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Fires, which are an intrinsic feature of southern African ecosystems, produce biogenic and pyrogenic losses of nitrogen (N) from plants and soils. Because of the long history of fires in these savannas, it was hypothesized that N2 fixation by legumes balances the N losses caused by fires. In this study, the N2 fixation activity of woody legumes was estimated by analyzing foliar delta15N and proportional basal area of N2 fixing species along experimental fire gradients in the Kruger National Park (South Africa). In addition, soil carbon (C) and N pools, foliar phosphorus (P) and gross N mineralization and nitrification rates were measured, to indicate the effects of fires on nutrient stocks and the possible N cycling processes modified by fires. Although observations of increased soil C/N and mineralization rates in frequently burned plots support previous reports of N losses caused by fires, soil %N did not decrease with increasing fire frequency (except in 1 plot), suggesting that N losses are replenished in burned areas. However, relative abundance and N2 fixation of woody legumes decreased with fire frequency in two of the three fire gradients analyzed, suggesting that woody legume N2 fixation is not the mechanism that balances N losses. The relatively constant %N along fire gradients suggests that these ecosystems have other mechanisms to balance the N lost by fires, which could include inputs by atmospheric deposition and N2 fixation by forbs, grasses and soil cyanobacteria. Soil isotopic signatures have been previously used to infer patterns of fire history. However, the lack of a relationship between soil delta15N and fire frequency found in this study indicates that the effects of fires on ecosystem delta15N are unpredictable. Similar soil delta15N along fire gradients may reflect the contrasting effects of increased N gaseous emissions (which increases delta15N) and N2 fixation other than that associated with woody legumes (which lowers delta15N) on isotopic signatures.

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We examined potential effects of herbivory on temperate forest ecosystems using FORET, a forest succession simulation model with the capacity for treating various hierarchical levels for long time periods. Two species of trees were chosen for evaluating herbivory effects: white oak (Quercus alba), a relatively slow-growing shade-tolerant species, and tulip poplar (Liriodendron tulipifera), a fast-growing shade-intolerant species. Both are dominants in the Southeastern U.S. forest system selected as a basis for this study. The study focused on four hierarchical levels, covering individual, phenotypic, interspecific, and community interactions. Simulations of herbivory were arrived at by stochastically varying annual incidence that herbivores were present on each simulation plot over a 500-yr period, and by modifying the proportion of energy allocated to either tree growth, or to its defense from herbivore attack. Two hypothetical tree phenotypes were considered, one that allocated specified amounts of energy to herbivore defense mechanisms at all times, and the other that made the allocation only when herbivores were present according to the stochastic determination made for a specific study period. Thus, one phenotype was fixed in its life history strategy; the other was given a facultative strategy where it switched its growth and defense tactics as a function of variation in herbivore presence. The results of the simulations suggest how deciduous forests may respond to long-term variations in the intensity of herbivore stress on two dominant tree species, and show the importance of hierarchical relationships within the community. White oak tended to show a greater sensitivity to interspecific interactions; tulip poplar showed a higher sensitivity to intraspecific interactions. Changes in growth rates associated with the switching strategies (an ultimate factor) were more important in answering variations in productivity than was impact imparted by annual changes in incidence of herbivore presence (a proximate factor) for these two species, although there were important differences in several statistical interactions. While our results suggest that herbivore stress can explain a larger degree of the variation in long-term community dynamics, ecological interactions between herbivore and climate effects must be more closely linked in such long-term studies.

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Predicting ecosystem response to climate change is a dynamic version of the classic problem of understanding vegetation-climate interrelations. Computer models can synthesize current knowledge and are important tools for understanding possible ecosystem dynamics under changed conditions. Models based on individual plant biology and natural history have been tested with respect to their ability to simulate vegetation response to changed climate, and are being applied to assess the potential effects of future climate change.

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The authors describe "a human ecosystem model, NUNOA, [which] simulates the yearly energy balance of individuals, families, and extended families in a hypothetical farming and herding community of Quechua Indians in the high Andes. The yearly energy demand of each family, based on the caloric requirements of its members, is computed by simulation of agricultural and herding activities in response to stochastic environmental conditions. The family energy balance is used in determining births, deaths, marriages, and resource sharing." The model can be used "to investigate the effect of changes in marriage patterns, resource sharing patterns, or subsistence activities on the ability of the human population to survive in the harsh Andean environment. Results from the model suggest that the substructuring of a population into extended families provides a mechanism for sheltering the population from control by exogenous influences. A population without substructures for resource sharing is shown to be unstable in such an unpredictable environment."

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The rate coefficients for species turnover (the proportion of species lost per unit time) for successional communities decrease as the communities approach some equilibrium state. This observation makes it possible to determine the parameters of a two-parameter model which quantifies the time variation of successional changes in the second derivative.

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The change in frequencies of D. melanogaster and D. simulans in competition experiments was used to measure the effect of radiation on the fitness of a population. A dose of 250 or 500 rads given to the males of highly inbred lines of D. simulans at the beginning of competition and every three weeks thereafter increased the relative frequency of the irradiated population. If the dose was increased to 1000 rads, the deleterious effects of radiation became too great a burden on the population, and the frequency of the irradiated population decreased. From these results it was concluded that below certain doses the introduction of radiation-induced mutations into a highly homozygous population would increase the fitness of the population.

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