Spider Abundance in Relation to Needle Density in Spruce

The effect of micro-habitat change, caused by needle-loss, on spiders living in spruce (Picea abies) was examined in a field experiment. Abundance and size distribution of spiders on spruce branches were recorded at the start and at the end of the experiment . All spiders and approximately 24% of the needles were removed from the experimental branches . In the control branches, the needle density was left unaltered but the spiders were removed. There were no initial differences in the spider community on the selected branches. Spiders were collected after seven weeks of colonization on control and needle-thinned branches, respectively . The mean density of spiders was significantly lower on needle-thinned branches (78%) than on control branches . This was shown to be an effect of reduced density of large (length >2 .5 mm) spiders, but not of small (<–2 . 5 mm) ones. However, the total size distributions did not differ between the branch categories . The field experimental data confirm earlier results in laboratory studies and natural populations. In Scandinavia, spiders are one of the dominating groups among the spruce-living (Picea abies) arthropods (Hagvar & Hagvar 1975 ; Askenmo et al . 1977 ; Jansson & von Bromssen 1981 ; Gunnarsson 1983, 1988, 1990 ; Suhonen et al . 1992) . Several studies on the spider fauna in SW Sweden have shown that Linyphiidae an d Thomisidae are predominant among families . Some common species are, e. g . , Pityohyphantes phrygianus, Neriene peltata, Philodromus spp . , Diaea dorsata . Other common families are Theridiidae, Araneidae, Tetrag,nathidae, Clubionidae, Dictynidae, and Uloboridae (refer to Norberg 1978 ; Gunnarsson 1983, 1988 for details) . However, there are reasons to suspect that long term changes in the spider community occur, an d that this process indirectly is caused by man (Gunnarsson 1988, 1990) . Coniferous forests in several parts of centra l and northern Europe are severely affected by air pollution (Blank 1985; Andersson 1986 ; Schulz e 1989). The accelerated needle-loss observed in polluted areas has been interpreted as a symptom of increasing stress . The architecture of the trees is altered by the decreasing needle density whic h makes the canopies and the branches "thinner " (Westman & Lesinski 1986). These changes may affect the tree-living fauna indirectly as plan t structures are important in providing living-spac e (Lawton 1986). Vegetation structure has been suggested as a major factor in determining th e 'To whom correspondence should be directed . abundance and diversity of plant-living arthropods (May 1978 ; Southwood 1978 ; Morse et al . 1985 ; Lawton 1986) . The relationship between vegetation structure and spider community is especially well documented (e. g . Duffey 1962 ; Col ebourn 1974 ; Halley & MacMahon 1980; Robinson 1981 ; Rypstra 1983 ; Greenstone 1984 ; Gunnarsson 1988, 1990, 1992). Indirect effects of air pollution, mediated by needle-loss in spruce, have been observed in natural spider populations in SW Sweden . Earlier studies showed that the abundance, size distribution and taxonomic composition were affected by the needle density, i . e. , indirectly related to air pollution effects (Gunnarsson 1988, 1990) . There is, however, no evidence for any toxic effects of acid precipitation on spiders (Gunnarsson & Johnsson 1989). Here, we examine the effects of needle density in spruce on the abundance of spiders in a fiel d experiment . The accelerated needle-loss was simulated by removal of approximately 25% of the needles from experimental branches . Any difference in spider abundance should be possibl e to attribute to the experimental manipulation o f needle density since all spiders were remove d from the branches at the start of the experiment. The prediction is : lower spider density on the needle-thinned branches than on the controls . An earlier two-month field experiment showed low densities on needle-thinned branches (Gunnarsson 1990) . However, this experiment wa s performed in fall and confounding factors could have influenced the results, e . g . , spiders may