; Niemela & from mere after-the-fact judgements of human actions to Halme, 1998). Small fragments (0.5-3.0 ha) had 18 spe making reliable predictions about the effects of such cies, large ones (9.6-21.5 ha) had 13 species, while con actions (Niemela, 1996; Niemela et al., 2000). tinuous, unfragmented forest only had 11 species. There The aim of this non-exhaustive review is to address the were more open-habitat species in the small fragments following questions: (1) what do carabids teach us about than in the large ones or continuous forest. This pattern the ecological effects of habitat fragmentation? (2) can was most likely attributed to the small fragments being carabids be used as indicators of fragmentation? and (3) more open and grassier, and thus more favourable for are carabids themselves in need of conservation as a carabid species primarily residing in the surrounding result ofhabitat fragmentation? grasslands. 127 Similar results have been reported from England. Bauer mentation. First, one can compare existing fragments with (1989) studied carabid occurrence on limestone outcrops a non-fragmented control site assuming that carabid surrounded by peatbog, and reported that there was an assemblages have been similar in the fragment and con inverse relationship between the size of the limestone out trol site before fragmentation. Second, stronger evidence crop and carabid species richness. Usher et al. (1993) can be derived from monitoring field manipulations in found that the overall species richness of carabids in which remnants of various sizes have been created by forest fragments surrounded by agricultural fields did not fragmenting a continuous habitat. This so called BACI correlate with fragment size. In both studies, the pattern approach (Before-After-Control-Impact, Underwood, was caused by an increasing influx of species from the 1991) is the recommended one because one can control matrix into the fragments with decreasing remnant size. for the environmental variation between sites and year-to- In addition, Usher et al. (1993) noted that the number of year fluctuations in conditions. However, the only study species was affected by the shape of the forest patch so using this method is that ofDavies & Margules (1998). that forest patches with high edge to area ratios contained The above studies also show that simply counting the more species because of higher invasion rate from the number of species provides little information about spe matrix. cific effects of fragmentation. Species richness used as a Contrary to the above studies, a study in Canada measure of biological conservation value may be mis showed that species richness of carabids increased with leading because disturbances may favour widespread and increasing forest fragment size (Burke & Goulet, 1998). abundant generalists leading to increased species The smallest fragment with forest core area of 0.3 hec richness. In contrast, analysing the responses of single tares had only two carabid species while there were species is more likely to provide an understanding of the 16-17 species in fragments > 30 ha and 22 species in processes that lead to extinction or persistence in frag continuous forest. However, isolation confounded the mented landscapes (Davies & Margules, 1998). In addi results so that the lowest species richness was found in tion, “rarity values” or “rarity scores” of sites (or species small, isolated fragments, while less isolated small frag assemblages) can be used in combination with species ments had a species richness comparable to that in large richness to provide a measure of the conservation value fragments. of a site (Blake et al., 1996; Eyre et al., 1996). Effects of fragmentation are manifested at the species Carabid metapopulations and dispersal in fragmented level. In the Finnish study, many forest species were landscapes absent from the small fragments, probably because of unsuitable habitat (Halme & Niemela, 1993). Species that occur in a fragmented landscape with sub Furthermore, the most specialized forest species occurred populations that are connected to each other by dispersal, only in the continuous forest (e.g., Carabus glabralus i.e. have a metapopulation structure, may be better buff Paykull, C. violaceus L.). These specialists are flightless, ered against local extinctions than species living in dis which probably hampers their movements across grass crete sub-populations. Although proper tests of whether land between forest patches. Radio-tracking has shown or not carabid populations are structured as metapopula that forest species are reluctant to cross open habitat tions are scarce, there are a few studies indicating that this (Riecken & Raths, 1996). Similarly, Bauer (1989) and may be the case in certain situations. Forest carabids Usher et al. (1993) reported that specialist species of the occupying hedges and small forest patches appear to form fragmented habitat type respond positively to fragment metapopulations in an agricultural landscape (Burel & size so that their species richness increased with Baudry, 1995). These forest-dwellers have poor dispersal increasing fragment size. ability and their survival depends on connections and A study from Australia using data from before and after short distances between suitable patches. fragmentation of Eucalyptus forest showed that carabid Dispersal ability is important for the survival of carabid species richness was not affected by fragment size

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