Niche construction by growth forms is as strong a predictor of species diversity as environmental gradients

We present a conceptual framework that describes how species belonging to a growth form collectively can be niche constructors (i.e. modify niches) and affect species diversity in plant communities. We use an empirical assessment of tundra plant communities to illustrate the framework's utility. In doing so, we make a first investigation of collective niche construction in ecological communities. In tundra plant communities, growth forms differently affect ecosystem process rates and cause environmental modifications; thus, growth forms are strong candidates for being niche constructors. To assess the impact of growth form niche construction on plant species diversity, we excluded the species of the growth form applied as niche constructor when estimating the community species diversity. We assessed niche construction in 70 tundra meadow communities and 1450 randomly selected tundra plant communities that are distributed along ecological gradients in temperature, resource availability, competitive interference and herbivory. These gradients allowed us to concomitantly assess to what extent the niche construction is independent of environmental conditions. Growth forms varied from strong positive to neutral predictors of both species richness and Simpson index in the order of forbs, grasses, sedges, deciduous shrubs and evergreen shrubs, suggesting that growth forms have important roles as niche constructors in tundra plant communities. Also, the environmental conditions were strong predictors of species diversity, but they did not interact with or confound the effects of growth forms. Forbs and grasses were the least abundant growth forms, yet they were the strongest positive predictors of species diversity. Therefore, our results suggest a particular niche-constructing role of these growth forms for enhancing species diversity in tundra plant communities.Synthesis. In this study, we provide conceptual and empirical evidence for collective niche construction as a powerful ecological process that affects species diversity and that can act independently of environmental conditions. Species sharing a single trait or species belonging to a growth form can act as collective niche constructors, and as exemplified for growth forms in this study, be important predictors of species diversity in ecological communities.