Rapids clubtail (Gomphus quadricolor) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Search effort 

In 2005, surveys of the historical Gomphus quadricolor sites on the Credit, Humber, and Thames rivers were conducted from June 9 to 14, as well as at three other potential sites on the Grand, Sydenham, and Ausable rivers, all of which appeared optimal on the respective rivers (Figure 4 in Harris and Foster 2006). Survey conditions were ideal throughout the survey period, and the ease of survey of adult Gomphus quadricolor at the Humber site suggests that extant populations do not exist on the other rivers. A survey of the Humber site on July 2, 1995 failed to find any Gomphus quadricolor but was hampered by cool, windy weather (Don Sutherland pers. comm. 2007). Catling and Brownell (2002) discovered Gomphus quadricolor at two sites on the Mississippi River in 2001 and surveyed three other sets of rapids on the same day without locating the species.

The Ontario Odonata Atlas (Ontario Odonata Atlas 2005) is an extensive database of Ontario odonate observations from published sources, institutional collections, and reports from amateur naturalists and professional entomologists. Of the over 12,000 Atlas records from June for Ontario south of the Canadian Shield including all of the Mixedwood Zone, no additional Gomphus quadricolor sites are recorded. Furthermore over 41,000 records of dragonflies in Ontario over the past 6 year period included this species only 8 times from two locations.

Although Gomphus quadricolor is believed to be extirpated on the lower Credit River, potential habitat may exist upstream at the Forks of the Credit. Stylurus scudderi, another gomphid associated with relatively unpolluted, swift rivers, has been recently observed there (Don Sutherland, pers. comm. 2007), but G. quadricolor has not been seen. Other unsurveyed Ontario rivers where Gomphus quadricolor could potentially occur include the Sydenham and Maitland rivers in southwestern Ontario (P. Pratt, pers. comm. 2007) and the Salmon River in eastern Ontario (P. Catling, pers. comm. 2007). These rivers have bouldery riffles and support Ophiogomphus rupinsulensis, suggesting that water quality is acceptable (Ontario Odonata Atlas 2005). Other Ontario rivers which may have suitable habitat include the Eramosa, Grand, Nith, Speed, Bayfield, North Saugeen, Rocky Saugeen, Beaver, Bighead, Boyne, Indian, Mad, Nottawasaga, Black, Crowe, Ganaraska, Gananoque, Moira, Napanee, Skootamatta, Tay, Trent, Madawaska, Fall, Indian Creek and Clyde (S. Thompson, D. Sutherland and A. Dextrase, pers. comm. 2007). However, it is important to remember that at least half of these rivers have been visited in June by experienced Odonatists and while other river–dwelling gomphids have been found, the obvious G. quadricolor has not.

Following its discovery at two locations on the Mississippi River near Ottawa in 2001, it was searched for elsewhere along the Mississippi, in the Rideau River and in rivers draining into the Ottawa River from Quebec (P. Catling, pers. comm.2008) and was not found. Other eastern Ontario rivers are well surveyed (Petawawa) and some that are well surveyed are also seriously polluted (South Nation and its tributaries). The Ottawa River, and rivers in Quebec draining into it, have been particularly well surveyed (Ménard 1996). Other flora and fauna extend north to the Ontario portion of the Ottawa valley but do not extend into Quebec (P.Catling, pers. comm.2008). Consequently there is little reason to expect that it is much more widespread in eastern Ontario.

Based on the fact that G. quadricolor is an obvious species, easy to observe where it occurs, and considering the extent of both directed and undirected search effort, and the fact that it is evidently confined to the Carolinian zone and its subunits, we conclude that if it does occur elsewhere in Ontario, it will continue to have a very small area of occupancy. Additionally since it has a ranking of S1 or S2 over most of its range to the south, suggesting habitat specialization, there seems little reason to expect it to be present at very many more sites in Ontario, if any.

Abundance

Estimating total population sizes for odonates is difficult (Corbett 1999). A rough estimate of the minimum number of individuals in Canada is provided in Table 3. Although only adult males were observed during population estimates at the extant sites, equal numbers of adult males and females were assumed to be present and at least two larvae for every adult (based on the assumption of a three year life cycle). The number of larvae is thus a minimum estimate. This gives a minimum Canadian population estimate of 318 individuals including 106 adults. The number of larvae is probably grossly underestimated. Although this population estimate is of interest, the level of uncertainty is such that it cannot be used in assessment.

Global abundance is estimated at 2500 - 10,000 individuals with an average population of more than 100 individuals in all life stages at each occurrence (NatureServe 2007). This estimate is based on a survey of biologists throughout the species range. According to NatureServe (2007), no range-wide changes in abundance, area occupied, or number of occurrences have been noted and the species is considered to be secure globally; however, declining populations have been noted in some U.S. states (in the east and south).

Fluctuations and trends

There are no data on fluctuations or trends of Gomphus quadricolor populations. The species is believed to be extirpated at two of its four known occurrences in Canada. Few concerted surveys for this species have been conducted and adult surveys are highly dependent on weather conditions and phenological development. Globally, the population is considered to be stable (unchanged or within +/- 10% fluctuation in population, range, area occupied, and/or number or condition of occurrences) (NatureServe2007).

Rescue effect

Given the high potential dispersal capablility of adult Gomphus quadricolor (estimated 3 km per day along the waterway), localized extirpations in some areas might be recolonized within a few years. However, Gomphus quadricolor appears not to move far from streams, suggesting that long-distance dispersal may be very infrequent. Extirpations at the periphery of the range, including all Canadian sites, would presumably take a very long time to be recolonized, or never happen. Genetic exchange between Canadian and U.S. populations is probably very infrequent, given the minimum distances of 300 km or more between them.

The largest number of populations of Gomphus quadricolor probably occurs in Minnesota and northern Wisconsin. The likelihood of natural invasion from U.S. populations is perhaps the greatest in northwestern Ontario near Thunder Bay where populations occur within about 200 km of the Canadian border. This area has more or less continuous forest cover and relatively unimpaired stream habitat, but has not been thoroughly surveyed for dragonflies.