The spatial distribution of seeds sets the template on which subsequent plant demographic processes take place. For plants dispersed by frugivores and herbivores, spatial patterns of recruitment are primarily influenced by the spatial arrangement and characteristics of adult plants, the feeding behaviour and movement patterns of the dispersers, and the structure of the habitat matrix. Traditional approaches to study dispersal have however ignored the spatial component. In contrast, a recent outburst of interest in the study of the factors determining dispersal distances and patterns of seed deposition is providing increasing evidence that animal movement patterns play a primary role in determining seed fate and the structure plant populations.
Our research focuses on understanding the factors that determine the distance and spatial patterns of deposition in endozoochorously dispersed seeds, with a specific focus on the role plaid by the animal dispersers.
We use a research framework in which such distances and patterns depend primarily on the interplay of three variables:
1. the gut passage time of plant seeds and its effects on seed viability and germination
2. the movement pattern of animal dispersers (speed and trajectory)
3. the home range of animal dispersers.
The relative importance of these three variables at each specific plant-disperser system defines a gradient of situations, which has a strong bearing on the approach to study and model its functioning. At one extreme of the gradient, dispersers show relatively short gut-passage times, slow movements and large home ranges. In such systems, dispersal distance is strongly influenced by the interplay between gut-passage time and movement speed, and dispersal patterns primarily depend on preferential movement directions over the habitat matrix. At the other extreme, dispersers show relatively long gut-passage times, fast movements and small home ranges. In such systems, dispersal distance and pattern will primarily depend on the dispersers´ home range size and their pattern of space use within it.
We are developing spatially-explicit studies of disperser movement, seed dispersal and (whenever possible) plant recruitment in a variety of plant-disperser systems, with the global aim of obtaining a general overview of the relative contribution of these three factors. At each system, however, we strive to identify applied questions for which our seed-dispersal models represent a significant contribution on its own. Examples of our study systems include:
- Dispersal of coastal shrubs by insular lizards (Podarcis lilfordi at Dragonera Island and Lacerta lepida at Cíes Islands).
- Dispersal of forest and scrub trees by Asian elephants, at Sri Lanka, Myanmar and Malaysia.
- Dispersal of fleshy-fruited epiphytes by the rare marsupial Dromyciops gliroides, at Chile’s austral forest.
- Dispersal of aquatic plants and invertebrates by waterfowl, at local (Doñana marshes, Spain) and continental scales (Western Europe).
Since recently, in cooperation with Anurag Agrawal, we are also studying the role of antagonists (herbivores and seed predators) in the evolution of seed dispersal. Our study focuses on the interplay between milkweeds and its suite of herbivores, in particular two species of milkweed bugs with sedentary and migratory life-cyces, respectively.
