FITNESS RESPONSES OF A CARNIVOROUS PLANT IN CONTRASTING ECOLOGICAL SCENARIOS

Abstract

This paper reports the results of a two-year field experiment on the deter- minants of fitness responses in Pinguicula vallisneriifolia (Lentibulariaceae), an endemic carnivorous plant of southeastern Spain. For the first time, in this experiment, we have considered irradiance as a factor, in combination with animal prey, for an array of natural field conditions. The goal was to determine how carnivory translates to fitness within different radiation regimes. For this, it was necessary to quantify an array of plant responses, such as survival, growth, reproduction (sexual and vegetative reproduction), as well as responses related to carnivory investment (leaf shape and mucilage secretion). Both irra- diance and animal food supply proved to be important limiting factors for P. vallisneriifolia under field conditions. Plants clearly decreased in performance from the sunny habitat to the deep-shade one, with plants growing in the least irradiance registering the lowest values for all variables analyzed. The clearest response to prey was the production of axillary buds. Most vegetative and reproductive responses depended heavily on the initial biomass of the plant before the experiments, the largest plants bearing the most leaves, flowers, stolons, and axillary buds. A gradient from less to more mucilage secretion appeared from deep-shade to sunny habitat, and within each habitat from prey exclusion to prey addition levels. Trapped prey stimulated digestive secretions in a positive feedback (the more prey, the more mucilage secretion) under all irradiance conditions. The curled, more secretary leaves of the sunny plants, in comparison with the nearly flattened, less secretary leaves of the deep-shade plants, illustrate the constraints imposed by differing scenarios on the capture of both prey and photons. With nonlimiting water availability, more irradiance and prey results in more survival, growth, and sexual and vegetative reproduction. Nevertheless, it is not usual to find an optimum combination of resources (i.e., irradiance, prey, and water) available in the same microhabitat during the Mediterranean summer. The spatial uncoupling of limiting resources progressively increased towards the extremes of the irradiance gradient (sunny and deep- shade habitats, respectively), and therefore the dual photosynthetic and carnivore functions of P. vallisneriifolia leaves were not equally efficient in all habitats. These opposing resource gradients determined all vegetative and reproductive plant responses. The perennial char- acter of this endemic plant, together with its vegetative form of propagation, allows the possibility of resisting extinction even in the absence of seedling recruitment or when vegetative growth is strongly limited.