Density effects at multiple scales in an experimental plant population

Abstract

1 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). 2 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. 3 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. 4 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. 5 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. 6 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. 7 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.