Feeding Behavior of a Plant: Differential Prey Capture in Old and New Leaves of the Pitcher Plant (Sarracenia purpurea)

Abstract

The functional response of the carnivorous pitcher plant (Sarracenia purpurea L.) was determined using Drosophila melanogaster as prey. These plants exhibit a sigmoid-shaped curve in response to an increase in prey availability. Plants with newly opened leaves captured significantly more Drosophila in 24 hr than plants without new leaves. The number of Drosophila caught was related to the size of the leaf opening in plants that did not have newly opened leaves. This relationship did not occur in plants that had newly opened leaves. There was no increase in the number of Drosophila caught as the number of leaves per plant increased. INTRODUCTION The response of predators to changes in the number of prey available has been widely studied. Soloman (1949) first noted two such responses: the numerical response in which there is a change in the density of predators and the functional response in which there is a change in the predator's capture rate. Holling (1959) categorized three types of functional responses typical of different animals: Type 1 curves are characteristic of animals which consume food at a rate proportional to the rate of encounter (e.g., filter feeders). This response increases linearly with increasing prey availability until it reaches an asymptote. Type 2, or the invertebrate curve, rises at a negatively accelerating rate and is characteristic of animals that require a certain amount of time to capture and digest their prey. Type 3, or sigmoid-shaped curves, are typical of animals which exhibit some form of learning behavior. The sigmoid curve results because the animals do not respond readily to changes in density when the prey is scarce; it is only when the prey is relatively abundant that the capture rate of the predators goes up quickly, and then the response is a curvilinear rise up to a plateau. While functional responses have been demonstrated for a wide range of animal species (Holling, 1959; Hassell, et al., 1977), the response to increased prey density has not been examined in carnivorous plants -plants that supplement their nutrition by capturing and digesting animal prey (Schnell, 1976). Pitcher plants (Sarracenia purpurea L.) capture insects passively by means of hollow tube-shaped leaves. The leaves grow for approximately 2-3 weeks before the opening of the leaf is revealed by the splitting open of the top or leaf hood. When the hood opens, insects are attracted to the leaf by a nectar secreted from glands on the internal surface of the leaf (Adams and Smith, 1977). They are then directed by downward-pointing hairs onto a slippery inner surface, causiig them to fall into the leaf liquid composed of rainwater and symbiotic organisms (Fish and Hall, 1978). Captured insects drown in the liquid and are digested by leaf-secreted enzymes (Heslop-Harrison, 1976) and bacterial exozymes. In this, the sarcophageal region of the leaf, foliar absorption of nutrients occurs (Plummer and Kethley, 1964). Previous work has shown that the prey-capture ability of a pitcherplant leaf is greatest in the 1st month after the flap opens (Fish and Hall, 1978). After 30 days, the number of prey captured by the leaf declines. The purpose of this laboratory study was to determine the functional response of pitcher plants to changes in prey density (Drosophila melanogaster) and how this response differs between plants with newly opened leaves ( < 30 days old) and those without 1 Present address: 222 Sheraton Drive, Montreal West, Quebec H4X IN4. Department of Botany, University of Toronto, Toronto, Ontario, Canada M55 7A7