X. Ben Wu

Differential Influence of Weather on Regional Quail Abundance in Texas

Although weather variables are known to influence quail abundance in some habitats, most studies have addressed only limited geographic areas and indices to weather conditions. The few replicated studies addressed relatively similar climate zones. We used 21 years (1978-98) of quail abundance data collected by the Texas Parks and Wildlife Department (TPWD) biologists to address the relationship between both simple precipitation and Palmer drought indices and Northern Bobwhite (Colinus virginianus) and Scaled quail (Callipepla squamata) abundance in 6 ecological regions of Texas. Three 12-month Palmer indices were more highly correlated with changes in Northern Bobwhite abundance in the South Texas Plains ecological region than was raw precipitation alone. The 12-month Modified Palmer Drought Severity Index (PMDI) was correlated (r s ≥ 0.78, P ≤ 0.001) with the mean number of Northern Bobwhites visually observed per survey route in the Rolling and South Texas Plains ecological regions, while a 12-month, raw precipitation index was correlated (r s = 0.64, P = 0.002) with Northern Bobwhite abundance in only the South Texas Plains. The PMDI and raw precipitation were correlated (r s ≥ 0.67, P ≥ 0.001 and r s ≤ 0.57, P ≤ 0.007, respectively) with the mean number Scaled Quail observed per survey route in the Edwards Plateau, South Texas Plains, and Trans-Pecos Mountains and Basins ecological regions. There was no relationship (P ≥ 0.437) between changes in quail abundance and the PMDI or raw precipitation in the Gulf Prairies and Marshes physiographic region, where precipitation was relatively high. The monthly PMDI was a better indicator of changes in both northern bobwhite and Scaled Quail abundance among years than was monthly precipitation alone. Both monthly and 12-month precipitation-based weather indices were more correlated with changes in Northern Bobwhite and scaled quail abundance among years in relatively dry as opposed to wet ecological regions. Our approach should help wildlife biologists and managers better account for annual variability in quail productivity in semi-arid environments so that long-term populations trends can be better elucidated.

Multiple-scale habitat modeling approach for rare plant conservation

Multiple-scale habitat assessment for rare plants is an important component of conservation and development planning. It is challenging, however, due to lack of information synthesis on the ecology of rare plants, lack of effective approaches for habitat assessment at multiple spatial scales, and lack of spatial data for relevant environmental attributes and scales. A multiple-scale habitat modeling approach was developed to meet this need. Regional-, landscape-, and site-scale habitat models were developed for eight rare plant species found in southern Texas, USA. The models were partially validated and used for planning of rare plant conservation and highway construction. Regional-scale habitat models were used to predict, based on coarse-scale geographic information system (GIS) data, spatial distribution of areas containing potential habitat of rare plant species and the probability of encountering potential rare plant habitats. Site-scale models, based on synthesis of the literature and field investigations, were developed for field survey and mapping of rare plant habitats to enable accurate assessment of potential and present habitat suitability of specific locations using fine-resolution field data on soil, landform and vegetation structure. The greatest need for assessing the presence and potential habitat of rare plants is at the landscape scales. Thus, landscape-scale models were developed for spatially explicit assessment of potential and present habitat suitability, based on site-scale models but using GIS and remote sensing-based data. These models can be used as effective tools for conservation planning, monitoring and management of rare plant habitat, as well as for reduction of land use conflicts and development cost. The processes of model development and application synthesizes the diffuse literature, identifies knowledge and data gaps to guide future research, and provides a framework for assimilating new information acquired in the future to improve habitat assessment.

Vegetation and water yield dynamics in an Edwards Plateau watershed

Woody cover, when expressed at the scale of the 207 km Cusenbary Draw basin, remained unchanged (~23%) from 1955 to 1990. When expressed at the scale of range sites, woody cover declined on sites with relatively high production potential and increased on sites with relatively low production potential. Change in woody cover distribution at sub-range site scales, increased low and high woody covers and decreased intermediate woody cover, would be expected to lead to increased water yield at the basin scale because there was an apparent threshold woody cover (~20%) above which simulated evapotranspiration (ET) changed little with increasing woody cover. This potential increase, however, was more than offset by the decreased water yield due to increased ET loss associated with compositional changes of woody vegetation from oak to juniper. A set of woody cover-ET regression curves was developed for different range sites based on simulation studies using the SPUR-91 hydrologic model. Based on these woody cover-ET regression curves and GIS analysis, no brush management would result in a 35% decrease in water yield, while a hypothetical brush management cost-share program would increase water yield by 43% over the 1990 level. Benefits in water yield and forage production from brush management differ in different range sites. A brush management cost-share program that preferentially allocated brush management to sites with deep soil and the highest forage production potential increased water yield by 50%, compared to a 100% increase if brush management were preferentially allocated on sites with shallow soil and highest water yield potential. These model results illustrate that the spatial scale of assessment and spatial distribution of brush management among range sites should be important concerns associated with developing and evaluating brush management policies.

Utilities of edge-based metrics for studying landscape fragmentation

Landscape fragmentation associated with human activities poses serious threats to conservation of the Earths natural ecosystems. A central issue in landscape fragmentation is the ecological effect of habitat edges, but edge characteristics and associated metrics have not been widely used in landscape fragmentation studies. This study explored the utility of edge-based metrics in quantifying landscape fragmentation using simulated artificial landscapes and a real-world example of landscape fragmentation in southwest Chinas Wolong Nature Reserve. Results from artificial landscape simulations demonstrate that edge-based landscape metrics are effective measures of landscape fragmentation, capturing important aspects of landscape fragmentation not detected by patch-based metrics. The landscape of the reserve has become more fragmented due mostly to effects of past reforestation efforts and of sustained human disturbances at lower elevations during the study period. This paper illustrates that the use of edge-based metrics, in conjunction with patch-based metrics, can provide more comprehensive information on the patterns and dynamics of landscape changes, and thus facilitate the assessment and monitoring of landscape fragmentation and restoration.

An initial exploration of a lacunarity-based segregation measure

It has been recognized that urban residential segregation is a scale-dependent phenomenon, and yet all existing structural or spatial indices measure only certain dimensions of segregation at one single scale. Inspired by new metrics developed by landscape ecologists to measure landscape heterogeneity, in this paper we explore the feasiblity of a multiscale, lacunarity-based segregation measure. We also develop a straightforward GIS-based procedure to calculate this new measure. Our initial simulation results show that lacunarity is an effective measure that can capture multiple dimensions of segregation patterns at multiple scales.

Assessing resilience and state‐transition models with historical records of cheatgrass Bromus tectorum invasion in North American sagebrush‐steppe

Summary 1. Resilience-based approaches are increasingly being called upon to inform ecosystem management, particularly in arid and semi-arid regions. This requires management frameworks that can assess ecosystem dynamics, both within and between alternative states, at relevant time scales. 2. We analysed long-term vegetation records from two representative sites in the North American sagebrush-steppe ecosystem, spanning nine decades, to determine if empirical patterns were consistent with resilience theory, and to determine if cheatgrass Bromus tectorum invasion led to thresholds as currently envisioned by expert-based state-and-transition models (STM). These data span the entire history of cheatgrass invasion at these sites and provide a unique opportunity to assess the impacts of biotic invasion on ecosystem resilience. 3. We used univariate and multivariate statistical tools to identify unique plant communities and document the magnitude, frequency and directionality of community transitions through time. Community transitions were characterized by 37–47% dissimilarity in species composition, they were not evenly distributed through time, their frequency was not correlated with precipitation, and they could not be readily attributed to fire or grazing. Instead, at both sites, the majority of community transitions occurred within an 8–10 year period of increasing cheatgrass density, became infrequent after cheatgrass density peaked, and thereafter transition frequency declined. 4. Greater cheatgrass density, replacement of native species and indication of asymmetry in community transitions suggest that thresholds may have been exceeded in response to cheatgrass invasion at one site (more arid), but not at the other site (less arid). Asymmetry in the direction of community transitions also identified communities that were ‘at-risk’ of cheatgrass invasion, as well as potential restoration pathways for recovery of pre-invasion states. 5. Synthesis and applications. These results illustrate the complexities associated with threshold identification, and indicate that criteria describing the frequency, magnitude, directionality and temporal scale of community transitions may provide greater insight into resilience theory and its application for ecosystem management. These criteria are likely to vary across biogeographic regions that are susceptible to cheatgrass invasion, and necessitate more in-depth assessments of thresholds and alternative states, than currently available.

An Assessment of State-and-Transition Models: Perceptions Following Two Decades of Development and Implementation

Abstract State-and-transition models (STMs) are being developed for many areas in the United States and represent an important tool for assessing and managing public and private rangelands. Substantial resources have been invested in model development, yet minimal efforts have been made to evaluate the utility of STMs for rangeland assessment and management. We interviewed 47 rangeland professionals, equally divided between managers and researchers, in four ecoregions to determine their perceptions of the purpose, development, and strengths and weaknesses of STMs to assess the status of the STM framework. Our analysis identified three primary perspectives regarding the purpose of STMs: a decision-making tool for land managers, a means to represent the complex dynamics of rangeland ecosystems, and an effective communication tool. These diverse views of STM purposes were associated with differing perspectives concerning model development that identified five major issues in need of further development and refinement: 1) the relative importance of management practices and ecological processes in driving transitions, 2) the criteria used to define thresholds, 3) the appropriate level of model complexity, 4) the respective roles of expert knowledge and ecological data in model development, and 5) processes for model review and revision. We recommend greater dialogue among researchers and managers to further clarify STM terminology and develop standard protocols for model development and validation. Mechanisms are critically needed to assure peer review and revision of existing models so that STMs are continually updated to reflect current understanding of rangeland dynamics.

Landscape-scale vegetation dynamics inferred from spatial patterns of soil δ13C in a subtropical savanna parkland

[1] Grasslands and savannas around the world have experienced woody plant encroachment during the past 100 years, but we know little regarding the manner in which woody plants spread across the landscape. We used soil δ 13 C, aerial photography, and geostatistics to quantify patterns of woody encroachment in a 160 x 100 m georeferenced grid subdivided into 10 × 10 m cells in a savanna parkland landscape in southern Texas. δ 13 C contour maps revealed that centers of closed contours coincided with centers of woody patches, and that larger woody patches developed from smaller woody plant clusters that spread laterally and coalesced. Areas where woody patches were expanding into grassland were characterized by low densities of soil δ 13 C contour lines, and indicated the direction and extent of woody encroachment. Conversely, areas with high contour densities represented grassland-woodland boundaries that were temporally stable. Indeed, aerial photos from 1930, 1941, 1982, and 2003 confirmed that woody patches with low spatial variability in δ 13 C corresponded to areas where woody plants had encroached during the past 30―75 years. While aerial photos can only record vegetation cover at the photo acquisition time, kriged maps of soil δ 13 C allowed us to accurately reconstruct long-term temporal dynamics of woody plant encroachment into grassland. This approach can reliably reconstruct landscape-scale vegetation changes in areas where historical aerial photography or satellite imagery are unavailable and provides a strong spatial context for studies aimed at understanding the functional consequences of vegetation change.

15N isoscapes in a subtropical savanna parkland: spatial‐temporal perspectives

Spatial patterns of soil δ15N reflect variation in rates of N-cycling processes across landscapes. However, the manner in which soil δ15N is affected by vegetation and topoedaphic properties under non-steady state conditions is understood poorly. Here we propose and evaluate a conceptual model that explains how soil δ15N values will respond to changes in disturbance regimes (intensification of grazing and removal of fire) and the resultant invasion of a subtropical grassland by woody vegetation dominated by Prosopis glandulosa (honey mesquite), a N-fixing tree legume. Spatially-specific sampling along a catena (hill-slope) gradient where woody plants are known to have displaced grasses over the past 100 years revealed a positive relationship between soil δ15N and δ13C, and a negative relationship between NDVI and soil δ15N on upland portions of the landscape, indicating that plant cover is a critical determinant of δ15N spatial patterns. Because the dominant woody invader is a N-fixer, its invasion has in...

The Grasslands of Inner Mongolia: A Special Feature

Grasslands of northern China are of great ecological, economic, and cultural importance (Kang et al. 2007). These immense grasslands cover 400 million ha or 40% of the land area of China and stretch 4 500 km northeast–southwest (lat 28 u N to lat 51 u N). They extend from the northeastern plains adjacent to Mongolia to the southern Tibetan Plateau and consist of four major types: meadow steppes, typical steppes, desert steppes, and alpine steppes (Sun 2005; Kang et al. 2007). Inner Mongolia has 87 million ha of natural grassland, which is a significant constituent of the Eurasian Steppe—the largest contiguous biome in the world (Li 1962, 1979; Wu and Loucks 1992). From east to west, meadow steppe, typical steppe, and desert steppe zones occur in response to the decreasing moisture gradient.