Ecological Methods and Research in Reptile Ecology-1999 Pit Trap Experiment

A preliminary investigation of the use of pittrapping in the Alvord Basin

Christopher J. Fabry, Giovanna Pizzin, Eric Scansen, and Laura Dingmon

Summer Session 1999 Field Biology Course, BIOL 417a,
Department of Biology, Western Washington University
Bellingham, WA 98225

Introduction

Methods

Results

Conclusions

Table

Introduction

A rough census of arthropods was conducted for a dune environment in the Alvord Desert Basin of the Pueblo Valley, Oregon.
Arthropod biomass and diversity were determined for three variables:
- Dune size
- Time of day
- Plant type
The census technique consisted of pitfall trapping with propylene glycol as an effective killing and preserving agent.
A set of three hypotheses were predicted prior to observational research:
1. Larger-sized dunes will have greater arthropod diversity in terms of number of Operational Taxonomic Units (OTU's)-i.e. orders, families, genera, species, etc. and will have greater arthropod biomass in terms of raw numbers of individuals per trap. We surmise this based simply because larger-sized dunes have greater surface area, thus allowing for larger and more varied arthropod trophic niches.
2. Traps open only during the daytime hours will have greater diversity and greater biomass due to more favorable environmental conditions during the night.
3. Traps placed beneath sage plants (Artemisia tridentata) will have greater diversity and greater biomass than those placed beneath greasewood plants (Sarcobatus vermiculatus) because 1) sage is the dominant plant species in this desert ecosystem, and 2) more species of arthropods will have evolved to specialize on or at least utilize this prevalent resource.

Methods

Pitfall trapping was conducted at a study site in the Pueblo Basin of the Alvord Desert in Southeast Oregon, from the 5th through the 12th of July, 1999.

Four sites were chosen, three pairs of dunes varying in size and height: small, medium, large, and one flat, non-dune. Both the medium and large dunes were 50 x 20 meters, and we set up ten 10 x 10 m quadrats on each dune. Small dunes were 45 x 15 m and we set up three 15 x 15 m plots on each small dune. The flats were three 50 x 50 m squares, and we set up four 25m x 25m plots. The three flat plots were separated by at least 50m of flat mesohabitat.

Traps were placed along four straight trap lines. The lines were evenly spaced along the width of every dune, on average one trap for every 25 m² on dunes (40 medium and large, 27 small). The flats had fifty total in one 50 x 50 plot. Each trapline started in the southwest corner of a plot and proceeded north until the end of the transect was reached, where it then doubled back approximately 5m to the east of the last line, and repeated as necessary.

Traps were further placed in four different locations, with sagebrush (Artemisia tridentata), black greasewood (Sarcobatus vermiculatus), a variable species, or an open spot (away from vegetation), all hosting traps. One of each category was represented in every 10 x 10 m² on medium and large dunes, with an evenly divided number of each category present in each third of the small dunes. The flats were essentially a sagebrush monoculture and traps placed there were under the sage or in the open, with only a few greasewood hosting the traps.

The traps were constructed out of 10ž ounce soup cans dug in level with the ground. Seven ounce wax covered drinking cups were placed inside and filled with one inch of propylene glycol which acted as both a killing agent and a preservative.

To test both the diversity and biomass of nocturnal vs. diurnal arthropod fauna in the dune regions, half of each dune's traps were randomly designated as day traps and the other half as night. To ensure that neither type of trap was working during their "off" hours, pert dishes and yogurt cups were used to cover specifically designated traps at the appropriate time. Traps for day capture were opened at 8 am and closed at 8 p.m. and vice versa for the night traps. Traps in the flats were left open for continuously just to get an idea of a general arthropod census.

Trap contents were collected on the night of the 12th, and brought back for analysis.

Data analysis consisted of categorizing all arthropods into their respective orders by hand at the end of the field study. Individuals and number of species were counted, and Operational taxonomic units (OTU's) were calculated by dividing the number of species captured by the order in which they belong. Data was entered into an Excel spreadsheet and then processed in Systat.

Results

The comparison between small, medium, and large dune diversity and biomass showed significant differences. With an acceptable p-value of less than 10%, biomass (p=0.097) was significantly greater for the larger dunes (particularly LD 2). Diversity (p=0.089), however, was greater for the medium dunes (particularly MD 2). (Figures 1 &2; Table 1)
Traps open at differing times of day failed to provide any significant difference for either biomass or diversity. Each p-value exceeded the 10% threshhold. (Figures 3 & 4; Table 1)
The placement of traps beneath either a sage or greasewood plant failed to provide any significant difference for either biomass or diversity. Each p-value exceeded the 10% threshhold, although the p-value for diversity was close (p=0.117). (Figures 5 & 6; Table 1)
Hymenopterans comprised the majority of arthropod fauna on the medium and small dunes in terms of overall percentage and sheer numbers. Dipterans were the principal insect on the large dunes. (Tables 2 & 3)
Between day and night traps, only five separate orders showed a trend towards significant difference. Orthoptera, Lepidoptera, and Coleoptera seemed to have more individuals present during the day. Araneae and Hymenoptera were collected more during the night. (Tables 2 & 3)

Conclusions

Of the six hypotheses prior to our research, we were proved incorrect in all but one-greater biomass in the large dune plots. Possible explanations and / or probable errors that contributed to these findings:
1. Because pitfall traps were the exclusive method of sampling, there was a bias towards terrestrial arthropods. While an occasional flying insect was found in the pitfall traps, a large diversity and number were absent. This possibility may explain no significant difference between day and night traps; there were considerably more flying insects during the day than at night.
2. The paper cups lining the tin cans were unable to effectively contain the propylene glycol, which collected in the bottom. Therefore, arthropods may have crawled in and out of the pitfall traps without suffering the debilitating effects of antifreeze.
3. Pitfall traps were not correlated with plant size. This may have allowed small plants of each species to have an unequal impact in terms of arthropod biomass. A larger plant would require additional pitfall traps to keep constant an area ratio of plant to trap.
4. Occasionally, traps were exposed at incorrect times or filled inadvertently. This may have been the result of excessive wind, sliding sand from above, human error in closing / opening traps, or influences by other organisms.
5. The number of day and night traps were unequal. Day traps numbered 218, whereas night traps numbered 241. The disparity may have skewed our end data; on average there was one more individual found per day trap for every night trap.
6. Traps were opened / closed on a 12D:12N schedule. This failed to account for the actual daylight vs. evening hours found during the middle of summer in this part of the world. As a result, the traps may have become contaminated slightly at the edges of the day / night.

Variable	Error Mean-Square	F-ratio	P value
Dune Biomass	292.949	2.092	0.097 *
Dune Diversity	64.252	2.157	0.089 *
Day/Night Biomass	346.077	0.709	0.406
Day/Night Diversity	77.917	0.160	0.692
Sage / Greasewood Biomass	72.883	0.072	0.790
Sage / Greasewood Diversity	0.567	2.485	0.117

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