Arguably, the awareness of natural scientists as regards indirect effects in the terrestrial environment can be traced back at least to the end of nineteenth century, when the school of thought founded by Dokuchaiev had developed a theory that soil was a product of complex interactions between climate and geological and biological components of the terrestrial landscape. To date, the importance of indirect interactions in the terrestrial environment is well recognized. Indirect effects in terrestrial ecosystems relate, for instance, to the dependence of plant nutrient supply on mineralization of nutrients by soil biota, and to the propagation of these effects through the food chain. Soil fauna may help to disperse microorganisms crucial for plant functioning and biogeo-chemical cycling, and physically modify the habitat, thus changing environmental conditions for all the biological community. Plants, in turn, modify the habitat for other organisms, for example, by producing litter, providing shade, shelter, etc. All in all, indirect effects in the terrestrial environment are widespread; below are just a few examples of their recent studies.
A number of studies conducted in the terrestrial environment (this includes both field experiments and soil microcosms) adopted experimental approach focusing on the density-manipulation experiments followed by analysis of the results obtained using parametric (e.g., ANOVA, Tukey's HSD) and nonparametric (e.g., Kruskal-Wallis and Mann-Whitney ¡7-tests) statistical tests. For instance, Miller used exclusion experiments to elucidate direct and indirect species interactions in a field plant community. Experimental results were analyzed by parametric and nonparametric techniques, which yielded interesting information on the ecological characteristics of the species involved. Particularly, it was established that species with a large competitive ability due to direct effects generally had almost as large indirect effects, so that the two effects almost cancelled each other.
A number of terrestrial studies used various mathematical methods to investigate indirect interactions. In particular, a good insight into specific indirect effects was gained using simulation modeling to interpret monitoring or experimental results. For example, Hunt and co-authors found that the increase in net N mineralization with precipitation is a consequence of not only the direct effect of moisture supply on decomposition, but also an indirect effect of changes in substrate supply and quality. de Ruiter and co-authors studied nitrogen mineralization conducted at a wheat field. The impact of microfaunal functional groups on N mineralization was evaluated by calculating the impact of group deletion. The results showed that the effect of the removal of a group may exceed the direct contribution of this group to N mineralization rather considerably, with amoebae and bacterivorous nematodes having values of 18% and 28%, and 5% and 12% for, respectively, direct contribution toward and impact of deletion upon overall N mineralization. Influence of the transitions of soil microorganisms between dormant and active stages was studied by Blagodatsky and co-authors. Such transitions were shown to be important for biogeochemical cycling and the rate of organic matter decomposition.
A combination of a detailed monitoring program, and statistical and simulation modeling has been used in a study of ecological patterns in the Heron Wood Reserve, located at the Dawyck Botanic Garden in Scotland. The suite of statistical techniques included ANOVA, ANCOVA, correlation analysis, CCA, factor analysis, and stepwise regression modeling. The study revealed a number of indirect effects resulting from a complex multivariate interplay among ecosystem components. For example, the results suggested that both direct negative and indirect positive effects of the microarthro-pod community on specific fungal groups appeared to take place. The relatively high local abundances of the dominant collembolan Folsomia might have caused local declines in ectomycorrhizal fungi, reflected, in turn, in the increase in pH (Whist this work was in press, Dr. Peter Shaw has checked identification of the dominant Folsomia species (previously referred to as F. candida) from the Dawyck ecosystem study, and has shown that it appears to fit the description of F. inoculata) However, for those samples where the dominant Folsomia were less abundant, overcompensatory fungal growth due to grazing by mites and other collembola was implicated. Complex effects were also shown for bacteria, nematodes, protozoa, plants, and soil properties.
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