Dispersal of Stemmadenia donnell-smithii (Apocynaceae) by Birds

Abstract

The dispersal ecology of Stemmadenia donnell-smithii was studied in the tropical dry forest zone of northwestern Costa Rica. Fruit was most abundant late in the dry season and was eaten by 22 species of birds, many of which are primarily insectivorous. Stomach-content analyses substantiated our observations. Total crop size per tree and rate of fruit opening were greatest in pasture-edge trees and smallest in forest trees. Rates and percent of seed germination were highest for seeds from which the surrounding aril was removed, scarified seeds, and seeds which had passed through the digestive tract of a bird. The mean compositions of the fruit tissues were, aril: 7.9% ash, 63.9% lipid, 10.9% protein, 16.8% carbohydrate; seed: 3.4% ash, 31.4% lipid, 10.9% protein, 54.2% carbohydrate; husk: 17.0% ash, 24.0% lipid, 11.2% protein, 47.6% carbohydrate. The foraging behavior of birds taking S. donnell-smithii included hovering, and perching and reaching. Rates of pulp utilization in each habitat were relatively constant; all pulp plus seeds were removed in only a few hours. Calculations of daily energy expenditures suggest that Stemmadenia may provide up to 25 percent of the total energy requirement of individuals of several bird species. Interspecific displacements of birds at fruit were rare. Characteristics of S. donnell-smithii that enhance dispersal include peak fruit availability in dry season, slow rate of opening, seed protection until maturation by husk, bright color of arils, relative accessibility and ease of separation of seed from husk and aril from seed, and high nutritive value of aril. Energetically the plant expends the most calories for protection (husk), followed by expenditures for germination (seed) and dispersal (aril). However, on a calorieper-gram, ash-free, dry-weight basis, the plant puts the greatest amount of energy into aril for dispersal, followed by seed for germination, and then husk for protection. Percent nutrient composition and caloric content of seeds and husk were relatively uniform, whereas lipid, protein, and caloric content of aril tissue varied among samples from different fruits, habitats, and years. This variation may be important in allowing the plant to maintain protection and seed quality while maximizing seed production under varying environmental conditions.