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Understanding complex ecological phenomena, such as the determinants of species richness, is best achieved by investigating their properties at different spatial scales. Factors significantly affecting the number of species occurring at one scale may not impact on richness at other scales. While this scale dependence has become increasingly recognized, there still remains a need to elucidate exactly how richness is structured across scales, and which mechanisms are influential for determining this important community property. This study explores how woody plant species richness varies in a fragmented system at multiple scales, and which factors are primarily responsible for these patterns. The study area is located in the Sonoran Desert within the bounds of metropolitan Phoenix, Arizona, which is the locus of the Central Arizona-Phoenix Long-Term Ecological Research (CAP-LTER) site. Estimates of local and fragment plant species richness were generated from field data collected from 22 sites. Independent variables describing fragment sites were also calculated, including area, habitat heterogeneity, density of individuals, mean elevation, and extent of isolation. Structural equation modelling, multiple regression, and analysis of covariance were used to assess the contribution of independent variables to richness at the fragment and local scales. Fragment species richness was significantly influenced by area, though not isolation, habitat heterogeneity, mean elevation, or density of individuals. Local richness was not significantly related to fragment area, but was positively related to fragment richness, plant density, and elevation. The fragment species-area effect resulted from larger remnants supporting higher numbers of individuals at comparable densities, increasing richness through either passive sampling of progressively less common species and/or lower extinction rates among larger populations. Without using multi-temporal data it is not possible to disentangle these mechanisms. We found that patterns evident at one scale are not necessarily apparent at other scales, as elevation and density of individuals significantly affected richness at the local scale but not at the fragment scale. These results lend support to the concept that mechanisms influencing the species richness of natural communities may be operable only within certain domains and that relevant scales should be specified.

Many changes wrought during the construction of “designer ecosystems” are intended to ensure—and often succeed in ensuring—that a city can provide ecosystem goods and services; but other changes have unintended impacts on the ecology of the city, impairing its ability to provide these critical functions. Indian Bend Wash, an urbanizing watershed in the Central Arizona–Phoenix (CAP) ecosystem, provides an excellent case study of how human alteration of land cover, stream channel structure, and hydrology affect ecosystem processes, both intentionally and unintentionally. The construction of canals created new flowpaths that cut across historic stream channels, and the creation of artificial lakes produced sinks for fine sediments and hotspots for nitrogen processing. Further hydrologic manipulations, such as groundwater pumping, linked surface flows to the aquifer and replaced ephemeral washes with perennial waters. These alterations of hydrologic structure are typical by-products of urban growth in arid and semiarid regions and create distinct spatial and temporal patterns of nitrogen availability.

The structure of urban environments is known to alter local climate, in part due to changes in land cover. A growing subset of research focuses specifically on the UHI in terms of land surface temperature by using data from remote sensing platforms. Past research has established a clear relationship between land surface temperature and the proportional area of land covers, but less research has specifically examined the effects of the spatial patterns of these covers. This research considers the rapidly growing City of Phoenix, Arizona in the United States. To better understand how landscape structure affects local climate, we explored the relationship between land surface temperature and spatial pattern for three different land uses: mesic residential, xeric residential, and industrial/commercial. We used high-resolution (2.4 m) land cover data and an ASTER temperature product to examine 90 randomly selected sample sites of 240 square-meters. We (1) quantify several landscape-level and class-level landscape metrics for the sample sites, (2) measure the Pearson correlation coefficients between land surface temperature and each landscape metric, (3) conduct an analysis of variance among the three land uses, and (4) model the determinants of land surface temperature using ordinary least squares linear regression. The Pearson’s correlation coefficients reveal significant relationships between several measures of spatial configuration and LST, but these relationships differ among the land uses. The ANOVA confirmed that mean land surface temperature and spatial patterns differed among the three land uses. Although a relationship was apparent between surface temperatures and spatial pattern, the results of the linear regression indicate that proportional land cover of grass and impervious surfaces alone best explains temperature in mesic residential areas. In contrast, temperatures in industrial/commercial areas are explained by changes in the configuration of grass and impervious surfaces.

Increased nitrogen (N) deposition threatens global biodiversity, but its effects in arid urban ecosystems are not well studied. In addition to altered N availability, urban environments also experience increases in other pollutants, decreased population connectivity, and altered biotic interactions, which can further impact biodiversity. In deserts, annual plant communities make up most of the plant diversity, support wildlife, and contribute to nutrient cycling and ecosystem processes. Functional trade-offs allowing coexistence of a diversity of annual plant species are well established, but maintenance of diversity in urban conditions and with increased availability of limiting nutrients has not been explored. We conducted a 13-yr N and phosphorus (P) addition experiment in Sonoran Desert preserves in and around Phoenix, Arizona, USA to test how nutrient availability interacts with growing season precipitation, urban location, and microhabitat to affect winter annual plant diversity. Using structural equation modeling and generalized linear mixed modeling, we found that annual plant taxonomic diversity was significantly reduced in N-enriched and urban plots. Water availability in both current and previous growing seasons impacted annual plant diversity, with significant interaction effects showing increased diversity in wetter years and greater responsiveness of the community to water following a wet year. However, there were no significant interactions between N enrichment and water availability, urban location, or microhabitat. Lowered diversity in urban preserves may be partly attributable to increased urban N deposition. Changes in biodiversity of showy species like annual wildflowers in urban preserves can have important implications for connections between urban residents and nature, and reduced diversity and community restructuring with N enrichment represents a challenge for future preservation of aridland biodiversity.

Urbanization is recognized as one of the greatest threats to biodiversity throughout the world. However, the vegetation within an urbanized landscape is diverse and includes a variety of native and exotic plant species. This variation allows for testing whether certain landscape designs outperform others in the support of native biodiversity. Residential yards represent a large component of an urban landscape and, if managed collectively for birds and other wildlife, could offset some of the negative effects of urbanization. In addition, many urbanites have their primary interaction with the natural world in their front and back yards. Therefore, ensuring positive wildlife experiences for them is essential in promoting urban biodiversity. At the Central Arizona-–Phoenix Long-Term Ecological Research site we tested the efficacy of native landscaping in residential yards in attracting native birds. We also explored the links between socioeconomic factors, landscape designs, and urban gradient measurements with the urban bird communities. A redundancy analysis suggested that native desert bird species increased in abundance in neighborhoods with desert landscaping designs, neighborhoods closer to large desert tracts, and higher-income neighborhoods. Variance partitioning showed that collectively these three sets of environmental variables explained almost 50%% of the variation in the urban bird community. Results suggested racial and economic inequities in access to biodiversity, whereby predominantly Hispanic and lower-income neighborhoods had fewer native birds. We also found that residents' satisfaction with bird diversity was positively correlated with actual bird diversity. Our study provides new insights into the relative importance of socioeconomic variables and common urban ecological measurements in explaining urban bird communities. Urban planners can use this information to develop residential landscapes that support the well-being of both birds and people.

The loss of habitat due to suburban and urban development represents one of the greatest threats to biodiversity. Conservation developments have emerged as a key player for reconciling new ex-urban residential development with ecosystem services. However, as more than half of the world population lives in urban and suburban developments, identifying conservation partners to facilitate retrofitting existing residential neighborhoods becomes paramount. Homeowner associations (HOA) manage a significant proportion of residential developments in the United States, which includes the landscape design for yards and gardens. These areas have the potential to mitigate the loss of urban biodiversity when they provide habitat for native wildlife. Therefore, the conditions and restrictions imposed upon the homeowner by the HOA could have profound effects on the local wildlife habitat. We explored the potential of HOAs to promote conservation by synthesizing research from three monitoring programs from Phoenix, Arizona. We compared native bird diversity, arthropod diversity, and plant diversity between neighborhoods with and without a HOA. Neighborhoods belonging to HOAs had significantly greater bird and plant diversity, although insect diversity did not differ. The institutional framework structuring HOAs, including sanctions for enforcement coupled with a predictable maintenance regime that introduces regular disturbance, might explain why neighborhoods with a HOA had greater bird diversity. For neighborhoods with a HOA, we analyzed landscape form and management practices. We linked these features with ecological function and suggested how to modify management practices by adopting strategies from the Sustainable Sites Initiative, an international sustainable landscaping program, to help support biodiversity in current and future residential landscapes.

As a part of the Central Arizona-Phoenix Long-Term Ecological Research project, we determined how land-use alteration influenced spider and harvestman diversity. We sampled spiders in six habitat types (desert parks, urban desert remnants, industrial, agricultural, xeric- and mesic-residential yards) and tested how habitat type and productivity affected spider diversity and abundance. As expected, agricultural fields and mesic yards were more productive than the other, xeric habitats. These more productive habitats were characterized by higher abundances but lower spider diversity and were dominated by Lycosidae (wolf spiders), followed by Linyphiidae (sheet-web weavers). The increase in wolf spider abundance was positively correlated with habitat productivity and negatively correlated with the abundance of other predatory arthropods that might compete with, or prey upon, wolf spiders. Temporal changes in productivity affected spider abundance. After an El-Niño winter (May 1998), spider abundance was five times higher than after an extremely dry winter (May 2000). The differences in spider abundance between agricultural fields and the four xeric habitats were profound in 2000 but moderate in 1998, suggesting an interaction between the effects of natural and anthropogenic factors on spider populations. Compared with xeric habitats, the El-Niño effect was less profound in agricultural sites, suggesting that human land modification mollifies seasonal effects. We suggest that habitat structure and productivity alteration may change community structure, as the urban or agricultural habitats favor one or a few preadapted taxa over many others. Incorporation of large fragments of natural habitats into future landscape planning in urban environments may be important for conservation of rich spider communities.

Ecologists increasingly use plot-scale data to inform research and policy related to regional and global environmental change. For soil chemistry research, scaling from the plot to the region is especially difficult due to high spatial variability at all scales. We used a hierarchical Bayesian model of plot-scale soil nutrient pools to predict storage of soil organic carbon (oC), inorganic carbon (iC), total nitrogen (N), and available phosphorus (avP) in a 7962-km² area including the Phoenix, Arizona, USA, metropolitan area and its desert and agricultural surroundings. The Bayesian approach was compared to a traditional approach that multiplied mean values for urban mesic residential, urban xeric residential, nonresidential urban, agricultural, and desert areas by the aerial coverage of each land-use type. Both approaches suggest that oC, N, and avP are correlated with each other and are higher (in g/m²) in mesic residential and agricultural areas than in deserts or xeric residential areas. In addition to traditional biophysical variables, cultural variables related to impervious surface cover, tree cover, and turfgrass cover were significant in regression models predicting the regional distribution of soil properties. We estimate that 1140 Gg of oC have accumulated in human-dominated soils of this region, but a significant portion of this new C has a very short mean residence time in mesic yards and agricultural soils. For N, we estimate that 130 Gg have accumulated in soils, which explains a significant portion of \"missing N\" observed in the regional N budget. Predictions for iC differed between the approaches because the Bayesian approach predicted iC as a function of elevation while the traditional approach employed only land use. We suggest that Bayesian scaling enables models that are flexible enough to accommodate the diverse factors controlling soil chemistry in desert, urban, and agricultural ecosystems and, thus, may represent an important tool for ecological scaling that spans land-use types. Urban planners and city managers attempting to reduce C emissions and N pollution should consider ways that landscape choices and impervious surface cover affect city-wide soil C, N, and P storage.

The Indian Bend Wash (IBW) flood-control project relies on a greenbelt to carry floods through Scottsdale, Arizona, USA. The greenbelt is characterized by a chain of shallow artificial lakes in a larger floodplain of irrigated turf, which has been protected from encroaching urban development. As such, this urban stream-floodplain complex can be divided into three subsystems: artificial lakes, channelized stream segments, and floodplain. We conducted experiments to evaluate which, if any, of these subsystems were important sites of denitrification, and to explore factors controlling denitrification rates. Denitrification enzyme activity (DEA) bioassays were conducted on sediments from eight lake and six stream segments as well as soil samples from eight floodplain transects. Mass-specific potential denitrification rates were significantly higher in lakes than in streams or floodplains. Nutrient limitation bioassays revealed that nitrate (NO 3 − ) limited denitrification in lake sediments, a surprising finding given that NO 3 − -rich groundwater additions frequently raised lake NO 3 − concentration above 1 mg N/L. Experiments on intact lake cores suggested that denitrification was limited by the rate NO 3 − diffused into sediments, rather than its availability in overlying water. Floodplain denitrification was limited by water content, not NO 3 − or C, and irrigation of soils stimulated denitrification. We constructed a N budget for the IBW stream-floodplain complex based on our experimental results. We found that both lakes and floodplains removed large quantities of N, with denitrification removing 261 and 133 kg N·ha −1 ·yr −1 from lake sediments and floodplain soils, respectively, indicating that lakes are hotspots for denitrification. Nevertheless, because floodplain area was >4.5 times that of lakes, floodplain soils removed nearly 2.5 times as much N as lake sediments. Given the desert's low annual precipitation, a finding that floodplain soils are active sites of denitrification might seem implausible; however, irrigation is common in urban landscapes, and it elevated annual denitrification in IBW. Based on our results, we conclude that construction of artificial lakes created hotspots while application of irrigation water created hot moments for denitrification in the stream-floodplain complex, demonstrating that management decisions can improve the ability of urban streams to provide critical ecosystem services like N retention.

Urbanization can have profound effects on the plant communities persisting in remnant habitats. That process can be explored by examining patterns of nestedness. Species composition for a set of communities exhibits a nested pattern if species present in progressively richer assemblages form a series of subsets. Nestedness can form as a result of the dynamic processes of extinction or colonization. It can also reflect a nested distribution of habitats among the sites or the differential abundance properties of species through passive sampling. This study investigated whether Sonoran Desert woody vegetation in remnant islands within metropolitan Phoenix is nested and explored which mechanisms are responsible for the pattern. It also examined whether vegetation is nested in similar habitat types across islands, and how species abundance relates to the nested pattern and hypothesized mechanisms. All data sets were significantly nested, indicating a nested pattern at the island and habitat levels. Community-level analyses did not indicate a primary mechanism leading to the nested pattern. Among species with abundances correlated with the nested rank-order of sites, abundance properties were significantly related to different variables. This suggests that individual taxa respond to divergent ecological mechanisms, leading to nestedness. Thus, nestedness in plant communities can result from a complex set of contributors and may not be attributable to a single factor.