Quantifying the effects of anthropogenic perturbations on calcium cycling and ecosystems is challenging because the effects are not instantaneous. Outcomes, such as deteriorating tree health (or die-offs) and declining bird populations due to calcium-depleted eggshells, may only be obvious after years of cumulative damage to the environment. On a hopeful note, grave and large-scale impacts such as these can inspire shifts in industrial practices; in response to the problems acidification was causing to terrestrial ecosystems, care has been taken in recent years by industrialized nations to lower emissions of sulfate and NOX (although emissions have not ceased entirely). Unfortunately, decreases in calcium-containing emissions have diminished the unintended anthropogenic amelioration of the calcium depletion caused by acidification.
Land-use changes also have mixed effects. Revegetation of areas may decrease their production of dust, but reforesting an area after repeated harvesting of crops accelerates the calcium depletion of the area as the calcium contained in the removed biomass has been lost. Accordingly, attempts at environmental remediation have been made through the application of calcium-rich compounds, such as lime or wollastonite. Although forest-scale manipulations have been set up to assess the effectiveness of these applications, these experiments are ongoing and, thus, conclusions about the effectiveness of these treatments cannot yet be made. Ecosystem models have also been employed to understand and predict impacts of these ecological manipulations on the terrestrial calcium cycle, but as our understanding of the complexities of the terrestrial calcium cycle is currently limited, these models primarily serve to provide broad guesses of future impacts.
As in terrestrial ecosystems, although the impacts of anthropogenic perturbations of the calcium cycle on marine ecosystems have been predicted and modeled, the extent of the impact of ocean acidification on ocean ecosystems remains uncertain. On one hand there are undoubtedly effects and variables that have not been considered, and on the other hand it is an open question to what extent calcifying organisms will adapt to and cope with the lower pHs, lower carbonate ion concentrations, and lesser degrees of saturation with respect to calcium carbonate minerals. Already stressed by pollution and overfishing, ecosystems centered around calcareous organisms like corals may collapse under the additional burden of acidification. Alternatively, although the rate of pH change is occurring at an unprecedentedly rapid timescale relative to evolution, genetic adaptation to the more acidic conditions could occur before widespread alteration of the ecosystems occurs. At the moment no experiments on appropriately long time frames have been conducted, in terms of either pollution or remediation, for the outcomes to be clear. Even where we immediately cease to perturb the calcium cycle, we have still made significant changes to the global calcium cycle and it will take some while for the full consequences of our inadvertent global-scale experiment on the calcium cycle to unfold.
See also: Acidification; Anthropospheric and Anthropogenic Impact on the Biosphere; Climate Change 2: Long-Term Dynamics; Deforestation; Forest Management; Global Change Impacts on the Biosphere; Pelagic Predators.
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