Plant trait ecology began from the wish to compare ecological strategies across species, more widely than was possible via comparing performance or distribution in shared habitat. Indeed since 1997, scattergrams of species in trait space have reached coverage of tens of thousands of species across all continents. However, assembly rules for vegetation (one of the original motivations for ecological strategy theory) remain to be clarified. An expectation that coexisting species will be overdispersed in trait-space arises from the classical idea that competition kernels might be approximately gaussian. Yet for the most part, vascular land plants use the same light, water and mineral nutrients as each other. Why should differences in (say) leaf mass per area or wood density map onto depleting different resources and thence to a competition kernel that weakens with distance across trait-space? We investigate a model where outcomes of competition for light emerge from traits and processes. As trait-combinations enter the assemblage and increase in abundance, the fitness landscape is drawn down until eventually all strategies present are found at trait-combinations with zero net increase, and the remainder of the trait landscape has negative increase and is not invasible.
Results/Conclusions
The model maintains size-structured populations, derives vegetative growth from traits and from light competition, and derives population increase in a successional metacommunity from competitive growth. For a given trait-constellation of occupants, this describes the fitness landscape. Over time invasion plus evolutionary adjustment lead to stable constellations in trait-space. One key outcome is that fitness drawdown is often distant in trait-space from the occupant. Spacing-out (“limiting similarity”) in trait space is not a consistent feature of the assemblage. This is in line with most field results in the literature. Another key outcome is that near-neutral fitness surfaces can be produced in specific regions of trait space. Along these ridges quite large numbers of species can coexist at low population density. Evolutionary adjustment of traits is important in maintaining the near-neutrality. Essentially coexistence in this model works by partitioning successional time, and the trait-constellations produced are such as to make that possible. This should not be a surprise, since time-of-use separation is the most straightforward way that light, water and nutrients can become separately-depletable resources. In general, we expect progress in community ecology to come from building models that capture the actual mechanisms by which traits influence competition.