A simple mechanistic model of seed dispersal, predation and plant establishment: Janzen-Connell and beyond

1. Although, in nature, seed dispersal usually declines with distance from the source, seedling establishment patterns are highly variable. An increase in seed survival can lead to either hump-shaped (Janzen-Connell (J-C) pattern) or declining (Hubbell pattern) establishment with distance from seed source, but declining establishment can also be generated if survival decreases with distance (McCanny pattern). Pathogens and seed predators are considered to be major mortality agents structuring recruitment patterns, but it is unclear how well predation alone can explain variation in these patterns. 2. We introduce a simple mechanistic model showing that distance and density-dependent seed predation can generate all of the observed recruitment patterns. Our approach provides the first mathematical reconstruction of conceptual models previously considered to be based on contrasting underlying mechanisms. Three easily measurable quantities (the proportion of seeds escaping predation at the source, and the mean distance from the source of dispersed seeds and of predators’ activity) can be used to test for consistency with the J-C pattern. The association between recruitment patterns and plant (dispersal) and animal (predation) characteristics is robust with respect to parameter values and various functional forms. 3. The model shows that the J-C pattern can occur only if the mean distance over which predators are active is lower than that over which seeds are dispersed, corresponding to a system with host-specific, or immobile, seed predators (often invertebrates) that are restricted to areas of high seed density near adult plants, and therefore selecting for longer dispersal distances of seeds. 4.The Hubbell pattern is generated by the model when dispersal and predation distances are of equivalent magnitudes. The McCanny pattern emerges if more generalized, or more mobile, seed predators (often vertebrates) are attracted to the adult trees but also tend to forage farther away, thereby selecting for short dispersal distances that generate high densities needed to satiate seed predators. 5.The model also predicts that the total number of seeds surviving predation is lowest at intermediate distances, suggesting that distance-dependent predation promotes either short or long dispersal distances, or both (dimorphism)

Espectro e distribuição vertical das estratégias de dispersão de diásporos do componente arbóreo em uma floresta estacional no sul do Brasil Spectrum and vertical distribution of diaspore dispersal modes in a seasonal forest in Southern Brazil

A dispersão de diásporos é um evento de elevada importância para as espécies vegetais, que dispõem de diversas estratégias para este fim. O presente estudo teve por objetivos conhecer e caracterizar o espectro das diferentes estratégias de dispersão do componente arbóreo em uma floresta estacional de Santa Maria, Rio Grande do Sul, Brasil (próximo às coordenadas 53º54'W e 29º38'S) e, a partir desses dados, investigar sua relação com os estratos verticais da floresta. Foram amostrados todos os indivíduos com perímetro à altura do peito > 15 cm, em 100 unidades amostrais de 10&times;10 m. A caracterização das estratégias de dispersão foi realizada por meio de observações a campo de frutos e potenciais dispersores, e consulta à bibliografia especializada. Foram amostradas 58 espécies pertencentes a 26 famílias botânicas. Dessas espécies, 74% apresentaram estratégia de dispersão zoocórica; 24%, estratégia de dispersão anemocórica e apenas Gymnanthes concolor Spreng. apresentou a estratégia autocórica. Analisando-se a proporção das estratégias de dispersão por meio da densidade relativa, verificou-se que 80% dos indivíduos são zoocóricos, 12% anemocóricos e 8% autocóricos. Para a floresta analisada foram encontradas diferenças significativas nos padrões de distribuição vertical das espécies agrupadas de acordo com a estratégia de dispersão. A zoocoria ocorreu em espécies de todos os estratos, enquanto a autocoria ficou limitada ao sub-bosque. Já a anemocoria foi mais importante entre as espécies com indivíduos emergentes.<br>Diaspore dispersal is presumed to have great fitness for plants, which could be enhanced by different dispersal modes. Our aim was to generate a dispersal spectrum and to seek for analyze relative ecological importance of tree-species dispersal modes in a seasonal forest in Santa Maria, Rio Grande do Sul, Brazil (approximately 53º54'W and 29º38'S) and, with these data, to investigate the relationship between vertical structure of the forest and dispersal strategies. The tree survey included all individuals with perimeter at breast height > 15 cm, in 100 sample units (10&times;10 m). We classified dispersal modes by diaspores and potential dispersers in the field, and when necessary through specialized bibliography. Fifty-eight tree species from 26 families were sampled. Of these species, 74% are zoochoric, 24% anemochoric and only Gymnanthes concolor Spreng. is autochoric. By analyzing the proportion of dispersal strategies based on relative density, we found that 80% of the trees are zoochoric, 12% anemochoric and 8% autochoric. In the study area, we found significant differences in vertical distribution patterns of species grouped by dispersal strategies. Zoochory occurred in all vertical strata while autochory was limited to the understorey. Anemochory occurred mainly in species with emergent trees