Counteracting Cascades Challenge the Heterogeneity-Stability Relationship.

Spatial environmental heterogeneity is widely assumed to enhance ecological stability by promoting refugia, biodiversity and asynchrony. Yet, we lack field experiments testing this fundamental relationship and its underlying mechanisms in naturally assembled multitrophic systems. To address this gap, we monitored experimental substrates replicating topographic heterogeneity on a rocky shore over 3 years. Contrary to theory, heterogeneity showed no net effect on community stability due to four counteracting pathways. Heterogeneity increased stability by (i) providing refugia that enhanced population stability and (ii) boosting species richness, which promoted asynchrony. At the same time, it decreased stability by (iii) reducing a dominant non-native species and (iv) suppressing consumers, both of which otherwise stabilised community composition. These opposing processes cancelled out the heterogeneity-stability relationship, highlighting the complex and multi-causal nature of this relationship. We caution against the assumption that increasing heterogeneity universally enhances stability, particularly in systems with strong consumer interactions and dominant species.