Effects of increasing landscape heterogeneity on local plant species richness: how much is enough?

Contemporary landscape ecology continues to explore the causes and consequences of landscape heterogeneity across a range of scales, and demands for the scientific underpinnings of landscape planning and management still remains high. The spatial distribution of resources can be a key element in determining habitat quality, and that in turn is directly related to the level of heterogeneity in the system. In this sense, forest habitat mosaics may be more affected by lack of heterogeneity than by structural fragmentation. Nonetheless, increasing spatial heterogeneity at a given spatial scale can also decrease habitat patch size, with potential negative consequences for specialist species. Such dual effect may lead to hump-backed shape relationships between species diversity and heterogeneity, leading to three related assumptions: (i) at low levels of heterogeneity, an increase in heterogeneity favours local and regional species richness, (ii) there is an optimum heterogeneity level at which a maximum number of species is reached, (iii) further increase in spatial heterogeneity has a negative effect on local and regional species richness, due to increasing adverse effects of habitat fragmentation. In this study, we investigated the existence of a hump-shaped relationship between local plant species richness and increasing forest landscape heterogeneity on a complex mosaic in the French Alps. Forest landscape heterogeneity was quantified with five independent criteria. We found significant quadratic relationships between local forest species richness and two heterogeneity criteria indicators, showing a slight decrease of forest species richness at very high heterogeneity levels. Species richness–landscape heterogeneity relationships varied according to the heterogeneity metrics involved and the type of species richness considered. Our results support the assumption that intermediate levels of heterogeneity may support more species than very high levels of heterogeneity, although we were not able to conclude for a systematic negative effect of very high levels of heterogeneity on local plant species richness.     

Is it sufficient? Assessment of two sampling methods for urban plant species richness investigations

Determining adequate sample size (ASS) and minimum plot size (MPS) are two fundamental issues in urban vegetation investigations. To estimate the ASS and MPS of urban plants, we introduced sampling completeness to the MPS calculation based on 54 samples in the urban area of Chongqing, China. Then, we examined the performance of the species-area curve method and CV_Jack1 (coefficient of variation (CV) for the first-order jackknife estimator (Jack1)) curve extrapolation method. We also tested the effectiveness and error distribution of extrapolating the ASS from CV_Jack1 curves using 180 samples from the urban area of Chongqing, China, and 222 samples from the urban area of Xiamen, China. The results of the urban plant analyses showed that (1) the constructed species-area accumulation curves fit a logistic function (R² > 0.990); (2) the MPS increased with sampling completeness, whereas unrecorded species exhibited the opposite trend; (3) the CV_Jack1 curves fit the allometric1 function (R² > 0.960) but might fail to reach a CV value of 0.05 for the calculation of ASS if the sample size was too small; and (4) the relative error values were below 5% when the error curves of extrapolation (by CV_Jack1 curve) approached a horizontal asymptote. The species-area accumulation curve with sampling completeness and the CV_Jack1 curve extrapolation method were promising approaches for determining the MPS and ASS of urban plants. The results and methods in this study provide a reference for determining the appropriate plot size and sample size for urban plant species richness investigations.     

Get larger or grow longer wings? Impacts of habitat area and habitat amount on orthopteran assemblages and populations in semi-natural grasslands

Landscape Ecology - Habitat loss and fragmentation are major threats to biodiversity and can change community composition, species traits and intraspecific morphology. Calcareous grasslands are...     

Enhancing connectivity in agroecosystems: focus on the best existing corridors or on new pathways?

Context

Restoring or establishing corridors between residual forest patches is one of the most adopted strategies for the conservation of animal populations and ecosystem processes in fragmented landscapes.

Objectives

This study aimed to assess whether it is more effective to focus restoration actions on existing corridors or to establish habitats in other strategic areas that can create new dispersal pathways to enhance connectivity.

Methods

We considered a real agroecosystem in northern Italy, based our analyses on graph-theory and habitat availability metrics, and focused on the Hazel Dormouse as the target species. We compared the connectivity increase resulting from (i) the simulated restoration of existing priority corridors, i.e., those with significant presence of forest but in which restoration actions would still result in considerable connectivity gains, or (ii) the simulated plantation of 30 hedgerows along new priority pathways, i.e., those areas with no current forest cover in which habitat creation would be more beneficial for connectivity.

Results

Implementing new priority pathways resulted in substantially larger connectivity gains (+?38%) than when restoration efforts were concentrated in improving already existing corridors (+?11%).

Conclusions

Establishing hedgerows along new pathways allowed enhancing the complementary and functionality of the full set of landscape corridors and proved more efficient than just strengthening the areas where dispersal flows were already concentrated. We demonstrated the importance of analytical procedures able to compare the effectiveness of different management strategies for enhancing connectivity. Our approach may be applied to multiple species sensitive to fragmentation in other heterogeneous landscapes and geographical contexts.

     

Bat richness and activity in heterogeneous landscapes: guild-specific and scale-dependent?

Context

The conversion of natural environments into agricultural land has profound effects on the composition of the landscape, often resulting in a mosaic of human-altered and natural habitats. The response to these changes may however vary among organisms. Bats are highly vagile, and their requirements often imply the use of distinct habitats, which they select responding to both landscape and local features.

Objectives

We aimed to identify which features influence bat richness and activity within Baixo Vouga Lagunar, a heterogeneous landscape located on the Central-North Portuguese coast, and to investigate if that influence varies across a gradient of focal scales.

Methods

We sampled bats acoustically, while simultaneously sampling insects with light traps. We assessed the relationships between species richness, bat activity, and activity of eco-morphological guilds with landscape and local features, across four scales.

Results

Our results revealed both scale- and guild-dependent responses of bats to landscape and local features. At broader scales we found positive associations between open-space foraging bats and habitat heterogeneity and between edge-space foraging bats and greater edge lengths. Woodland cover and water availability at an intermediate scale and weather conditions and insect abundance at a local scale were the factors that mostly influenced the response variables.

Conclusions

Globally, our results suggest that bats are sensitive to local resource availability and distribution, while simultaneously reacting to landscape features acting at coarser scales. Finally, our results suggest that the responses given by bats are guild-dependent, and some habitats act as keystone structures for bats within this mosaic.

     

Do corridors promote dispersal in grassland butterflies and other insects?

Ecological corridors are frequently suggested to increase connectivity in fragmented landscapes even though the empirical evidence for this is still limited. Here, we studied whether corridors, in the form of linear grass strips promote the dispersal of three grassland butterflies, using mark-recapture technique in an agricultural landscape in southern Sweden. We found no effects of the presence of corridors or of corridor length on inter-patch dispersal probabilities. Instead, dispersal probabilities appeared to be related to the quality, areas and population densities of the source and recipient patches. For two of the species, the density of captured individuals along corridors was better predicted by the corridor length than by the straight-line distance from a pasture, suggesting that short-distance movements within habitat patches result in a diffusion of individuals along corridors. A literature review revealed that only 16 published studies had explicitly studied the effect of corridors on insect movement. The context in which studies were performed appeared to affect whether corridors facilitated dispersal or not. All seven studies where the corridors consisted of open areas surrounded by forest showed positive effects, while only two out of six studies where corridors consisted of grassland surrounded by other open habitats showed positive effects of corridors. Our results clearly demonstrate that corridors do not always have positive effects on insect dispersal and that the effect seems to depend on the quality of the surrounding matrix, on the spatial scale in which the study is performed and on whether true dispersal or routine movements are considered. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seasonal and temporal changes in species use of the landscape: how do they impact the inferences from multi-scale habitat modeling?

Context

Multi-scale approaches to habitat modeling have been shown to provide more accurate understanding and predictions of species-habitat associations. It remains however unexplored how spatial and temporal variations in habitat use may affect multi-scale habitat modeling.

Objectives

We aimed at assessing how seasonal and temporal differences in species habitat use and distribution impact operational scales, variable influence, habitat suitability spatial patterns, and performance of multi-scale models.

Methods

We evaluated the environmental factors driving brown bear habitat relationships in the Cantabrian Range (Spain) based on species presence records (ground observations) for the period 2000–2010, LiDAR data on forest structure, and seasonal estimates of foraging resources. We separately developed multi-scale habitat models for (i) each season (spring, summer, fall and winter) (ii) two sub-periods with different population status: 2000–2004 (with brown bear distribution restricted to the main population nuclei) and 2005–2010 (with expanding bear population and range); and (iii) the entire 2000–2010 period.

Results

Scales of effect remained considerably stable across seasonal and temporal variations, but not the influence of certain environmental variables. The predictive ability of multi-scale models was lower in the seasons or periods in which populations used larger areas and a broader variety of environmental conditions. Seasonal estimates of foraging resources, together with LiDAR data, appeared to improve the performance of multi-scale habitat models.

Conclusions

We highlight that the understanding of multi-scale behavioral responses of species to spatial patterns that continually shift over time may be essential to unravel habitat relationships and produce reliable estimates of species distributions.

     

Small forest patches as pollinator habitat: oases in an agricultural desert?

Landscape Ecology - Small forest fragments are often the most abundant type of semi-natural habitat in intensive agricultural landscapes. Wild pollinators can use these forest patches as nesting or...     

Butterfly assemblages in residential gardens are driven by species’ habitat preference and mobility

Context

Understanding the factors contributing to maintaining biodiversity is crucial to mitigate the impact of anthropogenic disturbances. Representing large proportions of green area in highly modified landscapes, residential gardens are often seen as local habitats that can contribute to larger networks of suitable environments at the landscape scale.

Objectives

We investigated the impact of the landscape context on butterfly communities observed in residential gardens, taking into account garden characteristics, land-use types and presence of linear features in the surrounding landscape. We examined how species traits affected butterflies’ response to landscape context and habitat quality.

Methods

We performed a cross-scale study, based on citizen science data documenting butterfly species composition and abundance in 920 gardens across France. We examined the effect of garden quality, the area of different land-use types and the length of linear elements measured at three scales within the surrounding landscape. Species were grouped according to their habitat preference and mobility.

Results

Urbanization negatively affected total species richness and the abundance of butterfly in each group. This was related to declining habitat quality and reduced area of suitable habitat in the surrounding landscape. The magnitude of this effect, however, was negatively correlated with mobility, a trait related to habitat preference. The spatial scale at which landscape context best explained variation in butterfly abundance changed with species’ habitat preference.

Conclusions

This study highlights the importance of preserving high quality habitats in altered landscapes and considering species’ mobility and habitat preference when assessing the impact of landscapes on butterfly communities.

     

Road verges as invasion corridors? A spatial hierarchical test in an arid ecosystem

Disturbed habitats are often swiftly colonized by alien plant species. Human inhabited areas may act as sources from which such aliens disperse, while road verges have been suggested as corridors facilitating their dispersal. We therefore hypothesized that (i) houses and urban areas are propagule sources from which aliens disperse, and that (ii) road verges act as corridors for their dispersal. We sampled presence and cover of aliens in 20 plots (6 × 25 m) per road at 5-km intervals for four roads, nested within three localities around cities (n = 240). Plots consisted of three adjacent nested transects. Houses (n = 3,349) were mapped within a 5-km radius from plots using topographical maps. Environmental processes as predictors of alien composition differed across spatial levels. At the broadest scale road-surface type, soil type, and competition from indigenous plants were the strongest predictors of alien composition. Within localities disturbance-related variables such as distance from dwellings and urban areas were associated with alien composition, but their effect differed between localities. Within roads, density and proximity of houses was related to higher alien species richness. Plot distance from urban areas, however, was not a significant predictor of alien richness or cover at any of the spatial levels, refuting the corridor hypothesis. Verges hosted but did not facilitate the spread of alien species. The scale dependence and multiplicity of mechanisms explaining alien plant communities found here highlight the importance of considering regional climatic gradients, landscape context and road-verge properties themselves when managing verges. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Against the wall—Root growth and competition in four perennial winter hardy plant species grown in living walls

Plants in living walls face challenges from intraspecific and interspecific competition from plants around them, as well as water and nutrient availability in the growing media. This paper explores these challenges using four different species of hardy perennials.Campanula poscharskyana ‘Stella’, Geranium sanguineum ‘Max Frei’, Sesleria heufleriana and Veronica officinalis ‘Allgrün’ were grown in two types of vertical growing media, made of either coir or stone wool, in transparent boxes under greenhouse conditions. In the media, plants were placed above each other, two plants of same species, two plants of different species, or a plant grown alone. Root frequency was registered over 56 days and the activity of individual root systems was studied through uptake of ¹⁵N. In addition, plant dry weight and N content as well as water content in the growing media were measured at cessation of the experiment.Shoot and root growth as well as nutrient content in plants were higher in coir than in stone wool and plants placed at the top position had significantly higher biomass than at the bottom position. The stone wool media had significantly higher water content in the lower part of the media while the coir media had water more evenly distributed. Species differences in root frequency were found. Campanula and Geranium showed strong root growth and had root frequencies of up to 0.9, whereas Sesleria and Veronica had less root growth, in some cases only root frequencies around 0.3. The species reacted differently to root competition and planting position and there were differences in the competitive ability between the species. Campanula and Geranium were not affected by competition, whereas both Veronica and Sesleria showed altered root growth due to competition depending on the growing medium. When Geranium was grown above Veronica in stone wool, plant biomass and ¹⁵N uptake increased in Veronica indicating environmental modification, with one species improving the growing conditions for another.The results revealed that growing plants vertically in a living wall is complex, and that choice of growing medium and species composition is important for a successful living wall. Planting combinations should therefore be tested before being used in commercial applications.     

Relative influence of local- and landscape-level habitat quality on aquatic plant diversity in shallow open-water wetlands in Alberta’s boreal zone: direct and indirect effects

Reclamation usually involves modification of the local environment to achieve some biotic target, but if the influence of Landscape Condition on that target is great, we may fail to meet it despite efforts at the local-level. We sought to determine the relative influence of local- and landscape-level habitat on aquatic plant diversity in shallow open-water wetlands. Furthermore, we asked whether the influence of Landscape Condition should be attributed to direct (dispersal-related) effects, or to the indirect effect of landscape variables that influence local habitat quality. Finally, we asked if spatial scale (300–2000 m) would affect conclusions about the relative influence of local- and landscape-level effects. Using structural equation modeling, we found that Local Condition is consistently more influential than Landscape Condition. As landscape size increases, the relative importance of Landscape Condition declines and there is a trade-off between its direct and indirect components. At ≤500 m direct landscape effects were of greater importance than indirect effects, whereas indirect effects of Landscape Condition became more important at ≥1500 m. This suggests that the dominant mechanism by which land use influences diversity depends on the spatial extent of the landscape. We recommend that reclamation designs include a high proportion of wetland habitat and incorporate seeding/planting if diverse plant communities are desired. Additionally, we note that the influence of the landscape is strongest within 300 m. Thus, the focus of reclamation efforts should remain at the in-lake level and the immediate surroundings: this is where efforts will achieve the greatest effect on aquatic plant diversity.     

Poisonous and allergenic plant species in preschool’s and primary school’s yards in the city of Novi Sad

Presence of the poisonous and allergenic plant species was observed in 8 primary schools and 6 preschools in Novi Sad, Serbia. The aim of this research was to determine the extent to which pre-school and school-age children are exposed to harmful plant species and also to gain insight does the present landscaping take into account the appropriateness in the use of certain ornamental species in arranging preschool's and school's greenery. In the analyzed locations 22 poisonous plant species were noticed, mainly belonging to genera Thuja, Symphoricarpos, Cotoneaster, Juniperus, Berberis and Taxus, represented with 367 specimens. Along with poisonous, 21 allergenic plant species mainly belonging to genera Acer, Tilia, Betula, Populus, Platanus, Celtis, Aesculus, Thuja, Ulmus, Robinia and Quercus, represented with 675 specimens, were determined. The calculated allergenic index grouped most of the investigated species into strongly allergenic (calculated A.I. = 7) due to their prolonged phenantesic period, high abundance and possible cross-reactivity with other species noticed in the greenery. Establishment and enhancement of the collaboration between schools and experts from relevant institutions would result in the removal of the very most allergenic and poisonous landscape plants and ultimately toxic-free and allergy-friendly school yards. Gradual removal might temper the public reluctance to this measurement, but finally only joint efforts and complete abandonment of pointed species can convert unhealthy school yards into ones that are safe and allergy-friendly.     

Edge effects in tropical dry forests of Madagascar: additivity or synergy?

Context

The study of habitat fragmentation is complex because multiple, potentially synergistic, ecological processes may be acting simultaneously. Further, edge effects themselves may be complex in that additivity from multiple edges can give rise to heterogeneous nearest–edge gradients.

Objectives

We used heat diffusion as a proxy for additive edge effects in two study landscapes in order to test whether two key observations recently attributed to synergy between edge and area effects could be more simply explained by additivity; namely, steeper edge gradients in larger fragments and variation in slopes of species–area relationships as a function of distances to fragment edges.

Methods

We sampled forest structure in northwestern Madagascar at various distances from the edge in fragments and continuous forest and used an inverse modelling approach to parameterize the model. In addition, we applied the model to data from a published study of beetle communities in fragmented forests in New Zealand.

Results

With increasing proximity to edges, woody stem densities decreased and, as predicted, smaller fragments had lower stem densities and less steep edge gradients than larger ones. The model successfully predicted shifts in species–area relationships as a function of nearest–edge distances for beetle species, although observed richness for forest specialists in the smallest fragments was lower than predicted.

Conclusions

Two key observations attributed to synergy between edge and area effects were explained by edge additivity. The model is particularly useful in that it can help to disentangle the complex sets of processes acting in fragmented landscapes.

     

Species richness of hedgerow habitats in changing agricultural landscapes: are α and γ diversity shaped by the same factors?

Understanding the determinants of hedgerow plant diversity in agricultural landscapes remains a difficult task, because the potential drivers affect the complete range of biodiversity components (alpha to gamma diversity). We surveyed herbaceous plant communities (of a height <1.5 m) in 84 hedgerows in the Seine river floodplain of France. Two types of potential drivers for species richness, accounting for landscape mosaic and hedgerow network, were recorded at both hedgerow and site scale. The distribution of species richness through the components of alpha hedgerow diversity (i.e. the average diversity within a habitat) and gamma hedgerow diversity (i.e. the total diversity across habitats) were assessed using additive partitioning methods, while the relationship between species diversity and its potential landscape drivers at both scales was modeled using Generalized Additive Models. Our results indicated that gamma hedgerow diversity is explained by the heterogeneity of the landscape structure, which is correlated with the mosaic of agricultural land use. At this scale, intrinsic properties of the configuration of the hedgerow networks have a weak influence on species richness. Alpha hedgerow diversity is also explained by landscape variables, accounting for both the configuration of agricultural mosaics and hedgerow networks, but to a lesser extent. Time lags for species responses are shown at both scales, and for the two types of drivers. Extinction or colonization debt may be indicated at both scales, while the remnant effects of former practices may also be responsible for such patterns at a local scale. We suggest that hedgerow management should take the specific parameters of both scales into account. At a local scale, management actions should aim to decrease the influence of adjacent land use when the impact is negative, through the implementation of extended buffer zones, while at the landscape and farm scales, agri-environmental schemes should be dedicated to the conservation of specific agricultural land uses.     

Suburban habitats and their role for birds in the urban–rural habitat network: points of local invasion and extinction?

Suburban habitats in naturally forested areas present a conundrum in the urban–rural habitat network. Typically, these habitats contain less than half of the native woodland bird species that would exist at these sites if they were not developed. They also contain more total bird species than if these sites were left in a natural state. This apparent contradiction raises the question of “How do suburban habitats function in the urban–rural habitat network?” In this study, we analyze bird distributions on three rural-to-urban gradients in different ecoregions of the United States: Oxford, Ohio; Saint Paul, Minnesota; and Palo Alto, California. All three gradients exhibit similar patterns of extinction of native species followed by invasion of common species and subsequent biotic homogenization with urbanization. This patterning suggests that suburban land uses, those represented by the intermediate levels of development on the gradients, are a point of extirpation for woodland birds as well as an entry point for invasive species into urban systems. Furthermore, there are consistent patterns in the functional characteristics of the bird communities that also shift with intensifying urbanization, providing insight on the possible mechanisms of homogenization and community structure in urban ecosystems including an increase in the number of broods per year, a shift in nesting strategies, a decrease in insectivorous individuals, an increase in granivorous individuals, and a decrease in territoriality. Consequently, it appears that there are specific traits that drive the shift in community composition in response to urban and suburban land use. These results have significant implications for improving understanding of the mechanisms of suburban community ecology and conserving birds in urban habitat networks.     

The ‘habitat backbone’ as strategy to conserve pioneer species in dynamic port habitats: lessons from the natterjack toad (Bufo calamita) in the Port of Antwerp (Belgium)

Biodiversity conservation in economic areas like ports has recently become more important in the European Union due to a stricter interpretation of nature protection laws. In this study we develop a planning and design strategy—the ‘habitat backbone’—with which to support the long-term survival of pioneer species that occur in ports and have low dispersal abilities. For those species, long-term survival in port areas is uncertain because supply of their habitats (on vacant lots) is capricious and depends on land use dynamics. By gaining knowledge about spatial and temporal characteristics of these dynamics we were able to develop a solution to conserve such species. Our solution is based on the creation of permanent habitat—defined as a ‘backbone’—on (semi-) public land with an overall carrying capacity sufficient to support persistent populations. This best ensures long-term survival, and the backbone may also act as refugium. Satellite populations that emerge on adjacent vacant lots will thereby add to the persistence of the overall metapopulation. Management of permanent habitat is focused on retaining early-successional stages of vegetation. Implementing this strategy in the case of the natterjack toad in the Port of Antwerp taught us that realization of a habitat backbone is possible only if landowners, local governments and environmental NGOs cooperate. In the case at hand, such cooperation resulted in a plan that should ensure a coherent and persistent habitat network in which a chorus of some 1,400 natterjack toads could be accommodated—more than the number of toads currently observed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

20 years later: Does reduced soil area change overall tree growth?

Urban conditions have been thought to affect tree growth, but there is little conclusive evidence as to the severity of those influences or how various species respond differentially to urban stress. Reduced growth expectations are important to understand, because they affect design choices for the urban tree canopy, particularly as required by legislative mandate. Five tree species (Acer rubrum, Prunus serrulata, Pyrus calleryana, Quercus pallustris and Zelkova serrata) grown in parking lots ranging from 18 to 23 years old in central and northern New Jersey, USA were studied. Tree height, diameter at breast height (DBH), and canopy radius were measured, as was apparent plant available soil (nonpaved planting zone area). Tree DBH, commonly recorded for many municipal inventories, was found to be a useful predictor of canopy area. Data were normalized within site, to facilitate multiple site analysis. Across different parking lots, reductions in tree size were consistently associated with reduced apparent soil access. A previous study from Florida, USA was used for comparison of regional data, permitting conclusions on canopy reductions, relative to specification of design space for tree establishment.