| Sammanfattning: | We examined short-term differences in assemblage organization of surface-active arthropods from a restricted portion of a coastal dune ecosystem near Puerto Peñasco, Sonora, Mexico. To test the hypothesis that soil moisture gradients influence organization, we established three, 32-m transects running westward from real or simulated shorelines of 1)an estuary terminus ("estero") 2) a salt-water pond ("seep"), and 3) an intermediate dry desert site ("control"). Transect moisture gradients ranged, at the estero and seep sites respectively, from 15% and 11% to<1%, while soil moisture at the control site was <1% mead values to 15 cm depths). Four evenly spaced plots, each 3x7m, along each transect contained pitfall traps that captured arthropods continuously for eight (estero, seep), or six (control), 12-h periods. We trapped 558 arthropods comprising 50 species, excluding abundant ants of the genus Solenopsis. Rarefaction analysis for sample sizes of 80 gave expected species numbers, E(Sn), of 27 (estero), 21.5 (seep), and 18 (control) when the swamping effect of Uca latimanus (crabs) which were dominant at the estero, absent elsewhere) was removed. Arthropods other than crabs were twice as abundant at the estero and four time as abundant at night, respectively, as they were at the other two sites and during the day. Carnivore: detritivore species ratios at the three sites were approximately 2:1 (estero), 1:1 (seep), and 1:2 (control). Evenness (modified Hill's E5) values without Uca were similar at all sites; however, high diversity (Hill´'s N2) values for the estero assemblage confirmed the presence of a small number of abundant species (carabid beetles, lycosid spiders) there. We found no significant correlation between percent plant cover and totaldensities or species richness of trapped arthropods. Measurements of beta-diversity (Sorensen's community coefficient) reinforced the uniqueness of the estero assemblage. Our initial hypothesis received support in that species differences, individual abundances, and carnivore: detritivore rations all tended to increase along moisture gradients.
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