Spatial and Temporal Segregation Among Six Species of Coexisting Ophiogomphus (Odonata: Gomphidae) in the Aroostook River, Maine

Northeastern Naturalist, 2004 by Gibbs, K Elizabeth, Bradeen, Billie, Boland, Dan

Abstract - Spatial and temporal segregation of six coexisting species of Ophiogomphus (O. anomalus, O. aspersus, O carolus, O. howei, O. mainensis, and O. rupinsulensis) were studied in a 137.7 km reach of the Aroostook River in northern Maine. Collections of exuviae showed that O. mainensis was most abundant and dominated the assemblage at sites in the upper reaches of the study area and was in low numbers and percent of total Ophiogomphus at sites in the lower reaches of the study area. Ophiogomphus rupinsulensis was most abundant and dominated the assemblage at sites in the lower reaches of the study area but was absent or in low numbers and percent of total Ophiogomphus at sites in the upper reaches. Ophiogomphus anomalus and O. carolus were present at all sites and O. anomalus was generally more abundant and made up a higher percent of the total Ophiogomphus spp. than O. carolus. Ophiogomphus howei and O. aspersus were present at only a few sites in low abundance and percent of the total Ophiogomphus. Several species of Ophiogomphus coexisted as larvae in 0.20 m^sup 2^ areas of the substrate.

There was little evidence of temporal segregation among the six species. All species had short seasonal emergence periods in June. Diel emergence was during the morning and early afternoon. Timing of seasonal and diel emergence either differed little or not at all among the six species. Species that we were able to identify in all larval stages (O. mainensis, O. anomalus, and O. howei) were present in a wide range of instars on all sampling dates (May to October) suggesting overlapping generations and multivoltine life histories. In all six species, some larvae remained in the final instar for almost a year before emerging as adults.

Introduction

Maine has a diverse fauna of Odonata (dragonflies and damselflies) with Brunelle (1999) reporting 155 species in the state; recent collections of seven additional species (Paul-Michael Brunelle, 6044-1 Compton Avenue, Halifax, NS, Canada B3K 1E7, pers. comm.) bring this total to 162 species. Most larvae of Odonata are aquatic, inhabiting standing or running fresh water (Corbet 1999). The winged adults are terrestrial. Typically, reproductively mature adults aggregate near water where copulation and oviposition take place. Adults may spend extensive periods away from the water foraging for food both before and during the reproductive period. The larva that hatches from the egg usually undergoes 9 to 15 molts before reaching the final larval instar. During the final instar, the tissues within the larval cuticle change to those of the adult, a process known as metamorphosis. When metamorphosis is complete, and circumstances are favorable, the larva leaves the water and undergoes its final molt to a winged adult. This process is called emergence and, when the adult flies away, the caste larval skin, or exuviae, remains for a while on the surface from which emergence has occurred.

The study of assemblages of functionally similar or closely related aquatic insects allows us to examine the role of spatial, dietary, and temporal separation in reducing competition for food and space. Life history characteristics such as voltinism (number of generations completed within a year), phenology (seasonal timing), and degree of synchrony (simultaneous occurrence) of life history events can be important in determining population interactions among coexisting species (Wissinger 1988). Competitive interactions among coexisting species of odonate larvae may be reduced by habitat partitioning and seasonal segregation of life histories (Johnson and Crowley 1980). Interactions within and among species of odonates are complicated as odonate larvae may be major predators of other odonate larvae, including those of the same species. If such predation occurs, it is likely to have an important effect on the species composition and relative abundance of species in odonate assemblages (see review by Corbel 1999).

Seven species of the dragonfly genus Ophiogomphus occur in Maine (Brunelle 1999). Larvae of this genus are reported to occur in rivers and streams where they burrow into the substrate (Merrit and Cummins 1984, Needham et al. 2000). Ophiogomphus howei Bromley is listed as "threatened" in Maine and O. anomalus as of "special concern" (Brunelle 1999). The presence of six species in this genus (O. anomalus Harvey, O. aspersus Morse, O. carolus Needham, O. howei, O. mainensis Packard, and O. rupinsulensis [Walsh]) in the Aroostook River allowed us to compare the distribution and life histories of these species in a large river. We wished to determine if spatial and/or temporal separation exists among these species. Both emerging adults and larvae were sampled at sites on the Aroostook River along an upstream-downstream gradient to determine if species coexisted or occupied separate habitats. Seasonal and diel emergence of adults, and patterns of growth and development of larvae were used to determine the degree of temporal overlap or separation of these life history parameters.