Refocusing Habitat Fragmentation Research Using Lessons from the Last Decade

Activity-based measures of landscape fragmentation

Context

Landscape fragmentation, which has demonstrated links to habitat loss, increased isolation, a loss of connectivity, and decreased biodiversity, is difficult to quantify. Traditional pattern-based approaches to measuring fragmentation use landscape metrics to quantify aspects of the composition or configuration of landscapes.

Objective

The objective of this study was to examine the relative improvements of an alternative activity-based approach using the cost of traversing a landscape as a proxy for fragmentation and compare it with the traditional approach.

Methods

One thousand binary landscapes varying in composition and configuration were simulated, and least-cost path analysis provided the data to calculate the activity-based metrics, which were compared with computed traditional pattern-based metrics.

Results

Activity-based fragmentation assessments were sensitive to levels of landscape fragmentation, but offered improvements over exiting pattern-based methods in that some metrics varied monotonically across the spectrum of landscape configurations and thus makes their interpretation more holistically meaningful.

Conclusions

This study provides a modular conceptual framework for assessing fragmentation using activity-based metrics that offer functional improvements over existing pattern-based approaches. While we present a focused theoretical implementation, the process to be measured and the scale of observation can be altered to suit specific user requirements, ecosystems, or species of interest.

Overcoming confusion and stigma in habitat fragmentation research

Anthropogenic habitat loss is widely recognized as a primary environmental concern. By contrast, debates on the effects of habitat fragmentation persist. To facilitate overcoming these debates, here we: (i) review the state of the literature on habitat fragmentation, finding widespread confusion and stigma; (ii) identify consequences of this for biodiversity conservation and ecosystem management; and (iii) suggest ways in which research can move forward to resolve these problems. Confusion is evident from the 25 most-cited fragmentation articles published between 2017 and 2021. These articles use five distinct concepts of habitat fragmentation, only one of which clearly distinguishes habitat fragmentation from habitat area and other factors ('fragmentation per se'). Stigmatization is evident from our new findings that fragmentation papers are more charged with negative sentiments when compared to papers from other subfields in the environmental sciences, and that fragmentation papers with more negative sentiments are cited more. While most empirical studies of habitat fragmentation per se find neutral or positive effects on species and biodiversity outcomes, which implies that small habitat patches have a high cumulative value, confusion and stigma in reporting and discussing such results have led to suboptimal habitat protection policy. For example, government agencies, conservation organizations, and land trusts impose minimum habitat patch sizes on habitat protection. Given the high cumulative value of small patches, such policies mean that many opportunities for conservation are being missed. Our review highlights the importance of reducing confusion and stigma in habitat fragmentation research. To this end, we propose implementing study designs in which multiple sample landscapes are selected across independent gradients of habitat amount and fragmentation, measured as patch density. We show that such designs are possible for forest habitat across Earth's biomes. As such study designs are adopted, and as language becomes more precise, we expect that confusion and stigma in habitat fragmentation research will dissipate. We also expect important breakthroughs in understanding the situations where effects of habitat fragmentation per se are neutral, positive, or negative, and the reasons for these differences. Ultimately this will improve efficacy of area-based conservation policies, to the benefit of biodiversity and people.

Complex nonmonotonic responses of biodiversity to habitat destruction

It has typically been assumed that habitat destruction, characterized by habitat loss and fragmentation, has consistently negative effects on biodiversity. While numerous empirical studies have shown the detrimental effects of habitat loss, debate continues as to whether habitat fragmentation has universally negative effects. To explore the effects of habitat fragmentation, we developed a simple model for site-occupancy dynamics in fragmented landscapes. With the model, we demonstrate that a competition-colonization trade-off can result in nonlinear oscillatory responses in biodiversity to both habitat loss and fragmentation. However, the overall pattern of habitat loss reducing species richness is still established, in line with empirical observations. Interestingly, the existence of localized oscillations in biodiversity can explain the mixed responses of species richness to habitat fragmentation per se observed in nature, thereby reconciling the debate on the fragmentation-diversity relationship. Therefore, this study offers a parsimonious mechanistic explanation for empirically observed biodiversity patterns in response to habitat destruction.

Fragmentation effects on an endangered species across a gradient from the interior to edge of its range

Understanding how habitat fragmentation affects individual species is complicated by challenges associated with quantifying species-specific habitat and spatial variability in fragmentation effects within a species' range. We aggregated a 29-year breeding survey data set for the endangered marbled murrelet (Brachyramphus marmoratus) from >42,000 forest sites throughout the Pacific Northwest (Oregon, Washington, and northern California) of the United States. We built a species distribution model (SDM) in which occupied sites were linked with Landsat imagery to quantify murrelet-specific habitat and then used occupancy models to test the hypotheses that fragmentation negatively affects murrelet breeding distribution and that these effects are amplified with distance from the marine foraging habitat toward the edge of the species' nesting range. Murrelet habitat declined in the Pacific Northwest by 20% since 1988, whereas the proportion of habitat comprising edges increased by 17%, indicating increased fragmentation. Furthermore, fragmentation of murrelet habitat at landscape scales (within 2 km of survey stations) negatively affected occupancy of potential breeding sites, and these effects were amplified near the range edge. On the coast, the odds of occupancy decreased by 37% (95% confidence interval [CI] -54 to 12) for each 10% increase in edge habitat (i.e., fragmentation), but at the range edge (88 km inland) these odds decreased by 99% (95% CI 98 to 99). Conversely, odds of murrelet occupancy increased by 31% (95% CI 14 to 52) for each 10% increase in local edge habitat (within 100 m of survey stations). Avoidance of fragmentation at broad scales but use of locally fragmented habitat with reduced quality may help explain the lack of murrelet population recovery. Further, our results emphasize that fragmentation effects can be nuanced, scale dependent, and geographically variable. Awareness of these nuances is critical for developing landscape-level conservation strategies for species experiencing broad-scale habitat loss and fragmentation.

Obstruction of biodiversity conservation by minimum patch size criteria

Minimum patch size criteria for habitat protection reflect the conservation principle that a single large (SL) patch of habitat has higher biodiversity than several small (SS) patches of the same total area (SL > SS). Nonetheless, this principle is often incorrect, and biodiversity conservation requires placing more emphasis on protection of large numbers of small patches (SS > SL). We used a global database reporting the abundances of species across hundreds of patches to assess the SL > SS principle in systems where small patches are much smaller than the typical minimum patch size criteria applied for biodiversity conservation (i.e., ∼85% of patches <100 ha). The 76 metacommunities we examined included 4401 species in 1190 patches. From each metacommunity, we resampled species-area accumulation curves to evaluate how biodiversity responded to habitat existing as a few large patches or as many small patches. Counter to the SL > SS principle and consistent with previous syntheses, species richness accumulated more rapidly when adding several small patches (45.2% SS > SL vs. 19.9% SL > SS) to reach the same cumulative area, even for the very small patches in our data set. Responses of taxa to habitat fragmentation differed, which suggests that when a given total area of habitat is to be protected, overall biodiversity conservation will be most effective if that habitat is composed of as many small patches as possible, plus a few large ones. Because minimum patch size criteria often require larger patches than the small patches we examined, our results suggest that such criteria hinder efforts to protect biodiversity.

Landscape-scale habitat fragmentation is positively related to biodiversity, despite patch-scale ecosystem decay

Positive effects of habitat patch size on biodiversity are often extrapolated to infer negative effects of habitat fragmentation on biodiversity at landscape scales. However, such cross-scale extrapolations typically fail. A recent, landmark, patch-scale analysis (Chase et al., 2020, Nature 584, 238-243) demonstrates positive patch size effects on biodiversity, that is, 'ecosystem decay' in small patches. Other authors have already extrapolated this result to infer negative fragmentation effects, that is, higher biodiversity in a few large than many small patches of the same cumulative habitat area. We test whether this extrapolation is valid. We find that landscape-scale patterns are opposite to their analogous patch-scale patterns: for sets of patches with equal total habitat area, species richness and evenness decrease with increasing mean size of the patches comprising that area, even when considering only species of conservation concern. Preserving small habitat patches will, therefore, be key to sustain biodiversity amidst ongoing environmental crises.

Anthropogenic fragmentation increases risk of genetic decline in the threatened orchid Platanthera leucophaea

Protecting biodiversity requires an understanding of how anthropogenic changes impact the genetic processes associated with extinction risk. Studies of the genetic changes due to anthropogenic fragmentation have revealed conflicting results. This is likely due to the difficulty in isolating habitat loss and fragmentation, which can have opposing impacts on genetic parameters. The well-studied orchid, Platanthera leucophaea, provides a rich dataset to address this issue, allowing us to examine range-wide genetic changes. Midwestern and Northeastern United States. We sampled 35 populations of P. leucophaea that spanned the species' range and varied in patch composition, degree of patch isolation, and population size. From these populations we measured genetic parameters associated with increased extinction risk. Using this combined dataset, we modeled landscape variables and population metrics against genetic parameters to determine the best predictors of increased extinction risk. All genetic parameters were strongly associated with population size, while development and patch isolation showed an association with genetic diversity and genetic structure. Genetic diversity was lowest in populations with small census sizes, greater urbanization pressures (habitat loss), and small patch area. All populations showed moderate levels of inbreeding, regardless of size. Contrary to expectation, we found that critically small populations had negative inbreeding values, indicating non-random mating not typically observed in wild populations, which we attribute to selection for less inbred individuals. The once widespread orchid, Platanthera leucophaea, has suffered drastic declines and extant populations show changes in the genetic parameters associated with increased extinction risk, especially smaller populations. Due to the important correlation with risk and habitat loss, we advocate continued monitoring of population sizes by resource managers, while the critically small populations may need additional management to reverse genetic declines.

Connectivity conservation at the crossroads: protected areas versus payments for ecosystem services in conserving connectivity for Colombian carnivores

Protected areas (PAs) constitute one of the main tools for global landscape conservation. Recently, payments for environmental services (PES) have attracted interest from national and regional governments and are becoming one of the leading conservation policy instruments in tropical countries. However, the degree to which areas designated for PES overlap with areas that are critical for maintaining species' landscape connectivity is rarely evaluated. We estimated habitat distributions and connectivity for 16 of the 22 mammalian carnivores occurring in the Caribbean region of Colombia, and identified the overlap between existing PAs and areas identified as being important for connectivity for these species. We also evaluated the potential impact of creation of new PAs versus new PES areas on conserving connectivity for carnivores. Our results show that PAs cover only a minor percentage of the total area that is important for maintaining connectivity ( x = 26.8 % ± 20.2   s . d . ). On the other hand, PES, if implemented extensively, could contribute substantially to mammalian carnivores' connectivity ( x = 45.4 % ± 12.8   s . d . ). However, in a more realistic scenario with limited conservation investment in which fewer areas are set aside, a strategy based on implementing new PAs seems superior to PES. We argue that prioritizing designation of new PAs will be the most efficient means through which to maintain connectivity.

How Human Activity Has Changed the Regional Habitat Quality in an Eco-Economic Zone: Evidence from Poyang Lake Eco-Economic Zone, China

Human activities such as deforestation and urbanization have affected the regional habitat quality of the Poyang Lake area. To evaluate the evolution of habitat quality and its influencing factors in the area, we used Classification and Regression Trees (CART) to interpret the land-use status and used the InVEST (Integrated Valuation of Ecosystem Services and Tradeoffs) model to analyze the characteristics of changes in habitat quality in the Poyang Lake Eco-Economic Zone (PLEEZ) from 1988 to 2018. The results show that, from 1988 to 2018, land use in the PLEEZ underwent significant changes. The changes in land use led to a gradual increase in habitat degradation and a gradual decrease in habitat quality in the study area. Rapid urbanization notably decreased the habitat quality in the study area. However, at the same time, the ecological protection projects such as returning farmland to forests slowed the decline in habitat quality. Driven by these two factors, habitat quality in the PLEEZ gradually declined but the rate of its decline was suppressed. The findings of this study are of great significance for the coordinated development of social, economic, and ecological development in the PLEEZ and similar areas.

Mapping behaviorally relevant light pollution levels to improve urban habitat planning

Artificial nighttime lights have important behavioral and ecological effects on wildlife. Combining laboratory and field techniques, we identified behaviorally relevant levels of nighttime light and mapped the extent of these light levels across the city of Chicago. We began by applying a Gaussian finite mixture model to 998 sampled illumination levels around Chicago to identify clusters of light levels. A simplified sample of these levels was replicated in the laboratory to identify light levels at which C57BL/6J mice exhibited altered circadian activity patterns. We then used camera trap and high-altitude photographic data to compare our field and laboratory observations, finding activity pattern changes in the field consistent with laboratory observations. Using these results, we mapped areas across Chicago exposed to estimated illumination levels above the value associated with statistically significant behavioral changes. Based on this measure, we found that as much as 36% of the greenspace in the city is in areas illuminated at levels greater than or equal to those at which we observe behavioral differences in the field and in the laboratory. Our findings provide evidence that artificial lighting patterns may influence wildlife behavior at a broad scale throughout urban areas, and should be considered in urban habitat planning.

Land use change has stronger effects on functional diversity than taxonomic diversity in tropical Andean hummingbirds

Land use change modifies the environment at multiple spatial scales, and is a main driver of species declines and deterioration of ecosystem services. However, most of the research on the effects of land use change has focused on taxonomic diversity, while functional diversity, an important predictor of ecosystem services, is often neglected. We explored how local and landscape scale characteristics influence functional and taxonomic diversity of hummingbirds in the Andes Mountains in southern Ecuador. Data was collected in six landscapes along a land use gradient, from an almost intact landscape to one dominated by cattle pastures. We used point counts to sample hummingbirds from 2011 to 2012 to assessed how local factors (i.e., vegetation structure, flowering plants richness, nectar availability) and landscape factors (i.e., landscape heterogeneity, native vegetation cover) influenced taxonomic and functional diversity. Then, we analyzed environment – trait relationships (RLQ test) to explore how different hummingbird functional traits influenced species responses to these factors. Taxonomic and functional diversity of hummingbirds were positively associated with landscape heterogeneity but only functional diversity was positively related to native vegetation coverage. We found a weak response of taxonomic and functional diversity to land use change at the local scale. Environment‐trait associations showed that body mass of hummingbirds likely influenced species sensitivity to land use change. In conclusion, landscape heterogeneity created by land use change can positively influence hummingbird taxonomic and functional diversity; however, a reduction of native vegetation cover could decrease functional diversity. Given that functional diversity can mediate ecosystem services, the conservation of native vegetation cover could play a key role in the maintenance of hummingbird pollination services in the tropical Andes. Moreover, there are particular functional traits, such as body mass, that increase a species sensitivity to land use change.

Landscape-level effects on aboveground biomass of tropical forests: A conceptual framework

Despite the general recognition that fragmentation can reduce forest biomass through edge effects, a systematic review of the literature does not reveal a clear role of edges in modulating biomass loss. Additionally, the edge effects appear to be constrained by matrix type, suggesting that landscape composition has an influence on biomass stocks. The lack of empirical evidence of pervasive edge-related biomass losses across tropical forests highlights the necessity for a general framework linking landscape structure with aboveground biomass. Here, we propose a conceptual model in which landscape composition and configuration mediate the magnitude of edge effects and seed-flux among forest patches, which ultimately has an influence on biomass. Our model hypothesizes that a rapid reduction of biomass can occur below a threshold of forest cover loss. Just below this threshold, we predict that changes in landscape configuration can strongly influence the patch's isolation, thus enhancing biomass loss. Moreover, we expect a synergism between landscape composition and patch attributes, where matrix type mediates the effects of edges on species decline, particularly for shade-tolerant species. To test our conceptual framework, we propose a sampling protocol where the effects of edges, forest amount, forest isolation, fragment size, and matrix type on biomass stocks can be assessed both collectively and individually. The proposed model unifies the combined effects of landscape and patch structure on biomass into a single framework, providing a new set of main drivers of biomass loss in human-modified landscapes. We argue that carbon trading agendas (e.g., REDD+) and carbon-conservation initiatives must go beyond the effects of forest loss and edges on biomass, considering the whole set of effects on biomass related to changes in landscape composition and configuration.

Gut microbiomes of mobile predators vary with landscape context and species identity

Landscape context affects predator–prey interactions and predator diet composition, yet little is known about landscape effects on insect gut microbiomes, a determinant of physiology and condition. Here, we combine laboratory and field experiments to examine the effects of landscape context on the gut bacterial community and body condition of predatory insects. Under laboratory conditions, we found that prey diversity increased bacterial richness in insect guts. In the field, we studied the performance and gut microbiota of six predatory insect species along a landscape complexity gradient in two local habitat types (soybean fields vs. prairie). Insects from soy fields had richer gut bacteria and lower fat content than those from prairies, suggesting better feeding conditions in prairies. Species origin mediated landscape context effects, suggesting differences in foraging of exotic and native predators on a landscape scale. Overall, our study highlights complex interactions among gut microbiota, predator identity, and landscape context.