Vertical forest strata position and niche shifts between juvenile and adult spiders

Functional trait analyses have become a vital part of ecological and evolutionary research in recent years. Nevertheless, this progress highlights the persistent and significant gaps in our knowledge of species traits, a limitation known as the Raunkiæran shortfall. For spiders, the difficulty in properly identifying immature specimens has often contributed to discarding the contribution of these lifestages to intraspecific functional variability and community structure. Species microhabitat preferences along the vertical gradient in forest biomes are amongst the traits frequently unknown for spider juveniles, despite their relevance for multiple aspects of spiders' ecology. To bridge this knowledge gap, in this study, we used spider community data collected from the native forests on two islands belonging to the Azores Archipelago, a well-characterized and species-poor system ideal for trait-focused studies. Our goals were to compare the mean verticality and vertical range of adult and juvenile spiders belonging to different hunting guilds (hunters and web weavers) and ballooning propensity (frequent, occasional and rare ballooners). We did this using two-sample paired Wilcoxon signed rank tests and Kruskal-Wallis tests, followed by Dunn's tests to check for differences in the variation of adult and juvenile verticality from species belonging to different functional groups. Across 22 species sampled at 16 sites on two islands, adult and juvenile spiders did not differ in their mean vertical position within the forest strata. Unexpectedly, however, adults occupied a significantly broader vertical range than juveniles, indicating greater habitat flexibility than anticipated. The pattern observed for vertical range remained when looking at the two hunting guilds considered (hunters and web-weavers), although, in the case of hunters, adults tended to have lower mean verticality than juveniles. Finally, for the three categories of ballooning propensity (frequent, occasional and rare), we observed that juveniles of rare ballooners had higher mean verticality, while, for all categories, these tended to have lower values of vertical range. Our findings show the importance of including juveniles in microhabitat studies, as well as how this seems to vary across functional groups. It is hoped that this study will serve as a valuable baseline to future research aiming to better incorporate immature life stages in spider community ecology, particularly as new standardised methodologies are developed to reliably associate juveniles with their adult forms.