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Processes regulating threshold effects along environmental gradients.
Stevenson, R. Jan1, 1
ABSTRACT- Predator-prey interactions have been hypothesized to contribute to sudden changes in biomass along productivity gradients. Many other processes at multiple biological levels can also cause threshold effects, i.e. sudden changes in ecological conditions along uniformly changing environmental gradients. In two recent studies of nutrient effects on algae, thresholds were observed and related to lower level community processes. In one example, sudden changes in algal biomass occurred along a gradient in phosphorus concentrations in the Everglades. Sudden changes in algal species were not observed along the P gradient, so the changes in biomass were attributed to abrupt changes in the metabolism regulating calcium deposition, which provides structural integrity for Everglades periphyton. Experimental evidence does indicate that calcium deposition is regulated by phosphorus concentration. In another example, sudden changes in algal assemblages were associated with changes in species composition of algae. The ability of species to colonize habitats can occur at thresholds in ecological conditions. In streams, colonization by the large filamentous alga Cladophora occurs at a threshold in phosphorus concentration. Cladophora has great effects on stream habitat for other organisms. A conceptual model will be presented that relates processes generating threshold effects, generation times of organisms, and the spatial and temporal variability within habitats.
KEY WORDS: Thresholds, Nutrients, Algae