The elements of connectivity where corridor quality is variable

Small mammals in heterogeneous environments have been found to disperse along corridors connecting habitat patches. Corridors may have different survivability values depending on their size and the degree of cover they provide. This deterministic model tests the effects of varying corridor quality on the demographics of a metapopulation of Peromyscus leucopus. Two types of corridors are defined based on the probability of survival during a dispersal event. Results indicate that mortality during movement through corridors influences metapopulation demographics. We found that:

1. Any connection between two isolated patches is better than no connection at all in terms of persistence and population size at equilibrium.
2. Metapopulations with exclusively high quality corridors between patches have a larger population size at equilibrium than do those with one or more low quality corridors.
3. Increasing the number of high quality corridors between patches has a positive effect on the size of the metapopulation while increasing the number of low quality corridors has a negative effect.
4. The addition to a metapopulation of a patch connected by low quality corridors has a negative effect on the metapopulation size. This suggest the need for caution in planning corridors in a managed landscape.
5. There is no relationship between the number of corridors and the metapopulation size at equilibrium when the number of connected patches is held constant.
6. Geometrically isolated patches connected by low quality corridors are most vulnerable to local extinctions.

We conclude that corridor quality is an important element of connectivity. It contributes substantially to the effects of fragmentation and should be carefully considered by landscape planners.     

Matrix-dependent corridor effectiveness and the abundance of forest birds in fragmented landscapes

Corridor function for wildlife movement constitutes an important and desirable ecological characteristic of linear landscape structures. Changes in the matrix conditions, however, may result in substantial changes in the mechanisms responsible for the use of corridors by animals. I developed a model that describes the influence of matrix quality on the effectiveness of corridors for wildlife movement and the abundance of animals in the corridors. The model predicts that corridor effectiveness is maximized at intermediate matrix quality levels, while the abundance in the corridor increases asymptotically with matrix quality. I tested predictions of this model by comparing the expected and observed relative abundance of forest bird species in two landscape types of southern Chile. In nine out of 12 cases the model correctly predicted the relative abundance of forest birds. Riparian forest strips were expected to be effective functioning as corridors for five out of six studied species, although corridor effectiveness for each species varied between landscape types. A reasonable strategy to improve connectivity is to maintain (or to increase, if necessary) the matrix quality at a level such that corridors can function efficiently as both drift fences and movement conduits.     

Interactions between plant life span, seed dispersal capacity and fecundity determine metapopulation viability in a dynamic landscape

Classical metapopulation models do not account for temporal changes in the suitability of habitat patches. In reality, however, the carrying capacity of most habitat types is not constant in time due to natural succession processes. In this study, we modeled plant metapopulation persistence in a successional landscape with disappearing and emerging habitat patches, based on a realistic dune slack landscape at the Belgian–French coast. We focused on the effects of the variation of different plant traits on metapopulation persistence in this changing landscape. Therefore, we used a stage based stochastic metapopulation model implemented in RAMAS/Metapop, simulating a large variation in plant traits but keeping landscape characteristics such as patch turnover rate and patch lifespan constant. The results confirm the conclusions of earlier modeling work that seed dispersal distance and seed emigration rate both have an important effect on metapopulation persistence. We also found that high population growth rate or high recruitment considerably decreased the extinction risk of the metapopulation. Additionally, a long plant life span had a strong positive effect on metapopulation persistence, irrespective of the plant's dispersal capacity and population growth rate. Plant species that invest in life span require less investment in offspring and dispersal capacity to avoid extinction, even in dynamic landscapes with deterministic changes in habitat quality. Moreover, metapopulations of long-lived plant species were found to be much less sensitive to high levels of environmental stochasticity than short-lived species.     

Effects of patch attributes, barriers, and distance between patches on the distribution of a rock-dwelling rodent (Lagidium viscacia)

We tested whether size of habitat patches and distance between patches are sufficient to predict the distribution of the mountain vizcacha Lagidium viscacia a large, rock-dwelling rodent of the Patagonian steppe Argentina, or whether information on other patch and landscape characteristics also is required. A logistic regression model including the distance between rock crevices and depth of crevices, distance between a patch and the nearest occupied patch, and whether or not there was a river separating it from the nearest occupied patch was a better predictor of patch occupancy by mountain vizcachas than was a model based only on patch size and distance between patches. Our results indicate that a simple metapopulation analysis based on size of habitat patches and distance between patches may not provide an accurate representation of regional population dynamics if patches vary in habitat quality independently of patch size and features in the matrix alter connectivity. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Viability of Metapopulations: Individual Dispersal Behaviour Matters

Metapopulation models are frequently used for analysing species–landscape interactions and their effect on structure and dynamic of populations in fragmented landscapes. They especially support a better understanding of the viability of metapopulations. In such models, the processes determining metapopulation viability are often modelled in a simple way. Animals’ dispersal between habitat fragments is mostly taken into account by using a simple dispersal function that assumes the underlying process of dispersal to be random movement. Species-specific dispersal behaviour such as a systematic search for habitat patches is likely to influence the viability of a metapopulation. Using a model for metapopulation viability analysis, we investigate whether such specific dispersal behaviour affects the predictions of ranking orders among alternative landscape configurations rated regarding their ability to carry viable metapopulations. To incorporate dispersal behaviour in the model, we use a submodel for the colonisation rates which allows different movement patterns to be considered (uncorrelated random walk, correlated random walk with various degrees of correlation, and loops). For each movement pattern, the landscape order is determined by comparing the resulting mean metapopulation lifetime T_m of different landscape configurations. Results show that landscape orders can change considerably between different movement patterns. We analyse whether and under what circumstances dispersal behaviour influences the ranking orders of landscapes. We find that the ‘competition between patches for migrants’ – i.e. the fact that dispersers immigrating into one patch are not longer available as colonisers for other patches – is an important factor driving the change in landscape ranks. The implications of our results for metapopulation modelling, planning and conservation are discussed.     

Intermediate landscape disturbance maximizes metapopulation density

The viability of metapopulations in fragmented landscapes has become a central theme in conservation biology. Landscape fragmentation is increasingly recognized as a dynamical process: in many situations, the quality of local habitats must be expected to undergo continual changes. Here we assess the implications of such recurrent local disturbances for the equilibrium density of metapopulations. Using a spatially explicit lattice model in which the considered metapopulation as well as the underlying landscape pattern change dynamically, we show that equilibrium metapopulation density is maximized at intermediate frequencies of local landscape disturbance. On both sides around this maximum, the metapopulation may go extinct. We show how the position and shape of the intermediate viability maximum is responding to changes in the landscape’s overall habitat quality and the population’s propensity for local extinction. We interpret our findings in terms of a dual effect of intensified landscape disturbances, which on the one hand exterminate local populations and on the other hand enhance a metapopulation’s capacity for spreading between habitat clusters.     

Extinction risk of wood-living model species in forest landscapes as related to forest history and conservation strategy

Dead wood is a critical resource for biodiversity in boreal forests. We analysed the persistence of five model species inhabiting dead wood. By parameterising a metapopulation model (the incidence function model), the model species were all assigned characteristics that makes it likely that they have disappeared from some (20%) forest landscapes with a long history of forest management. In the metapopulation model, a forest stand (5 ha) was regarded as a habitat patch. The amount of habitat in each patch was obtained from models of dead wood dynamics of Norway spruce in central Sweden. Dead wood generated by altered management over the entire landscape was found to be less efficient in reducing extinction risks in comparison to the same amount of dead wood generated by protecting reserves. Because generation of dead wood by altered management is often less expensive than setting aside reserves, it is difficult to determine which conservation measure is most cost-efficient. In a landscape subjected to forestry for the first time, it was better to preserve a few large reserves than many small ones. However, in a managed, highly fragmented forest landscape it was better to set aside many small reserves. The reason for this was that small plots with high habitat quality could be selected, while large reserves originally contained habitats both of high and low quality, and the rate of habitat quality increase was low. A strategy for biodiversity conservation in a managed forest landscape should include information about the history of the landscape, the current amount and spatial distribution of forest habitats, and the potential for rapid restoration of forest habitats, both on managed and unmanaged forest land.     

Estimating the consequences of habitat fragmentation on extinction risk in dynamic landscapes

Analyzing the population dynamic consequences of spatio-temporal changes in landscape structure is a formidable challenge for spatial ecology. One key population dynamic process in fragmented landscapes is the influence of isolation on colonization rate and thereby on the occurrence of species in habitat fragments, but it is not obvious how isolation should be measured in landscapes that are affected by on-going habitat loss and fragmentation. We suggest the following procedure for the measurement of spatio-temporal isolation. First, a historical record of habitat loss and fragmentation in the landscape is prepared based on snapshots of the extent of the suitable habitat for the focal species. Second, a metapopulation model is used to simulate the occurrence of the species in this landscape, assuming the empirically observed landscape change. The model-predicted pattern of habitat occupancy at a particular point in time (usually the present time) is then compared with empirical observations on the occurrence of the species. We describe a metapopulation model that has been constructed for this purpose, and we apply it to a changing landscape of boreal forests in eastern Finland. We give an example on the occurrence of four threatened polyporous fungi in 18 small fragments of old-growth forest. In none of the species does the current isolation of the fragments nor the time since their isolation explain the occurrence of the species in the study fragments, but in three species the model-predicted occupancy probability had a significant effect on the observed abundance of the species. The model-predicted occupancy probabilities were also calculated by ignoring past landscape changes, that is, by assuming that the landscape had remained in the present configuration for a long time. These probabilities had a significant effect on the abundance of only one of the four species, suggesting that the occurrence of the species tracks landscapes changes with a noticable time lag.This revised version was published online in May 2005 with corrections to the Cover Date.     

Influence of agricultural landscape structure on a Southern High Plains,USA, amphibian assemblage

Landscape structure can influence demographics of spatially structured populations, particularly less vagile organisms such as amphibians. We examined the influence of agricultural landscape structure on community composition and relative abundance of the 4 most common amphibians in the Southern High Plains of central USA. Amphibian populations were monitored using pitfall traps and drift fence at 16 playa wetlands (8 playas/year) in 1999 and 2000. We quantified landscape structure surrounding each playa via estimating 13 spatial metrics that indexed playa isolation and inter-playa landscape complexity. Multivariate ordination and univariate correlations and regressions indicated that landscape structure was associated with community composition and relative abundance for 2 of the 4 amphibians. Spadefoots (Spea multiplicata, S. bombifrons) generally were positively associated with decreasing inter-playa distance and increasing inter-playa landscape complexity. Great Plains toads (Bufo cognatus) and barred tiger salamanders (Ambystoma tigrinum mavortium) usually were negatively associated with spadefoots but not influenced by landscape structure. Composition and relative abundance patterns were related to amphibian body size, which can influence species vagility and perception to landscape permeability. Spatial separation of these species in the multivariate ordination also may have been a consequence of differential competitive ability among species. These results suggest agricultural landscape structure may influence abundance and composition of spatially structured amphibian populations. This also is the first applied documentation that inter-patch landscape complexity can affect intra-patch community composition of amphibians as predicted by metapopulation theory. In the Southern High Plains, landscape complexity is positively associated with agricultural cultivation. Agricultural cultivation increases sedimentation, decreases hydroperiod, alters amphibian community dynamics, and negatively impacts postmetamorphic body size of amphibians in playa wetlands. Thus, conservation efforts should focus on preserving or restoring native landscape structure, hydroperiod, and connectivity among playas to maintain native amphibian populations and historic inter-playa movement.     

The importance of integrating landscape ecology in habitat models: isolation-driven occurrence of north island robins in a fragmented landscape

Although the role of habitat fragmentation in species declines is well recognised, the effect of habitat quality on species distributions is often studied using presence–absence models that ignore metapopulation dynamics. We compared three approaches to model the presence–absence of North Island robins in 400 sites among 74 fragments of native forest in a 15,000-ha agricultural landscape in New Zealand. The first approach only considered local habitat characteristics, the second approach only considered metapopulation factors (patch size and isolation), and the third approach combined these two types of factors. The distribution of North Island robins was best predicted by patch isolation, as their probability of occurrence was negatively correlated with isolation from neighbouring patches and from the closest major forests, which probably acted as a source of immigrants. The inclusion of habitat factors gave only a slight increase in predictive power and indicated that robins were more likely to occur in areas with tall canopy, tall understory and low density of young trees. We modelled the effect of isolation using an index of functional patch connectivity based on dispersal behaviour of radio-tracked juveniles, and this functional index greatly improved the models in comparison to classical indices relying on Euclidean distances. This study highlights the need to incorporate functional indices of isolation in presence–absence models in fragmented landscapes, as species occurrence can otherwise be a misleading predictor of habitat quality and lead to wrong interpretations and management recommendations.     

Spatial aspects of metapopulation survival – from model results to rules of thumb for landscape management

The role of spatial configuration for metapopulation survival is analyzed by using a stochastic metapopulation model. This model reveals conditions which must be satisfied by the species' ecology and the landscape settings before a metapopulation can persist over a long term. Taking this as a basis, initial rules of thumb for landscape management are deduced. The following results are highlighted: (1) the critical correlation length d_c of the extinction processes determines a spatial scale of the metapopulation dynamics. (2) Only species with a dispersal range d_r above the correlation length d_c are able to benefit from landscape management at all. (3) A certain metapopulation can only persist over a long term if no patch is inside the range of correlation of another one. (4) There is a hierarchy of importance in the characteristics of a spatial configuration (scale and type) and, hence, in the scopes of landscape management. To conclude, some general consequences for supporting species survival by management are discussed. Some prospects concerning the use of models for decision support in landscape planning are discussed.     

Plant strategies and agricultural landscapes: survival in spatially and temporally fragmented habitat

In agricultural landscapes many plant species are limited to the network of landscape elements that are not used for agricultural production. This habitat is fragmented in space and time due to anthropogenic, biotic and abiotic factors. Therefore, plant populations are spatially sub-divided and their persistence might be dependent on the spatial dynamics in the network of local populations. Dispersal characteristics and seed bank persistence are main determinants of colonization ability which in turn is a key determinant of metapopulation viability. We propose a conceptual model that relates plant population dynamics to habitat quality, configuration and dynamics. In this model, the habitat is arranged as a network of suitable and unsuitable patches,and the distribution of the patches is assumed to be dynamic in time. Based on dispersal and seed bank characteristics four plant strategies are distinguished:species having either long (> 100 m) or short (< 100m) distance dispersal and either a long (> 5 yr)or short (< 5 yr) term persistent seed bank. We expect that species with contrasting strategies have different survival probabilities in landscapes with contrasting habitat arrangement in space and time. We found few empirical studies for testing the hypotheses based on the model. Therefore the relation between landscapes and plant survival needs to be further explored,especially the quantitative aspects. We propose an iterative process of empirical and modelling research to determine this relation and to define management options for multifunctional farms in which biodiversity is one of the land use aims. This revised version was published online in July 2006 with corrections to the Cover Date.     

Within-patch habitat quality determines the resilience of specialist species in fragmented landscapes

Patch geometry and habitat quality among patches are widely recognized as important factors affecting population dynamics in fragmented landscapes. Little is known, however, about the influence of within-patch habitat quality on population dynamics. In this paper, we investigate the relative importance of patch geometry and within-patch habitat quality in determining population dynamics using a spatially explicit, agent-based model. We simulate two mobile species that differ in their species traits: one resembles a habitat specialist and the other a habitat generalist. Habitat quality varies continuously within habitat patches in space (and time). The results show that spatial variation in within-patch quality, together with patch area, controls population abundance of the habitat specialist. In contrast, the population size of the generalist species depends on patch area and isolation. Temporal variation in within-patch quality is, however, less influential in driving the population resilience of both species. We conclude that specialist species are more sensitive than generalist species to within-patch variation in habitat quality. The patch area-isolation paradigm, developed in metapopulation theory, should incorporate variation in within-patch habitat quality, particularly for habitat specialists.     

Landscape ecology: Population genetics at the metapopulation level

Distribution of genetic diversity in a landscape depends on both within and among population processes. Selective pressures within populations have traditionally been studied by population genetics, which usually assumes that populations are at equilibrium. However, when selection pressures within and among populations are different, landscape processes are required to define an equilibrium (landscape being defined as the habitat of a set of populations called a metapopulation, and populations will differ depending on their situation in the landscape, i.e. their age and the state of neighboring populations). We examine reproduction systems and life history traits, for which variation depends on landscape processes. Predictions of their states in a metapopulation are drawn from theoretical models, and confronted to observations collected in natural populations.     

Effects of corridor width and presence on the population dynamics of the meadow vole (Microtus pennsylvanicus)

We tested the effects of increased landscape corridor width and corridor presence on the population dynamics and home range use of the meadow vole (Microtus pennsylvanicus) within a small-scale fragmented landscape. Our objective was to observe how populations behaved in patchy landscapes where the animals home range exceeded or equaled patch size. We used a small-scale replicated experiment consisting of three sets of two patches each, unconnected or interconnected by 1-m or 5-m wide-corridors, established in an old-field community (S.W. Ohio). Control (0-m) treatments supported significantly lower vole densities than either corridor treatment. Females were the dominant resident sex establishing smaller home ranges (<150m²) than males (>450m²). Significantly more male voles dispersed between patches with corridors than between patches without corridors. However, no difference was observed regarding the number of male voles dispersing between patches connected by corridors when compared to the number dispersing across treatments. Dispersal between connected patches was restricted to corridors based on tracking tube data. Corridor presence was more important than corridor width regarding the movement of male voles within their home range.     

Measuring urban habitat fragmentation: an example from the Black Country,UK

The processes of urbanisation have left a fragmented mosaic of habitat patches of varying size, shape and character with the result that from location to location the number and quality of contacts between patches varies considerably. Traditional measurements of this habitat fragmentation, and its converse, connectivity, have rarely looked at the landscape as a whole but instead have simplified it to specific landscape subsets, or else have looked at area-to-area relationships through generalising the landscape into homogeneous pixels or grids. In this paper the character of the whole landscape is examined at scales appropriate to the spatial variability of the urban environment. Using a direct measurement of patch-to-patch contact all contacts between all patches are examined and the relationship between all contiguous and connecting habitats is quantified. This is further refined to look at connections between patches of different quality, a measure that highlights the adverse effects of urbanisation as a whole on landscape connections between quality habitats.     

Species Dynamics in Disturbed Landscapes: When does a Shifting Habitat Mosaic Enhance Connectivity?

Although landscape ecology emphasizes the effects of spatial pattern on ecological processes, most neutral models of species–habitat relationships have treated habitat as a static constraint. Do the working hypotheses derived from these models extend to real landscapes where disturbances create a shifting mosaic? A spatial landscape simulator incorporating vegetation dynamics and a metapopulation model was used to compare species in static and dynamic landscapes with identical habitat amounts and spatial patterns. The main drivers of vegetation dynamics were stand-replacing disturbances, followed by gradual change from early-successional to old-growth habitats. Species dynamics were based on a simple occupancy model, with dispersal simulated as a random walk. As the proportion of available habitat (p) decreased from 1.0, species occupancy generally declined more rapidly and reached extinction at higher habitat levels in dynamic than in static landscapes. However, habitat occupancy was sometimes actually higher in dynamic landscapes than in static landscapes with similar habitat amounts and patterns. This effect was most pronounced at intermediate amounts of habitat (p = 0.3?0.6) for mobile species that had high colonization rates, but were unable to cross non-habitat patches. Differences between static and dynamic landscapes were contingent upon the initial metapopulation size and the shapes of disturbances and the resulting habitat patterns. Overall, the results demonstrate that dispersal-limited species exhibit more pronounced critical behavior in dynamic landscapes than is predicted by simple neutral models based on static landscapes. Thus, caution should be exercised in extending generalizations derived from static landscape models to disturbance-driven landscape mosaics.     

Characterizing the importance of habitat patches and corridors in maintaining the landscape connectivity of a Pholidoptera transsylvanica (Orthoptera) metapopulation

Since the fragmentation of natural habitats is one of the most serious problems for many endangered species, it is highly interesting to study the properties of fragmented landscapes. As a basic property, landscape connectivity and its effects on various ecological processes are frequently in focus. First, we discuss the relevance of some graph properties in quantifying connectivity. Then, we propose a method how to quantify the relative importance of habitat patches and corridors in maintaining landscape connectivity. Our combined index explicitly considers pure topological properties and topographical measures, like the quality of both patches (local population size) and corridors (permeability). Finally, for illustration, we analyze the landscape graph of the endangered, brachypterous bush-cricket Pholidoptera transsylvanica. The landscape contains 11 patches and 13 corridors and is situated on the Aggtelek Karst, NE-Hungary. We characterize the importance of each node and link of the graph by local and global network indices. We show how different measures of connectivity may suggest different conservation preferences. We conclude, accordingly to our present index, by identifying one specific habitat patch and one specific corridor being in the most critical positions in maintaining connectivity.This revised version was published online in May 2005 with corrections to the Cover Date.     

Propagating sinks,ephemeral sources and percolating mosaics: conservation in landscapes

We present a framework that uses both sources and sinks as elements in the construction of a landscape matrix. We propose that the matrix be conceived as a collection of temporary habitats, some of which are sources, others of which are sinks, that form a landscape mosaic. The key element in this framing is that the sources are ephemeral and the sinks are propagating. A mean field approach is used to modify the classic metapopulation model, taking this new framework into account. Additionally a spatially explicit approach reveals different scaling rules for the percolation probability and the propagating probability.     

Altered functional connectivity and genetic diversity of a threatened salamander in an agroecosystem

Context

Amphibian metapopulations have become increasingly fragmented in the Midwestern United States, with wetland-breeding salamanders being especially dependent on intact, high-quality forested landscapes. However, the degree to which amphibian populations are isolated, the factors that influence dispersal and, ultimately, functional connectivity remain areas in need of investigation.

Objectives and methods

We combined population demographic and genetic approaches to assess how a landscape fragmented by agriculture influences functional connectivity and metapopulation dynamics of a locally threatened salamander (Ambystoma jeffersonianum).

Results

We found that the allelic richness and heterozygosity of this species was significantly related to the level of connectivity with other occupied breeding wetlands and that decreased connectivity resulted in increased genetic differentiation. We also found that effective population size appears to be declining and, while correlative, our focal landscape has experienced significant losses of forested upland habitats and potential wetland breeding habitats over the last 200 years.

Conclusions

By combining population and landscape genetic analyses with an assessment of regional wetland occupancy, our study has uniquely synthesized genetic and metapopulation processes, while also incorporating the effects of the landscape matrix on dispersal, connectivity, and population differentiation. The significant relationship between connectivity with heterozygosity, allelic richness, and genetic divergence observed in this study reinforces empirical observations of long distance dispersal and movements in ambystomatid salamanders. However, our results show that protection of core habitat around isolated wetlands may not sufficiently minimize genetic differentiation among populations and preserve critical genetic diversity that may be essential for the long-term persistence of local populations.