Do Landscape matrix features affect species movement? (systematic review)
Movement across the landscape is predicted to be important to species‟ regional population persistence and response to environmental change. Increased movement may increase the colonisation of newly-suitable patches, as well as recolonisation of patches after extinction events.
Many biodiversity policies, such as the European Union Habitats Directive and the England Biodiversity Strategy, include recommendations based on assumptions regarding the impact of the matrix (the landscape between habitat patches) on species movement. The empirical basis of these assumptions, particularly those within decision-support and landscape connectivity tools (particularly habitat networks) that have been built to support policy decisions have not been rigorously examined.
We aimed to examine the empirical evidence for whether, and how much, landscape features can affect species movement. A specific secondary aim was to summarise the evidence available for UK resident species (particularly UK Biodiversity Action Plan Priority Species and non-native species) and describe the spatial and temporal scales of relevant studies. Identifying knowledge gaps was also a key secondary objective.
Five online databases were searched using combinations of keywords related to the question, which were developed through scoping searches. Web search-engines were also utilised. In addition, the websites of statutory research organisations in the UK, USA, Canada and Australia were searched for relevant publications.
The review included any study presenting primary data concerning directly-measured movement (i.e. not inferred movement) in relation to:
- landscape features outside of habitat patches
- the shape of habitat patches.
Only studies with appropriate controls or comparators were included in the review. Specific exclusion criteria were:
- studies of colonisation where the source (i.e. distance moved) is not known
- studies of seed movement by animals or disease/parasite movement – any form of “lift-hitching‟.
Data collection and analysis
In total, 315 studies were included in the review and were divided into pools according to their study design and the landscape feature under investigation.
Meta-analyses were performed on measures of inter-patch movement or emigration, due to the reasonable number of available studies (27 in total, 8 – 20 per analysis), similarity of these studies and quality of data. Different meta-analyses were conducted to examine first, the impact of habitat corridors and second, matrix types, testing the hypothesis that matrix types with structural similarity to the organisms‟ home habitat would increase movement rate. Study design covariates were tested using meta-regression and sub-group analysis to explore factors affecting variation in effect size among studies.
To include a wider range of studies than those included in the meta-analysis, a narrative synthesis is presented for the 67 studies that included UK species. These studies were summarised by home habitat and by landscape feature tested.
All studies that were retrieved and met the inclusion criteria were on animals.
Meta-analysis found that, on average, corridors significantly increased inter-patch movement (risk ratio 1.60, 95% CI 1.19 to 2.15). However, there was high heterogeneity between studies, which we explored. Corridors with high structural contrast to the matrix were more likely to increase movement. Corridors were more likely to increase movement when the inter-patch distance was greater or the study period was shorter. Corridors were more likely to increase movement in studies where subjects in a patch were presented with a choice of matrix types than in studies of comparisons between separate patches, each surrounded by one matrix type.
Matrix types more structurally similar to home habitat tended to increase inter-patch movement compared to more dissimilar matrix types. However, the choice of test statistic affected whether the pooled effect was statistically significant and the sample of studies used in the analysis may have lacked adequate statistical power. Again, there was a great deal of heterogeneity in the results of different studies.
Analysis of data on emigration (i.e. patch-leaving) found that matrix type had a significant positive effect on the rates of individuals leaving patches (risk ratio 1.32, 95% CI 1.20 to 1.45), which was observed regardless of test statistic. Exploration of the considerable heterogeneity in study effect sizes found that comparisons of matrix types with contrasting vegetation structure (e.g. grass compared to forest, or water compared to a road) had significantly greater differences on the number of individuals leaving a patch. Choice studies showed a greater effect of matrix type on patch departure rates than did two-patch comparison studies, and studies with a longer duration showed a stronger impact of matrix type on patch emigration rates.
Studies in all three meta-analyses were restricted to mobile species studied over short distances and timescales. The possibility of publication bias was investigated using funnel plots. There was little evidence for bias in the corridor analysis but some suggestion of bias towards the publication of large negative studies in the matrix analyses (thought there were also small negative and small positive studies).
Qualitative, narrative synthesis of studies including UK species broadly supported the meta-analysis results. Species tended to move through matrix features more structurally similar to their home habitats. However, exceptions were common and appeared to reflect species-specific behaviours. Longer term studies tended to be less controlled and over larger areas, and there was a contrast between “experimental” studies, and radio-tagging of larger animals roaming over a greater area (“observational” studies).
Implications for managing landscapes for biodiversity
Landscape features outside of habitat patches are clearly important for some species as they can affect movement rates for mobile groups such as butterflies, birds and large herbivores. There is some quantitative and qualitative evidence that land cover features structurally similar to home habitat, and also habitat corridors, can increase movement rates of some species. For these species, providing habitat networks based on corridors and matrix elements with similar structure to the habitat should increase local dispersal. This could lead to greater population persistence but the evidence presented here does not test these implications.
Implications for research
Although some quantitative analyses are reported here, it is recommended that the potential for further quantitative analyses of the data is fully explored. There is a balance between furthering fundamental species-based information and furthering research synthesis. The search for fundamental information should be focussed on considering sources of heterogeneity in the response to matrix features (e.g. different species, different scales) or exploring the behavioural reasons for responses. In addition, a wider range of species (non-animals, slow movers) should be investigated.
In the context of commitments to halt the loss of biodiversity and meet other targets in the UK Biodiversity Action Plan, there is a need to consider the impacts of climate change on species, for understanding of their response and provision of potential adaptation measures (UK Biodiversity Partnership 2007). This is in addition to commitments in place which require reduction of the impacts of fragmentation. The EU Habitats Directive (EEC, 1992; transposed into law as the Habitats Regulations 1994) obliges the UK to endeavour to improve the ecological coherence of Natura 2000 sites (see Box 1) and maintain or restore favourable conservation status to species of community importance, many of which have been adversely affected by fragmentation. The England Biodiversity Strategy (EBS) Climate Change Adaptation workstream is currently engaged in promoting adaptation, and developing an adaptation strategy across all sectors of the EBS in recognition of the threat of climate change to meeting their biodiversity objectives.