The Minimum Carbon Theory: A Physiological Basis for Species Coexistence

Understanding how species coexist and why diversity varies across regions remains a central challenge in ecology. Although widely used theories such as niche theory, neutral theory, and modern coexistence theory have yielded key insights into coexistence and diversity, their reliance on abstract parameters lacking direct physiological grounding limits empirical validation and constrains cross-scale integration. To address this limitation, we propose the minimum carbon theory (cₘᵢₙ), grounded in the first principle that autotrophic organisms must maintain a net positive carbon balance to survive. The cₘᵢₙ defines the lowest rate of net carbon gain required for persistence, integrating the costs of maintenance respiration, structural investment, and stress responses, offset by facilitative interactions that reduce metabolic demand. By linking species persistence to physiological carbon sufficiency, cₘᵢₙ provides a unified, mechanistic explanation for patterns of distribution, coexistence, and diversity across environmental gradients. Reframing existing theories, including niche differentiation, neutral dynamics, modern coexistence models, and macroecological hypotheses such as the energy–diversity and water–energy dynamics theories through the lens of carbon physiology, cₘᵢₙ offers a coherent mechanism to integrate previously fragmented ecological paradigms. Because carbon is the universal currency of life, the cₘᵢₙ can be applied across taxa and scales, unifying questions of invasion, ecosystem function, and ecological stability. In a biosphere increasingly shaped by climate stress, the cₘᵢₙ theory provides a physiologically grounded foundation for a unified theory of diversity, offering mechanistic insight into how individual carbon thresholds scale up to shape the structure and functioning of ecological systems.