Rapids clubtail (Gomphus quadricolor) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
Habitat degradation is the most significant threat to Gomphus quadricolor, although accidental deaths through vehicle collisions and inbreeding are also potential limiting factors. Impoundment of running waters by dams, stream channelization leading to scour of microhabitats, pollution, and introduction of exotic species threatens the species in much of its range (NatureServe 2007). Threats appear minor over much of the species' range in the northern U.S., but habitat threat is probably significant in the south where loss of forest cover and pollution are more extensive (NatureServe 2007). In Canada, three out of four sites are in the heavily developed part of southern Ontario where continued urbanization threatens water quality in the riverine habitats and natural terrestrial vegetation is declining.
As described in Habitat Trends, all four Canadian rivers where the species has been recorded have numerous dams and other water control structures and are actively regulated for flood control. This results in changes to natural patterns of sediment accumulation,reduces new sediment accumulation and can alter water temperature regimes. Rapid drawdown of reservoirs for power generation can result in scouring of downstream habitats. The effects of water regulation on G. quadricolor larvae are unclear.
Water quality in most southern Ontario streams has been degraded (see Habitat Trends). High chloride, phosphorus, and nitrate levels and possibly pesticides may threaten Gomphus quadricolor larvae.
Invasive aquatic species are present in all four rivers. The upper Humber River supports chinook salmon, rainbow trout, brown trout, goldfish, and common carp (OMNR and TRCA 2005). Six other exotic species occur below a downstream dam, which acts as a dispersal barrier for most fish species. Eleven invasive fish species occur in the Credit River, including those listed above (Credit Valley Conservation 2002). Common carp are found throughout the Thames River and round goby and zebra mussels inhabit the river at and below Fanshawe Lake (downstream of the Gomphus quadricolor site) (A. Dextrase pers. comm. 2007). Round Goby is moving upstream in the Thames system. Zebra mussels and spiny waterflea occur in the Mississippi River watershed, but their status in the river itself is unknown. The impacts of these species, if any, on Gomphus quadricolor are unknown, but could include predation, competition, increased turbidity (common carp), and changes in the stream community structure. Other invasives are a continued threat especially to the Humber and Credit sites that are close to Lake Ontario. Rusty crayfish are not known to occur in any of the four Ontario rivers, but are spreading in Ontario and may pose a threat to Gomphus quadricolor by consuming aquatic vegetation, thereby destabilizing sediments.
Loss of riparian forest due to agriculture and residential development could threaten adult Gomphus quadricolor by exposing them to increased predation by birds and other dragonfly species. Females, which spend most of their three to four week life span in forest cover adjacent to the river, may be particularly vulnerable.
Collisions with cars could be source of adultmortality where road crossings fragment the stream habitat, as occurs in a population of Hines Emerald (Somatochlora hineana) (U.S. Fish and Wildlife Service 2001) but its significance has not been quantified in this or other species. Road kill could be a significant factor with Gomphus quadricolor when tenerals disperse to forest cover if forest cover is lacking adjacent to the stream. Roads with traffic speeds greater than 50 km / hour probably pose the greatest risk, although large highways with wide cleared areas tend to kill fewer odonates (P. Brunelle pers. comm. 2007). At the Humber River, there are five secondary highway crossings of the river with speed limits greater than 50 km / hour within 10 km of the Gomphus quadricolor population. About 10 other roads are near the river (within 100 m). Slower traffic crosses the bridge at the Mississippi River site. Currently the potential impact of vehicle-related mortality is unclear.
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