Recently, there has been strong renewed interest in how species interactions contribute to coexistence dynamics and diversity maintenance. Despite a widespread appreciation that many types of interactions occur in natural communities, few models of coexistence or diversity maintenance account for complex species interactions, with most including information about direct competition alone. Evidence has emerged, however, that non-additive ‘higher-order’ species interactions (HOIs) have a strong influence on individual fitness in real communities. Despite the potential importance of this finding, we still know little about how HOIs operate in natural systems. We build on base evidence of strong and widespread higher-order interactions impacting the fitness outcomes of annual plants in diverse natural communities. Using data on fitness outcomes of over 700 individual annual plants from six focal species living in natural plant communities in SW Western Australia’s York Gum woodlands, we answer the following questions: 1) Does the strength and direction of HOIs depend on the identity of the focal and neighbouring species? 2) Do HOIs mediated by species with similar sets of functional traits have generalizable impacts on fitness outcomes? And 3) can we use functional traits to simplify HOI-inclusive fitness models without losing the benefits of these more complex models?
We found minimal evidence that focal species identity was important to the fit of direct-interaction-only fitness models. When HOI terms were included, however, the identity of the focal species and neighboring species were significantly important for outcomes of both direct and higher-order interactions. The inclusion of species identity thus greatly improved the fit of HOI-inclusive fitness models. Using individual, commonly available morphological functional traits (e.g. height, specific leaf area, seed mass), we found variable outcomes of higher-order interactions mediated by species with similar trait values. When multiple traits were combined as proxies for complex life history strategies, results were more promising, providing some indication of how species may be grouped to reduce model complexity without losing the benefits of including HOIs in fitness models. Results suggest that distinct and significant non-additive species interactions occur among many species in high diversity plant communities. Our findings bring into question the common assumption that the majority of direct and indirect interactions among species in diverse communities are relatively unimportant to individual fitness outcomes and thus population dynamics and local patterns of diversity.