Ribble; 8:30 TR
April 27, 1999.
To-further an on going study of Government Canyon State Natural Area located North-West of Helotis, Texas, both Dr. David Ribble’s Ecology class and Dr. Tom Gardener’s Geology class of Trinity University camped out on the property to collect data the weekend of March 19-21, 1999. The focus of the study was to find out what differences existed between the north-facing and the south-facing slopes of the canyon. The geologists collected data on the slope of the canyon at various points along the creek bottom, measured the slope of the creek bottom, and measured soil moisture content at several sites on both the north-facing and the south-facing slopes. They also used a satellite linked total station to find the exact location of a particular pole in the canyon bottom to within a few centimeters. The ecologists trapped small mammals to get a population estimate as well as to find the species, sex, age, and breeding status for the different areas. A bird count was conducted to find the species living in the canyon and to discover any differences between the two sides. There was also a vegetation survey of trees, shrubs, and ground cover as well as a photo-radiation survey to show how much light was present at the various sites. The ecologists and geologists compared data to try to explain what factors might account for the observed differences in the data collected, especially the slopes on the North versus South slopes.
In this paper, the focus will be on the ecological data. The methods of collecting the geological data will be left up to the geologists to explain. The small mammals were caught in Sherman live catch traps, which were set up on transects along the canyon bottom as well as up the north- and south- facing sides. The transects on each side of the canyon were set on an East – West (90° on a compass) line, nearly parallel to the canyon bottom. There were twenty stations at each transect, twenty meters apart, and two traps set with rolled oats at each station. There were a total of eight transects: two on the canyon bottom and three on each side of the canyon. The transects going up each side of the canyon were about 75 meters apart. The traps were set on Friday and checked Saturday morning. The traps were reset on Saturday afternoon to be checked on Sunday morning. After the data was compiled, a population estimate was calculated using the Lincoln-Peterson method of mark-recapture population estimates.
Figure 1. A map of the Government Canyon study area showing the small mammal transects as well as the bird point count stations.
The bird count was conducted each morning at about 0700 hours. A student and a professional bird watcher went to each of the seventeen bird point count stations. Three stations are in the canyon bottom (15–17), six are on the south-facing slope (1-6), and eight are on the north-facing slope (7-14). At each of the stations the number of species heard, observed, or flying-by were recorded for five minutes. They were recorded on a map that was aligned with a compass. Bird species heard within 50 meters were recorded separately from birds further than 50 meters. The birds heard during the first three minutes were distinguished from birds heard during the last two minutes.
The data collection sheet for noting the species at the bird point count stations.
The vegetation survey was conducted at each of the 17 bird point count stations. At these bird point count stations, four classes of information were recorded. First, at each of the 17 stations, a computer printed a random set of orientation angles for quadrat samples. Using these randomized angles, a compass and a tape measure, a 10m X 10m square, with sides aligned with the randomized angles, was set out. Orange flags were used to demarcate the corners and the 5 meter mark. A flag was placed in the center of the larger square, which made essentially four 5m X 5m quadrats. In the larger 10m X 10m quadrat, all the trees were counted by species. A tree is defined as a living, woody plant that has a diameter greater than 2cm at a height of 30cm off the ground. The indicator species at each site was found by calculating the individual value (IndVal). The Simpson’s diversity index and equitability were also calculated for the canyon bottom, north-facing and south-facing slopes.
In the small 5m X 5m quadrat that touched the rebar stake demarcating the bird point count station, the shrubs were counted by species. A shrub is defined as a woody plant that is at least 30cm high, but its diameter is less than 2cm at 30cm off the ground. The diversity, equitability, and indicator species were again calculated for the shrubs of the canyon bottom, north-facing and south-facing slopes.
As well as counting the numbers of trees and shrubs at each location, the amount of canopy cover, ground cover, and sunlight reaching the ground were recorded. Unlike the plant counts, which were taken in randomly assigned quadrats, the ground cover canopy and solar radiation data were collected along the North, South, East, and West directions radiating from each bird count station. The method to collect the data was called the Step-Point Intercept Method. The Step-Point Intercept Method was done by starting at the bird station stake and walking out in one of the four directions: N, S, E, or W. Every 2 meters, for 10 meters, a wooden dowel was put to the ground, and what was at that exact point was recorded as litter, herb, soil/rock, or woody. Directly above that point, the type of canopy was recorded as either open with nothing directly over the point, shrub with some sort of cover less than 30cm off the ground, or canopy with plant material directly over the point that was above 30cm. The sunlight reaching the ground was recorded at the same points as were pointed to during the Step-Point Intercept ground cover count. The solar radiation data was recorded at ground level and at chest level, from various times between noon and 2pm when the sun was directly overhead. The data collected was grouped according to north- or south-facing slopes to determine if there is a difference in the amount of radiation reaching the north-facing slope versus the south-facing slope.
The species of small mammal that was trapped was Peromyscus pectoralis. The population estimates for the Peromyscus pectoralis was lowest on the South-facing slope, with approximately 40 individuals and highest on the North-facing slope with about 58 individuals. The canyon bottom was in between with 50 individuals estimated. All of the mice caught in the spring of 1999 were adult mice and none of the adults had started the breeding cycle which means the breeding season was starting late in the year. When the 95% confidence intervals were calculated, the ranges for all three areas over-lapped, so no conclusive data can be drawn from these results alone (see Figure 3 below).
The population estimates for Peromyscus pectoralis and their 95% confidence interval.
After the bird data was collected and the IndVal results were calculated, the indicator species were found. It was found that the Northern Cardinal, Tufted Titmouse, and Ruby-Crowned Kinglet were generalists. The Bewick’s Wren and Ladderbacked Wood Pecker are typical of the south-facing slope. On the north-facing slope, the typical bird species were the Scrub Jay and the Greater Roadrunner. The north-facing slope had a species richness of 15, while the south-facing had only 11 species.
The IndVal results showing which species are characteristic of the three habitats with the IndVal number in parenthesis after the species and the indicator species denoted for 1998 only. There were no Indicator species for 1999.
The tree data showed that while Ashe Juniper, Mountain Laurel, Persimmon, and Cedar Elm were distributed over all habitats, the Evergreen Sumac and Pink Mimosa were mainly found on the south-facing slope and Live Oak and the Shin Oak were common on the north-facing slope. The indicator species for the north-facing slope is Evergreen Sumac. The species richness for the south-facing slope is 9, and for the north-facing slope, there were 11 species.
The IndVal results showing which tree species are characteristic of the three habitats with the IndVal number in parenthesis after the species and the indicator species denoted for 1999.
The analysis of the shrub data shows us that Agarita and Live Oak were generalists and were equally likely to be found in any of the three habitats. The species who had their highest IndVal for the south-facing slope were Evergreen Sumac and Prickly Pear Cactus. For the north-facing slope, the most characteristic species were Ashe Juniper, Persimmon, and Possum Haw. The only indicator species occurred in the canyon bottom.
The IndVal results showing which shrub species are characteristic of the three habitats with the IndVal number in parenthesis after the species and the indicator species denoted for 1999.
The results of the Step-Point Intercept
ground and canopy cover are displayed in Figure 7. It shows that the south-facing slope had less canopy cover, which
meant more open spaces for sunlight to hit the ground. The south-facing slope also had more woody
and herbaceous ground cover, which naturally leads to less exposed soil and
rock. The solar radiation data
collected using the photometers showed that indeed the amount of radiation
hitting the South slope was more consistent.
The standard deviation for the data on the north-facing slope was larger
which is a representation of the increased shady
patches due to increased canopy cover.
The results of the Step-Point Intercept habitat study
An enormous amount of information was collected during the weekend field trip by both the ecologists and geologists. According to the measurements taken by the geologists, the south-facing slope showed a little lower slope although the standard errors and T-test showed that there was no significant difference between the two slopes. If we try to explain the measured difference by using the biological data, we could take several approaches. First the lower slope on the south-facing side must have come from an erosion level that was greater than the erosion on the north-facing slope. The number of trees and shrubs on the north-facing slope was more than twice that than on the south-facing slope (see power point presentation Richness and Diversity slide). With a greater number of plants and greater diversity on the north-facing slope, the root system would do a better job of holding in the soil and therefore lessening the rate of erosion. To answer why more woody plants would grow on the north-facing side one must look at the climate of this part of Texas. The sun would hit the south-facing slope far more than the north-facing slope. The sun would dry out the soil faster and make it harder for plants to survive there. The lessened amount of plant life allows for more erosion and a biological explanation of the differences in slope.
Ribble; 8:30 TR
April 27, 1999.