A scoping review of evidence for the effects of seven global deer species on woody vegetation

Rapid expansion of deer (Cervidae) populations is a concern for forest ecosystems. Despite extensive reviews on how deer affect forests, variation in effects across deer species has received less attention. A lack of focus on species-specific effects may lead to oversights and failure to achieve desired management outcomes. We used a systematic approach to compile data on the extent to which the effects of seven deer species on woody vegetation have been studied. We focused on the six deer species present in Britain and Ireland, and elk (Cervus canadensis). A total of 455 worldwide studies were included. Red deer (Cervus elaphus) (n = 163) and elk (n = 158) were the most studied species, while Reeve’s muntjac (Muntiacus reevesi) (n = 18) and Chinese water deer (Hydropotes inermis) (n = 5) were the least researched. Fifty-four percent of studies (n = 245) used fenced exclosures to assess deer impacts. Research mainly focused on defoliation via browsing and grazing (n = 424), while debarking (n = 44), defecation (n = 8), and trampling (n = 5) were less frequently studied. Vegetation density (n = 235), height (n = 189), and diversity (n = 135) were the most common metrics used, while fewer studies focused on vegetation mortality (n = 74), structural variability (n = 28), and condition (n = 15).

Methods
Peer-reviewed journal articles were obtained by searching the Web of Science (WoS) Core Collection for articles published between 1970 and 2024. The searches were restricted to the six deer species present in Britain and Ireland, and elk, including any subspecies. No geographic limits were imposed. We also searched the online thesis database Opengrey and the UK government website GOV.UK, which includes literature from Natural England, the Department for Environment and Rural Affairs, and the Forestry Commission.

When selecting Outcome search terms, each potential term was first entered into the Web of Science (WoS) Core Collection along with the Population terms. Each Outcome search term was recorded with the date, the total number of hits, and whether relevant hits were returned. Outcome search terms were selected for the final search when they gave relevant results that were not already being generated by other terms. Search terms were put together in one additive search. Search terms were discussed among the authors to ensure the search encompassed everything the review aimed to address. Some terms that generated many irrelevant sources were excluded.

Study relevance was sequentially assessed using our finalised inclusion criteria and the ROSES flowchart. Studies were first assessed based on title, abstract, and keywords. The second screening stage involved reading the full text. Any articles not entirely published in English were excluded. Reviews identified by the WoS search published from 2010 onwards were screened for relevant articles through bibliographical back-searching. Following the final search at the end of 2021, a WoS weekly search alert was used to monitor for new publications until the end of 2024. The GOV.UK website was revisited in December 2024 to search for final updates to the literature. The same could not be completed for Opengrey, as the database had since been closed.

Data were extracted on the study location, deer species reported to be present, any additional herbivores reported, whether the study focused on captive or wild deer, use of exclosures, population monitoring methods – if any – used to assess relative deer species abundance or density, effect mechanism studied (browsing, grazing, bark removal, trampling), and effects(s) of deer activity for forest, woodland or woody vegetation.