Reachable Habitat Research Articles

Rapid loss in landscape connectivity after the peace agreement in the Andes-Amazon region

There is a complex interplay of criminal groups’ control over land, illicit activities, and forest cover change in the Colombian Andes-Amazon region. This area is dealing with diverse forms of conflict and peace, directly impacting landscape connectivity. While many studies have documented rapid deforestation after the peace agreement in 2016, we know little about the effect of these socio-political changes on the state of landscape connectivity. We disentangle habitat from connected habitat in forest ecosystems using the Landsat archive and landscape connectivity indices. We find that in the Andes-Amazon region during 2000–2020, connected habitat loss reached 18%, while habitat loss was 13%. This result is worrisome because it indicates that well-connected patches are more fragmented and isolated, affecting the natural connections between the Andes and Amazon biogeographical regions and the movement ability of species. The Colombian government should conduct a strategic peacebuilding process incorporating structural changes that prevent the increase of large-scale extractive activities that are often illegal in the region. While finding a balance between extractive activities and conservation remains a big challenge, legal land tenure, census/taxation, and specific agreements with local actors can initially prevent deforestation. We discourage localized military actions and the return of aerial fumigation of coca fields, which rather than stop deforestation might exacerbate land cover change deeper into pristine forests. • De-escalation of conflict coincides with intra-annual deforestation dynamics in Colombia. • Connected habitat loss (18%) is higher than habitat available (13%) during 2000–2020. • Rapid fragmentation of well-connected forest patches can affect the biological connection between the Andes-Amazon biomes. • Reachable habitat for species is now less than the habitat available. • Conservation strategies should focus on developing agreements with local communities.

Importance of climate‐induced tree species composition changes in forecasting the amount of reachable habitat for forest birds

AbstractAimBird and tree species distributions will shift along with future climatic conditions through direct effects related to species responses to bioclimatic variables. In addition, tree species composition changes should indirectly drive changes in forest bird habitats. Here, we predicted the effects of climate changes on the amount of reachable habitat for forest birds and teased apart the role of the indirect effects.LocationFrance.MethodsWe projected tree and bird species distributions (2020 and 2050) using generalized additive models which accounted for climatic conditions at a large scale (France) and local environmental conditions at regional scale. The projections combined two scenarios of climate changes (RCP 2.6 and 8.5) and two scenarios of forest management (increase in either deciduous or coniferous stands). We modelled the amount of reachable habitat with landscape graphs and the equivalent connectivity index (EC) for five birds associated to the main forest types of the study area. To differentiate the sources of EC change between 2020 and 2050, we compared two future landscape graphs including only the indirect effect, or the direct and indirect effects of climate changes.ResultsFor birds with low and intermediate thermal maxima, climate‐based distributions were nearly stable over time (absence of direct effects), EC change was only attributed to indirect effects. In contrast, for bird species with high thermal maximum, EC change was mostly driven by direct effects (more favourable climatic conditions) and little by indirect effects.Main conclusionsUsing an innovative modelling framework that bind species distribution and connectivity models with climate and land use changes in forest, we showed that EC dynamics could be completely due to climate‐driven changes in tree species composition. Our results highlight the importance of accounting for the indirect effects of climate changes, especially in forests where major shifts in tree species composition are expected.

Assessing the influence of the amount of reachable habitat on genetic structure using landscape and genetic graphs.

Genetic structure, i.e. intra-population genetic diversity and inter-population genetic differentiation, is influenced by the amount and spatial configuration of habitat. Measuring the amount of reachable habitat (ARH) makes it possible to describe habitat patterns by considering intra-patch and inter-patch connectivity, dispersal capacities and matrix resistance. Complementary ARH metrics computed under various resistance scenarios are expected to reflect both drift and gene flow influence on genetic structure. Using an empirical genetic dataset concerning the large marsh grasshopper (Stethophyma grossum), we tested whether ARH metrics are good predictors of genetic structure. We further investigated (i) how the components of the ARH influence genetic structure and (ii) which resistance scenario best explains these relationships. We computed local genetic diversity and genetic differentiation indices in genetic graphs, and ARH metrics in the unified and flexible framework offered by landscape graphs, and we tested the relationships between these variables. ARH metrics were relevant predictors of the two components of genetic structure, providing an advantage over commonly used habitat metrics. Although allelic richness was significantly explained by three complementary ARH metrics in the best PLS regression model, private allelic richness and MIW indices were essentially related with the ARH measured outside the focal patch. Considering several matrix resistance scenarios was also key for explaining the different genetic responses. We thus call for further use of ARH metrics in landscape genetics to explain the influence of habitat patterns on the different components of genetic structure.

Conservation planning on China's borders with Myanmar, Laos, and Vietnam.

Transboundary conservation is playing an increasingly important role in maintaining ecosystem integrity and halting biodiversity loss caused by anthropogenic activities. However, lack of information on species distributions in transboundary regions and understanding of the threats in these areas impairs conservation. We developed a spatial conservation plan for the transboundary areas between Yunnan province, southwestern China, and neighboring Myanmar, Laos, and Vietnam in the Indo‐Burma biodiversity hotspot. To identify priority areas for conservation and restoration, we determined species distribution patterns and recent land‐use changes and examined the spatiotemporal dynamics of the connected natural forest, which supports most species. We assessed connectivity with equivalent connected area (ECA), which is the amount of reachable habitat for a species. An ECA incorporates the presence of habitat in a patch and the amount of habitat in other patches within dispersal distance. We analyzed 197,845 locality records from specimen collections and monographs for 21,004 plant and vertebrate species. The region of Yunnan immediately adjacent to the international borders had the highest species richness, with 61% of recorded species and 56% of threatened vertebrates, which suggests high conservation value. Satellite imagery showed the area of natural forest in the border zone declined by 5.2% (13,255 km2) from 1995 to 2018 and monoculture plantations increased 92.4%, shrubland 10.1%, and other cropland 6.2%. The resulting decline in connected natural forest reduced the amount of habitat, especially for forest specialists with limited dispersal abilities. The most severe decline in connectivity was along the Sino‐Vietnamese border. Many priority areas straddle international boundaries, indicating demand and potential for establishing transboundary protected areas. Our results illustrate the importance of bi‐ and multilateral cooperation to protect biodiversity in this region and provide guidance for future conservation planning and practice.

Explicit integration of dispersal-related metrics improves predictions of SDM in predatory arthropods

Fishing spiders (Dolomedes spp.) make an interesting model to predict the impact of global changes because they are generalist, opportunistic predators, whose distribution is driven mostly by abiotic factors. Yet, the two European species are expected to react differently to forthcoming environmental changes, because of habitat specialization and initial range. We used an original combination of habitat and dispersal data to revisit these predictions under various climatic scenarios. We used the future range of suitable habitat, predicted with habitat variables only, as a base layer to further predict the range or reachable habitat by accounting for both dispersal ability and landscape connectivity. Our results confirm the northward shift in range and indicate that the area of co-occurrences should also increase. However, reachable habitat should expand less than suitable habitat, especially when accounting for landscape connectivity. In addition, the potential range expansion was further limited for the red-listed D. plantarius, which is more of a habitat specialist and has a lower ability to disperse. This study highlights the importance of looking beyond habitat variables to produce more accurate predictions for the future of arthropods populations.

Influence of land-use/land-cover change on landscape connectivity for an endemic threatened amphibian (Argenteohyla siemersi pederseni, Anura: Hylidae)

Modifications in natural landcover generally result in a loss of habitat availability for wildlife and it’s persistence will depend largely on their spatial configuration and functional connections. Argenteohyla siemersi is a threatened and endemic amphibian whose habitat is composed of forest patches near rivers and water bodies edges. This study aimed to analyse the accessible habitat for this species and identify key elements to maintain its ecological network in two different types of land uses: an anthropized area with extensive cattle raising and a protected area. The structural and functional characteristics of both landscapes were analyzed. The connectivity at landscape level and the contribution of each habitat patch were evaluated through simulation models with different dispersion distances in the context of the graph theory. In both landscapes, nine types of landcover were identified with different compositions. Remarkable differences were found in habitat connectivity for this amphibian species between both landscapes. As the percentage of dispersion distance increases, reachable habitat increases as well, although with higher percentages in the protected area. Two corridors were identified in the protected landscape and one in the rangeland one; patches and key links constituted all of them. The present work provides spatially explicit results with a quantitative basis. It could be useful as a tool for the development of management plans aimed at guaranteeing the functionality of the ecological network for this endangered species and, therefore, contribute to its long-term conservation.

Importance of spatio–temporal connectivity to maintain species experiencing range shifts

Climate change can affect the habitat resources available to species by changing habitat quantity, suitability and spatial configuration, which largely determine population persistence in the landscape. In this context, dispersal is a central process for species to track their niche. Assessments of the amount of reachable habitat (ARH) using static snap‐shots do not account, however, for the temporal overlap of habitat patches that may enhance stepping‐stone effects. Here, we quantified the impacts of climate change on the ARH using a spatio–temporal connectivity model. We first explored the importance of spatio–temporal connectivity relative to purely spatial connectivity in a changing climate by generating virtual species distributions and analyzed the relative effects of changes in habitat quantity, suitability and configuration. Then, we studied the importance of spatio–temporal connectivity in three vertebrate species with divergent responses to climate change in North America (grey wolf, Canadian lynx and white‐tailed deer). We found that the spatio–temporal connectivity could enhance the stepping‐stone effect for species predicted to experience range contractions, and the relative importance of the spatio–temporal connectivity increased with the reduction in habitat quantity and suitability. Conversely, for species that are likely to expand their ranges, spatio–temporal connectivity had no additional contribution to improve the ARH. We also found that changes in habitat amount (quantity and suitability) were more influential than changes in habitat configuration in determining the relative importance of spatio–temporal connectivity. We conclude that spatio–temporal connectivity may provide less biased and more realistic estimates of habitat connectivity than purely spatial connectivity.

Environmental mitigation hierarchy and biodiversity offsets revisited through habitat connectivity modelling

Biodiversity loss is accelerating because of unceasing human activity and land clearing for development projects (urbanisation, transport infrastructure, mining and quarrying …). Environmental policy-makers and managers in different countries worldwide have proposed the mitigation hierarchy to ensure the goal of “no net loss (NNL) of biodiversity” and have included this principle in environmental impact assessment processes. However, spatial configuration is hardly ever taken into account in the mitigation hierarchy even though it would greatly benefit from recent developments in habitat connectivity modelling incorporating landscape graphs. Meanwhile, national, European and international commitments have been made to maintain and restore the connectivity of natural habitats to face habitat loss and fragmentation.Our objective is to revisit the mitigation hierarchy and to suggest a methodological framework for evaluating the environmental impact of development projects, which includes a landscape connectivity perspective. We advocate the use of the landscape connectivity metric equivalent connectivity (EC), which is based on the original concept of “amount of reachable habitat”. We also refine the three main levels of the mitigation hierarchy (impact avoidance, reduction and offset) by integrating a landscape connectivity aspect.We applied this landscape connectivity framework to a simple, virtual habitat network composed of 14 patches of varying sizes. The mitigation hierarchy was addressed through graph theory and EC and several scenarios of impact avoidance, reduction and compensation were tested.We present the benefits of a habitat connectivity framework for the mitigation hierarchy, provide practical recommendations to implement this framework and show its use in real case studies that had previously been restricted to one or two steps of the mitigation hierarchy. We insist on the benefits of a habitat connectivity framework for the mitigation hierarchy and for ecological equivalence assessment. In particular, we demonstrate why it is risky to use a standard offset ratio (the ratio between the amount of area negatively impacted and the compensation area) without performing a connectivity analysis that includes the landscape surrounding the zone impacted by the project. We also discuss the limitations of the framework and suggest potential improvements. Lastly, we raise concerns about the need to rethink the strategy for biodiversity protection. Given that wild areas and semi-natural habitats are becoming scarcer, in particular in industrialised countries, we are convinced that the real challenge is to quickly reconsider the current vision of “developing first, then assessing the ecological damage”, and instead urgently adopt an upstream protection strategy that would identify and protect the land that must not be lost if we wish to maintain viable species populations and ecological corridors allowing them the mobility necessary to their survival.

Maximizing habitat connectivity in the mitigation hierarchy. A case study on three terrestrial mammals in an urban environment

Environmental policies and the objective of no net loss highlight the importance of preserving ecological networks to limit the fragmentation of natural habitats and biodiversity loss, especially due to urbanization. In the environmental impact assessment context, habitat connectivity and the spatio-temporal dynamics of biodiversity are crucial to obtaining reliable predictions that can support decision-making. We propose a methodological framework 1) to quantify the overall impact of a development project on the functioning of an ecological network, and 2) to select the best locations for implanting new habitat patches intended to enhance landscape connectivity. The amount of reachable habitat concept was applied to three representative terrestrial mammal species: the red squirrel, the Eurasian badger and the European hedgehog. All three species are recognized as vulnerable to human pressures and potentially affected by the construction of a new stadium in our study site, Lyon (Southern France). The method combines the species distribution model Maxent with the landscape functional connectivity model Graphab. The results showed that using any one of the avoidance and reduction measures on its own was unsuccessful in achieving the objective of no net loss when habitat connectivity is considered. However, the combination of new habitat patches and corridors offered a higher gain than distinct measures. This is especially important in the short term, when new hedgerow plantations have not yet developed enough to be used by the target species. Our findings indicate, first, the need to take the temporal scale into account in environmental impact assessment. We also show that applying the optimal scenario, constructed using a cumulative patch addition followed by a similar process testing a set of potential land-use changes, maximizes habitat connectivity. Our methodology provides a useful tool to increase target species’ habitat connectivity within the mitigation hierarchy and to enhance development project design for increased environmental efficiency.

Host functional connectivity and the spread potential of Lyme disease

In the ecology of Lyme disease emergence, it remains unclear to what extent spread of the tick vector (Ixodes scapularis) and the pathogen (Borrelia burgdorferi) are dependent upon the dispersal of vertebrate hosts in spatially heterogeneous landscapes. Yet, empirical measure of these complex ecologically driven spread processes present conceptual and methodological challenges despite important public health implications. To examine the relationship between landscape characteristics and tick-borne disease spread, we modeled the influence of landscape connectivity for a simplified vertebrate host community (white-footed mouse—Peromyscus leucopus, American robin—Turdus migratorius, white-tailed deer—Odocoileus virginianus) on the potential spread of the tick population compared to the pathogen in a spatially-structured landscape. We parameterized a hybrid demographic-dispersal connectivity model by combining a series of reported host dispersal and tick burden estimates with empirically-measured tick abundance and pathogen prevalence sampled from a Lyme-endemic island landscape in Thousand Islands National Park (Ontario, Canada) and simulated several tick- and pathogen-spread scenarios. The extent of tick spread by mice [amount of reachable habitat (ARH) = 18.0%] is considerably similar to that of robins (ARH = 18.7%), while deer support the greatest tick spread extent (ARH = 82.0%). Infected mice carrying ticks support the highest pathogen spread (ARH = 19.8%). Short-distance pathogen spread and long-distance tick spread were facilitated by intermediate stepping stone habitat fragments. We provide evidence that host functional connectivity mediates tick spread differently than pathogen spread, and depends strongly on landscape configuration. Our study therefore emphasizes the importance of landscape spatial heterogeneity on the ecological processes that influence regional tick-borne disease spread.

The challenge of assaying landscape connectivity in a changing world: A 27-year case study in the southern Great Plains (USA) playa network

Many habitat resources fluctuate in availability due to natural environmental variability and anthropogenic influences. These fluctuations pose challenges to organisms attempting to move from one habitat patch to another, and also pose challenges to detecting and managing factors impacting landscape connectivity. Our understanding of these relationships is further hampered by lack of precedence on how to quantify dynamic connectivity. The ephemeral freshwater wetlands of the southern Great Plains of the USA (playas) form a dynamic habitat network that serves as a case study of these challenges and allows us to propose a suite of connectivity metrics to monitor changes in network topology and evaluate the management importance of individual wetlands. We used satellite imagery to examine inundation patterns of >7000 playas in a 29,083 km2 portion of Texas on 80 dates from 1984 to 2011. Based on historic locations of playa basins, approximately 85% of playas (particularly those ≤10 ha) have lost the capacity to hold water even during regionally wet times, resulting in a ∼69% reduction of surface water area. These losses were associated with proximity to cropland, with total cropland acreage increasing by 0.07–17.34% of county land area during our focal time span in 10 of the 14 counties in our study area. We examined connectivity at wetland and whole-network scales to determine effects of playa losses on network topology and thus on connectivity. We evaluated 11 metrics for this purpose, which quantified the number of wet playas present on each date, their degree of connectedness, their clustering, path redundancy within the network, overall network topology, the importance of individual playas in various roles, and the size of a single playa that would provide equivalent connectivity (amount of reachable habitat) as in the actual network. Topology has thinned over the past three decades with playa losses, reflected in increasing graph density, average path length, degree of connectivity for highly linked hubs, and average number of cutpoints. Similarly, graph diameter is currently less than half of the historic potential maximum, and the equivalent connected area has declined by over 23% since 1984 (and by over 82% relative to historic values). These patterns suggest that path redundancy through the network has declined such that dispersers currently have fewer connectivity options compared to a few decades ago. Relatively high transitivity scores indicate that the playa network is still populated with a large (but diminishing) number of wetlands, and the dwindling surface water present in the remaining playas is not compensating for playa losses over time. Average coalescence distances are currently higher than the dispersal capacity of many organisms, meaning that the playa network is fragmented such that only an extremely vagile disperser (capable of moving at least 18–45 km) would be able to traverse the landscape via the remaining wetlands, even if all were wet simultaneously. These findings illustrate the importance of using multiple indicators in assaying dynamic connectivity and provides a framework of possible metrics to use for monitoring and assessment of any dynamic habitat network.

Islands, mainland, and terrestrial fragments: How isolation shapes plant diversity.

The fragmentation of natural habitats is a major threat for biodiversity. However, the impact and spatial scale of natural isolation mechanisms leading to species loss, compared to anthropogenic fragmentation, are not clear, mainly due to differences between fragments and islands, such as matrix permeability. We studied a 500 km2 Mediterranean region in France, including urban habitat fragments, continuous habitat, and continental‐shelf islands. On the basis of 295 floristic relevés, we built species–area relationships to compare isolation in fragments after urbanization, with continuous habitat and continental‐shelf islands. We assumed either no dispersal, infinite dispersal, or estimated intermediate levels of habitat reachability through graph theory. Isolation mechanisms occurred in fragments but with a lower strength than in near‐shore islands, and most importantly affected perennial plants. Annual plants were less affected, probably due to their smaller size and shorter life cycle. Isolation occurred at landscape level in fragments and at patch level in islands. The amount of reachable habitat (accounting for spatial configuration) explained local species richness in both systems, but the amount of habitat (no consideration of spatial configuration) was already a good predictor. These results suggest an important role of habitat amount around fragments in mitigating the isolation effects observed in near‐shore islands, and the importance of carefully considering different functional groups.

Spatio‐temporal connectivity: assessing the amount of reachable habitat in dynamic landscapes

SummaryLandscape heterogeneity and habitat connectivity affect species movements, playing an important role in determining the likelihood of species persistence. However, landscape connectivity is usually evaluated using static snap‐shots, which do not account for the sequential interactions among habitat patches through time.We developed a network‐based model of landscape dynamics, and corresponding connectivity metrics, to account for the reachable habitat across space and time. We illustrate the behaviour of these metrics, using fragmented forested landscapes in the Atlantic Forest of Brazil. We parametrized the models using the dispersal capacities of selected bird and small mammal species.We found that when considering spatio‐temporal links, connectivity is estimated to be on average 30% higher (with a maximum of 150% higher) than what is estimated from purely spatial models. This higher degree of spatio‐temporal connectivity arises due to connections through temporal stepping‐stone patches that appear (habitat gain) and disappear (habitat loss) over time. Species with short dispersal distances (<1000 m) particularly benefited from the spatio‐temporal connections. The contribution of spatio‐temporal connectivity to habitat reachability increased with higher habitat loss rates. Moreover, it depended on the amount of habitat in the landscape, being higher at intermediate habitat amounts (∼30%).We showed that accounting for spatio‐temporal connectivity is critical for understanding ecological patterns and processes in dynamic landscapes, and that a series of purely spatial connectivity metrics underestimates the actual connectivity patterns across time. The proposed spatio‐temporal connectivity approach and metrics can be applied to evaluate the effective connectivity patterns and trends in a variety of dynamic landscapes, avoiding the potential overestimates of population isolation and extinction probabilities that may result from widely used purely spatial connectivity models.

Assessing the importance of individual habitat patches as irreplaceable connecting elements: An analysis of simulated and real landscape data

Habitat loss and fragmentation are considered to be severe threats to biodiversity, and maintaining natural levels of landscape connectivity may be one of the best responses to these issues. Graph-based habitat availability (reachability) metrics have been shown to be an appropriate method for a multifaceted but coherent landscape connectivity assessment. These metrics can be partitioned into three commensurate fractions (intra, flux, connector) that quantify the different ways in which habitat patches contribute to the overall landscape connectivity and habitat availability. In particular, the connector fraction measures the contribution of patches to the connectivity between other habitat areas as irreplaceable connecting elements or stepping stones. Because many conservation efforts and initiatives are focused on conserving or restoring corridors and other linkages between habitat areas, it is critical to understand more thoroughly the conditions under which investing in these connecting elements is an efficient management strategy. For this purpose, we analysed the contribution of the connector fraction in different simulated habitat patterns under different levels of habitat amount and fragmentation and in natural habitats for endangered forest bird species in Catalonia (Spain). We show that a prominent role of individual stepping stone patches as irreplaceable providers of habitat connectivity and availability arises only under a relatively narrow set of conditions, characterised by low habitat amount, high habitat fragmentation and modest to intermediate species dispersal abilities. We suggest that to support connectivity-related investments, it is necessary to focus on those few species or dispersal distance ranges that are likely to be both most dependent on and most benefited by the conservation or restoration of stepping stone patches. We conclude that the total amount of reachable habitat for a particular species is rarely determined by the contribution of individual connectors as the only dominant factor. Therefore, conservationists should be cautious not to overemphasise the importance of connectivity investments and to balance them with other conservation alternatives and strategies to promote species conservation in heterogeneous landscapes.