Perennial plant models to study species coexistence in a variable environment
The maintenance of species diversity is a fundamental yet especially puzzling question in ecology. It is unclear why so many species, especially plants, coexist despite being very similar in their resource needs. There is even more uncertainty in how diversity changes as environment changes. Theories studying species coexistence in variable environment exist which provide a framework to quantify different coexistence mechanisms arise in variable environment. However, key models used to develop these theories are very simple. Simple models reduce the life histories of species to the bare essence. It is difficult to match these models to empirical systems where species have complicate life histories, leaving more challenges to investigate these mechanisms empirically. Questions remain whether a complicate model is needed. Identifying which aspects of the life histories matter for species coexistence is critical for understanding the maintenance of species diversity.
Using forest trees as an illustration, we developed a size-structured model in which tree growth can be sensitive to environment and competition as the seedling recruitment does. In addition, size also allows detailed formulation of life history strategy and its change through ontogeny. We demonstrate that quantifying coexistence mechanisms is essentially no more difficult for species with complex life histories than for species with simple life histories. Our study revealed that variation in seedling recruitment and variation in tree growth have a symmetric role in promoting stable coexistence. At least two coexistence mechanisms are present: the storage effect and the consequent shift in size structures. A clear distinction lies between the two mechanisms in terms of the different aspects of life histories that affect them. The storage effect is important if the sensitivity to environmental variation and relative importance to population growth of a process are aligned. Shift in mean structure will be promoting coexistence if a population in low density increases the proportion of individuals with demographic advantages. In contrast, if in constant environment with only one limiting resource, differences in life histories can only affect average fitness-differences between species, and potentially act as equalizing mechanisms.