Author: Jillian Vaught
Location: Kessler Atmospheric and Ecological Field Station near Purcell, OK and Lake Thunderbird in Norman, OK
Date: November 25, 2014
Habitat disturbance, both above- and below-ground, can have negative impacts on plant biodiversity. In a biological conservation study conducted on fragmented costal ecosystems it was found that patterns of land-use activity, such as trails and agriculture, had a negative impact on floral species richness (Laurance et al., 2012). In our ecological study, we investigated a dirt pathway in a grassland area and a dirt and gravel mixed pathway in a wooded area. In our investigation we looked at how these pathways affected plant diversity as we surveyed away from the pathway. We define disturbance as activity along a pathway that has led to erosion of the pathway and biodiversity as the richness of species found within a 1 meter quadrat. To hypothesize our results we looked at a previous study on disturbances which concluded that basal species and plants were most likely to flourish under intermediate disturbance conditions (Wootton, 1998). Another study on disturbance rates found that increased exposure to disturbance always decreased diversity (Santos et al., 2010). We tested our hypothesis that species of plants nearing the pathway will become less diverse due to the pathway disturbance such as physical traffic along the path and changes in nutrients.
In addition to above-ground diversity, we looked at how distance from disturbance could affect soil pH. Studies indicate that the type and magnitude of soil disturbance can influence soil pH (Sleutel et al., 2012). Levels of pH are a determining factor for microbial biomass and communities structure in the soil (Pietri and Brookes, 2009). A change in microorganisms in the soil could be a mechanism for changes in plant biodiversity across sample plots. To further investigate this mechanism, we looked at the relationship between soil pH and plant biodiversity. We predicted that soil acidity would become more neutral as we moved away from the pathway.
To determine the effects of disturbance on plant species diversity, we distributed 1 m2 quadrats at various distances from a disturbance path at Kessler Atmospheric and Ecological Field Station (KAEFS) and at Lake Thunderbird. To account for a diversity of habitats in our sampling, we used KAEFS to represent an ecological sample from a grassland and Lake Thunderbird to represent a sample from a wooded area.
We established 10 meter transects, 2 perpendicular to one side of the path and 2 perpendicular to the other side of the path. We placed 5 quadrats along each transect. Each quadrat started at the edge of the path and was separated from the next quadrat by 1 meter. A total of 40 quadrats were sampled, 20 from each location. We used JMP version 10 to generate a general linear regression analysis to determine the relationship between disturbance distance and plant richness.
Within each quadrat we measured the number of plant species present. In addition to species diversity, we also collected a soil sample from within each of the quadrats using a metal probe. We then stored the soil at ambient temperatures in plastic bags until we could analyze the pH in the lab.
Across both sites our results suggest as distance from the path increased, our species diversity significantly increased (p-value = 0.0024; Figure 1). As pH increased, species diversity significantly declined (p-value = 0.0081; Figure 2). Using a 2 variable model for a multiple regression we get an r2 value of .336305 (Figure 3).
Species diversity at the Lake Thunderbird site, was not significantly correlated with increasing pH value (p-value = 0.7167; Figure 4). However, as our distance from the path increased our species diversity increased significantly with a p value of 0.0080 (Figure 5). Our multiple regression model for Lake Thunderbird gives us an r2 value of 0.357229.
Our KAEFS site indicates that increasing pH cannot significantly determine species diversity with a p value of 0.2149 (Figure 6). However, as distance from the path increases our species diversity also significantly increases with a p value of 0.0058 (Figure 7). The multiple regression model for Lake Thunderbird gives us an r2 value of 0.381445 (Figure 8).
The multiple regression model across both sites returned the most significant results. Our results indicate that both pH and distance explains 33% of our species diversity. In both sites, as distances from the pathway increased, the number of species also significantly increased. A previous study on disturbance rates found that increased exposure to disturbance always decreased diversity (Santos et al., 2010). Studies like this reaffirm our hypothesis and results that an increased proximity to disturbance such as a pathway may cause harsher treading conditions or changes in the soil community that negatively affects plant biodiversity.
Alternative to our hypothesis, pH is not significantly correlated with distance from the pathway. Interestingly, pH is significantly related to species biodiversity. As our pH increases across both sites, our species biodiversity decrease is significant. Sources indicate that deciduous forests and temperate grasslands of North America such as our sample sites have an optimal soil pH ranging from 3.4 to 6.5 (Baskin, 2014). This optimal pH is consistent with our results which indicate that species biodiversity at both our locations was greatest under slightly acidic conditions. A study with similar results conducted at Rutgers New Jersey Agricultural Experiment Station show that disturbances caused by man can affect soil quality and filtration. In their study, their pH near a man made path caused the pH to rise to 8.3 which negatively affected the surrounding agriculture crops (Gimenez et. al., 2010).
Considering both sites separately, our data indicated that distance from the path at both locations was a significant explanation for species biodiversity. However, pH when both sites were considered separately was not significant enough to explain species biodiversity. Although other sources validate the negative affects of disturbance on plant diversity, one study finds that species of native and non-native plants actually prosper near roadsides. This study goes on to infer that roadsides offer intermediate levels of disturbance that may actually be a refuge for maintaining species diversity (Zeng et. al., 2010). More studies need to be done to clarify the relationship between levels of disturbance and plant biodiversity.
We thank the staff of Kessler Atmospheric and Ecological Field Station for access to the station. We would also like to thank Dr. Lara Souza and Nicole Poe for their instruction in developing and presenting our study.
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