Harold E. Balbach

Developmental instability in Rhus copallinum L.: Multiple stressors, years, and responses

Developmental instability, as assessed by leaf fluctuating asymmetry and stem internode allometry, was examined at nine sites, representing three levels of disturbance, over multiple years. Site selection was based on land‐use disturbance classes related to training of mechanized infantry and other land management activities at Fort Benning, Georgia. Developmental instability varied among sites and years, and there was a strong site‐by‐year interaction for many traits. Indeed, depending on the year, the same site could be ranked as having the greatest and least amount of leaf fluctuating asymmetry. Burning a site the year prior to collecting the leaves profoundly influenced measures of leaf fluctuating asymmetry. In the absence of recent burning, leaf fluctuating asymmetry declined with increasing disturbance, but burning the year prior to collecting the leaves reversed this trend. Total plant cover, proportion of bare ground, and amount of plant litter influenced the amount of leaf asymmetry in a site‐dependent manner. Overall, burning influenced the levels of developmental instability more than either disturbance or microhabitat variables such as total plant cover, which should reflect competition in a plant’s immediate neighborhood.

Photosynthesis and Fluctuating Asymmetry as Indicators of Plant Response to Soil Disturbance in the Fall‐Line Sandhills of Georgia: A Case Study Using Rhus copallinum and Ipomoea pandurata

We examined net photosynthesis, transpiration, stomatal conductance, and leaf fluctuating asymmetry on two species (Rhus copallinum and Ipomoea pandurata) as indicators of stress at nine sites across a gradient of soil disturbance at Fort Benning, Georgia. There were three sites for each of three disturbance levels. Physical habitat disturbance was caused by activities associated with infantry training, including mechanized elements (tanks and personnel carriers) and foot soldiers. In addition, we examined the influence of prescribed burns and microhabitat effects (within meter‐square quadrats centered about the plant) on these measures of plant stress. Net photosynthesis declined with increasing disturbance in the absence of burning for both species. However, when sites were burned the previous year, net photosynthesis increased with increasing disturbance. Developmental instability in Rhus, as measured by fluctuating asymmetry, also declined with increasing disturbance in the absence of burning but increased with disturbance if sites were burned the previous year. Developmental instability was much less sensitive to burning in Ipomoea and in general was lowest at intermediate disturbance sites. Microenvironmental and microhabitat effects were weakly correlated with measures of plant stress when all sites were combined. However, higher correlations were obtained within site categories, especially when the recent history of prescribed burning was used as a category. Finally, using all of the combined data in a discriminant function analysis allowed us to correctly predict the disturbance level of more than 80% of the plants. Plant stress is responsive to both large‐scale perturbations, such as burning, and microhabitat parameters. Because of this, it is important to include macro‐ and microhabitat parameters when assessing stress. Similarly, we found a combination of developmental and physiological indicators of stress was superior to using them separately.

Using Demographic Sensitivity Testing to Guide Management of Gopher Tortoises at Fort Stewart, Georgia: A Comparison of Individual-Based Modeling and Population Viability Analysis Approaches

Population Viability Analysis (PVA) models have been used as a decision-support tool for managing wildlife populations, both game and nongame species. However, PVA models require extensive population-level data; without such data, an individual-based model (IBM) may be a more appropriate tool. The at-risk gopher tortoise (Gopherus polyphemus) is one species of interest about which numerous individual studies have been published, but with little published documentation of population dynamics. Using NetLogo (http://ccl.northwestern.edu/netlogo/), the authors developed a spatially explicit IBM for the gopher tortoise population that inhabits Fort Stewart, GA, a US Army installation. The model was used to perform demographic sensitivity analyses and compare the results to sensitivity analyses conducted using a PVA model based on the same combinations of demographic parameters. The comparison showed a significant congruence in results from the two approaches. Several parameters—particularly juvenile and egg-to-age 1 mortality—appeared to disproportionately affect simulation results and are likely to be influenced by habitat manipulation. Based on their results, the authors conclude that IBMs can be useful to perform demographic sensitivity analysis and evaluate the capacity for habitat manipulation alone to provide the means for ensuring long-term persistence of gopher tortoise populations at Fort Stewart.