Darwin's Naturalization Conundrum in the Azores: Scale-Dependent Patterns of Arthropod Invasion

Abstract

Aim

To test the mechanisms governing the assembly of exotic arthropods within endemic-rich island communities by evaluating Darwin's naturalization conundrum, which predicts that invasion success occurs through either functional dissimilarity from native species (Naturalization Hypothesis) or functional similarity with them (Pre-adaptation Hypothesis).

Location

Azores archipelago, Portugal, North Atlantic Ocean.

Methods

We analysed long-term spatial and temporal data from two standardized monitoring programmes in Azorean native forests: the BALA project (spatial dataset) and the SLAM project (temporal dataset). The dataset included more than 100 species of spiders and beetles sampled across multiple islands over two decades. Using functional trait approaches, we quantified exotic–endemic community structure across spatial and temporal scales and compared observed patterns with null expectations to assess functional similarity or dissimilarity.

Results

Across both datasets, most Standard Effect Sizes (SES) values clustered around zero, indicating that exotic assemblages were largely indistinguishable from null expectations relative to endemic communities. This neutral pattern was consistent for exotic spiders across all spatial and temporal scales, providing no support for either Darwinian hypothesis. Exotic beetles, however, showed weak but detectable functional overdispersion at broader spatial scales and through time, suggesting limited and context-dependent support for the Naturalization Hypothesis. Overall, the pattern found is consistent with stochastic processes that appear to dominate exotic–endemic functional relationships, although this cannot be demonstrated directly from SES values alone.

Main Conclusions

Our results suggest that deterministic competition plays a minor role in structuring exotic arthropod assemblages in Azorean native forests. Instead, landscape-level processes, particularly the pressure from surrounding anthropogenic habitats and environmental filtering linked to forest fragmentation, likely shape invasion dynamics. These findings highlight the importance of human-modified landscapes in influencing community assembly and provide insights into how invasion processes operate in insular ecosystems.