Oecologia (Berl) (198I) 48:1-6 Oecologia 9 Springer-VerIag 1981 Niche Breadth and Resource Partitioning by Four Sympatric Species of Bark Beetles (Coleoptera: Scolytidae) T.D. Paine, M.C. Birch, and P. Svihra* Department of Entomology, University of California, Davis, California 95616, USA Summary. Standing loblolly pines in southeastern North America are colonized by four sympatric species of bark beetles: Dendroc- tonus frontalis (Zimm.), Ips calligraphus (Germ.), L grandicollis (Eichh.) and L avulsus (Eichh.). The beetles compete for the limited amount of phloem tissue used as a site for reproduction. Using indices of niche breadth and niche overlap determined from the surface areas attacked, the interaction of colonizing beetle species in partitioning resources in entire trees and within each sample level was examined. The broadest niche breadth was exhibited by L avulsus, while L grandicollis had the narrow- est. D. frontalis dominated the lower bole and overlapped primar- ily with L calligraphus. The upper bole was similarly dominated by L avulsus, which overlapped only slightly with D. frontalis, but overlapped extensively with L calligraphus. Within tree spe- cies diversity was highest in the mid-bole sections and declined progressively toward the stump and top. Increasing species diver- sity showed a strong positive correlation with increasing mean niche overlap. Introduction Four species of scolytid bark beetles colonize phloem tissue in loblolly pine (Pinus taeda L.) and other pines in southeastern North America. These species; Dendroctonus frontalis (Zimm.), Ips calligraphus (Germ.), L grandicollis (Eichh.) and L avulsus (Eichh.), lay eggs along distinctive galleries excavated under the bark. The lower tree bole is colonized by D. fi'ontalis, while the three Ips species generally colonize upper bole tissue (Svihra et al. 1980). A fifth species, D. terebrans (Oliv.) which colonizes pines at the base of the bole, was eliminated from this study. This species is often found in the base of living trees and is not closely associated with mass colonization of southern pines. Previous studies of colonization of southern pines have em- phasized that portion of the tree occupied by the principal pest species D. frontalis, "the southern pine beetle" (Coulson et al. 1976; Mayyasi et al. 1976, Pulley et al. 1977). Vit6 et al. (1964, 1978) examined the relationships among four species of sympat- tic southern pine beetles based on their responses to synthetic pheromones. More recently, Birch et al. (1980) described natural olfactory interactions among these four species during coloniza- tion of the host, and showed that these interactions may partially explain the temporal and spatial patterns of colonization by all species at the heights characterized by Svihra et al. (1980). * Present address: University of California Cooperative Extension 137 Giannini Hail University of California, Berkeley, California 94720, USA The present paper quantifies the ecological relationships among the four species to explain the behavioral interactions, and to- gether with Svihra et al. (1980) and Birch et al. (1980) should provide the basis for a more realistic interpretation of bark beetle infestations by a complex of species than has hitherto been attempted. The interactions between species sharing a similar environ- ment have been frequently examined (Cody 1968; Pianka 1969; Price 1971; Schoener 1974; Denno and Cothran 1975; McClos- key 1976 ; Rathcke 1976 ; Hanson 1978 ; Whitham 1978). Species utilizing common resources may interact in many ways, including competition (Colwell and Fuentes 1975). These four species colo- nize a suitable host tree within a short period of time. The indices of niche breadth and niche overlap can be used to reflect the ecological specialization of the species within their particular environments and the potential for competition with co-occurring organisms (Levins 1968). Niche overlap may also reflect the absence of competition among coexisting species (Colwell and Futuyma 1971; Vandermeer 1972). The niche measures can also be used to describe the division of a common scarce resource within the community. This information can be integrated with behavioral relationships described by Birch et al. (1980) to par- tially explain the resource partitioning and avoidance of competi- tive interactions under the bark. Materials and Methods A total of 27 mature second growth P. taeda were felled between J~ne, 1975 and June, 1976, near Nacogdoches in southeast Texas. Trees were sampled after the lower trunk was colonized. Each tree was divided into ten sections of equal length. A 30 cm bolt was removed from the top of each section (levels 1-10). Branch samples were re- moved from the upper and lower crown (levels 11 and 12) as described by Svihra et al. (1980). Assuming that the environmental factors in- fluencing the distribution of each species form a gradient from the stump to the top, spacing the samples evenly along this gradient avoided problems of range, spacing and non-linearity discussed by ColwelI and Futuyma (197I). Bark was removed from sample bolts and the area encompassing each gallery system was measured. These areas were transformed to a proportion of the total area of each sample section. Treated in this manner, the size of each sample is discounted, and infested areas can be directly compared between sample sections. Surface areas occu- pied by each species were measured rather than the number of individu- als or number of gallery systems since the resource being exploited is the cambial area under the bark. Also, the size of individual gallery systems may vary with different attack densities (McMullen and Atkins 1961; Dudley 1971; Berryman 1974), even though the total cambial area occupied may be similar. 0029-8549/81/0048/0001/$01.20