Seed mass diversity along resource gradients: the role of allometric growth rate and size-asymmetric competition

The large variation in seed mass among species inspired a vast array of theoretical and empirical research attempting to explain this variation. So far, seed mass variation was investigated by two classes of studies: one class focuses on species varying in seed mass within communities, while the second focuses on variation between communities, most often with respect to resource gradients. Here, we develop a model capable of simultaneously explaining variation in seed mass within and between communities. The model describes resource competition (for both soil and light resources) in annual communities and incorporates two fundamental aspects: light asymmetry (higher light acquisition per unit biomass for larger individuals) and growth allometry (negative dependency of relative growth rate on plant biomass). Results show that both factors are critical in determining patterns of seed mass variation. In general, growth allometry increases the reproductive success of small-seeded species while light asymmetry increases the reproductive success of large-seeded species. Increasing availability of soil resources increases light competition, thereby increasing the reproductive success of large-seeded species and ultimately the community (weighted) mean seed mass. An unexpected prediction of the model is that maximum variation in community seed mass (a measure of functional diversity) occurs under intermediate levels of soil resources. Extensions of the model incorporating size-dependent seed survival and disturbance also show patterns consistent with empirical observations. These overall results suggest that the mechanisms captured by the model are important in determining patterns of species and functional diversity

North American Breeding Bird Survey Underestimates Regional Bird Richness Compared to Breeding Bird Atlases

Standardized data on large-scale and long-term patterns of species richness are critical for understanding the consequences of natural and anthropogenic changes in the environment. The North American Breeding Bird Survey (BBS) is one of the largest and most widely used sources of such data, but so far, little is known about the degree to which BBS data provide accurate estimates of regional richness. Here, we test this question by comparing estimates of regional richness based on BBS data with spatially and temporally matched estimates based on state Breeding Bird Atlases (BBA). We expected that estimates based on BBA data would provide a more complete (and therefore, more accurate) representation of regional richness due to their larger number of observation units and higher sampling effort within the observation units. Our results were only partially consistent with these predictions: while estimates of regional richness based on BBA data were higher than those based on BBS data, estimates of local richness (number of species per observation unit) were higher in BBS data. The latter result is attributed to higher land-cover heterogeneity in BBS units and higher effectiveness of bird detection (more species are detected per unit time). Interestingly, estimates of regional richness based on BBA blocks were higher than those based on BBS data even when differences in the number of observation units were controlled for. Our analysis indicates that this difference was due to higher compositional turnover between BBA units, probably due to larger differences in habitat conditions between BBA units and a higher likelihood of observing geographically restricted species. Our overall results indicate that estimates of regional richness based on BBS data suffer from incomplete detection of a large number of rare species, and that corrections of these estimates based on standard extrapolation techniques are not sufficient to remove this bias. Future applications of BBS data in ecology and conservation, and in particular, applications in which the representation of rare species is important (e.g., those focusing on biodiversity conservation), should be aware of this bias, and should integrate BBA data whenever possible

Dispersal in time and space: a combinaded analytical-experimental approach

Many plants disperse their seeds in both space (Via dispersal) and time (via dormancy j. Theoretical models indicate that seed dispersal and between-year dormancy are alternative mechanisms that may contribute to the stabilization of populations in fluctuating environments, to the coexistence of competing species, to species diversity, and to the maintenance of populations in unfavorable habitats where local reproduction is insufficient to balance mortality. Unfortunately, most of these models contain variables and parameters which are extremely difficult to measure. As a consequence, there has been virtually no experimental calibration of these models, or tests of their predictions in natural plant populations. In an attempt to bridge this gap, a new approach combining a theoretical model and an experimental procedure has been developed for quantifying population dynamic functions of seed dispersal and dormancy. The approach is based on the identification of three potential sources of recruitment to plant populations: (I) input from local reproduction in the previous year (2) input from the soil seed bank. and (3) input from dispersal. The quantitative relationships among these three sources of recruitment determine the contribution of seed dispersal and between-year dormancy to population density. The approach developed allows one to derive mathematical expressions for the three sources of recruitment, and to estimate their values in natural plant populations. Results from a field study based on this approach indicate that input of seeds produced in relatively favorable habitats may interact with seed dormancy in promoting the persistence of annual plant populations in less favorable habitats

Ron et al. 2017 JE data

plot level, habitat level and regional level species diversity for plots of four different soil depth

Data from: The role of species pools in determining species diversity in spatially heterogeneous communities

1. The 'habitat-specific species pool hypothesis' proposes that differences between habitats in the sizes of their species pools are the main drivers of diversity responses to habitat heterogeneity. Empirical tests of this hypothesis are not trivial since species might be missing from ecologically suitable habitats due to limited dispersal, while others may occur in unsuitable habitats by means of source-sink dynamics and mass effect. 2. We tested the habitat-specific species pool hypothesis in a local, environmentally heterogeneous community of annual plants using a novel 'ecological selection' experiment. Mixtures of seeds representing the whole community were sown in each habitat, and the emerging species were exposed to six generations of selection by environmental filtering and competition while being blocked from dispersal. A comparison of the total number of species that were able to survive in each habitat (i.e., to pass the selection test) with data on species richness in the natural community allowed us to test the degree to which observed differences in species richness between habitats could be explained by differences in the sizes of the respective species pools. 3. Results supported the species pool hypothesis, showing that differences in the sizes of the habitat-specific species pools were important in determining diversity responses to habitat heterogeneity. Moreover, species richness showed a unimodal response to local-scale gradients in habitat productivity, and this response could be attributed to underlying differences in species-pool sizes. Both results were robust to properties of the data and the method of analysis. 4. Synthesis. Our results provide a strong experimental evidence that differences in the sizes of habitat-specific species pools might be important in shaping the diversity of local communities. Future theoretical and empirical studies in community ecology should explore the potential sources and implications of such differences

LIGHT ASYMMETRY EXPLAINS THE EFFECT OF NUTRIENT ENRICHMENT ON GRASSLAND DIVERSITY

Data used for all analyses in the pape