Ecological Consequences Of Habitat Fragmentation Research Articles

The diverse effects of habitat fragmentation on plant–pollinator interactions

Habitat fragmentation affects a wide variety of biological variables including species’ abundance and richness (population demography), phenology, male and female reproductive fitness, and it also affects the degree of specialization versus generalization of pollination networks. Evidence is accumulating that suggests that habitat fragmentation can have significant impacts on plant–pollinator interactions. In this article, we review the literature on habitat fragmentation effects on plants, pollinators, and the pollination network. We also discuss pollination network mechanisms that may be affected by habitat fragmentation. Habitat fragmentation isolates populations and affects ecological properties at both population and community levels. Evidence shows that habitat size and connectivity directly or indirectly influence the abundance of both plant and pollinator species. In general, plant and pollinator diversity and population size decrease with the decreasing size and habitat connectivity. Habitat fragmentation of plant communities can shift plant phenological patterns, contract flowering periods and increase the risk of local pollinator extirpation. Fragmentation has the potential to influence pollination dynamics by altering pollinator or plant densities and by altering pollinator behavior. However, evidence for the impact of habitat fragmentation on plant species’ flowering phenology is relatively limited, and little is known about the effect of habitat fragmentation on the phenology of pollinators. Habitat fragmentation also leads to reduced reproduction in many species. In contrast, other species showed neutral or positive responses to habitat fragmentation in female reproductive fitness, especially in plants regularly affected by pollen limitation and pollination limitation which lead to plants’ experience selection for increased autogamy in isolated habitats. Habitat fragmentation often leads to the extirpation of specialist species and results in an influx of generalists. However, studies have shown that pollinators tend to be more generalized as habitat fragmentation increases. The reason is that habitat fragmentation changes the composition of the flora, and scatters floral resources, so any remaining pollinators may need to behave as generalists in order to survive. However, the knowledge of potential ecological consequences of habitat fragmentation is limited, especially regarding the effects on a long-term scale and at landscape scales. We propose experiments involving long-term monitoring, permanent samples of flowering plants, pollinators, and their interactions at large spatial and temporal scales.

Rapid human-induced divergence of life-history strategies in Bahamian livebearing fishes (family Poeciliidae).

Human-induced rapid environmental change (HIREC) can have dramatic impacts on ecosystems, leading to rapid trait changes in some organisms and extinction in others. Such changes in traits signify that human actions can lead to cases of increased phenotypic diversity and consequently can strongly impact population-, community- and ecosystem-level dynamics. Here, we examine whether the ecological consequences of habitat fragmentation have led to changes in the life histories of three native species of mosquitofish (Gambusia spp.) inhabiting tidal creeks on six different Bahamian islands. We address two important questions: (i) How predictable and parallel are life-history changes in response to HIREC across islands and species, and (ii) what is the relative importance of shared (i.e. parallel) responses to fragmentation, differences between species or islands and species- or island-specific responses to fragmentation? Phenotypic differences between fragmentation regimes were as great or greater than differences between species or islands. While some adult life histories (lean weight and fat content) showed strong, shared responses to fragmentation, offspring-related life histories (embryo fat and fecundity) exhibited idiosyncratic, island-specific responses. While shared responses to fragmentation appeared largely driven by a reduction in piscivorous fish density, increased conspecific density and changes in salinity, we found some evidence that among-population variation in male reproductive investment and embryo fat content may have arisen via variation in conspecific density. Our results suggest that phenotypic responses to HIREC can be complex, with the predictability of response varying across traits. We therefore emphasize the need for more theoretical and empirical work to better understand the predictability of phenotypic responses to human-induced disturbances.

Temporal ecology in the Anthropocene.

Two fundamental axes - space and time - shape ecological systems. Over the last 30 years spatial ecology has developed as an integrative, multidisciplinary science that has improved our understanding of the ecological consequences of habitat fragmentation and loss. We argue that accelerating climate change - the effective manipulation of time by humans - has generated a current need to build an equivalent framework for temporal ecology. Climate change has at once pressed ecologists to understand and predict ecological dynamics in non-stationary environments, while also challenged fundamental assumptions of many concepts, models and approaches. However, similarities between space and time, especially related issues of scaling, provide an outline for improving ecological models and forecasting of temporal dynamics, while the unique attributes of time, particularly its emphasis on events and its singular direction, highlight where new approaches are needed. We emphasise how a renewed, interdisciplinary focus on time would coalesce related concepts, help develop new theories and methods and guide further data collection. The next challenge will be to unite predictive frameworks from spatial and temporal ecology to build robust forecasts of when and where environmental change will pose the largest threats to species and ecosystems, as well as identifying the best opportunities for conservation.

Very high resolution Earth Observation features for testing the direct and indirect effects of landscape structure on local habitat quality

Modelling the empirical relationships between habitat quality and species distribution patterns is the first step to understanding human impacts on biodiversity. It is important to build on this understanding to develop a broader conceptual appreciation of the influence of surrounding landscape structure on local habitat quality, across multiple spatial scales. Traditional models which report that ‘habitat amount’ in the landscape is sufficient to explain patterns of biodiversity, irrespective of habitat configuration or spatial variation in habitat quality at edges, implicitly treat each unit of habitat as interchangeable and ignore the high degree of interdependence between spatial components of land-use change. Here, we test the contrasting hypothesis, that local habitat units are not interchangeable in their habitat attributes, but are instead dependent on variation in surrounding habitat structure at both patch- and landscape levels. As the statistical approaches needed to implement such hierarchical causal models are observation-intensive, we utilise very high resolution (VHR) Earth Observation (EO) images to rapidly generate fine-grained measures of habitat patch internal heterogeneities over large spatial extents. We use linear mixed-effects models to test whether these remotely-sensed proxies for habitat quality were influenced by surrounding patch or landscape structure. The results demonstrate the significant influence of surrounding patch and landscape context on local habitat quality. They further indicate that such an influence can be direct, when a landscape variable alone influences the habitat structure variable, and/or indirect when the landscape and patch attributes have a conjoined effect on the response variable. We conclude that a substantial degree of interaction among spatial configuration effects is likely to be the norm in determining the ecological consequences of habitat fragmentation, thus corroborating the notion of the spatial context dependence of habitat quality.

Neutral communities in fragmented landscapes

Habitat fragmentation displays a crucial role in conservation biology. Despite this, little is known about the detailed ecological consequences of habitat fragmentation due to the scarce number of controlled experimental surveys. The species–area relationship, a fundamental concept in ecology, requires the understanding of the fragmentation effects in a long term perspective, which turns this task even harder. Here we address the spatial patterns of species distribution in fragmented landscapes, assuming a neutral community model. We study the species area relationship and how its shape changes as the landscape becomes more fragmented. Recent investigations, based on extensive computer simulation, have contributed to establish some definite conclusions in the study of non‐fragmented landscapes: the existence of a three‐regime or two‐regime scenario for the species–area relationship, the emergence of a power‐law regime at intermediate scales and the augment of the species–area exponent z with the speciation rate. Despite the recent efforts, some other questions remain, such as the dependence of z in the whole range of the speciation rate. Questions like these are currently debated but generalizations cannot be drawn. This is the first paper, to our knowledge, that uses the coalescence method and neutral theory to examine biodiversity on more complex spatial structures. Our simulation results corroborate that the fragmentation plays a crucial role in shaping the species–area relationship, by determining the existence and extension of the power‐law regime associated with small and intermediate areas. On the other hand, when individuals are allowed to disperse over longer distances the species–area relationship now displays the classic triphasic pattern, and the intermediate regime, which is well described by a power‐law, is established even for highly fragmented landscapes.

Effects of small-scale habitat fragmentation, habitat corridors and mainland dispersal on soil decomposer organisms

Habitat corridors have been suggested to be one possible way to reduce the often negative effects of habitat fragmentation. In the present experiment, we focused on small habitat fragments (humus patches) inhabited by soil decomposer organisms. These fragments were either unconnected or connected with each other by habitat (humus) corridors. Moreover, these systems were either isolated from the mainland by a dispersal barrier (plastic “walls”) or were open to dispersal from the mainland. The fragments and corridors were embedded in a matrix of mineral soil that was expected to be uninhabitable or at least an unpreferred habitat for the organisms studied. Undisturbed forest soil functioned as the mainland. The results showed that habitat fragmentation decreased the abundance of microarthropods (Acari). Presence of corridors affected positively the abundance of mites, but only after the first 4 months of the experiment: later on, this effect disappeared. Furthermore, this “corridor rescue effect” was only seen in the systems isolated from the mainland, whereas in the open systems, dispersal from the mainland apparently overwhelmed the effect of corridors. Also soil fungi benefited from the corridors. The results indicate that soil decomposer organisms, microarthropods in particular, may be useful as model organisms in studies focusing on the ecological consequences of habitat fragmentation. It is also suggested that connecting otherwise isolated small habitat fragments with each other by habitat corridors may increase the persistence time of fragment-inhabiting populations.

Dispersal success on fractal landscapes: a consequence of lacunarity thresholds

Habitat fragmentation is expected to disrupt dispersal, and thus we explored how patch metrics of landscape structure, such as percolation thresholds used to define landscape connectivity, corresponded with dispersal success on neutral landscapes. We simulated dispersal as either a purely random process (random direction and random step lengths) or as an area-limited random walk (random direction, but movement limited to an adjacent cell at each dispersal step) and quantified dispersal success for 1000 individuals on random and fractal landscape maps across a range of habitat abundance and fragmentation. Dispersal success increased with the number of cells a disperser could search (m), but poor dispersers (m<5) searching via area-limited dispersal on fractal landscapes were more successful at locating suitable habitat than random dispersers on either random or fractal landscapes. Dispersal success was enhanced on fractal landscapes relative to random ones because of the greater spatial contagion of habitat. Dispersal success decreased proportionate to habitat loss for poor dispersers (m=1) on random landscapes, but exhibited an abrupt threshold at low levels of habitat abundance (p<0.1) for area-limited dispersers (m<10) on fractal landscapes. Conventional metrics of patch structure, including percolation, did not exhibit threshold behavior in the region of the dispersal threshold. A lacunarity analysis of the gap structure of landscape patterns, however, revealed a strong threshold in the variability of gap sizes at low levels of habitat abundance (p<0.1) in fractal landscapes, the same region in which abrupt declines in dispersal success were observed. The interpatch distances or gaps across which dispersers must move in search of suitable habitat should influence dispersal success, and our results suggest that there is a critical gap-size structure to fractal landscapes that interferes with the ability of dispersers to locate suitable habitat when habitat is rare. We suggest that the gap structure of landscapes is a more important determinant of dispersal than patch structure, although both are ultimately required to predict the ecological consequences of habitat fragmentation.

DEMOGRAPHIC RESPONSES TO HABITAT FRAGMENTATION: EXPERIMENTAL TESTS AT THE LANDSCAPE AND PATCH SCALE

We tested the ecological consequences of habitat fragmentation by comparing the density, population growth rate, survivorship, and recruitment of Microtus pennsylvanicus populations within a 20-ha fragmented landscape with those of populations in a 20-ha unfragmented landscape. We also tested for fragment-size effects by comparing the same measures of demographic performance across three fragment sizes (0.06, 0.25, and 1.0 ha). During 17 censuses between June 1993 and October 1994, we recorded 10020 captures of 3946 individuals and found strong landscape differences but weak fragment-size effects. Although fragmentation reduced the habitable area by 72%, density and adult recruitment were significantly higher on the fragmented landscape relative to the control. With the exception of adult recruitment (higher on small relative to medium and large patches), no significant demographic differences existed among patches of different size. Low rates of between-population movement and differential juvenile growth rates suggested that higher recruitment rates on the fragmented landscape likely resulted from enhanced local reproduction rather than from immigration. Thus, despite the fact that populations in the fragmented landscape experienced severe habitat loss, some individuals on fragments accrued important reproductive advantages (possibly as a result of diminished social costs or enhanced food resources). That population and individual responses to fragmentation could differ so dramatically provides a novel result that illustrates the importance of using hierarchical field designs in tests of population responses to large-scale habitat alteration. We conclude that controlled, large-scale field tests can serve as an important intermediary between the inherent abstraction of simulation modeling and what is observed in the real world.

Ecological consequences of habitat fragmentation: implications for landscape architecture and planning

Habitat loss and isolation associated with land conversion for human activities constitute the most serious threat to the Earth's biological diversity. The study of habitat fragmentation provides an important link between the concepts and principles of landscape ecology and the practice of landscape architecture and planning. Here I review ecological literature to examine current understanding of the ecological consequences of habitat fragmentation, and briefly suggest ways in which the results of these studies may guide decision-making by landscape architects and planners. Two theoretical developments in ecology have informed studies of habitat fragmentation and have provided testable hypotheses for empirical studies: island biogeography theory and metapopulation dynamics. Ecologists have examined the influences of habitat fragment size, shape, degree of isolation, context, and habitat quality or heterogeneity on plant and animal population persistence, community composition, and ecosystem processes. Disruption of continuous habitat usually results in an increase in the length of the boundary between fragments and their surrounding habitats. Newly created edges experience shifts in microclimatic characteristics, which may significantly alter the native plant and animal communities present. The size of a habitat fragment markedly influences the ecological processes occurring therein, largely due to the changes induced by these habitat edges. In general, species richness declines as fragment area decreases. Vegetated corridors may facilitate the movement of plants and animals among habitat fragments, however, more information is needed regarding the efficacy of corridors in reducing species loss from fragmented habitats. Fragments with highly irregular, convoluted boundaries will likely have greater exchange of nutrients, materials, and organisms with adjacent habitats than will those with less convoluted boundaries. Adjacent habitat types, land management regimes, and intensity of human activities influence boundary permeability and thus flow among habitat fragments. Large fragments are likely to be more heterogeneous than small fragments; they contain a greater variety of soil types, greater topographic variation, and a greater number of habitat types. An integrated view of the spatial characteristics of habitat fragments and their ecological consequences improves our ability to predict the outcomes of, and to design, particular patterns of land conversion.

Using Landscape Indices to Predict Habitat Connectivity

The ecological consequences of habitat fragmentation include the direct effects of habitat loss and the indirect effects of reduced inter-patch dispersal. In particular, habitat patches that survive the process of fragmentation become increasingly isolated from one another, and this can cause species' declines in excess of predictions based strictly on reductions in habitat area. To quantify the hindrance of dispersal caused by habitat frag- mentation, landscape ecologists have invented the notion of habitat connectivity. Indices of landscape pattern are frequently used to estimate habitat connectivity, but whether they actually do so remains undocumented. If indices of habitat pattern do indeed estimate habitat connectivity, then these indices should correlate well with predictions of dispersal success. To test this possibility, I looked for correlations between nine common indices of habitat pattern and the results of a simulated dispersal process conducted using GIS data on the distribution of old-growth forest through the Pacific Northwest. The nine indices of habitat pattern that I examined were only weakly correlated with the results from the dispersal modeling, but I identified a new pattern index, termed patch cohesion, for which the fit was much better. Moreover, I found patch cohesion to be insensitive to the details and artifacts of the dispersal model. The methodology described here will be useful to inves- tigators using indices of landscape pattern to quantify habitat fragmentation.