Academic journal article The American Biology Teacher

Flying Dragons: A Colorful Field Experiment in Resource Partitioning

Academic journal article The American Biology Teacher

Flying Dragons: A Colorful Field Experiment in Resource Partitioning

Article excerpt

[ILLUSTRATION OMITTED]

Stroll near a pond on a warm sunny day between late spring and early autumn and you will undoubtedly see the shimmering wings of darting, dancing dragonflies. On closer inspection you will probably see several species of dragonflies dashing across the pond and along the perimeter; and in the reedy vegetation at the margins you will spy delicate damselflies floating from one stalk to another. You may first be struck by the beauty of these colorful animals and the apparent chaos of their acrobatic flights. But a small amount of time and careful study will reveal interesting patterns relevant to important ecological principles such as niche partitioning and territoriality For example, many species of dragonflies perch at different heights (Worthen & Patrick, 2004; Worthen & Jones, 2006, 2007). Here, I describe a field-based laboratory activity that I developed from this research. I have used this activity in ecology courses and introductory biology courses for majors and nonmajors at the university level, but I think that aspects of the activity could be used throughout K-12. Students observe perch-height selection by different dragonfly species. They test for perch-height preferences with simple chi-square tests. Mounted "decoys" can be added to the design to examine interspecific and intraspecific aggression and territorial behavior. Finally, if dragonflies are netted and photographed, differences in size and wing morphology can be correlated with differences in perch-height preferences. It is a simple system that can address interesting and important questions in ecology and the biomechanics of flight in a short time, using beautiful and charismatic organisms that students enjoy watching and handling.

Male dragonflies perch along the margins of ponds and lakes. Almost any water body--even catchment basins or borrow pits along roadways--will attract a variety of dragonfly species. Dragonflies perch to thermoregulate (May, 1978) and to scan their territory for intruders, prey and mates (Moore, 1952; Ottolenghi, 1987; Gorb, 1995). Several species also "display" from their perches, raising their abdomens high above their heads to ward off conspecific males and attract mates. Acquiring and maintaining a territory attractive to females for egg laying is critical to male reproductive success (Parr, 1983; Switzer, 2002), so males often battle for territories in spectacular dogfights, and for perches that provide the best access to these territories. If high-quality perches are a limiting resource, ecological theory suggests that competing species might partition this resource, with different species preferring different types of perches to decrease the energetic and physical costs of interspecific competition. If taller perches provide a better view of the surrounding area, and if competitive ability correlates with size, then progressively smaller species would be relegated to progressively shorter perches in a competitive hierarchy. There have long been anecdotal observations suggesting that dragonfly species may partition this resource by perching at different heights, with larger species perching higher than smaller species (Warren, 1964).

However, several other factors could contribute to the relationship between body size and perch height. Because wind speed increases with height above the ground, large species may prefer tall perches to gain lift. Or, because they must fly faster to remain aloft, large species may avoid dangerous landings on short perches that are precariously close to the water. Conversely, small species may gain some protection from larger predators (larger dragonflies and birds) by flying and perching close to the water's surface. Or dragonfly size may be correlated with prey size, and their prey may fly at different heights. Although the predation hypothesis has not been addressed, wind speed does not seem to be a contributing factor (Worthen & Jones, 2007). …

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