Are fluvial islands 'real' islands for arboreal mammals? Uncovering the effect of patch size under the species-area relationship.

Aim We tested the 'habitat amount hypothesis', which predicts that the effect of patch size on number of species results from a 'sample area effect' rather than an 'island effect'. Specifically, we (1) compared parameters of the species-area relationship ( SAR) of arboreal mammals in forested fluvial islands and continuous forest, and (2) separated the effects of island size and of landscape-scale habitat amount on species richness. Location Middle-Solimões River region, Central Amazon. Methods We surveyed arboreal mammals along line-transects on 15 fluvial floodplain islands and nine transects in continuous floodplain forest. Transect length varied according to island size. Transects of similar length were established in continuous forest. We used power transformed (log-log) models to construct a SAR for the set of island transects and another SAR for the set of continuous forest transects. We compared slope and intercept between the two SARs using ANCOVA. We used multiple regressions to separate the effects of island size and of habitat amount on the rarefied number of species across multiple spatial scales (500-6,000 m). Results The two species-area curves showed similar slopes, but the intercept was lower for islands. Multiple regressions showed the best fit at the 5,500 m spatial scale. At this scale, habitat amount predicted species richness, whereas island size did not. Main conclusions We conclude that the apparent effect of patch size on the number of species may be simply due to the sample area effect and that no island effect operates on this patchy system. Accordingly, island size per se does not increase the number of species at a sample site. Fluvial islands should not be treated as 'real' islands from the island biogeography perspective. In sum, we found support for the habitat amount hypothesis for predicting the richness of arboreal mammals on fluvial islands. [ABSTRACT FROM AUTHOR]