Sarah Walker and Dr. C. Riley Nelson, Biology

The stonefly Pictetiella expansa has been acknowledged as a species since the 1920s, yet little is known about this rare species of Perlodidae stonefly. The purpose of our study was to document the growth rates and feeding behaviors of Pictetiella expansa in the American Fork River as well as to document its current geographical distribution.

Over the summer of 2007, extensive field work was performed for a life history study of P. expansa. Field studies began in May 2007 after young P. expansa nymphs were located at a previously unstudied area. Samples where thenceforth taken every other week as follows. A crew of two or three students, including myself, went to both the target areas, the planned South Fork site of the American Fork River, as well as the Granite Flats Campground area above the Tibble Fork Reservoir of the same river. At each site, water and air temperature readings were taken or recorded from continuously running probes. To test for substrate electivity, surber samples were taken three to five times across the stream width. As the surber samples were taken water depth, approximate flow, and substrate ratios of each microhabitat were taken. Kick samples were also taken as surber samples were not always effective at collecting abundant numbers of P. expansa, thus documenting the presence of the nymph in both a quantitative (surber) and qualitative (kick) manner. The highest abundances of nymphs were on medium to large rock substrates, habitats conducive to the typical predator behaviors of perlodids.

The nymphs from the bi-monthly surber and kick sampling at Granite Flats and South Fork are currently undergoing lab analysis. The femur, interocular, thorax, and other morphological features are being measured to the nearest hundredth of a millimeter to calculate growth rates of each of the instars. This information will yield an allometric equation for predicting body size, document the number and length of instars by cohort splitting, and demonstrate when sexual dimorphism in size becomes apparent during the life cycle. From on-site observations it is clear that the species is univoltine (developing and emerging within a one-year period), as all individuals collected were of approximately the same time (if there were multiple generations, very small and much larger individuals would coexist). Abundance numbers are tallied as these size measurements are taken to examine the population fluctuations throughout the season. Gut content analysis will also be performed to document the diet of P. expansa, who is presumed to be a predator like other perlodid stoneflies (Stewart and Stark 1988), but may also feed on diatoms and algae during earlier life stages. Other species representative of the aquatic community were also collected with P. expansa and will be examined to confirm species available for predation as well as community members that may have influenced P. expansa behavior. It is anticipated that the presence of Megarcys signata, a similarly sized perlodid with a summer time emergence will impact P. expansa diet and possibly depress growth rates.

To document adult presence and length of emergence, pitfall trapping was employed heavily when nymphs were sufficiently large to document length of emergence and relative abundances of emerging adults. At each site, 10 pitfall traps were placed flush with the ground no more than 5 feet from the stream banks to allow emergent adults to walk into the traps. After adults began to appear in these traps, we also carefully scanned the stream banks and likely emergence areas of moist rocks and shaded tree trunks for newly emerged adults. Adult emergence extended from late August to early September, approximately two weeks in length which is relatively short for Perlodidae stoneflies.

When P. expansa nymphs were approaching emergent size, series of ten nymphs were taken to our lab for rearing. The nymphs were placed in three treatments to elucidate rearing, but various problems prevented the ideal emergence habitat. However, one adult emerged under each condition, none of which were considered ideal which demonstrates the difficulty of rearing P. expansa due to its sensitivities but also the surprising ability of a few nymphs to acclimate to adverse conditions. Length of survival and sizes of all nymphs from rearing experiments were recorded and may become important in future analysis.

Important species location information was recorded on P. expansa in a variety of areas within the western United States. Because P. expansa is a relatively isolated species, it is susceptible to local extinction especially when metapopulations are small. Unfortunately, this was observed at the type locality of P. expansa in Grant, Colorado where it is apparent that the influences of mine trailings and other impoundments from upstream establishments has caused a depression in the community richness and appears to have wiped out the P. expansa population. Examination of an isolated Idaho population above Soda Springs at Timber Creek reconfirmed P. expansa presence in that area. Nymphs collected from Idaho (Caribou County) and Salt Lake County areas demonstrate accelerated growth rate at more northern latitudes. Quite unexpectedly, P. expansa was documented near the Timpooneke campground 5 miles upstream (700 meters elevation increase) from the main South Fork study site, a location where P. expansa has never been documented as a nymph or adult. Only one nymph was found at this location despite multiple samplings. This information from my field work expands previous information available from studies of western stoneflies as it extends the elevational range of P. expansa from above 3000 meters (Baumann et al. 1977) to as low as 1000 meters, with quite robust populations at 1500 meters. Because exact distribution and abundances are not known for this species, it is unclear if this indicates downstream movement over historical time or if it is merely a result of undersampling in previous years.

The existence of P. expansa as small isolated populations rather than widespread distributions raises questions answered by metapopulation hypotheses. Four prominent models have been formulated to explain this isolation; all of which have evolutionary history premises and which have yet to be investigated for validity. Long term studies should be done to examine the speciation of the few isolated populations of P. expansa, and could likely be accomplished by examining genetic diversity of the subpopulations as well as the validity of proposed metapopulation theories by such tests of important functional traits present in modern populations that would promote colonization and of competition abilities in both controlled and natural environments to infer competitive abilities.

References

• Baumann, R.W., A.R. Gaufin, and R.F. Surdick. 1977. The Stoneflies (Plecoptera) of the Rocky Mountains. The American Entomological Society, Philadelphia. 208 pp.
• Stewart, K. W. and B. P. Stark, 1988. Nymphs of North American stonefly genera (Plecoptera). Thomas Say Found. Publ.