A Tropical Hawkmoth Community: Costa Rican Dry Forest Sphingidae

Abstract

Hawkmoths (Sphingidae) constitute a major class of pollinators in the lowland dry forest of Costa Rica. In our study area, hawkmoths are the primary pollinators of about 10 percent of the tree species; they also pollinate various species of shrubs, herbs, lianas, and epiphytes. We found a total of 65 hawkmoth species and 31 native plant species that were adapted primarily for hawkmoth pollination. Abundance of both hawkmoths and hawkmoth flowers peaked in the first half of the wet season (May-July). Numbers of hawkmoths decreased through the dry season (December to April) with lowest numbers during April. Species of hawkmoth flowers also decreased through the dry season, but several species began flowering before the onset of the wet season and before the sudden increase in hawkmoth numbers that followed shortly after the first rains. Tube lengths of hawkmoth flowers ranged from 0 to 19 cm (x = 5.1 cm), matched by the range in length of hawkmoth tongues (1-20 cm, x = 4.9 cm). Hawkmoth tongue lengths were correlated with body size (wing length) (r = .70). Long flower tubes restricted access to nectar to long-tongued moths. However, long-tongued moths did not restrict their visits to long-tubed flowers. In fact, hawkmoths visited many species of flowers adapted for other pollinator types-e.g., hummingbirds, bees, and bats-along with hawkmoth flowers whose tubes were much shorter than their tongue lengths. The attractiveness of hawkmoth flower species varied greatly: some flowers were always heavily visited, other species were virtually ignored. In less attractive flowers, nectar accumulated during the night and attracted a variety of diurnal visitors the following morning. Similarly, hawkmoth flowers that opened before dark attracted diurnal and crepuscular visitors (bees, wasps, hummingbirds, butterflies) that robbed nectar and pollen, possibly decreasing flower attractiveness to hawkmoths. HAWKMOTHS (SPHINGIDAE) ARE RECOGNIZED as a major pollinator type in the tropical dry forest of Costa Rica (Frankie 1975, Haber & Frankie 1982, Frankie et al. 1983, Opler 1983), along with other animals such as large bees, butterflies, hummingbirds, and bats (Heithaus et al. 1975; Heithaus 1979a, b, c; Frankie & Haber 1983). The marked seasonality of this forest has an overriding effect on the animals that live there (Janzen 1983, 1984). The forest is largely leafless during part of the year as a result of a 6-mo dry season. Because larval hawkmoths eat leaves, the dry season strongly influences their life histories. A predictable pattern of seasonal flowering also occurs in the dry forest (Janzen 1967, Frankie et al. 1974, Opler et al. 1980), with a consequent fluctuation in availability of resources for pollinators, induding adult hawkmoths. In this paper we describe characteristics and interactions of hawkmoths and the plants they exploit for nectar. In contrast to most previous studies of hawkmoth pollination (e.g., Gregory 1963, Cruden et al. 1976), we focused on hawkmoth-plant relationships at a single site. Here we describe the members of the hawkmoth and flower communities, their seasonal patterns of abundance and floral specificity, and we discuss some key characteristics of the moths and plants as they may relate to pollination interactions. STUDY SITES AND CLIMATE STUDY SITES.-Research was centered at Cafias, Guanacaste Province, Costa Rica (Hacienda La Pacifica, Hacienda Ciruelas, and the MAG station at Taboga). The natural vegetation in the area is classified as tropical dry forest (Holdridge et al. 1971). This is a closed-canopy, broadleaf forest containing about 180 tree species that occur in two principal associations: mostly deciduous species on well-drained slopes and hills, and an evergreen forest along permanent rivers and flood plains. For detailed descriptions of the area see Holdridge et al. (1971), Frankie et al. (1974), Janzen and Liesner (1980), Opler et al. (1980), and Hartshorn (1983). CLIMATE.-Rainfall in the study area is highly seasonal (Fig. 1). The average wet season extends from mid-May to mid-November, interrupted by a 2-wk dry period in July or August (Hagnauer 1980). During dry season (November to May) precipitation is restricted to rare brief showers. Annual rainfall at Caiias ranged from 947 to 2962 mm (x = 1790) for the years 1920 to 1976 (Opler et al. 1976). I Received 14 October 1986, revision accepted 20 December 1987. 2 Corresponding author. 3 Includes Lomas Barbudal Biological Reserve and the two refuges of Palo Verde. BIOTROPICA 21(2): 155-172 1989 155 This content downloaded from 157.55.39.138 on Thu, 19 May 2016 06:27:24 UTC All use subject to http://about.jstor.org/terms