Diversity and Distribution Patterns of Ground-Dwelling Arthropods in the Santa Catalina Mountains, Arizona

Abstract

Mountains, with their rapid climatic and topographic variations along elevational gradients, offer ideal settings for investigating general patterns and factors associated with species diversity. While some common patterns and factors associated with species diversity may emerge across different systems, taxonomic groups are expected to exhibit differences along the same environmental gradient due to their distinct ecological requirements. However, relatively few studies have simultaneously examined multiple taxonomic groups within the same system, and it remains unclear whether commonalities exist in how species diversity varies with elevation and key environmental factors. In the Santa Catalina Mountains, elevational patterns of species richness were found to vary in different ground-dwelling arthropod groups. In line with this finding, combinations of different environmental factors are associated with variation in species richness in different ground-dwelling arthropods, suggesting that different taxonomic groups might not respond in the same way to the same environmental gradient. At the same time, average temperature and temperature range explained most of the variation in species richness in most groups, highlighting their predictive value in some but not all ground-dwelling arthropod groups. According to the climatic variability hypothesis, species range extent and distribution are determined by the climatic fluctuations a species can tolerate. Along gradients of climatic variability, species adapted to stable conditions are expected to not survive in areas with greater climatic fluctuations, and therefore present smaller range extents. In contrast, species inhabiting more variable climates are predicted to have broader range extents, as they can tolerate both climatically variable and stable regions. Since climatic variability generally increases with elevation, species range extents are expected to increase with elevation, a pattern known as Rapoport’s rule. Based on Rapoport’s rule, species richness is also expected to decline with increasing climatic variability, and consequently with elevation, since climatic specialists are gradually filtered out at higher elevations. Here, the validity of Rapoport’s corollaries is explored by establishing the relationship between species range extent, species richness, elevation, and climatic variability in different groups of ground-dwelling arthropods on the Santa Catalina Mountains. Although species range extent seemingly increases with increasing climatic variability as expected by the climatic variability hypothesis, this pattern does not result in elevational patterns of species range extent and species richness concordant with Rapoport’s rule. Contrary to species richness, variation in community structure is consistent across ground-dwelling arthropod groups. Pure turnover is the dominant component of beta diversity for all taxa, as beta diversity mostly varies according to biome identity along this elevational gradient. In line with this finding, environmental distances, but not geographic distances, correlate with turnover and overall beta diversity in all taxa, possibly suggesting that local environmental factors play a key role in shaping communities characterized by distinct species. This similarity in community structure and variation along environmental distance is further highlighted in Canonical Correspondence Analysis (CCA), where average temperature emerges as the main factor associated with patterns in community composition across all taxa. Based on these observations, we propose that environmental distance and average temperature might serve as a shared proxy for community structure differences across various arthropod groups in the Santa Catalina Mountains, and potentially other ranges within the Madrean Sky Island Archipelago.