Understanding mechanisms of species coexistence has lured many into field of Ecology. Skilled theoreticians have come up with many forms of coexistence – Janzen-Connell, storage effect, n-resource competition. These models predict how species differences lead to stable coexistence. However, now as we are moving from species-level parameterization toward trait-based approaches, the species coexistence question in concept becomes a different theoretical challenge.
As we use functional traits to free our models from the need for species-specific parameterization, we are left with the question of how diversity of functional traits persists in the face of evolutionary and ecological community assembly. For example, why do plant species coexist that differ in leaf mass per unit area, hydraulic safety margins, or even vegetative form? This is a challenge for large models which rely on relatively simple, tractable ecology to give general answers. But understanding coexistence among species through a trait-based approach is not just a modeling effort. With this relaxation from species consideration to functional traits, we are able to probe ecological and evolutionary principles to understand the coexistence of specific traits themselves.
Results/Conclusions
How do first principles of ecology – resource limitation, environmental variability, interdependence of spatial and temporal scales, and biophysical and evolutionary constraints – lead to coexistence? Here I will present lessons from a few attempts at finding trait-based coexistence within an adaptive dynamics modeling framework.
We will look at the effect of rainfall variability on plant hydraulic strategies as well as attempts to understand tree/grass coexistence. In these examples, we can see that although species do outcompete one another under the extremes of the environmental variability and will coexist if their traits are held constant, they are excluded by intermediate strategies when evolution by small mutations is allowed.
We have found coexistence that is stable to eco/evolutionary dynamics however. One example comes about by an assumed physiological connection between functional traits of leaf mass per unit area (LMA) and leaf longevity (Weng et al. 2016). And one arises from an assumption of individual limits to plasticity in leaf nitrogen and its relationship to photosynthetic rate (Dybzinski et al. 2013).
Together these examples show the power of trait-based approached to understanding coexistence in refining our knowledge of physiological constraints and dynamics across scales.