Causes and effects of small-scale spatial structure in plant populations

Introduction If plants in a community were located in space independently of one another, and if each small part of the space had the same probability of being occupied,the community would have no spatial structure. Such randomness is unusual. Plants in mesic environments are commonly clustered together in groups of conspecifics, and the spatial pattern of plants in arid environments can sometimes have the opposite property — a striking overdispersion of individuals (Cody 1986). Spatial structure (a departure from a homogeneous Poisson process) is the norm. The existence of spatial structure has profound implications for plant ecology because plants interact primarily with their close neighbours. The effects of neighbours of a plant are most evident above ground through shading, and are also felt below ground through the uptake of nutrients and water. Individuals outside the neighbourhood have relatively little effect on these processes — in particular, there is no reason to suppose that density averaged over some large spatial region, the so-called ‘mean-field’ assumption (Law et al. 2000), is of any significance (Mack & Harper 1977). These points might seem too trivially obvious to mention were it not for the great difficulty plant ecologists have had in constructing a theory of population dynamics that holds them properly in place. Historically,plant population dynamics has taken its framework from animal ecology, together with the mean-field approximation widely used there. Yet this is not enough: somehow the information on local spatial structure, on which the growth of plants and ultimately their vital rates crucially depend, has to be accounted for. Without this, models at the theoretical core of plant ecology are dynamically insufficient (Lewontin 1974,page 8),and predictions about future states are liable to be seriously in error. This chapter is concerned with small-scale spatial structure in plant communities, the readiness with which such structure is generated, how it can be built into plant–community dynamics,and the implications of such structure for plant population genetics. The thread that runs through all this is the need to replace the mean-