Ants were surveyed in three habitats at Mount St. Helens in 2008. The area most impacted by the 1980 eruption is the Pumice Plain. Less impacted is the Blowdown Zone where trees were toppled due to the blast. Two habitats were surveyed in the Pumice Plain varying in vegetation density (Pumice Plain Low-Vegetation (PPLV) and Pumice Plain High-Vegetation (PPHV)), and one habitat was surveyed in the Blowdown Zone (BDZ). Ten ant species were collected with the most species collected from the BDZ habitat and the least from the PPLV habitat. Ant abundance was higher at the BDZ and PPHV habitats than at the PPLV habitat. Ant biodiversity was highest at the BDZ habitat than at the PPHV and PPLV habitats. Significant correlations between ant community parameters and plant community parameters were also found. Few plants in the PPLV habitat may contribute to the lack of ants. High ant species richness at the BDZ habitat may be due to complex plant architecture. Results from this study suggest that ants are important focal species in tracking biotic recovery following disturbances.
Mount St. Helens erupted on May 18, 1980, strongly impacting a 600 km2 area as a result of a northern lateral blast of the volcano [
Since 1980, numerous ecological studies have recorded the impact of the volcanic eruption on plant and animal communities and ecosystem dynamics [
Among the pioneer arthropod colonists found on the Pumice Plain were winged ants (Formicidae) [
After extensive hiking throughout the Pumice Plain in July 2008, it was clear that it was not a homogeneous environment. Two distinct habitat types were apparent and consisted of areas lush with vegetation along the edge of the Pumice Plain (referred to as the Pumice Plain-High Vegetation (PPHV) habitat type) and areas devoid of almost all vegetation in the center of the Pumice Plain (referred to as the Pumice Plain-Low Vegetation (PPLV) habitat type).
Two study sites were randomly selected within each of the Pumice Plain habitat types (Table
Description of 6 study sites*.
Site name** | Site location | UTM coordinates and datum |
---|---|---|
PPLV | ||
1a | Center of pumice plain | 562988E, 5121705N, Z10 |
1b | 562983E, 5121729N, Z10 | |
1c | 562982E, 5121754N, Z10 | |
1d | 562973E, 5121780N, Z10 | |
2a | 562953E, 5122158N, Z10 | |
2b | 562954E, 5122183N, Z10 | |
2c | 562950E, 5122203N, Z10 | |
2d | 562951E, 5122227N, Z10 | |
PPHV | ||
1a | Eastern fringe of pumice plain | 564847E, 5121574N, Z10 |
1b | 564853E, 5121601N, Z10 | |
1c | 564859E, 5121624N, Z10 | |
1d | 564862E, 5121648N, Z10 | |
2a | 564342E, 5121613N, Z10 | |
2b | 564347E, 5121638N, Z10 | |
2c | 564351E, 5121660N, Z10 | |
2d | 564353E, 5121686N, Z10 | |
BDZ | ||
1a | Windy ridge vicinity east of pumice plain | 573269E, 5124328N, Z10 |
1b | 573293E, 5124342N, Z10 | |
1c | 573301E, 5124363N, Z10 | |
1d | 573323E, 5124374N, Z10 | |
2a | 572996E, 5124151N, Z10 | |
2b | 572995E, 5124125N, Z10 | |
2c | 572997E, 5124102N, Z10 | |
2d | 573004E, 5124081N, Z10 |
*Designations of 1a–1d and 2a–2d indicate ant survey sites located 25 m apart along 75 m transects at each study site.
**PPLV: Pumice Plain-Low Vegetation habitat type, PPHV: Pumice Plain-High Vegetation habitat type, BDZ: Blowdown Zone habitat type.
Representative photographs of (a) Pumice Plain-Low Vegetation (PPLV) habitat type, (b) Pumice Plain-High Vegetation (PPHV) habitat type, and (c) Blowdown Zone (BDZ) habitat type, where ants were collected at Mount St. Helens National Volcanic Monument. Images were taken between 22 and 25 of July 2008.
To provide a comparison to the habitat types within the Pumice Plain, an additional habitat type was surveyed outside of the Pumice Plain to the east in the Blowdown Zone in the Windy Ridge vicinity (referred to as the Blowdown Zone (BDZ) habitat type). Two study sites were randomly selected within this third habitat type (Table
A survey of ant species at each of the study sites was conducted between 22 and 25 July, 2008. During this time period air temperature maximums ranged from 15.6 to 25.9°C, and air temperature minimums ranged from 5.1 to 9.1°C; there was no precipitation, some low lying fog early in the mornings, clouds and sun the rest of the day, and at times breezy conditions [
Statistical analyses were performed to evaluate differences in the ant communities between habitat types at Mount St. Helens National Volcanic Monument. Richness and abundance of the ant communities were compared among habitat types using a nested ANOVA with study site nested within habitat type. Statistical analyses of richness and abundance of ant communities were performed on untransformed data because all ANOVA assumptions were met. Following ANOVA analyses, Tukey’s pairwise comparison tests were used to elucidate differences in richness and abundance of the ant communities collected at the different habitat types.
The data for the Shannon-Wiener Biodiversity Indices of ant communities, plant richness, and percent ground cover by plants were not normally distributed. No transformations normalized the data, so nonparametric statistical tests were performed. Specifically, Kruskal-Wallis tests were conducted to look for differences in the Shannon-Wiener Biodiversity Indices of ant communities, plant richness, and percent ground cover by plants among habitat types (an equivalent nonparametric test does not exist for a nested ANOVA [
The relationships between plant communities (richness and percent ground cover by plants) and ant communities (richness, abundance, and Shannon-Wiener Biodiversity Indices) were explored by performing Spearman rank-order correlations. Nonparametric correlations were used instead of Pearson product-moment correlations because the data for both plant richness and percent ground cover by plants were not normally distributed.
A total of 987 ants belonging to ten different species were collected during the course of the survey (Table
List of ant species and numbers collected in each habitat type at Mount St. Helens National Volcanic Monument between 22 and 25 July 2008.
Genus | Species | Habitat type* | Total number | ||
---|---|---|---|---|---|
PPLV | PPHV | BDZ | |||
|
|
1 | 479 | 49 | 529 |
|
|
10 | 11 | 314 | 335 |
|
|
0 | 40 | 0 | 40 |
|
|
0 | 0 | 39 | 39 |
|
|
0 | 3 | 12 | 15 |
|
|
0 | 0 | 13 | 13 |
|
|
0 | 3 | 4 | 7 |
|
|
0 | 0 | 6 | 6 |
|
|
0 | 0 | 2 | 2 |
|
|
0 | 0 | 1 | 1 |
* PPLV: Pumice Plain-Low Vegetation, PPHV: Pumice Plain-High Vegetation, BDZ: Blowdown Zone.
The richness (Table
Overall nested ANOVA of the effect of habitat type and study site on richness of ant communities (
Source | SS | df |
|
|
---|---|---|---|---|
Habitat type | 68.250 | 2 | 30.560 | <0.001 |
Study site (habitat type) | 1.042 | 1 | 0.930 | 0.346 |
Error | 22.333 | 20 |
Overall nested ANOVA of the effect of habitat type and study site on abundance of ants (
Source | SS | df |
|
|
---|---|---|---|---|
Habitat type | 19564.600 | 2 | 22.170 | <0.001 |
Study site (habitat type) | 748.200 | 1 | 1.700 | 0.208 |
Error | 8824.600 | 20 |
Mann-Whitney
Habitat type* comparison | Mann-Whitney |
|
---|---|---|
BDZ > PPLV | 99 | 0.0014 |
BDZ > PPHV | 36 | 0.0009 |
PPHV = PPLV | 51 | 0.0754 |
*PPLV: Pumice Plain-Low Vegetation, PPHV: Pumice Plain-High Vegetation, BDZ: Blowdown Zone.
**With a Bonferroni correction, only
Richness (mean number of ant species ±1 SE) of ant communities in three habitat types at Mount St. Helens National Volcanic Monument sampled between 22 and 25 July 2008. Different letters above the bars indicate significant differences (
Abundance (mean number of individuals ±1 SE) of ants in three habitat types at Mount St. Helens National Volcanic Monument sampled between 22 and 25 July 2008. Different letters above the bars indicate significant differences (
Shannon-Wiener Biodiversity Indices (mean biodiversity index ±1 SE) for ant communities in three habitat types at Mount St. Helens National Volcanic Monument sampled between 22 and 25 July 2008. Different letters above the bars indicate significant differences (
The study sites within the PPLV habitat type were almost entirely devoid of plants leading to clear differences in plant species composition (Table
List of plant species and numbers found in each habitat type at Mount St. Helens National Volcanic Monument between 22 and 25 July 2008*.
Genus | Species | Habitat type** | Total number | ||
---|---|---|---|---|---|
PPLV | PPHV | BDZ | |||
Unidentified grasses*** | 0 | 90 | 110 | 200 | |
|
|
0 | 125 | 15 | 140 |
|
|
0 | 115 | 0 | 115 |
|
|
0 | 40 | 1 | 41 |
|
|
0 | 1 | 27 | 28 |
|
|
0 | 0 | 13 | 13 |
|
|
0 | 0 | 10 | 10 |
|
|
1 | 8 | 1 | 10 |
|
spp. | 0 | 0 | 9 | 9 |
|
spp. | 0 | 0 | 8 | 8 |
|
|
0 | 7 | 0 | 7 |
|
|
0 | 6 | 0 | 6 |
Unidentified ferns | 0 | 0 | 3 | 3 | |
|
|
0 | 2 | 0 | 2 |
|
|
0 | 0 | 1 | 1 |
|
spp. | 0 | 0 | 1 | 1 |
|
|
1 | 0 | 0 | 1 |
|
|
0 | 0 | 1 | 1 |
* Only plants that fell within the 0.5 m2 quadrats placed in the center of each ant survey circle were included.
**PPLV: Pumice Plain-Low Vegetation, PPHV: Pumice Plain-High Vegetation, BDZ: Blowdown Zone.
***The number of grass plants is an estimate due to the difficulty of distinguishing one individual from another.
Mann-Whitney
Plant community characteristic | Habitat type* comparison | Mann-Whitney |
|
---|---|---|---|
Plant richness | BDZ > PPLV | 99 | 0.0014 |
PPHV > PPLV | 36 | 0.0009 | |
BDZ = PPHV | 51 | 0.0754 | |
| |||
Percent ground cover by plants | BDZ > PPLV | 99.5 | 0.0011 |
PPHV > PPLV | 36 | 0.0006 | |
BDZ = PPHV | 51 | 0.0809 |
* PPLV: Pumice Plain-Low Vegetation, PPHV: Pumice Plain-High Vegetation, BDZ: Blowdown Zone.
**With a Bonferroni correction, only
Richness (mean number of plant species ±1 SE) of plant communities in three habitat types at Mount St. Helens National Volcanic Monument sampled between 22 and 25 July 2008. All plant species, including unidentified ferns and grasses (which were categorized to morphospecies), found within the 0.5 m2 sampling quadrats were used in plant richness calculations. Different letters above the bars indicate significant differences (
Percent ground cover by plants (mean percent ground cover ±1 SE) in three habitat types at Mount St. Helens National Volcanic Monument sampled between 22 and 25 July 2008. Different letters above the bars indicate significant differences (
Several positive correlations between plant community and ant community characteristics were found in this survey (Table
Spearman rank-order correlations between ant community and plant community characteristics.
Correlations | Spearman rank correlation coefficient |
|
---|---|---|
Ant abundance and percent ground cover by plants | 0.787 | <0.001 |
Ant abundance and plant richness | 0.784 | <0.001 |
Ant richness and percent ground cover by plants | 0.550 | 0.005 |
Ant richness and plant richness | 0.508 | 0.011 |
Ant biodiversity and percent ground cover by plants | 0.491 | 0.015 |
Ant biodiversity and plant richness | 0.365 | 0.079 |
Relationship between percent ground cover by plants and ant abundance from samples collected at Mount St. Helens National Volcanic Monument between 22 and 25 July 2008. This relationship represents a significant positive correlation (Spearman rank correlation coefficient = 0.787,
Relationship between plant richness and ant abundance from samples collected at Mount St. Helens National Volcanic Monument between 22 and 25 July 2008. This relationship represents a significant positive correlation (Spearman rank correlation coefficient = 0.784,
A total of 10 different ant species were collected at three habitat types in Mount St. Helens National Volcanic Monument during this study, which was conducted over a 4-day period in July 2008. The most ant species (total of 9) were collected in the BDZ habitat type, whereas the least ant species (total of 2) were collected in the PPLV habitat type. Interesting comparisons can be made between the ant communities found in this study to those found in previous studies at Mount St. Helens to better understand the trajectory of recovery of this important biotic community.
The most interesting comparison would be to compare the ant species collected from Mount St. Helens in this study to those in the area prior to the 1980 eruption to determine whether the ant communities are recovering to their original composition or to a composition that is totally different. Unfortunately, there is little documentation of the biological communities at Mount St. Helens prior to 1980 [
The next interesting comparison to make is between the ant communities presently found at Mount St. Helens and those found immediately following the 1980 eruption. Detailed studies of ant communities at Mount St. Helens were conducted by Sugg [
In addition to collecting ants on the Pumice Plain, Sugg [
The most commonly collected ants in this study came from the genus
The least commonly collected ant species in this survey was
Of the 10 ant species collected during this study, only
The distribution of
Both ant species richness and ant abundance were significantly higher at the BDZ and PPHV habitat types than at the PPLV habitat type. Additionally, there were overall significant correlations between ant parameters (richness and abundance) and plant parameters (richness and percent ground cover). These results seem to fall in line with the “taxonomic diversity hypothesis” put forward by Murdoch et al. [
Another interesting finding from this survey was the positive correlation between percent ground cover by plants and ant species richness and abundance. Two explanations for this observation involve the importance of shelter and shade for ants. Andersen [
As previously discussed, ant species richness was significantly higher at the BDZ and PPHV habitat types than at the PPLV habitat type. Upon closer investigation, it turns out that the BDZ habitat had significantly higher ant species richness when compared to the PPHV habitat type. At first glance there is no clear explanation for this difference considering that both the plant species richness and percent ground cover by plants are not significantly different between the BDZ and PPHV habitat types. However, representative images of the PPHV and BDZ habitat types (Figures
In conclusion, this study documents the ecological response of ants (Formicidae) to the 1980 eruption of Mount St. Helens by recording species distributions in three habitat types in 2008. Positive correlations between ant richness and abundance and plant richness and percent ground cover were found at Mount St. Helens suggesting that there is a strong connection between ant and plant community recovery. Further studies exploring ant-plant interactions may help us better understand the complex mechanisms driving the recovery of biotic communities following natural disasters such as volcanic eruptions.
The author would like to thank Dr. William F. Fagan for the opportunity to pursue research at Mount St. Helens. She also thanks Dr. Ted Suman (of the Smithsonian Institute) for identifying the ant specimens to species. In addition, the author greatly appreciated the advice and help in the field from Dr. John Bishop, Elise Larsen, Chris Che-Castaldo, and Ray Yurkewycz. The author wishes to express her sincere gratitude to Jeanne and Walt Ratterman for providing a welcome home away from home and to Shane Ratterman for all of his support. The author also thanks two anonymous reviewers whose comments improved the quality of this paper. This project was supported by the NSF Division of Environmental Biology through LTREB award 0614263.