Elizabeth J. Bankhead and Dr. C. Riley Nelson, Biology
A hot topic in ecology right now is the Mid-Domain Effect (MDE) as presented by Robert K. Colwell (2001). This null model states that the greatest species diversity along an elevation gradient will be found at the incline’s mid-point, if no other environmental gradients interfere. Another hypothesis involving elevation gradients is an abundance null-model created by John Terborgh (1971). The research set forth in this paper seeks to understand abundance and richness along an elevation gradient in a riparian system and help validate MDE.
Sampling on Mount Timpanogos along the American Fork River in Utah was done bi-monthly in 2005. Malaise traps were set up from March to November at 6 sites spanning a climb of 1000 meters. During part of the summer the river at site 5 and 6 is intermittent. The aquatic orders Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies) (EPT) were taken from the samples. They were identified and analyzed by Sarah W. Judson in her honor’s thesis (2008). Using the samples from the year 2005, I sorted out the fly family Empididae (dance flies). I then identified each specimen, which was over 11,500, to genus using a microscope and a dichotomous key (McAlpine, 1981). I then separated each genus into morphospecies. I used Estimate S 8.0 to quantitatively measure richness and abundance. The data was used to find altitudinal distribution, seasonality, and Mid-Domain Effect. These analyses for the empidid flies were then compared to the same analyses for EPT.
Richness found in the family Empididae was 62 species. In the combined three orders of Ephemeroptera, Plecoptera, and Trichoptera 71 species were found. By using Estimate s 8.0, an expected number of species was able to be predicted using different indices. A graph displaying the altitudinal distribution of this diversity (graph 2) shows that EPT’s greatest richness is at site 3 (1862 msl) and the empidid’s peak at site 4 (2264 msl). A seasonal abundance curve showed that Empididae generally emerged late in the summer and carried over to the autumn. Trichoptera followed this same trend which is opposite to the early summer emergence of Plecoptera. Ephemeroptera numbers were too low to compare due ineffective trapping methods. The greatest abundance in dance flies is at site 5 (2291 msl), whereas for mayflies, stoneflies and caddisflies is at site 4 (2264 msl).
As mentioned above, the diversity found in the Empidid family was remarkable with 62 species, especially when compared to Rhyacophila (richest EPT family) which had 9. According to the Choa 2 index, around 70 species is predicted. Due to a lack of keys that go to species in empidid literature, morphospecies were necessary to create. Some of these morphospecies are suspected to be unidentified species. Two collections of empidids from the sub family Hybotinae cannot be identified to genus and are possibly two new genera. This is an area open for taxonomic research, expecially in Rhamphomyia, a very convoluted and unorganized genus.
In Terborgh’s paper of Peruvian birds he sets forth a null-model for abundance along an elevation gradient. The model focused on habitat discontinuities called ecotones. He hypothesized that abundance would drop drastically at an ecotone only to bounce back up. Along the American Fork River we have an ecotone at site 4 (Timpooneke). From just below Timpooneke down to the Canyon’s Mouth (site 1), the river has a canopy of vegetation, whereas above Timpooneke, there is a meadow. As can be seen in graph 1, the abundance of dance flies drops at site 4. This data parallels Terborgh’s null-model. In EPT’s abundance graph though, the ecotone at Timpooneke has no dramatic drop off and in fact has the greatest abundance. At site 5 there is a drop and then a small increase at the last site. The curve follows Terborgh’s model, but the placement does not. This is an interesting phenomenon that could be answered by niche variation, overlap, or lack of competition. This is an area that can receive more attention in research and analysis.
The Mid-Domain Effect predicts that the greatest diversity will be found at the mid-point elevation. The greatest richness in Empididae as well as EPT was found at mid-elevation ranges, but at different places. The same gradient was used for all our collecting, yet the peak of dance flies was higher on the mountain than the aquatic orders’ peak. The reason for this discrepancy can be a number of possibilities. One cause is due to the life cycles of the insects and their correlation to the intermittent part of the river from sites 5 to 6. Because EPT life-cycles are dependant on water, for part of the year they are bound to only the lower portion of the river. Empididae, on the other hand, have aquatic stages in some species, semi-aquatic in others and completely terrestrial life cycles in other species. Because of the versatile life-cycles in the dance flies, within one family many habitats are able to be occupied. Essentially these two groups of insects have different gradients which accounts for a difference in their richness peaks.
Understanding the American Fork River and its many variations provides us with information needed for monitoring and therefore preservation of the riparian environment. The research gathered on Empididae and Ephemeroptera, Plecoptera and Trichoptera has given us species lists, seasonality reports, and locality. This research has also helped validate Terborgh’s null-models as well as Colwell’s pressing topic, the Mid-Domain Effect.
References
- Terborgh, John. 1971. Distribution on Environmental Gradients: Theory and a Preliminary Interpretaion of distributional Patterns in the Avifauna of the Ocrdillera vilcabama, Peru. Ecology. Vol. 52:01. pp 23-40
- Colwell, Robert K; C. Rahbek; and N.J. Gotelli. 2005. The Mid-Domain Effect: There’s a Baby in the Bathwater. The American Naturalist. 166:05. pp 149-54.
- Walker, Sarah. 2008. Altitudinal and Seasonal Survey of Ephemeroptera, Plecoptera, and Trichoptera in American Fork Canyon, Utah. Honors Thesis Brigham Young University.
- McAlpine, J.F., B.V. Peterson, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, and D.M. Wood (eds.). 1981. Manual of Nearctic Diptera, Vol. 1. Research Branch, Agriculture Canada, Monographs 27. pp479-602.