Seed dispersal and tree species diversity

Less fit Lamium amplexicaule plants produce more dispersible seeds

Theory predicts that less fit individuals would disperse more often than fitter ones (Fitness Associated Dispersal, FAD hypothesis). To test this prediction under laboratory conditions, an entire life cycle of Lamium amplexicaule plants and the preferences of its dispersal agent, Messor ebeninus ants, were tracked. Characterization of individual L. amplexicaule plant revealed high variability in spot cover on the surface of the seeds, where less fit plants produce “unspotted seeds” (see Fig. 1 in Introduction). Unspotted L. amplexicaule seeds showed higher variation in germination time and lower germination rate. Moreover, M. ebeninus ants preferably collected these unspotted seeds. Our results show that low fitness L. amplexicaule plants produce seeds with higher potential for dispersal, supporting the FAD hypothesis in a plant-animal system.

Reconciling empirical ecology with neutral community models

Neutral community models embody the idea that individuals are ecologically equivalent, having equal fitness over all environmental conditions, and describe how the spatial dynamics and speciation of such individuals can produce a wide range of patterns of distribution, diversity, and abundance. Neutral models have been controversial, provoking a rush of tests and comments. The debate has been spurred by the suggestion that we should test mechanisms. However, the mechanisms and the spatial scales of interest have never clearly been described, and consequently, the tests have often been only peripherally relevant. At least two mechanisms are present in spatially structured neutral models. Dispersal limitation causes clumping of a species, which increases the strength of intraspecific competition and reduces the strength of interspecific competition. This may prolong coexistence and enhance local and regional diversity. Speciation is present in some neutral models and gives a donor-controlled input of new species, many of which remain rare or are short lived, but which directly add to species diversity. Spatial scale is an important consideration in neutral models. Ecological equivalence and equal fitness have implicit spatial scales because dispersal limitation and its emergent effects operate at population levels, and populations and communities are defined at a chosen spatial scale in recent neutral models; equality is measured relative to a metacommunity, and this necessitates defining the spatial scale of that metacommunity. Furthermore, dispersal has its own scales. Thorough empirical tests of neutral models will require both tests of mechanisms and pattern-producing ability, and will involve coupling theoretical models and experiments.