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Indicator summary

Summary of indicator structure and function

Definition and/or background

The following is from Fulton et al 2004a -

Just as system throughput measures system growth under the network analysis methodology developed by Ulanowicz (1986), Relative Ascendency is a measure of system organisation or development. This measure has been shown to be correlated with maturity and as such may be used as an indicator of large-scale system change resulting from the effects of fishing. Relative Ascendency is a model-derived measure calculated from Ascendency and Developmental Capacity. Ascendency (A) is the product of total system throughput and an index of the “information content” of the ecosystem and therefore is calculated using:

where the throughput (T) is calculated as follows (Christensen et al. 2000):

and the “information content” I is:

with f_ij the fraction of the total flow from j:

where T_ij is the flow from j  to i, and Q_i is the probability that a unit of energy  passes through i given by:

Developmental Capacity is the upper bound for Ascendency – that is the value of A if the system overheads were zero (or close to it, as thermodynamics does not allow for overheads to ever reach zero). System overheads include inputs, exports, respiration and redundancy (multiple flows between components) and they reflect a system’s “reserves” that can be drawn upon to combat the effects of unexpected perturbations (Ulanowicz and Norden 1990). Developmental Capacity (C) is calculated using:

Relative Ascendency is then simply the ratio of A to C (often given as a percentage). Recent research suggests that changes in ascendency (or at least relative ascendency) are commensurate with trends in species richness, resource internalisation, trophic specialisation and succession (Christensen 1994, Ulanowicz and Abarca-Arenas 1997). Therefore, it has a lot of potential as an ecological indicator of the effects of fishing.

Attribute

Ecosystem structure and function

Purpose

Fisheries

Data required

The following is from Fulton et al 2004a -

• Biomass per species (or group) recorded (preferably at least for the main components of the ecosystem and through time or vs. some reference area)
• Taxonomic data (or at least categorisation of the main components of the system)
• Catch data of species in fishery
• Mortality estimates for each group
• Consumption / Biomass estimates for each group
• Diet data

Ecosystem applicability

The following is from Fulton et al 2004a -

Should be applicable in all ecosystems.

Robustness

The following is from Fulton et al 2004a -

Medium to high: theoretical and model studies have suggested that Relative Ascendency has the potential to be very informative, with regard to system state (Ulanowicz 1986, Christensen 1994, Ulanowicz and Abarca-Arenas 1997). Its exact degree of robustness is unclear at present, as there is still much to learn about all of its properties and it is a model-derived indicator, which makes it reliant on the assumptions used to construct the underlying model and the data used to parameterise and initialise the model. However, as part of an integrated set of indicators (e.g. in conjunction with empirical indicators or other model-derived indicators such as total system throughput, which is a measure of system growth), it could be very useful and robust tool for characterising systems and how they have changed.

References

Fulton, E.A., Smith, A.D.M., Webb, H., and Slater, J. (2004a) Ecological indicators for the impacts of fishing on non-target species, communities and ecosystems: Review of potential indicators. AFMA Final Research Report, report Number R99/1546.

References that Fulton et al uses for this indicator:

Christensen, V. 1994. On the behaviour of some goal functions for ecosystem development. Ecological Modelling75/76: pp 37-49.

Christensen, V., C.J. Walters, and D. Pauly. 2000. ECTOPATH with ECOSIM: a user’s guide. October 2000 edition. Fisheries Centre, University of British Columbia, Vanouver, Canada and International Centre for Living Aquatic Resources Management, Penang, Malaysia.

Ulanowicz, R.E. 1986. Growth and development: ecosystem phenomenology. Springer-Verlag, New York, 203pp.

Ulanowicz, R. E., and L. G. Abarca-Arenas. 1997. An informational synthesis of ecosystem structure and function. Ecological Modelling95: pp 1-10.

Ulanowicz, R.E., and J.S. Norden. 1990. Symmetrical overhead in flow and networks. International Journal Systems Science 21: 429-437.

Background reading

Fulton, E.A., Fuller,M., Smith, A.D.M., and Punt, A. (2004) Ecological indicators of the ecosystem effects of fishing: Final report. AFMA Final Research Report, report Number R99/1546.

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