Spider and vascular plant assemblages in subarctic peat bogs are complementary ecological indicators of variation in local and landscape factors

Despite their specific biodiversity and the ecosystem services they furnish, peat bogs remain poorly understood, especially in terms of their responses to changes at multiple spatial scales. Many subarctic peat systems face acute risks, due to current global change. In this study, using a multi-metric approach, we assessed the relative value of plants and spiders as ecological indicators in contrasted peat bogs of a French archipelago (St. Pierre-etMiquelon), located in the North Atlantic. In 2021, pitfall trapping and suction sampling collected spiders while phytosociological releve ' s were used to document vascular plants. A total of 69 species of spiders and 38 species of vascular plants were identified, and both taxonomic (TD) and functional (FD, based on traits related to dispersal, size and ecosystem functioning) diversity were then estimated for three levels of species relative abundance (Hill numbers q = 0, 1 and 2) using the iNEXT 3D package for alpha diversity as well as iNEXT beta3D for beta diversity. As expected, patterns of TD and FD were highly correlated for each Hill number, but also between Hill numbers for spiders, indicating a surprisingly low effect of species abundance on (spider) diversity patterns. On the contrary, differences between sites were more visible for plants species when species coverage was considered (q = 1 and q = 2), for both TD and FD, and for alpha and beta diversity. RLQ and Fourth corner analyses indicated differences based on functional traits between sites which were significantly associated with relationships between dispersal modes and mostly local factors for plant assemblages. Finally, variance partitioning on assemblage composition showed the importance of an interaction between local and landscape factors for both plants and spiders, with a relatively high importance of local factors alone for plants. Our study thus revealed the high complementary of spider vs. plant assemblage-based metrics as indicators of changing local conditions in peat bogs, confirming these both experience rapid change in subarctic climates.