First Order Detritivory

First-order detritivory is the consumption of primary producer detritus. First-order detritivores include microbial decomposers (i.e., bacteria and fungi) and detritivorous invertebrate (and very rarely vertebrate) organisms that feed upon primary producer detritus and attached micro-bial decomposers. The ingestion of producer detritus by microbial decomposers and invertebrates is difficult to measure and thus, direct values of first-order detritivory are rare in the literature. Alternatively, first-order detritivory is often estimated as the temporal rate of loss of producer detritus mass incubated in containers or mesh bags (i.e., 'detritus incubation' method), which can be measured much more easily. This method has received criticisms, mainly because some of the producer detritus mass lost in mesh bags is not consumed by the first-order detritivores enclosed in the bags but instead is flushed out of the bags. However, rates derived in this manner are suspected to hold for comparisons across ecosystems because natural variation in first-order detritivory is expected to exceed the methodological error.

For many decades it has been clear to researchers that richer producer detritus (i.e., producer detritus with higher concentrations of nitrogen and phosphorus) decays faster than poorer detritus, which indicates that richer producer detritus is subject to higher rates of consumption by detritivores. Nevertheless, comparisons between producer detritus decay rates and nitrogen and phosphorus concentrations in the detritus encompassing a broad range of ecosystems have not been done until recently. The comparisons have shown that faster decay rates, expressed as the percentage of detritus mass lost per day, are associated with higher nitrogen and phosphorus concentrations in the detritus, albeit not strongly, across ecosystems (Figure 4). The unexplained variance in this

Producer detritus nitrogen or phosphorus concentrations but..

Producer detritus nitrogen or phosphorus concentrations

At the long-term scale

Producer detritus nitrogen or phosphorus concentrations

At the long-term scale

Producer detritus nitrogen or phosphorus concentrations

Figure 4 Patterns of first-order detritivory and implications on the size of the producer detrital pool, observed across a broad range of ecosystems.

relationship is not surprising given the many environmental factors that can affect detritus decay rates, such as temperature, humidity, redox conditions, type and size of the detritivore population, and predation intensity on the detritivore population. These broad comparisons demonstrate that detritus in aquatic ecosystems tends to have a higher percentage of mass lost per day than do detrital pools in terrestrial ecosystems (Table 1).

As with herbivory, when rates of producer detritus decay are expressed in absolute terms (i.e., quantity of producer detritus mass lost per square meter per year), decay rates and nitrogen and phosphorus concentrations in the detritus are often unrelated across a broad range of ecosystems (Figure 4). The reasons are twofold: first, absolute producer detritus production (i.e., quantity of producer detritus generated per square meter per year) is unrelated to the nitrogen and phosphorus concentrations in the detritus across a broad range of ecosystems; second, absolute producer detritus production varies more broadly than does the percentage consumed by detritivores. Thus, absolute rates of producer detritus decay, which result from the product between absolute producer detritus production and the percentage consumed by detritivores, will often be more closely associated with absolute producer detritus production than with the percentage consumed when a broad range of ecosystems are compared and, by extension, will often be unrelated to the nitrogen and phosphorus concentrations in the producer detritus. This logic is identical to that presented above for herbivory since it is the lack of a relationship between nutrient content and primary production driving the differences between relative and absolute consumption rates (as herbivory or detritivory).

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