Harvestman (Arachnida: Opiliones) species distribution along three Neotropical elevational gradients: an alternative rescue effect to explain Rapoport's rule?

Abstract

Aim Relationships between elevation and litter-dweller harvestman (Arachnida: Opiliones) species richness along three elevational gradients in the Brazilian Atlantic Forest were evaluated. Specifically, three candidate explanatory factors for the observed patterns were tested: (1) the mid-domain effect, (2) the Rapoport effect, and (3) the influence of environmental variables on species density and specimen abundance.Location Cuscuzeiro, Corcovado and Capricó rnio mountains, in Ubatuba (23°26¢ S, 45°04¢ W), a coastal municipality in São Paulo state, south-eastern Brazil.Methods We recorded harvestman species and abundance through active sampling using 8 · 8-m plots in both summer and winter. At each plot we measured the temperature, humidity and mean litter depth. Harvestman species richness per elevational band was the sum of all species recorded in each band, plus the species supposed to occur due to the interpolation of the upper and lower elevational records. Differences between observed and expected species richness per elevational band, based on the mid-domain effect, were examined through a Monte Carlo simulation. The Rapoport effect was evaluated using both the midpoint method and a new procedure proposed here, the 'specimen method'. We applied multiple regression analysis to evaluate the contribution of each environmental variable (elevation, temperature, humidity and litter depth) on species density and specimen abundance per plot.Results Harvestman abundance and species richness decreased at higher elevations in the three mountains. The decrease in species richness was not monotonic and showed a plateau of high species richness at lower elevations. The number of harvestman species per elevational band does not fit that predicted by the mid-domain effect based solely on geometric constraints assuming hard boundaries. Species with their midpoints at higher elevations tended to cover broader elevational range sizes. Both the midpoint method and the specimen method detected evidence of the Rapoport effect in the data. At fine spatial scales, temperature and humidity had positive effects on species density and specimen abundance, while mean litter depth had no clear effect. These relationships, however, were not constant between seasons.Main conclusions Our results suggest that harvestman species density declines at higher elevations due to restrictions imposed by temperature and humidity. We found a pattern in species range distribution as predicted by the elevational Rapoport effect. However, the usual rescue effect proposed to explain the Rapoport effect does not apply in our study. Since the majority of harvestman

Diet of the toad Rhinella icterica (Anura: Bufonidae) from Atlantic Forest Highlands of southeastern Brazil

“…This variation in diet richness found among these studies may reflect differences in food availability between areas, periods of collection or even altitudes. In fact, it is well known that invertebrate availability decreases with an increase in altitude (Leakey & Proctor 1987, Brühl et al 1999, Almeida-Neto et al 2006) and thus, we do not know in which extent a supposedly lower availability of arthropods in the highlands of PNI could explain part of these observed differences in number of prey size consumed among R. icterica populations. However, the similarities are that ants and beetles showed the most values items in the diet of the species in both localities.…”

Diversity of harvestmen (Arachnida, Opiliones) in Parque da Onça Parda, southeastern Brazil

“…Apparently this occurs due to their being highly sensitive to environmental conditions, especially humidity (SantoS, 2007). Species richness declines at higher elevations in the tropics due to restrictions imposed by temperature and humidity (alMeida-neto et al, 2006). Moreover, not only the diversity, but also the level of endemism is especially high in tropical forests, where several species present restricted distributions.…”

Diversity and Habitat Use of Neotropical Harvestmen (Arachnida: Opiliones) in a Costa Rican Rainforest

“…The most diverse harvestmen communities occur in neotropical rainforests, where richness may exceed 25 species [1,9,11]. The highest diversity at a single site was recorded in the Brazilian Atlantic forest with a richness of 64 species [12].…”

“…Individuals may undertake daily vertical migrations, descending to the leaf litter at dusk to forage [8,17,18]. Abiotic factors, especially temperature and humidity, significantly influence the distribution, habitat selection, and activity patterns of harvestmen [4,6,8,19] as well as community structure along elevational gradients [11,[20][21][22]. Other studies have examined variation in smallscale spatial distributions of harvestmen in response to biotic factors, such as vegetation and habitat structure [10,[23][24][25][26].…”