PHENOTYPIC PLASTICITY FOR FITNESS COMPONENTS INPOLYGONUMSPECIES OF CONTRASTING ECOLOGICAL BREADTH

Abstract

The ecological distribution of species is influenced by individual patterns of response to environment for traits that contribute to fitness. Comparative data on fitness responses to complex environments are particularly valuable for understanding the relation of plasticity to ecological breadth. This study compares individual phenotypic plasticity for components of fitness in four congeneric annual plant species with contrasting ecological distributions (Polygonum cespitosum, P. hydropiper, P. lapathifolium, and P. persicaria). Replicate seedlings of 32 inbred lineages drawn from five natural populations per species were grown to maturity under controlled conditions in all 12 possible combinations of low and high light; dry, moist, and flooded soil; and poor vs. rich nutrients. Timing of reproduction, total reproductive output, offspring size, and allocation to reproduction were determined for each plant. The data revealed highly complex differences among the species in patterns of plastic response for fitness traits (i.e., high-order species-by-environment interaction effects). These plasticity differences illuminate the species' known differences in ecological distribution. Individuals of the broad ecological generalist P. persicaria maintained fecundity and offspring size in poor conditions and also reproduced at extremely high levels when given plentiful resources. In contrast, P. lapathifolium plants showed high fitness in favorable treatments but sharply delayed and decreased reproduction as well as offspring size when deprived of light and other resources; this species is restricted in nature to high-light, moist sites. Conversely, P. hydropiper plants increased reproductive output relatively little in resource-rich environments, which may explain why this species is not an invasive colonizer. Although other factors evidently limit P. cespitosum to shaded habitats, the ability shown by plants in this species to maintain offspring size and output across a range of environments may be a factor in its extremely rapid spread. These results confirm that ecological breadth of distribution may reflect not an equable, constant pattern of fitness response, but rather the ability to both maintain fitness in resource-poor environments and opportunistically maximize fitness in favorable conditions. These results contribute three important insights to our understanding of the relation of phenotypic plasticity to ecological breadth: ecologically important species differences in fitness plasticity may entail (a) multiple environmental factors, as well as (b) a number of distinct fitness components; furthermore (c) neither reproductive plasticity nor constancy per se is necessarily associated with ecological breadth.