Micro-Refuges or Ecological Traps: Context-Dependent Effects of Rock Pools on Intertidal Biodiversity Across Latitudes

Aim
We investigated how local-scale environmental heterogeneity influences biodiversity patterns across broad biogeographic gradients, using intertidal microhabitats as a model system within one of the most environmentally stressful ecosystems on Earth.

Location
Intertidal habitats at 26 locations (two rocky shore sites per location) across six continents, spanning 98° of latitude (38°S to 60°N).

Time Period
2019–2022.

Major Taxa Studied
Algae, sessile and mobile invertebrates.

Methods
We compared biodiversity and thermal environments across contrasting microhabitats (rock pools and adjacent emergent rock) along a latitudinal gradient, sampling during environmentally ‘milder’ and ‘harsher’ periods. Biodiversity was quantified using multiple richness metrics (mean, total, unique taxa) and functional diversity.

Results
Microhabitat differences strongly influenced biodiversity patterns across latitude. Rock pools consistently supported higher taxonomic and functional diversity than emergent rock, irrespective of sampling period, reflecting their ability to buffer thermal extremes, particularly under harsher conditions. Mean species richness exhibited a non-linear, s-shaped latitudinal pattern, with lowest values near the equator and higher richness at mid-latitudes, diverging from classical Latitudinal Diversity Gradient expectations. Biodiversity differences between microhabitats were greatest in temperate regions and diminished at low latitudes, where extreme conditions constrained diversity across habitats.

Main Conclusions
Local environmental heterogeneity can substantially modify, and in some cases obscure large-scale biodiversity patterns. By mediating exposure to environmental stress, intertidal microhabitats provide insight into how fine-scale variability interacts with latitudinal stress gradients to shape biodiversity distributions. Incorporating microhabitat variability into biogeographic frameworks is important for understanding global biodiversity patterns and predicting ecological responses to climate change.