Ranking ecological contingencies from high-order factorial data demonstrate tidy control of biodiversity from facilitation cascades in estuaries on the South Island of New Zealand

Community ecology has been described as a ‘mess' because ecological processes vary in space, time, and across species traits, resulting in myriads of ecological contingencies and low scientific predictability. Here, we aimed to identify and rank ecological contingencies and improve predictability using fully crossed high-order factorial mensurative and manipulative experiments across axes of spatiotemporal variability that may influence the strength of facilitation cascades on the South Island of New Zealand. Facilitation cascades, arising from chains of positive interactions, are prevalent in intertidal sedimentary estuaries, where biogenic habitat-formers, such as bivalves and attached seaweed, provide hard substrates, food, hiding places, and reduce environmental stress for small animals. Specifically, we measured facilitation of > 65 000 small mobile invertebrates across eight archetypical contingencies, i.e. within and between seasons, latitudes, sites with different distances to the open ocean, vertical intertidal elevations, wider habitat matrix (bare sediment vs seagrass beds), and between small-scale habitat-forming species (the endemic cockle Austrovenus and attached cosmopolitan seaweeds, Ulva and Gracilaria spp.) and their sizes. Overall, our multifactorial tests revealed that most higher-order interactions (three-way or more) were not important ecologically and that many important lower-order interactions (two-way) were ‘simple', demonstrating that facilitation can increase when and where the baseline biodiversity is higher. Furthermore, most of the main test factors were significant and ecologically important, suggesting that facilitation of animals, generally and across other factors, was strongest on large and morphologically complex seaweeds, at near-ocean sites and deeper intertidal elevations, and in warmer summer months. Our case study highlights a relatively tidy – not messy – control of biodiversity of intertidal epifauna, and that high-order factorial sampling can help unravel and rank co-occurring spatiotemporal drivers to better understand ecological contingencies. Finally, our results may also inform management of habitat-forming species to preserve estuarine biodiversity and maintain their secondary production.