Density effects at multiple scales in an experimental plant population

Abstract

In sessile organisms, each individual in a population experiences a different local conspecific density, and this density itself will vary depending on the scale at which it is measured (converging on a 'mean field' population density only at coarse scales). The effects of density on any ecological process (e.g. competition, herbivory, mortality) may therefore be scale-specific, and different processes may respond to density at different spatial scales. A field experiment was set up in which focal Silene latifolia plants were subjected to a range of densities at each of four spatial scales (squares with sides of 16 cm, 80 cm, 4 m and 20 m); data were collected on growth, herbivory, flowering and survival. The Akaike information criterion was used to determine which scale displayed the strongest relationship between density and each response variable. The variables showed peak responses to density at different scales and with various relationships (positive, negative and non-monotonic). Some (e.g. herbivory by mammals) showed contrasting responses at different scales. Growth rate showed surprisingly little response to conspecific density, presumably owing to balancing effects of intra-specific and inter-specific competition from the surrounding vegetation. However, 25% of variation in first-year survival was explained by a model including a positive effect of density at the 80-cm scale. Herbivory was mainly due to slugs and showed negative responses to density at the 4-m and 20-m scales, in accordance with hypotheses about resource limitation, mobility and spatial scaling. Mammal herbivory responded with contrasting patterns at the 80-cm and 20-m scales. Congruence between the responses of herbivory and survival suggest that herbivory was an important cause of mortality. Plant population patterns may thus reflect density effects interacting at different characteristic scales. Generalizations about density-dependence in spatially structured populations may be misleading if they fail to recognize that density is scale-dependent. © 2007 The Authors Journal compilation © 2007 British Ecological Society.