Interactions of Seeds and their Insect Predators/Parasitoids in a Tropical Deciduous Forest

Abstract

In this paper I will touch on some of the patterns and processes that have come to light in an on-going study of how the insects that eat seeds in tropical vegetation influence the structure of that vegetation and the characteristics of its individual members, and how the characteristics of the plants influence the insects. This study began in 1964 in the lowlands of the state of Veracruz, Mexico. While trying to understand the population dynamics and interdependency of obligate acacia-ants and their acacias (Janzen 1967), I was struck by the thoroughness with which the bruchid Acanthoscelides oblongoguttatus decimated the seed crops of Acacia cornigera. This swollen-thorn acacia was extremely common as a result of farming and grazing practices, and it produced far more fruits than the local community of dispersal agents would eat (a thorough study of dispersal agents was not made, but at least the Plain-tailed Brown Jay, Psilorhinus mexicanus, Black- headed Saltator, Saltator atriceps, and Grayish Saltator, Saltator coerulescens, ate seeds along with the pulp and defecated living seeds). While most of the A. cornigera population bore flowers in the later part of the dry season (March, April) and mature fruits about ten months later, individuals in almost any reproductive stage could be found at any time of year in an area as small as a few hectares. The pods remained on the acacia in a stage susceptible to bruchid oviposition for as long as two to three months. Attack was heavy and continuous, both from beetles that could have emerged from other pods on the same acacia, and from beetles coming from other A. cornigera out of fruiting synchrony. Only very broad synchrony of flowering and fruiting occurred in the highly mixed and variously disturbed habitats (roadsides, back yards, various age corn fields, pastures, fencerows, creekbanks, marshes).