Niche complementarity enhances ecosystems by:
Niche complementarity occurs when species differ in where, when, or how they obtain resources. One plant may use shallow soil water while another accesses deeper water; pollinators may forage at different times; microbes may process different substrates. These differences allow a community to capture a larger fraction of available resources than any one species could, often increasing total productivity, nutrient retention, or stability. Complementarity does not eliminate all competition, but it reduces direct overlap in the most limiting dimensions. It therefore provides a mechanism by which biodiversity can improve ecosystem functioning. Increased competition or overlap would work against complementarity, and reducing biodiversity removes potentially distinct functional strategies. Experimental biodiversity studies distinguish complementarity effects from selection effects, in which a diverse mixture performs well mainly because it contains one highly productive species. True complementarity is supported when species perform better together through partitioning or facilitation than expected from their monocultures. The principle links niche differences at the species level to more complete and efficient resource use at the ecosystem level.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 13