Ascendency Applications

Principles of ascendency, as they have been shown here, have been applied to compare similar ecosystems (e.g., estuaries), or the same ecosystems over a period of time including the response of systems to disturbances. Examples of such applications are the description of spatial and temporal change of ascendency in marine microbial systems. They revealed that ascendency is strongly related to the functionality of the microbenthic loop. Important parameters determining the value of ascendency were the decomposition activity and the capacity for resource exploitation. Ascendency was found to be a useful indicator for the health assessment of marine benthic ecosystems over space and time.

Ascendency has also been applied to establish ecosystem responses to eutrophication and other anthropogenic system alterations of carbohydrates, proteins, lipids, and carbon biopolymers in various parts of the globe. Whereas ascendency is, in general, believed to rise with eutrophication due to an increase in TST, this is not always the case. Depending on the extent and frequency of the eutrophication event, it might disturb the system to an extent where ascendency reflects a decrease in ecosystem stability through a decrease in AMI and TST. Another case of system perturbation was described for pesticide-perturbed microcosms, using an index called 'scope for change in ascendency' (SfCA). SfCA is an analogy to

scope for growth of an organism and is the balance of the ascendency of individual compartment inputs and outputs. SfCA was hypothesized to decrease in the presence of a disturbance and was ultimately found to be a useful indicator for the short-term assessment of perturbations in herbicide-treated microcosms.

Ascendency has also been used to assess the whole ecosystem impacts of severe freshwater abstractions from an estuarine catchment. The interdecadal comparison between light and severe freshwater abstraction and the consequential reduction in sustained and pulsing freshwater inflow into the Kromme estuary revealed a decrease in ascendency under the present, freshwater-starved condition. The spatial comparison with other, similar, estuaries that do not have such severe freshwater abstractions in the catchment shows a higher ascendency in estuaries with higher freshwater inflow that ensures sustained renewal of the nutrient pool to fuel primary production.

Since ascendency is very often influenced by a change in the magnitude of TST, the organization of a system is frequently reported as a ratio of ascendency/development capacity (A/C), which cancels out the influence of TST. Also the AMI is used as an unscaled index in a comparative way. In general, it is advised to take the behavior of other indicators of ecosystem health (e.g., exergy) into account in combination with ascendency to arrive at a representative assessment of ecosystem state. Ascendency has been shown to vary with the degree of aggregation of the network. In general, ascendency decreases in highly aggregated networks, even if the TST is the same. The type of aggregation, that is, which compartments are aggregated, also significantly affects the value of ascendency. This is equally true for the aggregation of living and nonliving components of the network.

The biomass inclusive version of ascendency and the sensitivities of the individual flows were determined for the Chesapeake Bay system to identify the limiting nutrient in the ecosystem and bottlenecks in carbon, nitrogen, and phosphorus transfers. The comparison over four seasons revealed that, in general, the primary producers were nitrogen limited, which was in concordance with previous studies on these groups. However, the nitrogen limitation on the primary producer level was not propagated throughout the entire web, but all nekton was found to be phosphorus limited. The type of nutrient limitation changed over the course of the year for a few primary producers and invertebrates, but not for the nekton. It is important to note that nutrient limitations in a trophic flow network are not determined by the type of limitation of the primary producer, since the various organisms have different stoichiometric requirements.

See also: Autocatalysis; Ecosystem Health Indicators; Emergent Properties; Goal Functions and Orientors; Indirect Effects in Ecology; Limiting Factors and Liebig's Principle; Resilience.

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