Within‐Habitat β Diversity Increases With Elevation in Tropical Forest Spider Assemblages

ABSTRACT

While between-habitat comparisons are commonplace in ecology, we know very little about the changes among species assemblages within habitats. Here we aimed to examine within-habitat processes in spider assemblages across three elevations in tropical forests, using a design tailored both geographically and methodologically for this purpose. We hypothesised greater within-habitat βTD (caused by lower connectivity), decreasing body sizes (because of lower temperatures) and less frequent ballooning (due to greater isolation) with increasing elevation. We collected spiders at five within-habitat (horizontal) distances at each elevation in the Udzungwa Mountains, Tanzania, applying standardised protocols. We assessed differences among assemblages using taxonomic and functional β-diversity measures and their components. We applied PerMANOVAs to test for different β between elevations, PERMDISP to evaluate within-habitat variability, exponential and power-law models to examine within-habitat distance-decay, and community weighted values of traits to capture functional patterns for each trait. As predicted, within-habitat βTD was greater at higher elevations, with stronger distance-decay relationships (taxonomic and functional). Also, as predicted, high elevation assemblages appeared to show smaller body sizes and lower ballooning frequency. Our results demonstrate the uniqueness of mountain assemblages and that the Udzungwa Mountains, as well as other mountainous areas, should receive increasing conservation attention, as they provide unmissable opportunities to conserve, through small additions or connections, countless small-range or habitat-restricted endemic species.