Pools of Producer Biomass and Detritus

The tendency toward larger percentages of gross primary production consumed by herbivores with higher producer nitrogen and phosphorus concentrations suggests that, all else being equal, ecosystems composed of nutrient-rich primary producers will have lower producer biomass and accumulate primary detritus (derived from primary producer tissue) more slowly than systems dominated by nutrient-poor producers. This expectation will hold if gross primary production varies to a lesser extent than does the percentage consumed by herbivores across the ecosystems compared. If, in contrast, gross primary production varies to a greater extent than does the percentage consumed by herbivores across the ecosystems compared, then the quantity of matter available for storage as producer biomass will be more closely associated with rates of gross primary production than with the percentage consumed by herbivores. Unfortunately, too few comparisons across ecosystems have been done to conclude with rigor whether these expectations hold. However, one recent comparison including a broad range of aquatic and terrestrial ecosystems demonstrated that ecosystems with nutrient-rich producers tended to have smaller pools of producer biomass (Figure 3). Interestingly, in the ecosystems compared in this study gross primary production varied more broadly than did the percentage of production consumed by herbivores. This observation is a direct contradiction to the arguments presented above which predict that biomass pools should track primary production rates rather than patterns in producer nutrient content.

Evolutionary constraints may provide an answer to this apparent contradiction. Primary producers with higher nitrogen and phosphorus content tend to grow faster, have shorter life spans, and higher natural mortality rates (i.e., senescence) than their low-nutrient counterparts. Thus, when a broad range of ecosystems are compared, nutrient-rich producers have both larger proportions of their biomass consumed by herbivores and higher natural mortality rates (Figure 3). Both mechanisms lead to smaller pools of producer biomass under high-nutrient conditions across a broad range of aquatic and terrestrial ecosystems despite large differences in gross primary production. Indeed, recent modeling advances have confirmed this prediction.

The moderate tendency toward higher percentages of producer detritus mass lost per day with higher nitrogen and phosphorus concentrations in the detritus observed when a broad range of ecosystems are compared suggests that, if the differences in absolute producer detritus production do not exceed the differences in the percentage consumed by detritivores, ecosystems with richer producer detritus should also tend, at least moderately, to have smaller pools of producer detritus. The rationale is analogous to the case of producer biomass pools discussed above. The observation that aquatic ecosystems, which tend to have richer primary detritus and higher percentages of producer detritus production consumed by detritivores, generally have smaller pools of producer detritus when compared to terrestrial ecosystems is consistent with this hypothesis (Table 1).

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