UMBC Center for Urban Environmental Research and Education (CUERE)

Permanent URI for this collectionhttp://hdl.handle.net/11603/7776

CUERE’s mission is to advance the understanding of the environmental, social and economic consequences of the transformation of the urban landscape through research, conferences and symposia, support of university teaching programs and assistance to K-12 education. CUERE fosters interdisciplinary approaches to environmental science, engineering and public policy.

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Shaping Baltimore’s urban forests: past insights for present-day ecology
(Springer Nature, 2024-07-29) Sonti, Nancy F.; Baker, Matthew; Lagrosa, John; Allman, Michael; Grove, J. Morgan; Katoski, Michelle
Context Land use history of urban forests impacts present-day soil structure, vegetation, and ecosystem function, yet is rarely documented in a way accessible to planners and land managers. Objectives To (1) summarize historical land cover of present-day forest patches in Baltimore, MD, USA across land ownership categories and (2) determine whether social-ecological characteristics vary by historical land cover trajectory.MethodsUsing land cover classification derived from 1927 and 1953 aerial imagery, we summarized present-day forest cover by three land cover sequence classes: (1) Persistent forest that has remained forested since 1927, (2) Successional forest previously cleared for non-forest vegetation (including agriculture) that has since reforested, or (3) Converted forest that has regrown on previously developed areas. We then assessed present-day ownership and average canopy height of forest patches by land cover sequence class.ResultsMore than half of Baltimore City’s forest has persisted since at least 1927, 72% since 1953. About 30% has succeeded from non-forest vegetation during the past century, while 15% has reverted from previous development. A large proportion of forest converted from previous development is currently privately owned, whereas persistent and successional forest are more likely municipally-owned. Successional forest occurred on larger average parcels with the fewest number of distinct property owners per patch. Average tree canopy height was significantly greater in patches of persistent forest (mean = 18.1 m) compared to canopy height in successional and converted forest patches (16.6 m and 16.9 m, respectively).Conclusions Historical context is often absent from urban landscape ecology but provides information that can inform management approaches and conservation priorities with limited resources for sustaining urban natural resources. Using historical landscape analysis, urban forest patches could be further prioritized for protection by their age class and associated ecosystem characteristics.
Evaluation of green infrastructure using hydrologic modeling and high performance computing
(2015) Andino-Nolasco, Elvis; Welty, Claire
To develop a coupled groundwater-surface water model to evaluate the impacts of green infrastructure on groundwater resources in Philadelphia.
A three-dimensional integrated hydrologic model of the Chesapeake Bay Watershed using High Performance Computing
Seck, Alimatou; Welty, Claire; Maxwell, Reed M.
Leaf diversity alters litter breakdown in a Piedmont stream
(University of Chicago Press, 2004-01) Swan, Christopher; Palmer, Margaret A.
Work in terrestrial ecosystems has suggested that the breakdown rate of leaf litter may not change predictably with increasing plant species richness. Indeed, it may be that certain combinations of leaf species demonstrate significant non-additive effects on breakdown rates, mediated by the presence of a single key species. Such effects have not been explored in running-water ecosystems despite the strong interest in the conservation and restoration of riparian systems. We documented the magnitude and species composition of leaf litter inputs, the species richness and composition of leaf litter on the streambed, and estimated the breakdown rate of mixed litter in both the summer and autumn in a warmwater stream. We found that leaf species richness of litter packs on the streambed varied from 1 to 11 species, and leaf species composition reflected the composition of litter inputs from May through November. We did not find a general relationship between breakdown rate and leaf litter species richness. However, we did find a strong effect of species composition of leaf packs on the breakdown rate during the summer. Overall, breakdown rates of mixed-species leaf packs were non-additive during the summer, but very predictable in the autumn. In particular, leaf mixtures containing American sycamore always exhibited slower breakdown rates than predicted in summer. One explanation for the discrepancy between summer and autumn results may be decreased temperature in autumn; reduced temperature may have slowed breakdown rates across treatments to the extent that any non-additive effects found in summer were masked by the effect of temperature. Given the importance of detritus to stream food webs, the simplification of plant assemblages along intact or restored streams may have important implications for stream ecosystems.
Leaf litter diversity leads to non-additivity in stream detritivore colonization dynamics
(Sciendo, 2005-01) Swan, Christopher; PALMER, MARGARET A.
Resource heterogeneity in the lake littoral zones and streams often takes the form of leaf litter diversity. Since resource quality varies between leaf species, diversity of leaf litter inputs to these systems may alter detritivore dynamics. We challenged the assumption that stream detritivores view their leaf resources independently, and that shredder colonization dynamics were additive. We developed a null model that suggests the temporal coefficient of variation in abundance of detritivores declines as leaf diversity increases. We then conducted an experiment in which leaf composition was manipulated in a stream and shredder abundance was followed. Shredder stability was low on single leaf species, increased at intermediate levels, then declined at maximum levels. The null model did not predict the low stability observed, suggesting other ecological factors influenced shredder abundance. Non-additive patterns in colonization support the notion that leaf diversity leads to emergent properties, altering consumer dynamics in this ecosystem.
Loss of foundation species: consequences for the structure and dynamics of forested ecosystems
(Wiley, 2005-11-01) Ellison, Aaron M.; Bank, Michael S.; Clinton, Barton D.; Colburn, Elizabeth A.; Elliott, Katherine; Ford, Chelcy R.; Foster, David R.; Kloeppel, Brian D.; Knoepp, Jennifer D.; Lovett, Gary M.; Mohan, Jacqueline; Orwig, David A.; Rodenhouse, Nicholas L.; Sobczak, William V.; Stinson, Kristina A.; Stone, Jeffrey K.; Swan, Christopher; Thompson, Jill; Holle, Betsy Von; Webster, Jackson R.
In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.
Methods to study litter decomposition: a practical guide
(University of Chicago Press, 2007-06) Swan, Christopher
Consequences of non-random species loss for decomposition dynamics: experimental evidence for additive and non-additive effects
(Wiley, 2008-02-01) Ball, Becky A.; Hunter, Mark D.; Kominoski, John S.; Swan, Christopher; Bradford, Mark A.
1. Although litter decomposition is a fundamental ecological process, most of our understanding comes from studies of single-species decay. Recently, litter-mixing studies have tested whether monoculture data can be applied to mixed-litter systems. These studies have mainly attempted to detect non-additive effects of litter mixing, which address potential consequences of random species loss – the focus is not on which species are lost, but the decline in diversity per se. 2. Under global change, species loss is likely to be non-random, with some species more vulnerable to extinction than others. Under such scenarios, the effects of individual species (additivity) as well as of species interactions (non-additivity) on decomposition rates are of interest. 3. To examine potential impacts of non-random species loss on ecosystems, we studied additive and non-additive effects of litter mixing on decomposition. A full-factorial litterbag experiment was conducted using four deciduous leaf species, from which mass loss and nitrogen content were measured. Data were analysed using a statistical approach that first looks for additive identity effects based on the presence or absence of species and then significant species interactions occurring beyond those. It partitions non-additive effects into those caused by richness and/or composition. 4. This approach addresses questions key to understanding the potential effects of species loss on ecosystem processes. If additive effects dominate, the consequences for decomposition dynamics will be predictable based on our knowledge of individual species, but not statistically predictable if non-additive effects dominate. 5. We found additive (identity) effects on mass loss and non-additive (composition) effects on litter nitrogen dynamics, suggesting that non-random species loss could significantly affect this system. We were able to identify the species responsible for effects that would otherwise have been considered idiosyncratic or absent when analysed by the methods used in previous work. 6. Synthesis. We observed both additive and non-additive effects of litter-mixing on decomposition, indicating consequences of non-random species loss. To predict the consequences of global change for ecosystem functioning, studies should examine the effects of both random and non-random species loss, which will help identify the mechanisms that influence the response of ecosystems to environmental change.
Interaction between Urbanization and Climate Variability Amplifies Watershed Nitrate Export in Maryland
(ACS, 2008-07-09) Kaushal, Sujay S.; Groffman, Peter M.; Band, Lawrence E.; Shields, Catherine A.; Morgan, Raymond P.; Palmer, Margaret A.; Belt, Kenneth T.; Swan, Christopher; Findlay, Stuart E. G.; Fisher, Gary T.
We investigated regional effects of urbanization and land use change on nitrate concentrations in approximately 1,000 small streams in Maryland during record drought and wet years in 2001−2003. We also investigated changes in nitrate-N export during the same time period in 8 intensively monitored small watersheds across an urbanization gradient in Baltimore, Maryland. Nitrate-N concentrations in Maryland were greatest in agricultural streams, urban streams, and forest streams respectively. During the period of record drought and wet years, nitrate-N exports in Baltimore showed substantial variation in 6 suburban/urban streams (2.9−15.3 kg/ha/y), 1 agricultural stream (3.4−38.9 kg/ha/y), and 1 forest stream (0.03−0.2 kg/ha/y). Interannual variability was similar for small Baltimore streams and nearby well-monitored tributaries and coincided with record hypoxia in Chesapeake Bay. Discharge-weighted mean annual nitrate concentrations showed a variable tendency to decrease/increase with changes in annual runoff, although total N export generally increased with annual runoff. N retention in small Baltimore watersheds during the 2002 drought was 85%, 99%, and 94% for suburban, forest, and agricultural watersheds, respectively, and declined to 35%, 91%, and 41% during the wet year of 2003. Our results suggest that urban land use change can increase the vulnerability of ecosystem nitrogen retention functions to climatic variability. Further work is necessary to characterize patterns of nitrate-N export and retention in small urbanizing watersheds under varying climatic conditions to improve future forecasting and watershed scale restoration efforts aimed at improving nitrate-N retention.
Geologic and Edaphic Controls on a Serpentine Forest Community
(Eagle Hill Institute, 2009-06-01) Burgess, Jerry L.; Lev, Steven; Swan, Christopher; Szlavecz, Katalin
This study examined woody vegetation, edaphic factors, bedrock geochemistry, petrography, and outcrop structure to evaluate some of the community-structuring factors in an ultramafic terrain of Maryland. Analyzing the dynamic nature of combined geological and ecological processes can detect correlative relationships between factors that are typically considered as independent such as tectonically driven bedrock fracturing and ecological community interaction. This study provides evidence for structural variation in fracture density of bedrock as a partial control on tree species distribution in an ultramafic woodland/forest ecosystem. Increases in the number of bedrock fractures correlates negatively with plot-level volumetric soil moisture. Additionally, the degree of serpentinization of the ultramafic parent material results in compositional variation in Ca, Mg, and Ni of parent materials and soils. The combination of these factors provides a significant level of control on the distribution of xeric tree species.
Processing of transgenic crop residues in stream ecosystems
(Wiley, 2009-11-26) Swan, Christopher; Jensen, Peter D.; Dively, Galen P.; Lamp, William O.
1. Research in agricultural ecosystems is uncovering how the management of crop fields leads to the delivery of transgenic crop residues to adjacent waterways. Aquatic consumers encountering this material may be reduced in abundance and/or limit their feeding activity, subsequently altering organic matter breakdown rate, which is a key ecosystem process in streams. 2. We investigated the effect of the transgenic nature of senesced corn (Zea mays L.), tissue on breakdown rates, invertebrate abundance and invertebrate community composition in nine streams draining agricultural fields over 2 years (2004–2006). We studied corn tissue modified to express protein toxins from the bacterium Bacillus thuringiensis (Bt) from four hybrid families, each with its single, stacked and non-Bt near-isoline. 3. In 2004, we identified two instances whereby Bt leaf litter degraded slower (67–68%) than corresponding near isolines. At one site this was associated with significantly fewer individuals of Pycnopsyche sp., a leaf-chewing caddisfly. In 2005–2006, no differences in breakdown were found between Bt and non-Bt near isolines. Multivariate analysis of invertebrate communities found no difference associated with Bt treatment. 4. Principle components analysis identified important abiotic factors as explanatory variables influencing breakdown, but no interaction was found between these and Bt treatment. Breakdown was strongly related to total invertebrate abundance occurring on experimental litter bags, but this did not interact with Bt treatment across all hybrid × isoline × site combinations. 5. Synthesis and applications. Ecological interactions facilitate breakdown of allochthonous detritus, and understanding the potential disruption of these interactions is important to the management of ecosystem processes. The results from our study suggests that corn tissue breakdown is unlikely to be altered by Bt, but more so by hybrid- and site-specific factors such as nutrients. Management of agricultural streams will need to consider multiple sources of stress at larger scales, such as nutrient loading and temperature, which probably overwhelm the potential for consumer mediation of ecosystem processes in these ecosystems.
Exposure and Nontarget Effects of Transgenic Bt Corn Debris in Streams
(Oxford University Press, 2010-04-01) Jensen, Peter D.; Dively, Galen P.; Swan, Christopher; Lamp, William O.
Corn (Zea mays L.) transformed with a gene from the bacterium Bacillus thuringiensis (Bt) comprises 49% of all corn in the United States. The input of senesced corn tissue expressing the Bt gene may impact stream-inhabiting invertebrates that process plant debris, especially trichopteran species related to the target group of lepidopteran pests. Our goal was to assess risk associated with transgenic corn debris entering streams. First, we show the input of corn tissue after harvest was extended over months in a stream. Second, using laboratory bioassays based on European corn borer [Ostrinia nubilalis (Hübner)], we found no bioactivity of Cry1Ab protein in senesced corn tissue after 2 wk of exposure to terrestrial or aquatic environments. Third, we show that Bt near-isolines modify growth and survivorship of some species of invertebrates. Of the four nontarget invertebrate species fed Bt near-isolines, growth of two closely related trichopterans was not negatively affected, whereas a tipulid crane fly exhibited reduced growth rates, and an isopod exhibited reduced growth and survivorship on the Cry1Ab near-isoline but not on the stacked Cry1Ab + Cry3Bb1 near-isoline. Because of lack of evidence of bioactivity of Bt after 2 wk and because of lack of nontarget effects on the stacked near-isoline, we suggest that tissue-mediated differences, and not the presence of the Cry1Ab protein, caused the different responses among the species. Overall, our results provide evidence that adverse effects to aquatic nontarget shredders involve complex interactions arising from plant genetics and environment that cannot be ascribed to the presence of Cry1Ab proteins.
Dendritic network structure constrains metacommunity properties in riverine ecosystems
(Wiley, 2010-03-22) Brown, B. L.; Swan, Christopher
1. Increasingly, ecologists conceptualize local communities as connected to a regional species pool rather than as isolated entities. By this paradigm, community structure is determined through the relative strengths of dispersal-driven regional effects and local environmental factors. However, despite explicit incorporation of dispersal, metacommunity models and frameworks often fail to capture the realities of natural systems by not accounting for the configuration of space within which organisms disperse. This shortcoming may be of particular consequence in riverine networks which consist of linearly -arranged, hierarchical, branching habitat elements. Our goal was to understand how constraints of network connectivity in riverine systems change the relative importance of local vs. regional factors in structuring communities. 2. We hypothesized that communities in more isolated headwaters of riverine networks would be structured by local forces, while mainstem sections would be structured by both local and regional processes. We examined these hypotheses using a spatially explicit regional analysis of riverine macroinvertebrate communities, focusing on change in community similarity with distance between local communities [i.e., distance-decay relationships; (DDRs)], and the change in environmental similarity with distance. Strong DDRs frequently indicate dispersal-driven dynamics. 3. There was no evidence of a DDR in headwater communities, supporting our hypothesis that dispersal is a weak structuring force. Furthermore, a positive relationship between community similarity and environmental similarity supported dynamics driven by local environmental factors (i.e., species sorting). In mainstem habitats, significant DDRs and community × environment similarity relationships suggested both dispersal-driven and environmental constraints on local community structure (i.e., mass effects). 4. We used species traits to compare communities characterized by low vs. high dispersal taxa. In headwaters, neither strength nor mode (in-network vs. out of network) of dispersal changed our results. However, outcomes in mainstems changed substantially with both dispersal mode and strength, further supporting the hypothesis that regional forces drive community dynamics in mainstems. 5. Our findings demonstrate that the balance of local and regional effects changes depending on location within riverine network with local (environmental) factors dictating community structure in headwaters, and regional (dispersal driven) forces dominating in mainstems.
Metacommunity theory as a multispecies, multiscale framework for studying the influence of river network structure on riverine communities and ecosystems
(University of Chicago Press, 2011-01-18) Brown, Bryan L.; Swan, Christopher; Auerbach, Daniel A.; Grant, Evan H. Campbell; Hitt, Nathaniel P.; Maloney, Kelly O.; Patrick, Christopher
Explaining the mechanisms underlying patterns of species diversity and composition in riverine networks is challenging. Historically, community ecologists have conceived of communities as largely isolated entities and have focused on local environmental factors and interspecific interactions as the major forces determining species composition. However, stream ecologists have long embraced a multiscale approach to studying riverine ecosystems and have studied both local factors and larger-scale regional factors, such as dispersal and disturbance. River networks exhibit a dendritic spatial structure that can constrain aquatic organisms when their dispersal is influenced by or confined to the river network. We contend that the principles of metacommunity theory would help stream ecologists to understand how the complex spatial structure of river networks mediates the relative influences of local and regional control on species composition. From a basic ecological perspective, the concept is attractive because new evidence suggests that the importance of regional processes (dispersal) depends on spatial structure of habitat and on connection to the regional species pool. The role of local factors relative to regional factors will vary with spatial position in a river network. From an applied perspective, the long-standing view in ecology that local community composition is an indicator of habitat quality may not be uniformly applicable across a river network, but the strength of such bioassessment approaches probably will depend on spatial position in the network. The principles of metacommunity theory are broadly applicable across taxa and systems but seem of particular consequence to stream ecology given the unique spatial structure of riverine systems. By explicitly embracing processes at multiple spatial scales, metacommunity theory provides a foundation on which to build a richer understanding of stream communities.
Reconstructing the assembly of a stream-insect metacommunity
(University of Chicago Press, 2011-01-18) Patrick, Christopher J.; Swan, Christopher
Dispersal rates and the diversity of the regional species pool strongly affect community assembly in habitat patches. Incorporating these elements mechanistically into a model of community assembly requires adoption of a metacommunity paradigm. We developed a hierarchical model of community assembly for stream insects that incorporates regional effects (distance to and generic richness of other stream reaches) and local effects (water quality and community composition). We tested our model with a unique data set detailing changes in stream-insect community composition over 6 sampling periods across a 27-y period of watershed recovery from anthropogenic effects. α and γ richness increased greatly over the time period, whereas β richness declined strongly. Generic richness of individual stream reaches was significantly related to dispersal distance and generic richness of surrounding immigrant pools in preceding years. However, the strength of the relationship declined over time indicating that distance to potential colonists played a major role only early in community assembly. Water quality, characterized by an ordination of pH, temperature, conductivity, dissolved O₂, NO₃, NH₄, and orthophosphate, was correlated with generic richness at all time periods during the community-assembly sequence. The functional diversity (diversity of functional attributes present in an assemblage of species) of entire communities was lower than expected from random simulations in all sampling years. However, functional diversity of individual functional feeding groups varied through time and amongst themselves. Our results suggest that both deterministic and random processes are important in metacommunity assembly, and their relative strengths vary throughout the assembly process.
Advancing theory of community assembly in spatially structured environments: local vs regional processes in river networks
(University of Chicago Press, 2011-01-18) Swan, Christopher; Brown, Bryan L.
Consumer presence and resource diversity independently induce stability of ecosystem function in a Piedmont stream
(Wiley, 2011-12-29) Swan, Christopher
With the acceleration of species loss across multiple ecosystems, the mechanisms explaining subsequent changes in ecosystem processes are under continuing investigation. In detritus-based ecosystems, such as soils, small streams, and wetlands, one consequence of tree species loss is the shift in the species composition of leaf litter resources to consumers. Given substantial variation in resource quality among senesced leaf species, organic matter processing rates are known to change with litter species loss as both microbial and invertebrate consumers respond to loss of resource diversity. While the effects on processing rates are now well documented, the implications for such species loss on the stability of organic matter processing have not been explicitly tested. In a field experiment, leaf litter diversity was manipulated as single- and mixed-species treatments in a full-factorial design with the presence/absence of a functionally important leaf-shredding consumer, the caddisfly Pycnopsyche gentilis. It was hypothesized that in the absence of the consumer, loss of leaf litter species would result in higher variability (i.e., lower stability) in organic matter processing rates, owing to the portfolio effect commonly observed in plant communities. However, compensatory feeding by the consumer should offset the effect of leaf litter species loss. The results showed higher variation in litter processing among single-species leaf treatments compared to diverse mixtures. When P. gentilis had access, variation among single-species litter treatments was significantly reduced (i.e., stability increased), and was statistically indistinguishable from high diversity litter treatments. In small streams, which comprise >70% of stream miles in river drainages and often rely on allochthonous resources from riparian vegetation, how loss of stream-side forest species influences stability of in-stream organic matter processing can be independent of important detritivorous consumers.
Forecasting functional implications of global changes in riparian plant communities
(Wiley, 2013-08-26) Kominoski, John S; Shah, Jennifer J Follstad; Canhoto, Cristina; Fischer, Dylan G; Giling, Darren P; González, Eduardo; Griffiths, Natalie A; Larrañaga, Aitor; LeRoy, Carri J; Mineau, Madeleine M; McElarney, Yvonne R; Shirley, Susan M; Swan, Christopher; Tiegs, Scott D
Riparian ecosystems support mosaics of terrestrial and aquatic plant species that enhance regional biodiversity and provide important ecosystem services to humans. Species composition and the distribution of functional traits – traits that define species in terms of their ecological roles – within riparian plant communities are rapidly changing in response to various global change drivers. Here, we present a conceptual framework illustrating how changes in dependent wildlife communities and ecosystem processes can be predicted by examining shifts in riparian plant functional trait diversity and redundancy (overlap). Three widespread examples of altered riparian plant composition are: shifts in the dominance of deciduous and coniferous species; increases in drought-tolerant species; and the increasing global distribution of plantation and crop species. Changes in the diversity and distribution of critical plant functional traits influence terrestrial and aquatic food webs, organic matter production and processing, nutrient cycling, water quality, and water availability. Effective conservation efforts and riparian ecosystems management require matching of plant functional trait diversity and redundancy with tolerance to environmental changes in all biomes.
Using rarity to infer how dendritic network structure shapes biodiversity in riverine communities
(Wiley, 2014-05-15) Swan, Christopher; Brown, Bryan L.
Dispersal of organisms connects physical localities, but the strength of connection varies widely. Variability in the influence of dispersal can be predictable in sharply defined networks like river systems because some sections of the network are more isolated, leading to different balances of local (i.e. environmental filtering, species interactions) and regional (i.e. dispersal-driven) processes in structuring communities. We examined the influence of spatial isolation on the relative contributions of α- and β-diversity to regional (γ) diversity, and examined how that influence differed between common and rare species in stream macroinvertebrate communities. One explanation for rarity on a regional scale is that common species are habitat generalists while rare species are specialists. Therefore, common species should be influenced more by dispersal-driven processes while rare species should be more influenced by local processes. We predicted that for rare taxa, β-diversity should represent a higher fraction of γ-diversity in isolated headwaters but that differences between rare and common taxa with regard to the contribution of β-diversity to γ-diversity should be less distinct in well-connected mainstem habitats. To test these predictions, we used macroinvertebrate communities from 634 sites across 22 watersheds. Regardless of rarity, β- and γ-diversity were higher in headwaters compared to mainstems. However, α-diversity was similar regardless of isolation for rare assemblages. But contrary to our predictions, common assemblages of predators and herbivores did exhibit differences in α-diversity between locations. Our predictions were strongly supported for two guilds of consumers, the detritivores and collectors, but less so for herbivores and predators. However, these results make sense considering differences in life histories between the groups. For detritivores and collectors, species turnover (β-diversity) was higher in isolated regions in river networks, and rarity exacerbated this effect, resulting in higher regional diversity of rare species, supporting the general theory that rarity reflects habitat specialization.
Latitudinal gradient of nestedness and its potential drivers in stream detritivores
(Wiley, 2015-05-14) Boyero, Luz; Pearson, Richard G.; Swan, Christopher; Hui, Cang; et al
Understanding what mechanisms shape the diversity and composition of biological assemblages across broad-scale gradients is central to ecology. Litter-consuming detritivorous invertebrates in streams show an unusual diversity gradient, with α-diversity increasing towards high latitudes but no trend in γ-diversity. We hypothesized this pattern to be related to shifts in nestedness and several ecological processes shaping their assemblages (dispersal, environmental filtering and competition). We tested this hypothesis, using a global dataset, by examining latitudinal trends in nestedness and several indicators of the above processes along the latitudinal gradient. Our results suggest that strong environmental filtering and low dispersal in the tropics lead to often species-poor local detritivore assemblages, nested in richer regional assemblages. At higher latitudes, dispersal becomes stronger, disrupting the nested assemblage structure and resulting in local assemblages that are generally more species-rich and non-nested subsets of the regional species pools. Our results provide evidence that mechanisms underlying assemblage composition and diversity of stream litter-consuming detritivores shift across latitudes, and provide an explanation for their unusual pattern of increasing α-diversity with latitude. When we repeated these analyses for whole invertebrate assemblages of leaf litter and for abundant taxa showing reverse or no diversity gradients we found no latitudinal patterns, suggesting that function-based rather than taxon-based analyses of assemblages may help elucidate the mechanisms behind diversity gradients.

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