Exposure: herbivory, graz*, brows*, trampl*
Agent: reindeer, caribou, Rangifer
The terms within each of the categories ‘exposure’ and ‘agent’ will be combined using the Boolean operator ‘OR’. The two categories will then be combined using the Boolean operator ‘AND’. An asterisk (*) indicates ‘wildcard’ truncation.
Searches will also be made for Swedish, Norwegian, Finnish and Russian counterparts of the above terms. The following search strings will be used (although they in some cases will have to be simplified as some sites do not allow wildcards or Boolean operators):
English: (herbivory OR graz* OR brows* OR trampl*) AND (reindeer OR caribou OR Rangifer)
Swedish: renbet* OR ((herbivori OR bet* OR tramp*) AND (renar OR caribou OR Rangifer))
Norwegian: reinbeit* OR renbeit* OR ((beit* OR gressing OR tramp*) AND (*rein OR *ren OR reinsdyr OR rensdyr OR karibu OR caribou OR Rangifer))
Finnish: (herbivoria OR laidun* OR tallata OR talloa OR polkea) AND (poro OR karibu OR Rangifer)
Russian: (травоядные OR пастбище OR пастись OR выпасать OR выбирать OR высматривать OR вытаптывать) AND (олень OR карибу)
No time, language or document type restrictions will be applied.
In addition to the exposure and agent terms mentioned above, the following terms for ‘subject’ have been tested during a scoping exercise:
vegetation, vascular, plant*, herb*, forb*, gramin*, lichen*, moss*, bryophyte*, flora, shrub*, tree*, forage, tundra, alpine, subalpine, arctic, subarctic, heath*, pasture*, rangeland*
However, it was found that searches using the exposure and agent terms alone were specific enough to return a quite reasonable amount of articles. Including the above subject terms would restrict the search and reduce the number of hits by a factor of about two. The subject terms were therefore excluded – the loss of specificity was judged to be less important than the increase of sensitivity.
The search aims to include the following online publication databases:
Directory of Open-Access Journals
Other literature searches
Relevant literature will also be searched for in bibliographies of literature reviews such as those by Moen & Danell , Linkowski & Lennartsson , Suominen & Olofsson  and Forbes & Kumpula .
Study inclusion/exclusion criteria
Articles found by searches in databases will be evaluated for inclusion at two successive levels. First they will be assessed by title and abstract. In cases of uncertainty, the reviewer will tend towards inclusion. A subset consisting of at least 10% of the articles will be assessed by at least two reviewers. A kappa statistic relating to the assessments will be calculated. If this statistic indicates that the reviewers are inconsistent in their assessment (κ < 0.5), discrepancies will be discussed and the inclusion criteria will be clarified or modified.
Next, each article found to be relevant on the basis of title and abstract will be judged for inclusion by reviewers studying the full text. Again, the reviewers will tend towards inclusion in cases of uncertainty.
Studies or datasets found by other means than database searches may be entered at any of the two stages in this screening process.
A list of studies rejected on the basis of full-text assessment will be provided in an appendix to the review together with the reasons for exclusion.
Each study must pass each of the following criteria in order to be included at any of the two screening stages:
Relevant subject(s): Vegetation in alpine/subalpine areas or arctic/subarctic tundra, including the forest-tundra ecotone. Reindeer may also occur in boreal coniferous forests, but studies of vegetation in such regions will not be included.
Relevant types of exposure: Grazing, browsing or trampling by reindeer. Modern reindeer husbandry may also affect vegetation through disturbances caused by reindeer herders’ all-terrain vehicles, but such impacts will not be considered by this review.
Relevant types of comparator: Lower or no grazing, browsing or trampling.
Relevant types of outcome: Change in cover (abundance), biomass, diversity (including species richness), structure, composition or productivity of vegetation.
Relevant types of study: Any primary field study (observational or manipulative) comparing vegetation in areas and/or time periods with different degrees of reindeer herbivory. Remote-sensing studies will also be included, but not simulation-modelling studies or field studies of simulated herbivory since these do not represent direct impacts of reindeer.
Potential effect modifiers and reasons for heterogeneity
The following potential effect modifiers will be considered and recorded:
Latitude and longitude
Elevation and topography
Local climate (e.g. temperature, precipitation and snow conditions) and quantified climate change
Vegetation type (e.g. species present)
Quality of vegetation as forage (contents of nutrients, proteins, herbivore-defence compounds etc.)
Reindeer subspecies involved
Seasonality of grazing (whether reindeer are present permanently or only during parts of the year)
Domestication status of the reindeer
Presence of other herbivores
Presence and species identity of predators
Grazing history of the site (e.g. whether formerly used by cattle or sheep)
History of herd (e.g. whether native or introduced, or affected by large-scale exclosures etc.)
Variation in husbandry (e.g. supplemental feeding)
Presence and history of other land management activities in the area
Proximity to other human activities
Presence of fences and other artificial barriers to migration
Study and intervention timescale and seasonality
Further modifiers and causes of heterogeneity will be identified and defined in an iterative process.
Study quality assessment
Most studies in this field compare vegetation in areas that for a long time have been subject to different levels of reindeer herbivory, or vegetation inside and outside areas that for a number of years have been fenced to exclude reindeer. Thus, they are usually ‘CI’ (Comparator/Intervention) studies describing effects of various levels of reindeer herbivory relative to a control site similar to the intervention site in all aspects other than the variable of interest. Some studies may alternatively present data on vegetation before and after fencing or throughout a period when herbivory has changed; ‘BA’ (Before/After) studies. A few studies combine these two approaches in ‘BACI’ (Before/After/Comparator/Intervention) designs, where site control and intervention comparisons are made both before and after herbivory has changed. Randomised control trials (RCT) are also possible within this field of research – some investigators have applied a certain element of randomisation e.g. when selecting locations for exclosures.
A general problem is that data on reindeer density are usually very uncertain. Many studies simply describe areas as subject to ‘heavy grazing’ or ‘no/light grazing’, with no further attempt of quantification being made. In some cases, however, reindeer densities have actually been estimated, e.g. using trampling indicators or counts of reindeer droppings.
As a result of these differences in study quality and susceptibility to bias, the following factors will be assessed and used to categorise studies as having high, medium, or low susceptibility to bias:
Selection of plot locations
Study design (BA/CI/BACI/RCT)
Temporal extent of study
Methodological detail (e.g. number of plots, number of visits, data on reindeer density)
Accounting for confounding variables
Appropriate use of statistics and statistical analysis
Detailed reasoning will be recorded in a transparent manner. Study quality will be critically appraised by one reviewer, but a subset of at least 25% of studies will be appraised by a second reviewer. Conclusions will be compared, and where reviewers differ, discrepancies will be discussed and reconciled individually.
A list of studies rejected on the basis of quality assessment will be provided in an appendix to the review together with the reasons for exclusion.
Data extraction strategy
Means and measures of variation (standard deviation, standard error, confidence intervals) will be extracted from tables and graphs, using image analysis software when necessary. If only raw data are provided, summary statistics will be calculated. Data on potential confounding variables or effect modifiers will also be extracted.
In addition to extracting data from articles, it may be useful to ask authors of relevant articles for access to unpublished primary data, since the articles usually present only a fraction of all vegetation data that have been collected. Thus, it may be possible to get information on total species richness even from studies where published data refer only to biomass or abundance or to the species richness within certain groups of vegetation. Similarly, some data on reindeer density may be available even if they have not been published. Authors will be contacted for this information where possible, and a time limit on the acceptance of responses set before data are synthesised.
Data synthesis and presentation
A narrative synthesis of data from all studies included in the review will describe the quality of the results along with the findings of studies of sufficient quality. Tables will be produced to summarise these results. Where studies report similar outcomes meta-analysis may be possible, and in these cases effect sizes will be standardised (using standardised mean effect size) and weighted according to inverse variance. Precise details of the quantitative analysis will only be known when full texts have been assessed for their contents and quality.
Separate analyses will be undertaken for studies that report reindeer density as categorical (high or low) and those that quantify reindeer density in some way. Meta-analysis of heterogeneity in effect size will take the form of random-effects models, and meta-regression will be performed where effect modifiers cause significant heterogeneity between studies. Subgroup analysis of categories of studies will also be performed where sufficient studies report common sources of heterogeneity. Publication bias and sensitivity analysis will also be carried out where possible. Overall effects of reindeer herbivory will be presented visually in plots of mean effect sizes and variance.