Matthew E. Baker

How novel is too novel? Stream community thresholds at exceptionally low levels of catchment urbanization

Novel physical and chemical conditions of many modern ecosystems increasingly diverge from the environments known to have existed at any time in the history of Earth. The loss of natural land to urbanization is one of the most prevalent drivers of novel environments in freshwaters. However, current understanding of aquatic community response to urbanization is based heavily upon aggregate indicators of community structure and linear or wedge-shaped community response models that challenge ecological community theory. We applied a new analytical method, threshold indicator taxa analysis (TITAN), to a stream biomonitoring data set from Maryland to explicitly evaluate linear community response models to urbanization that implicitly assume individual taxa decline or increase at incrementally different levels of urbanization. We used TITAN (1) to identify the location and magnitude of greatest change in the frequency and abundance of individual taxa and (2) to assess synchrony in the location of change points as evidence for stream community thresholds in response to percent impervious cover in catchments. We documented clear and synchronous threshold declines of 110 of 238 macroinvertebrate taxa in response to low levels of impervious cover. Approximately 80% of the declining taxa did so between 0.5% and 2% impervious cover, whereas the last 20% declined sporadically from 2% to 25% impervious cover. Synchrony of individual responses resulted in distinct community-level thresholds ranging from < or = 0.68% (mountains), 1.28% (piedmont), and 0.96% (coastal plain) impervious cover. Upper limits (95% confidence intervals) of community thresholds were < 2% cover in all regions. Within distinct physiographic classes, higher-gradient, smaller catchments required less impervious cover than lower gradient, larger catchments to elicit community thresholds. Relatively few taxa showed positive responses to increasing impervious cover, and those that did gradually increased in frequency and abundance, approximating a linear cumulative distribution. The sharp, synchronous declines of numerous taxa established a consistent threshold response at exceptionally low levels of catchment urbanization, and uncertainty regarding the estimation of impervious cover from satellite data was mitigated by several corroborating lines of evidence. We suggest that threshold responses of communities to urban and other novel environmental gradients may be more prevalent than currently recognized.

Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients

Abstract The goal of this paper is to help managers better understand implications of using aggregate community metrics, such as taxon richness or Indices of Biotic Integrity (IBI), for detecting threshold responses to anthropogenic environmental gradients. To illustrate, we offer an alternative analytical approach, Threshold Indicator Taxa ANalysis (TITAN), geared toward identifying synchronous changes in the distribution of multiple taxa as evidence of an ecological community threshold. Our approach underscores the fundamental reality that which taxa are affected by stressors is important, both from a conservation standpoint and because taxon-specific life-history traits help us understand relevant mechanisms. First, we examine macroinvertebrate community response to an impervious cover gradient using a well-studied biomonitoring data set to show that representative community metrics are relatively insensitive to synchronous threshold declines of numerous individual taxa. We then reproduce these response relationships using a simulated community data set with similar properties to demonstrate that linear or wedge-shaped responses of community metrics to anthropogenic gradients can occur as an artifact of aggregating multiple taxa into a single value per sampling unit, despite strong nonlinearity in community response. Our findings do not repudiate the use of community metrics or multimetric indices, but they challenge assumptions that such metrics are capable of accurately reflecting community thresholds across a broad range of anthropogenic gradients. We recommend an alternative analysis framework that begins with characterization of the responses of individual taxa and uses aggregation only after distinguishing the magnitude, direction, and uncertainty in the responses of individual members of the community.

Extraction of hydrological proximity measures from DEMs using parallel processing

Land surface topography is one of the most important terrain properties which impact hydrological, geomorphological, and ecological processes active on a landscape. In our previous efforts to develop a soil depth model based upon topographic and land cover variables, we derived a set of hydrological proximity measures (HPMs) from a Digital Elevation Model (DEM) as potential explanatory variables for soil depth. These HPMs are variations of the distance up to ridge points (cells with no incoming flow) and variations of the distance down to stream points (cells with a contributing area greater than a threshold), following the flow path. The HPMs were computed using the D-infinity flow model that apportions flow between adjacent neighbors based on the direction of steepest downward slope on the eight triangular facets constructed in a 3 x 3 grid cell window using the center cell and each pair of adjacent neighboring grid cells in turn. The D-infinity model typically results in multiple flow paths between 2 points on the topography, with the result that distances may be computed as the minimum, maximum or average of the individual flow paths. In addition, each of the HPMs, are calculated vertically, horizontally, and along the land surface. Previously, these HPMs were calculated using recursive serial algorithms which suffered from stack overflow problems when used to process large datasets, limiting the size of DEMs that could be analyzed. To overcome this limitation, we developed a message passing interface (MPI) parallel approach designed to both increase the size and speed with which these HPMs are computed. The parallel HPM algorithms spatially partition the input grid into stripes which are each assigned to separate processes for computation. Each of those processes then uses a queue data structure to order the processing of cells so that each cell is visited only once and the cross-process communications that are a standard part of MPI are handled in an efficient manner. This parallel approach allows efficient analysis of much larger DEMs than were possible using the serial recursive algorithms. The HPMs given here may also have other, more general modeling applicability in hydrology, geomorphology and ecology, and so are described here from a general perspective. In this paper, we present the definitions of the HPMs, the serial and parallel algorithms used in their computation and their potential applications.

Determinants of urban tree canopy in residential neighborhoods: Household characteristics, urban form, and the geophysical landscape

The aesthetic, economic, and environmental benefits of urban trees are well recognized. Previous research has focused on understanding how a variety of social and environmental factors are related to urban vegetation. The aim is often to provide planners with information that will improve residential neighborhood design, or guide tree planting campaigns encouraging the cultivation of urban trees. In this paper we examine a broad range of factors we hypothesize are correlated to urban tree canopy heterogeneity in Salt Lake County, Utah. We use a multi-model inference approach to evaluate the relative contribution of these factors to observed heterogeneity in urban tree canopy cover, and discuss the implications of our analysis. An important contribution of this work is an explicit attempt to account for the confounding effect of neighborhood age in understanding the relationship between human and environmental factors, and urban tree canopy. We use regression analysis with interaction terms to assess the effects of 15 human and environmental variables on tree canopy abundance while holding neighborhood age constant. We demonstrate that neighborhood age is an influential covariate that affects how the human and environmental factors relate to the abundance of neighborhood tree canopy. For example, we demonstrate that in new neighborhoods a positive relationship exists between street density and residential tree canopy, but the relationship diminishes as the neighborhood ages. We conclude that to better understand the determinants of urban tree canopy in residential areas it is important to consider both human and environmental factors while accounting for neighborhood age.

Comparison of Automated Watershed Delineations: Effects on Land Cover Areas, Percentages, and Relationships to Nutrient Discharge

We compared manual delineations with those derived from ten automated delineations of 420 watersheds in four physiographic provinces of the Chesapeake Basin. Automated methods included commercial DEM-based routines and different parameterizations of four enhanced methods: stream burning, normalized excavation, surface reconditioning, and normalized reconditioning. Un-enhanced methods resulted in individual watershed boundaries with some gross discrepancies in watershed size relative to manual delineations (error rate of 0.22 � 25 percent difference compared to manual) and significantly different watershed size distributions (Mann-Whitney U p � 0.012). Integrating mapped streams through enhanced methods substantially improved correspondence with manual watersheds (error rates of only 0.08‐0.02 � 25 percent difference). Analysis of cropland area among methods showed a significant difference between manual estimates and un-enhanced estimates (p � 0.049) that was corrected using enhanced algorithms. Subsequent analysis of percent cropland revealed that measurements of land cover proportions were not always affected by delineation errors. However, differences were large enough to influence regressions with stream nitrate-N at the 90 percent confidence level within one physiographic province. Enhanced delineations produced statistical relationships between percent cropland and nitrate-N concentrations consistent with manual delineations. The results provide support for enhanced automated watershed delineation within the Chesapeake Basin and suggest that normalized excavation can be an effective augmentation of existing stream burning and reconditioning procedures.

Of TITAN and straw men: an appeal for greater understanding of community data

Abstract.  Cuffney and Qian (2013) performed numerous simulations to demonstrate potential flaws in Threshold Indicator Taxa Analysis (TITAN), a method for interpreting taxon contributions to community change along novel environmental gradients. Based on their simulations, they concluded that: 1) TITAN is not an effective method for detecting different types of statistical thresholds in trend lines, 2) permutation results in highly significant p-values even for splits that are not thresholds, and 3) coincident change points may arise as an artifact of inaccuracies, imprecision, and systematic bias in both change-point estimation and TITANs bootstrap. The critique raises some important concerns, but because of significant misunderstanding, it is based on analyses that violate basic assumptions of both TITAN and indicator species analysis (IndVal), and thus, constitutes a straw man that cannot be used to evaluate their performance. We demonstrate that the critique: 1) fundamentally misrepresents TITANs primary goals; 2) simulates taxon abundances based on unrealistic statistical models that fail to represent important empirical patterns present in Cuffney and Qians own published data sets (i.e., negative binomial distributions, frequent absences a function of the predictor); 3) tests TITANs ability to identify breaks in trend lines distorted by log-transformation that do not match the greatest change in the simulated response, leading to misinterpretation of expected and previously documented behavior by TITAN as errors; 4) misinterprets TITANs use of p-values while ignoring diagnostic indices of purity and reliability for identifying robust indicator taxa; and 5) asserts that bootstrapped change-point quantiles in TITAN are too narrow despite published results to the contrary. Last, in contrast to the claim that change-point synchrony may be an artifact of the technique, we show that: 6) analysis of published data using completely independent methods (i.e., scatterplots of abundance data or generalized additive models) also reveals synchrony in the nonlinear decline of numerous taxa in corroboration of TITAN and its underlying conceptual model. Thus, Cuffney and Qian have not identified any serious limitations of TITAN because their critique is based on misinterpretation of TITANs assumptions and primary objectives. However, their critique does highlight the need for clarification of the appropriate uses, potential misuses, and limitations of TITAN and other methods for ecological analysis.

Ecological resistance in urban streams: the role of natural and legacy attributes

Urbanization substantially changes the physicochemical and biological characteristics of streams. The trajectory of negative effect is broadly similar around the world, but the nature and magnitude of ecological responses to urban growth differ among locations. Some heterogeneity in response arises from differences in the level of urban development and attributes of urban water management. However, the heterogeneity also may arise from variation in hydrologic, biological, and physicochemical templates that shaped stream ecosystems before urban development. We present a framework to develop hypotheses that predict how natural watershed and channel attributes in the pre-urban-development state may confer ecological resistance to urbanization. We present 6 testable hypotheses that explore the expression of such attributes under our framework: 1) greater water storage capacity mitigates hydrologic regime shifts, 2) coarse substrates and a balance between erosive forces and sediment supply buffer morphological changes, 3) naturally high ionic concentrations and pH pre-adapt biota to water-quality stress, 4) metapopulation connectivity results in retention of species richness, 5) high functional redundancy buffers trophic function from species loss, and 6) landuse history mutes or reverses the expected trajectory of eutrophication. Data from past comparative analyses support these hypotheses, but rigorous testing will require targeted investigations that account for confounding or interacting factors, such as diversity in urban infrastructure attributes. Improved understanding of the susceptibility or resistance of stream ecosystems could substantially strengthen conservation, management, and monitoring efforts in urban streams. We hope that these preliminary, conceptual hypotheses will encourage others to explore these ideas further and generate additional explanations for the heterogeneity observed in urban streams.

Predicting Fish Growth Potential and Identifying Water Quality Constraints: A Spatially-Explicit Bioenergetics Approach

Anthropogenic impairment of water bodies represents a global environmental concern, yet few attempts have successfully linked fish performance to thermal habitat suitability and fewer have distinguished co-varying water quality constraints. We interfaced fish bioenergetics, field measurements, and Thermal Remote Imaging to generate a spatially-explicit, high-resolution surface of fish growth potential, and next employed a structured hypothesis to detect relationships among measures of fish performance and co-varying water quality constraints. Our thermal surface of fish performance captured the amount and spatial-temporal arrangement of thermally-suitable habitat for three focal species in an extremely heterogeneous reservoir, but interpretation of this pattern was initially confounded by seasonal covariation of water residence time and water quality. Subsequent path analysis revealed that in terms of seasonal patterns in growth potential, catfish and walleye responded to temperature, positively and negatively, respectively; crappie and walleye responded to eutrophy (negatively). At the high eutrophy levels observed in this system, some desired fishes appear to suffer from excessive cultural eutrophication within the context of elevated temperatures whereas others appear to be largely unaffected or even enhanced. Our overall findings do not lead to the conclusion that this system is degraded by pollution; however, they do highlight the need to use a sensitive focal species in the process of determining allowable nutrient loading and as integrators of habitat suitability across multiple spatial and temporal scales. We provide an integrated approach useful for quantifying fish growth potential and identifying water quality constraints on fish performance at spatial scales appropriate for whole-system management.

Multiscale control of flooding and riparian-forest composition in Lower Michigan, USA

Despite general agreement that river-valley hydrology shapes riparian ecosystems, relevant processes are difficult to distinguish and often inadequately specified in riparian studies. We hypothesize that physical constraints imposed by broad-scale watershed characteristics and river valleys modify local site conditions in a predictable and probabilistic fashion. To test this hypothesis, we employ a series of structural equations that decompose occurrence of riparian ecotypes into regional temperature, catchment storm response, valley hydraulics, and local site wetness via a priori specification of factor structure and ask (1) Is there evidence for multiscale hydrologic control of riparian diversity across Lower Michigan? (2) Do representations of key constraints on flood dynamics distinguish regional patterns of riparian vegetation? (3) How important are these effects? Cross-correlation among geospatial predictors initially obscured much of the variation revealed through analysis of semipartial variance. Causal relationships implied by our model fit with observed variation in riparian conditions (chi-square P = 0.43) and accounted for between 84% and 99% of the occurrence probability of five riparian ecotypes at 94 locations. Results suggest strong variation in the effects of regional climate, and both the relative importance and spatial scale of hydrologic factors influencing riparian vegetation through explicit quantification of relative flood frequency, duration, intensity, and relative overall inundation. Although climate and hydrology are not the only determinants of riparian conditions, interactions of hydrologic sourcing and flood dynamics described by our spatial models drive a significant portion of the variation in riparian ecosystem character throughout Lower Michigan, USA.

Soil Methane and Carbon Dioxide Fluxes from Cropland and Riparian Buffers in Different Hydrogeomorphic Settings.

Riparian buffers contribute to the mitigation of nutrient pollution in agricultural landscapes, but there is concern regarding their potential to be hot spots of greenhouse gas production. This study compared soil CO and CH fluxes in adjacent crop fields and riparian buffers (a flood-prone forest and a flood-protected grassland along an incised channel) and examined the impact of water table depth (WTD) and flood events on the variability of gas fluxes in riparian zones. Results showed significantly ( < 0.001) higher CO emission in riparian areas than in adjoining croplands (6.8 ± 0.6 vs. 3.6 ± 0.5 Mg CO-C ha yr; mean ± SE). Daily flux of CO and soil temperature were significantly related ( < 0.002), with Q values ranging between 1.75 and 2.53. Significant relationships ( < 0.05) were found between CH daily flux and WTD. Flood events resulted in enhanced CH emission (up to +44.5 mg CH-C m d in a swale) under warm soil conditions (>22°C), but the effect of flooding was less pronounced in early spring (emission <1.06 mg CH-C m d), probably due to low soil temperature. Although CH flux direction alternated at all sites, overall the croplands and the flood-affected riparian forest were CH sources, with annual emission averaging +0.04 ± 0.17 and +0.92 ± 1.6 kg CH-C ha, respectively. In the riparian forest, a topographic depression (<8% of the total area) accounted for 78% of the annual CH emission, underscoring the significance of landscape heterogeneity on CH dynamics in riparian buffers. The nonflooded riparian grassland, however, was a net CH sink (-1.08 ± 0.22 kg CH-C ha yr), probably due to the presence of subsurface tile drains and a dredged/incised channel at that study site. Although these hydrological alterations may have contributed to improvement in the CH sink strength of the riparian grassland, this must be weighed against the water quality maintenance functions and other ecological services provided by riparian buffers.