Are Current Management Recommendations for Conserving Saproxylic Invertebrates Effective? (systematic review)
Throughout Europe, saproxylic insects have been identified as a highly threatened community of invertebrates. Conservation management recommendations for saproxylic invertebrates advocate the continuous provision of dead and decaying wood microhabitats that they require for survival. In addition to protecting veteran trees, this can be achieved by leaving fallen dead and decaying wood in situ on sites, providing supplementary coarse woody material (CWM), inducing decay in mature trees and strategic planting in order to maintain a balanced age structure of trees in both space and time. Such site-based management interventions may be of importance when used as a conservation tool to bridge gaps between dead wood generations.
The primary objective was to systematically collate and synthesise published and unpublished evidence in order to address the question “Are current management recommendations for conserving saproxylic invertebrates effective?”. The systematic review aimed to examine whether site-based manipulation of biotic and abiotic factors, such as the provision of supplementary CWM, inducing decay in younger trees or altering the degree of sun-exposure within a stand of trees, can benefit the saproxylic fauna. It was anticipated that the review would draw attention to areas where primary research, or long-term monitoring, would be valuable in order to substantiate the current management guidelines and to initiate evidence-based best practice in saproxylic conservation.
Relevant studies were identified through computerised searches of the following electronic databases: ISI Web of Knowledge (including ISI Web of Science and ISI Proceedings), JSTOR, Science Direct, Directory of Open Access Journals (DOAJ), Copac, Scirus, Scopus, Index to Theses Online, Digital Dissertations Online, Agricola, CAB Abstracts, English Nature’s “WildLink” and the Countryside Council for Wales (CCW) library. Web searches were conducted using the internet meta-search engines Alltheweb and Google Scholar, in addition to inspecting the following statutory organisation websites: UK Department for Environment, Farming and Rural Affairs (Defra), Northern Ireland Department of Agriculture and Rural Development (DARD) and European Union portal (Europa). The specialist publication “Coleopterist” was searched by hand for any appropriate information. No specific non-English language searches were conducted. Bibliographies of traditional literature reviews and articles accepted into the systematic review at the full text stage were examined for studies that had not yet been identified by any other means.
Study Selection Criteria
The criteria, which studies had to meet for inclusion into the final stage of the systematic review, were:
- Subject: any saproxylic invertebrate population or assemblage.
- Intervention: any site-based management action.
- Outcome: desired primary outcomes were change in population density for a target species or change in species richness within assemblages. Nonetheless, studies were not rejected on the basis of outcome.
- Type of study: any.
Data Collection and Data Analysis
Study inclusion assessments were performed and the observed agreement between the two independent reviewers was deemed to be “substantially good”. Due to a lack of high quality data (i.e., long-term fully replicated and controlled field-based experiments or investigations) on changes in the long-term persistence of populations, or in species richness within assemblages, no meta-analysis was undertaken. In addition, the variation in type of investigation and range of outcome measures adopted in the studies precluded the use of formal statistical techniques.
The available evidence is insufficient to critically appraise the effectiveness of any site- based conservation interventions for saproxylic species or communities in the long-term. However, there is a range of studies describing changes in the saproxylic fauna in response to a variety of habitat management interventions, observed over relatively short periods of time. The research suggests that incorporating a variety of different management actions, such as the retention of dead and decaying wood or the provision of supplementary CWM, into site management plans will increase microhabitat heterogeneity and therefore the diversity of species present on a site.
In the absence of robust, high quality evidence, recommendations relating to the use of specific site-based management interventions would be speculative. However, it is acknowledged that general proposals for the maintenance of suitable microhabitats, such as the protection of veteran trees within the landscape, are based on sound ecological principles and should not be discouraged even though experimentally rigorous evidence is lacking. Further primary research (including long-term monitoring) is required to fill the gaps in our ecological knowledge that potentially weaken the case for the effectiveness of current saproxylic invertebrate conservation action.
The evidence that is available suggests that management priorities should be to improve the diversity, quantity and continuity of dead and decaying wood on sites. Optimising microhabitat heterogeneity by artificially manipulating the orientation and type of CWM on a site may increase number of saproxylic species present and help to buffer populations against environmental change. Conducting such interventions within an experimental framework, and subsequently monitoring the saproxylic fauna throughout the lifetime of the dead wood, will generate important information on the relative value of different CWM constructions for target species/assemblages at each progressive stage of decay. Detailed information on the distribution and autecology of species, particularly those of conservation concern, must continue to be collated and disseminated in order to allow practitioners to allocate conservation resources effectively.
Saproxylic invertebrates particularly Coleoptera make up one of the largest groups of red-listed species in Fennoscandian, and European countries (Speight 1989, Berg et al. 1995, Jonsell et al. 1998, Martikainen & Kaika 2004). A lack of dead wood and low numbers of deciduous trees resulting from forest management practices have been identified as major factors contributing to the loss of saproxylic biodiversity especially in Fennoscandian forests (Esseen et al. 1997). More sympathetic management regimes have been advocated to address this decline. These include the creation of man- made stumps and retention of natural stumps (Jonsell et al. 2004), maintenance of over-mature trees (tree surgery, reduction of competition), leaving fallen wood under light Bracken or Bramble cover and lying in water, inducing decay in young trees where over- mature trees are absent (Alexander et al.), leaving large diameter fallen wood where possible, pollarding young trees, leaving woodpiles in preference to removing timber and creating large piles of tightly packed brash rather than small piles of open brashings (Kirby 1992). The National Trust (NT) is interested in ascertaining the efficacy of these interventions for the maintenance of saproxylic invertebrate diversity.
The effectiveness of these management interventions may be modified by the species and age of timber, as well as the species of invertebrate (Jonsell et al. 2004). Light levels may modify invertebrate response to woodpile colonisation; with partial shade thought to be desirable (Alexander et al., Kirby 1992). Diameter of wood source and size of brash piles are also important factors (Kirby 1992). The length of follow up period, scale and geographical location over which responses are measured are also likely to modify the impact of the interventions. The impact of these potential effect modifiers requires investigation.
An explicit systematic review methodology will be used to retrieve data pertaining to the impact of sympathetic management on saproxylic invertebrates. The review will limit bias through the use of comprehensive searching, specific inclusion criteria and formal assessment of the quality and reliability of the studies retrieved. Subsequent data synthesis will summarise empirical evidence guiding the formulation of appropriate evidence-based management guidelines and highlighting gaps in research evidence. The review should be of use to practitioners in government and non- statutory conservation agencies, informing decisions over regional and national management guidelines. It also has a wider international relevance with the potential to inform management decisions at European and Fennoscandian level.