Graph models of habitat mosaics Urban, Dean L; Minor, Emily S; Treml, Eric A ...
Ecology letters,
March 2009, Volume:
12, Issue:
3
Journal Article
Peer reviewed
Open access
Graph theory is a body of mathematics dealing with problems of connectivity, flow, and routing in networks ranging from social groups to computer networks. Recently, network applications have erupted ...in many fields, and graph models are now being applied in landscape ecology and conservation biology, particularly for applications couched in metapopulation theory. In these applications, graph nodes represent habitat patches or local populations and links indicate functional connections among populations (i.e. via dispersal). Graphs are models of more complicated real systems, and so it is appropriate to review these applications from the perspective of modelling in general. Here we review recent applications of network theory to habitat patches in landscape mosaics. We consider (1) the conceptual model underlying these applications; (2) formalization and implementation of the graph model; (3) model parameterization; (4) model testing, insights, and predictions available through graph analyses; and (5) potential implications for conservation biology and related applications. In general, and for a variety of ecological systems, we find the graph model a remarkably robust framework for applications concerned with habitat connectivity. We close with suggestions for further work on the parameterization and validation of graph models, and point to some promising analytic insights.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Connectivity of habitat patches is thought to be important for movement of genes, individuals, populations, and species over multiple temporal and spatial scales. We used graph theory to characterize ...multiple aspects of landscape connectivity in a habitat network in the North Carolina Piedmont (U.S.A).. We compared this landscape with simulated networks with known topology, resistance to disturbance, and rate of movement. We introduced graph measures such as compartmentalization and clustering, which can be used to identify locations on the landscape that may be especially resilient to human development or areas that may be most suitable for conservation. Our analyses indicated that for songbirds the Piedmont habitat network was well connected. Furthermore, the habitat network had commonalities with planar networks, which exhibit slow movement, and scale-free networks, which are resistant to random disturbances. These results suggest that connectivity in the habitat network was high enough to prevent the negative consequences of isolation but not so high as to allow rapid spread of disease. Our graph-theory framework provided insight into regional and emergent global network properties in an intuitive and visual way and allowed us to make inferences about rates and paths of species movements and vulnerability to disturbance. This approach can be applied easily to assessing habitat connectivity in any fragmented or patchy landscape.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Plantings in residential neighborhoods can support wild pollinators. However, it is unknown how effectively wild pollinators maintain pollination services in small, urban gardens with diverse floral ...resources. We used a 'mobile garden' experimental design, whereby potted plants of cucumber, eggplant, and purple coneflower were brought to 30 residential yards in Chicago, IL, USA, to enable direct assessment of pollination services provided by wild pollinator communities. We measured fruit and seed set and investigated the effect of within-yard characteristics and adjacent floral resources on plant pollination. Increased pollinator visitation and taxonomic richness generally led to increases in fruit and seed set for all focal plants. Furthermore, fruit and seed set were correlated across the three species, suggesting that pollination services vary across the landscape in ways that are consistent among different plant species. Plant species varied in terms of which pollinator groups provided the most visits and benefit for pollination. Cucumber pollination was linked to visitation by small sweat bees (Lasioglossum spp.), whereas eggplant pollination was linked to visits by bumble bees. Purple coneflower was visited by the most diverse group of pollinators and, perhaps due to this phenomenon, was more effectively pollinated in florally-rich gardens. Our results demonstrate how a diversity of wild bees supports pollination of multiple plant species, highlighting the importance of pollinator conservation within cities. Non-crop resources should continue to be planted in urban gardens, as these resources have a neutral and potentially positive effect on crop pollination.
As our world becomes increasingly urbanized, cities are often where we come into contact with the natural world—not just in parks and urban nature preserves, but in more familiar places like ...residential yards. We conducted bird surveys and social surveys in Chicago-area residential landscapes near forest preserves (primarily in middle- and high-income areas) to examine residents' perceptions of the birds that co-inhabit their neighborhoods and the relationship of those perceptions with characteristics of the bird community. We found that residents value many aspects of neighborhood birds, especially those related to aesthetics and birds' place in the ecosystem. Our results indicate that while birds were generally well liked and annoyances were minor, several common and visible urban species, such as the House Sparrow (Passer domesticus), European Starling (Sturnus vulgaris), and Blue Jay (Cyanocitta cristata), may attract attention for their negative qualities, such as their sounds and effects on personal property. The results also indicate that residents' valuations of ecosystem services are linked to their perceptions of bird species richness rather than the actual species richness, and people may perceive only a subset of the birds in their neighborhoods. Although birds provide many important ecosystem services, perhaps one of their most important roles in cities is as a relatable and likable connecting point between city dwellers and the broader environment.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Vacant lots are typically viewed as urban blight but are also green spaces that provide wildlife habitat and ecosystem services in urban landscapes. Vacant lot vegetation results from interacting ...biophysical and social forces, and studying vacant lot ecology is an opportunity to examine urban socio-environmental intersections. Here, we assess vegetation patterns in vacant lots across Chicago, IL (USA), and ask two questions: 1) How does diversity and structure vary, and 2) how do social and biophysical drivers contribute to this variation? We conducted vegetation surveys in 35 vacant lots in the summer of 2015. In each lot, we identified all herbaceous plants (excluding turf grasses) and woody seedlings and measured species richness, evenness, vegetation height, and total vegetated area. We used field sampled data about human activities and land use in vacant lots (e.g., presence of a path, trash and turf), coupled with sociodemographic data (e.g., income, ethnicity), and fine-scale land cover to construct two models for each vegetation measure: a best-fit biophysical model and a best-fit social model. We then used variation partitioning to compare the relative strength of these models and any overlap between them. In total, we identified 109 plant species. Species evenness was high, suggesting that there are few rare species in this system. Species richness and vegetation height were better explained by social models, while vegetated area and evenness were better explained by biophysical models. We saw evidence of overlapping explanatory power between the social and biophysical domains. The amount of trash in a lot was the most significant variable, explaining three of our vegetation measures. Lots with higher amounts of trash had higher richness and evenness, and lower vegetated area. This assessment of patterns of vegetation in Chicago's vacant lots provides insight into how habitat differs across the city and informs urban conservation paradigms.
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•Assessed plant richness, evenness, vegetated area, and height in 35 vacant lots.•Richness and evenness are positively related; species in lots are unlikely to be rare.•Social and biophysical models both explained vegetation, and sometimes overlapped.•Trash in a lot was an important explanatory variable for three vegetation measures.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
As urbanization accelerates, urban biodiversity conservation is becoming a great concern for the maintenance of urban ecosystem functions. In particular, forest bird communities in urban areas have ...been recognized as a conservation target because of their functions in food webs and ecosystem services. But our understanding of which local- and landscape-scale factors influence native bird communities within urban green spaces is still insufficient to provide managers with information for effectively planning biodiversity management programs. Here we examine how local habitat characteristics, human disturbance, and habitat connectivity influence the diversity of forest bird communities in 44 small forest patches (0.5–20.0 ha) embedded in an urbanized landscape. Patch size exerted a positive influence on the diversity of most bird functional groups, and it had the greatest effects on total abundance and species richness. The second most important factor was human disturbance. Remnant patches with lower levels of human disturbance had higher diversity than newly established patches where intense human activities occurred more frequently. In addition, vegetation complexity and habitat connectivity were positively related to total species richness and abundance, respectively, but they were less important. Management strategies for the conservation of urban forest birds, therefore, should consider not only local improvements in habitat structure – through increased patch size, reduced human disturbance, and increased vegetation complexity – but also the maintenance of habitat connectivity.
Residential yards comprise a substantial portion of urban landscapes, and the collective effects of the management of many individual yards may "scale up" to affect urban biodiversity. We conducted ...bird surveys and social surveys in Chicago-area (Illinois, USA) residential neighborhoods to identify the relative importance of yard design and management activities for native birds. We found that groups of neighboring yards, in the aggregate, were more important for native bird species richness than environmental characteristics at the neighborhood or landscape scale. The ratio of evergreen to deciduous trees in yards and the percentage of yards with trees and plants with fruits or berries were positively associated with native bird species richness, whereas the number of outdoor cats had a negative association. The number of birdfeeders was not an important predictor for native species richness. We also found that migratory birds were observed on transects with more wildlife-friendly features in yards, and nonnative birds were observed on transects with greater numbers of outdoor cats and dogs. Our results highlight the potential importance of residential matrix management as a conservation strategy in urban areas.
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BFBNIB, FZAB, GIS, IJS, INZLJ, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZRSKP
Urban landscapes are often florally rich due to extensive management of cultivated plants around the city. High abundance and diversity of these floral resources has been linked to more abundant and ...diverse pollinator communities, but little is known about how urban pollinators select from the wide variety of available flowers. This study provides unique insight into urban plant-pollinator interactions by examining how many plant taxa, and which taxa, are visited by pollinators. Over a three-year period, we observed foraging of urban pollinators across 63 neighborhoods in Chicago, Illinois (USA). We observed 1815 plant-pollinator interactions between 24 pollinator morpho-types and 106 plant taxa. An additional 57 plant taxa were flowering but not visited by pollinators. On average, each pollinator morpho-type visited 19 plant taxa, and most pollinators showed generalist tendencies. We identified 42 plant taxa that were visited by more pollinator morpho-types than their floral abundance would predict; we call these ‘highly attractive’ plants. In general, perennial and native plants received more pollinator visits than their counterparts, and ornamental plants were visited by more species than unintentional ‘weeds’. However, the two most-visited plant taxa were non-native, perennial weeds. Our results suggest that many flowering plants in cities are not visited by pollinators. Furthermore, the plant-pollinator network could be reduced to only four plant taxa without losing any pollinator morpho-types. To enhance urban pollinator conservation, urban residents can select ornamental plants from our list of ‘highly attractive’ plant taxa, or can allow some of the highly attractive ‘weeds’ to persist in their gardens.
Despite increasing concern regarding broad-scale declines in insects, there are few published long-term, systematic butterfly surveys in North America, and fewer still that have incorporated the ...influence of changing climate and landscape variables. In this study, we analyzed 20 years of citizen science data at seven consistently monitored protected areas in Illinois, U.S.A. We used mixed models and PERMANOVA to evaluate trends in butterfly abundance, richness, and composition while also evaluating the effects of temperature and land use. Overall butterfly richness, but not abundance, increased in warmer years. Surprisingly, richness also was positively related to percent impervious surface (at the 2 km radius scale), highlighting the conservation value of protected areas in urban landscapes (or alternately, the potential negative aspects of agriculture). Precipitation had a significant and variable influence through time on overall butterfly abundance and abundance of resident species, larval host plant specialists, and univoltine species. Importantly, models incorporating the influence of changing temperature, precipitation, and impervious surface indicated a significant overall decline in both butterfly abundance and species richness, with an estimated abundance decrease of 3.8%/year and richness decrease of 1.6%/year (52.5% and 27.1% cumulatively from 1999 to 2018). Abundance and richness declines were also noted across all investigated functional groups except non-resident (migratory) species. Butterfly community composition changed through time, but we did not find evidence of systematic biotic homogenization, perhaps because declines were occurring in nearly all functional groups. Finally, at the site-level, declines in either richness or abundance occurred at five of seven locations, with only the two largest locations (>300 Ha) not exhibiting declines. Our results mirror those of other long-term butterfly studies predominantly in Europe and North America that have found associations of butterflies with climate variables and general declines in butterfly richness and abundance.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Urban greening increases vegetation and can restore ecological functions to urban systems. It has ties to restoration ecology, which aims to return degraded land to diverse, functional ecosystems. ...Both practices can be applied to maximizing ecosystem services and habitat in vacant lots, which are abundant in post-industrial cities, including Chicago, Illinois (USA), where our study took place. We tested four methods for increasing native plant diversity in vacant lots, ranging from low input to resource-intensive: seed bombing, broadcast seeding, planting plugs, and gardening. After three growing seasons, we assessed the growth of eight target native species and all non-target species. We expected that intensive treatments would have more target species stems and flowers and fewer non-target species, but we found that less-intensive options often produce equal or better results. From this, we recommend broadcast seeding as a viable, low-cost method for improving habitat and biodiversity in vacant lots.
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