Biology Department, Mars Hill College
Mars Hill, N.C. 28754
Arthropods represent the bulk of the animal species. Arthropods play an important role
in nutrient cycling by breaking down coarse woody debris (CWD) and leaf litter.
They supply the forest with nutrients and also serve as food for other animals.
Arthropods use CWD and herbaceous plants for shelter. Herbivorous species use
these plants for food.
Arthropod abundances and diversity may be controlled by habitat characteristics.
Some arthropods prefer sites where coarse woody debris, leaf litter, and vegetation are
abundant. Depending on an arthropod's ecological niche, the habitat can be the deciding factor on whether or not it is present in a given area.
We studied the abundances of four arthropod families in small (<25 ha) patches of forest in the French Broad River Basin near Mars Hill, NC. These families are common and widespread throughout this area. The families were
We hypothesized that these families would be most abundant at sites with greater amounts of coarse woody debris and greater herbaceous cover, because these two habitat features provide the trophic base to support arthropod populations.
Study Areas
Nine sites were chosen in patches of temperate deciduous forest near Mars Hill, North Carolina (Figure 1, Click on the image to the right). All of these sites were second growth forest stands more than 40 years old and had experienced similar levels of anthropogenic disturbance from agricultural uses such as woodland grazing and from past timber harvest (see Photo 1).
Population Sampling
A square grid of 25 pitfall traps was established at each of nine sites. There were 25 traps spaced at 10-m intervals in each grid. The pitfall traps which were 1-l cups fill half-full with water and preservative. Trapping was done for two one-week periods: June 12-26, 1998. Specimens were removed from traps at the end of each week, preserved in alcohol, and taken to the laboratory for identification.
Microhabitat Measurements
Techniques for habitat measurements were modeled after those used by Dueser and Shugart (1976). Five variables were measured: 1) herbaceous vegetation, 2) leaf litter, 3) understory, 4) overstory, and 5) coarse woody debris.
At each site, percent coverage of herbaceous vegetation was measured using forty 0.5 m2 quadrants (Photo 4). Depth of leaf litter was measured at each
quadrat. The volume of coarse woody debris (CWD) (Photo 2) was estimated by measuring the length and average diameter of down logs >7.5 cm in diameter. The location of major pieces of CWD was mapped for each grid. This mapping was done in order to determine whether the proximity of CWD affected capture rates of the nearby traps.
Coarse Woody Debris
No relationship was found between the abundances of animals and the amount of coarse woody debris. There was less variation in the total amount of herb cover and coarse woody debris (CWD) among the study sites than expected (Figure 2). This lack of variation hindered our ability to detect any effect of CWD. We also analyzed the effect of CWD on catch rates of groups of traps within each site. This analysis also showed no significant relationship between CWD and capture rates. Thus, these data fail to support the hypothesis of a positive correlation between CWD and arthropod abundance (Figure 3).
Herbaceous Cover
There was a negative relationship between herb coverage and beetles, spiders, and ants. The abundances were similar for lower levels of herb cover. Above coverages of 60%, the abundances drop. These data led us to reject the hypothesis of a positive relationship between arthropod abundance and herbaceous cover.
Correlation between families
The abundances of some of the four families were positively correlated. There was a strong correlation between ants and ground spiders. There was a weak correlation between carabid beetles and ants. Millipedes and carabid beetles, spiders and millipedes and millipedes and ants showed no correlation (Table 1). There may be a correlation between carabid beetles and spiders because they prefer the same type of habitat, which is under bark, logs, and rocks. Carabid beetles and spiders are both predators in their feeding habits and feed on larvae, other insects, and even other spiders (Dillon and Dillon 1961). Variations in microhabitat could drive correlations in arthropod abundance. For example, humidity is an important factor in the habitat choice of millipedes and other forest arthropods (Cloudsley-Thompson 1968). However, our sampling procedure was not designed to pick up fine-scale (<5 m) variation in these habitat features.
Table 1. Pearson correlation coefficients (r) between families of arthropods. Bold r values are significant at P<0.05. See also Figure 3.
| Carabid beetles | Ground Spiders | Millipedes | |
| Ants | 0.30 | 0.57 | 0.30 |
| Carabid beetles | - | 0.59 | 0.14 |
| Ground Spiders | - | - | 0.10 |
Habitat variables can greatly affect animals in a certain environments and influence the number of animals that live there. The types of vegetation and species of trees are two variables that affect animal abundance. Our working hypothesis was that coarse woody debris (CWD) and herb cover would influence abundance and diversity in these forested habitats.
Other studies have found a relationship and characteristics of the forest floor. For example, Dueser and Shugart (1978) found that shrews responded to variation in debris on the forest floor. Moreover, differences in habitat preferences among species were apparent in their data. The proximity of CWD and shrubs, depth of leaf litter, and vegetation type have been shown to affect the capture of beetles in studies similar to this one (e.g., Ahearn 1971, Greenberg and McGrane 1996, Shure and Phillips 1987, Thomas and Sleeper 1974).
When studying the relationship between CWD and the families, we thought that there would be more animals found where the volume of CWD is greater. Figure 3.A-C suggests that there does not seem to be a direct relationship. Similar numbers of animals were found next to the same amount of debris. For all four families, the numbers dropped between CWD volumes of 10-15 units, therefore rejecting the hypothesis that there would be more animals found around more CWD. There was a relationship found between the percentage of herbaceous cover and animal abundance.(Figure 3). Greenberg et al. (1995) found a lack of effect of forest managment practices that affect CWD on beetle assemblages.
Many families can be found in abundance near each other either because they prefer the same habitats or food, or they are food for another family. Therefore, the abundances of different taxa may be correlated. For example, carabid beetle and spiders are both predators They feed on larvae and other small arthropods (Dillon and Dillon 1961). We found that only ants and spiders and carabids and spiders are correlated to each other. The other pairs from the four families did not demonstrate statistically significant correlations (Table 1).
This research was supported by grant from the Research Experience for Undergraduates (REU) Program of the National Science Foundation. The grant was awarded to the Coweeta LTER Program. Several private landowners graciously provided assistance and access to sampling sites: J. Hood, N. Harmon, T. Plaut, and A. Smith. Skilled assistance with the field sampling and lab identification was provided by L. P. Allen, K. A. Lee, A. E. Panikowski, K. L. Pyatte, L. Jacobson, and M. Kerr.