Oviposition site preferences and spatial segregation in two species of stream-dwelling waterstriders (Hemiptera: Gerridae).
A major goal of ecology is to understand mechanisms that mediate competition between species (Ricklefs and Schluter, 1993). Resource partitioning has received much study as a mechanism that reduces competition and thus enhances competitive coexistence (Schoener, 1974; Pianka, 1981). Most studies on niche segregation emphasize partitioning of diets, habitat use or temporal patterns of activity (Spence, 1981; Grant, 1986; Streams, 1992).
Habitat partitioning in adult, pond-dwelling waterstriders is well-established (Spence, 1981). Segregation of juvenile pond-dwelling gerrids by microhabitat use has been documented as well (Vepsalainen and Jarvinen, 1974). In contrast, very little has been reported on habitat partitioning in stream-dwelling waterstriders (but see Calabrese, 1977). In part, this may be due to the fact that very few streams in North America and Europe contain more than one species of waterstrider (J. R. Spence, pers. comm.). The exception to this appears to be in the southeastern region of the U.S., where two species of waterstriders, Aquarius remigis and A. conformis (Hemiptera: Gerridae, hence, referred to as gerrids) coexist.
The species. - Gerrids are semiaquatic insects commonly found skating on the surfaces of ponds, streams, rivers and oceans. Adults and juveniles are both scavengers and predatory, feeding on insects that become trapped on the water surface. The range of Aquarius conformis is uncertain, but specimens have been collected from both the NE and SE regions of the United States (Smith, 1988; Andersen, 1990). Aquarius remigis is widely distributed across the United States; consequently, it extensively overlaps the range of A. conformis. Both species are abundant in central Kentucky.
Adult gerrids mate repeatedly throughout the spring and summer breeding season. Aquarius remigis typically begin mating during the 1st wk in March, continuing until the 2nd wk of June. Aquarius conformis begin mating the 1st wk of May and continue into August (K. E. Haskins, pers. observ.). Thus, both species are actively mating for the entire month of May. Following copulation, females oviposit every 1-2 days (Spence et al., 1980; Fairbairn, 1988) on substrates submerged or floating in water. Female A. remigis are known to store sperm (Rubenstein, 1989), and observations of A. conformis (K. E. Haskins, pers. observ.) suggest that they too can store sperm. Juveniles hatch from eggs after 2-3 wk depending on environmental conditions (Spence et al., 1980). Before becoming adults in ca. 2 mo, juveniles go through five instars (Spence and Andersen, 1994). Latitudinal variation in voltinism, the number of generations produced per season, is known to occur in A. remigis. The general trend is for northern populations to be univoltine and southern populations to be bivoltine (Blanckenhorn, 1994). In contrast, A. remigis in central Kentucky tends to be univoltine, overwintering as adults before reproducing the following spring or summer (Tramontin and Sih, 1995). Aquarius conformis appears to be univoltine as well (K. E. Haskins, pers. observ.).
Aquarius remigis is usually found in smaller streams that consist of riffles, runs and pools, whereas A. conformis is usually found in large streams and rivers (Haskins, 1995). For this study, smaller streams are defined as those of stream order 1 or 2, whereas larger streams are of order 3 and higher, according to classification methods described by Strahler (1952). Generally, small streams in central Kentucky have rocky bottoms and banks and do not support emergent vegetation. In contrast, larger bodies of water tend to support emergent vegetation (primarily Justicia sp.), but rocks are mostly inaccessible to gerrids due to the deep water. Streams in which both species are found are usually of medium size (stream order 3) and contain physical characteristics of both large and small streams (i.e., emergent vegetation and shallow, rocky riffles, respectively). In such habitats, each species will typically be found in the area that most closely resembles the habitat where each species is found alone. Despite micro-habitat segregation within these streams, there is still potential for interspecific interaction because adults move frequently among microhabitat types (K. E. Haskins, pers. observ.).
Although Aquarius remigis and A. conformis inhabit the same streams and share a common food source, competition is probably not a driving force in habitat partitioning. However, it is well known that male A. remigis exhibit aggressive behavior intraspecifically (Krupa et al., 1990; Krupa and Sih, 1993). Recent work has determined that male A. remigis are also aggressive interspecifically towards single males and females (Haskins, 1995). For these reasons, A. conformis should avoid interactions with A. remigis, and choosing different oviposition sites is one way in which segregation may be accomplished. Differences between species in oviposition preferences may suggest spatial segregation among adults and juveniles of each species. I tested the hypothesis that the species differ significantly in their choice of oviposition site.
Aquarius remigis was collected from a tributary of Boone Creek in Clark County, approximately 22.5 km E of the University of Kentucky campus in Lexington. Aquarius conformis came from Indian Creek in the Daniel Boone National Forest in Menifee County, 96.5 km E of the University of Kentucky campus. Before the experiment, male and female A. conformis were held together but separately from male and female A. remigis. They were held for 6 days in gray plastic tubs (45 x 34 x 15 cm) that contained 14 liters of deionized water and an airstone to break any surface film. Pieces of Styrofoam in the tubs substituted for the natural rocks and grass as resting places. On day 1 of the experiment groups of three randomly chosen females of one or the other species were placed in experimental arenas. Males were not introduced in the experimental arenas so that females could oviposit without harassment. Each arena held a clump of orchard grass (Dactylis glomerata) taken from the yard of the University of Kentucky's aquatic research facility. The grass filled a space that extended 10-12 cm from one end of the tub. An equal space at the opposite end was filled with rocks. Both rocks and grass were half in and half out of the water so gerrids had resting areas at either end. Nylon screen tops were glued on top of the arenas to prevent escape. The experiment was replicated 12 times for each species.
Temperature was maintained at 23 C (plus or minus 1 C), and the light regime was 13.5 h day: 10.5 h night. These conditions reflect the season in which the experiment was conducted (11-16 May). Gerrids were fed one cricket (Gryllus sp.), approximately 1 cm long, per getrid per day. Every other day uneaten crickets were removed to keep the arenas clean. Gerrids were held under these conditions for 5 days; on day 6 all females were removed and the numbers of eggs oviposited on grass and rocks were counted. Gerrid eggs are approximately 2 mm long and are easy to census visually.
I used a Student t-test to test the hypothesis that Aquarius conformis and A. remigis differ in their oviposition site preferences. Because gerrids lay eggs in batches, not individually, I treated tubs as independent data points. In addition, I used Student t-tests to determine if the proportion of eggs laid on each substrate differed from a random expectation of 0.50. The value 0.50 was chosen as a random expectation based on equal use of the two substrates. Also, there was sufficient space provided on each substrate to support all eggs of three females (K. E. Haskins, pers. observ.), so proportion of available substrate should not be a factor. It should be noted that all other surfaces within the arena were checked for eggs and only three eggs were found on a substrate other than rocks or grass. The calculated proportions were arcsine-square root transformed before being subjected to t-tests. Statistical tests were performed using the S.A.S. (1985) statistical package.
Nearly all (99.9%) of the eggs laid were on either grass or rocks; i.e., only 3 of 3272 eggs were laid on another substrate (airhose). Aquarius remigis and A. conformis differed significantly in their oviposition site preferences (t = 6.71, P = 0.0001). The mean percent of eggs laid on the grass by A. conformis was 85% (n = 1590; SD = [+ or -] 47.12) and 20% (n = 158; so = [+ or -] 10.64) by A. remigis. Aquarius remigis showed a significant preference for rocks (t = 4.14, P = 0.0016). Aquarius conformis showed a significant preference for grass (t = 5.42, P = 0.0002).
Choice of an appropriate oviposition site is crucial because it will influence juvenile survival and thus, population dynamics (Nummelin et al., 1988; Resetarits and Wilbur, 1989; Sih and Maurer, 1992). In order to maximize juvenile survival, the following factors should be assessed when choosing an oviposition site: risk of egg dehydration, risk of parasitism, risk of predation, risk of competition and interference and risks associated with food availability (Haskins, 1995; Kaitala et al., 1989; Nummelin et al., 1984; Spence, 1986a, b). Because first and second instars of stream-dwelling gerrids in central Kentucky tend to move very little from the site where they hatch (J. J. Krupa, pers. comm.) , juveniles have little opportunity to compensate for mistakes made by adults.
Waterstrider eggs must be wet to survive (Kaitala et al., 1989). Therefore, females should choose sites that have a low probability of desiccation, to increase the likelihood that the eggs hatch. The laboratory experiment showed that Aquarius conformis strongly prefers to oviposit on grass, and A. remigis strongly prefers to oviposit on rocks. These oviposition site preferences make adaptive sense in their respective habitats. In larger streams (A. conformis habitat), rocks in the streambed are inaccessible, but emergent or overhanging vegetation is. In contrast, small streams (A. remigis habitat) do not support vegetation because they are more susceptible to desiccation, but they are associated with rocky bottoms. Sites where both species occur do support emergent vegetation, which is available for at least the last half of the mating season of A. remigis. However, I never observed A. remigis ovipositing on vegetation when it was available.
The evolutionary basis of the differences in oviposition preferences between Aquarius conformis and A. remigis remains to be explored. One possibility is that interspecific competition has driven the evolution of divergent oviposition site preferences. Alternatively, it might be that different, ancestral site preferences "pre-adapted" the species for coexistence via niche segregation. Phylogenetic analyses are necessary to shed further light on these evolutionary scenarios (Brooks and McLennan, 1993).
Acknowledgments. - Funding for this work came from NSF grants IBN 92-21697 and BSR 90-20870 to A. Sih and J. J. Krupa, and the NSF/KY/EPSCoR program. I would also like to give sincere thanks to J. J. Krupa for assistance with the care and maintenance of the gerrids and to A. Sih for his many helpful comments on this manuscript.
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K. E. HASKINS, Center for Ecology, Evolution and Behavior, University of Kentucky, Lexington 40506. Submitted 19 June 1996; accepted 20 August 1996.
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|Publication:||The American Midland Naturalist|
|Date:||Apr 1, 1997|
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