Abstract
1. Community indicators are used to assess the state of ecological communities and to guide management. They are usually calculated from monitoring data, often collected annually. Since any given community indicator provides a univariate summary of complex multivariate phenomena, different changes in the community may lead to the same response in the indicator. Sampling variation can also mask ecologically important trends. 2. This study addresses these challenges for community indicators, with a focus on the large fish indicator (LFI), internationally used to report status of marine fish communities. The LFI expresses ‘large’ fish biomass as a proportion of total fish biomass and is calculated from species–size–abundance data collected on trawl surveys. We develop new methods to decompose the contributions of species, sampling locations and season to trends over time in the LFI, and highlight consequences for assessment and management. 3. Our results showed that both species and locations made divergent contributions to overall trends in the LFI indicator, with contributions differing by several orders of magnitude and in sign. Only small proportions of species and locations drove overall LFI trends, and their contributions changed with season (spring and autumn surveys). To assess significance of component trends, a resampling method was developed. Our method can be generalised and applied to many other community indicators based on survey data. 4. Synthesis and applications. Our new method for decomposing community indicators and generating confidence intervals makes it possible to extract much more information on what drives a ‘headline’ indicator, providing a solution to challenges arising from multiple possible interpretations of changes in the indicator, and from sampling variation. Analysis of the effects of indicator components on headline indicator values is recommended, because the results allow assessors and managers to identify and interpret how divergent factors (e.g. species, sampling locations and seasons) contribute to the headline indicator value.
Highlights
Ecological indicators are indices of the state of an ecosystem and are widely used for environmental assessment, reporting and management support (Rice 2003).Indicators typically provide information on the status and trends of ecosystem components and attributes, usually those which are sensitive to human and environmental impacts.Ecological indicators are often used in marine systems to describe the effects of climate or fishing pressure on the composition or function of a marine community or part884 G
This study addresses these challenges for community indicators, with a focus on the large fish indicator (LFI), internationally used to report status of marine fish communities
The LFI expresses ‘large’ fish biomass as a proportion of total fish biomass and is calculated from species–size–abundance data collected on trawl surveys
Summary
Ecological indicators are indices of the state of an ecosystem and are widely used for environmental assessment, reporting and management support (Rice 2003).Indicators typically provide information on the status and trends of ecosystem components and attributes, usually those which are sensitive to human and environmental impacts.Ecological indicators are often used in marine systems to describe the effects of climate or fishing pressure on the composition or function of a marine community or part884 G. Ecological indicators are indices of the state of an ecosystem and are widely used for environmental assessment, reporting and management support (Rice 2003). Reuman of that community (Shin et al 2005, 2010) Indicators range from those that summarize responses of certain species to those describing changes in properties resulting from direct and indirect interactions of species. To provide reliable information on the effects of human and environmental pressures on communities, the response of the indicator should be sensitive and specific to the pressures (Fulton, Smith & Punt 2005; Rice & Rochet 2005; Rombouts et al 2013). Best practice would require testing the properties and performance of indicators before they are used to report changes in the state of the environment and guide management (e.g. Rice 2003; Jennings 2005; Rice & Rochet 2005)
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