Diversity-stability relationships across organism groups and ecosystem types become decoupled across spatial scales

The relationship between biodiversity and stability, or its inverse, temporal variability, is multidimensional and complex. Temporal variability in aggregate properties, like total biomass or abundance, is typically lower in communities with higher species diversity (i.e., the diversity-stability relationship [DSR]). At broader spatial extents, regional-scale aggregate variability is also lower with higher regional diversity (in plant systems) and with lower spatial synchrony. However, focusing exclusively on aggregate properties of communities may overlook potentially destabilizing compositional shifts. It is not yet clear how diversity is related to different components of variability across spatial scales, nor whether regional DSRs emerge across a broad range of organisms and ecosystem types. To test these questions, we compiled a large collection of long-term metacommunity data spanning a wide range of taxonomic groups (e.g., birds, fish, plants, invertebrates) and ecosystem types (e.g., deserts, forests, oceans). We applied a newly developed quantitative framework for jointly analyzing aggregate and compositional variability across scales. We quantified DSRs for composition and aggregate variability in local communities and metacommunities. At the local scale, more diverse communities were less variable, but this effect was stronger for aggregate than compositional properties. We found no stabilizing effect of γ-diversity on metacommunity variability, but β-diversity played a strong role in reducing compositional spatial synchrony, which reduced regional variability. Spatial synchrony differed among taxa, suggesting differences in stabilization by spatial processes. However, metacommunity variability was more strongly driven by local variability than by spatial synchrony. Across a broader range of taxa, our results suggest that high γ-diversity does not consistently stabilize aggregate properties at regional scales without sufficient spatial β-diversity to reduce spatial synchrony.

Current street tree communities reflect race-based housing policy and modern attempts to remedy environmental injustice

Humans promote and inhibit other species on the urban landscape, shaping biodiversity patterns. Institutional racism may underlie the distribution of urban species by creating disproportionate resources in space and time. Here, we examine whether present-day street tree occupancy, diversity, and composition in Baltimore, MD, USA, neighborhoods reflect their 1937 classification into grades of loan risk-from most desirable (A = green) to least desirable (D = "redlined")-using racially discriminatory criteria. We find that neighborhoods that were redlined have consistently lower street tree α-diversity and are nine times less likely to have large (old) trees occupying a viable planting site. Simultaneously, redlined neighborhoods were locations of recent tree planting activities, with a high occupancy rate of small (young) trees. However, the community composition of these young trees exhibited lower species turnover and reordering across neighborhoods compared to those in higher grades, due to heavy reliance on a single tree species. Overall, while the negative effects of redlining remain detectable in present-day street tree communities, there are clear signs of recent investment. A strategy of planting diverse tree cohorts paired with investments in site rehabilitation and maintenance may be necessary if cities wish to overcome ecological feedbacks associated with legacies of environmental injustice.

The effects of local filtering processes on the structure and functioning of native plant communities in experimental urban habitats

Despite a growing literature-base devoted to document biodiversity patterns in cities, little is known about the processes that influence these patterns, and whether they are consistent over time. In particular, numerous studies have identified the capacity of cities to host a rich diversity of plant species. This trend, however, is driven primarily by introduced species, which comprise a large proportion of the urban species pool relative to natives. Using an experimental common garden study, we assessed the relative influence of local assembly processes (i.e., soil environmental filtering and competition from spontaneous urban species) on the taxonomic and functional diversity of native plant communities sampled over four seasons in 2016-2018. Taxonomic and functional diversity exhibited different responses to local processes, supporting the general conclusion that species- and trait-based measures of biodiversity offer distinct insights into community assembly dynamics. Additionally, we found that neither soil nor competition from spontaneous urban species influenced taxonomic or functional composition of native species. Functional composition, however, did shift strongly over time and was driven by community-weighted mean differences in both measured traits (maximum height, Hmax; specific leaf area, SLA; leaf chlorophyll a fluorescence, Chl a) and the relative proportions of different functional groups (legumes, annual and biennial-perennial species, C4 grasses, and forbs). By contrast, taxonomic composition only diverged between early and late seasons. Overall, our results indicate that native species are not only capable of establishing and persisting in vacant urban habitats, they can functionally respond to local filtering pressures over time. This suggests that regional dispersal limitation may be a primary factor limiting native species in urban environments. Thus, future regreening and management plans should focus on enhancing the dispersal potential of native plant species in urban environments, in order to achieve set goals for increasing native species diversity and associated ecosystem services in cities.

Loss of phylogenetic diversity under landscape change

Habitat alteration and destruction are primary drivers of biodiversity loss. However, the evolutionary dimensions of biodiversity loss remain largely unexplored in many systems. For example, little is known about how habitat alteration/loss can lead to phylogenetic deconstruction of ecological assemblages at the local level. That is, while species loss is evident, are some lineages favored over others? Using a long-term dataset of a globally, ecologically important guild of invertebrate consumers, stream leaf "shredders," we created a phylogenetic tree of the taxa in the regional species pool, calculated mean phylogenetic distinctiveness for >1000 communities spanning >10 year period, and related species richness, phylogenetic diversity, and distinctiveness to watershed-scale impervious cover. Using a combination of changepoint and compositional analyses, we learned that increasing impervious cover produced marked reductions in all three measures of diversity. These results aid in understanding both phylogenetic diversity and mean assemblage phylogenetic distinctiveness. Our findings indicate that, not only are species lost when there is an increase in watershed urbanization, as other studies have demonstrated, but that those lost are members of more distinct lineages relative to the community as a whole..

The application of metacommunity theory to the management of riverine ecosystems

River managers strive to use the best available science to sustain biodiversity and ecosystem function. To achieve this goal requires consideration of processes at different scales. Metacommunity theory describes how multiple species from different communities potentially interact with local-scale environmental drivers to influence population dynamics and community structure. However, this body of knowledge has only rarely been used to inform management practices for river ecosystems. In this paper, we present a conceptual model outlining how the metacommunity processes of local niche sorting and dispersal can influence the outcomes of management interventions and provide a series of specific recommendations for applying these ideas as well as research needs. In all cases, we identify situations where traditional approaches to riverine management could be enhanced by incorporating an understanding of metacommunity dynamics. A common theme is developing guidelines for assessing the metacommunity context of a site or region, evaluating how that context may affect the desired outcome, and incorporating that understanding into the planning process and methods used. To maximize the effectiveness of management activities, scientists and resource managers should update the toolbox of approaches to riverine management to reflect theoretical advances in metacommunity ecology.

Latitude dictates plant diversity effects on instream decomposition

Running waters contribute substantially to global carbon fluxes through decomposition of terrestrial plant litter by aquatic microorganisms and detritivores. Diversity of this litter may influence instream decomposition globally in ways that are not yet understood. We investigated latitudinal differences in decomposition of litter mixtures of low and high functional diversity in 40 streams on 6 continents and spanning 113° of latitude. Despite important variability in our dataset, we found latitudinal differences in the effect of litter functional diversity on decomposition, which we explained as evolutionary adaptations of litter-consuming detritivores to resource availability. Specifically, a balanced diet effect appears to operate at lower latitudes versus a resource concentration effect at higher latitudes. The latitudinal pattern indicates that loss of plant functional diversity will have different consequences on carbon fluxes across the globe, with greater repercussions likely at low latitudes.

The interaction between spatial variation in habitat heterogeneity and dispersal on biodiversity in a zooplankton metacommunity

It is hypothesized that biodiversity is maintained by interactions at local and regional spatial scales. Many sustainability plans and management practices reflect the need to conserve biodiversity, yet once these plans are implemented, the ecological consequences are not well understood. By learning how management practices affect local environmental factors and dispersal in a region, ecologists and natural resource managers can better understand the implications of management choices. We investigated the interaction of local and regional scale processes in the built environment, where human-impacts are known to influence both. Our goal was to determine how the interaction between spatial variation in habitat heterogeneity in algal management of urban ponds and dispersal shape biodiversity at local and regional spatial scales. A twelve-week mesocosm study was conducted where pond management and dispersal were manipulated to determine how spatial variation in habitat and dispersal from various source pools influence zooplankton metacommunities in urban stormwater ponds. We hypothesized that dispersal from managed or unmanaged source pools will lead to community divergence and local management practices will act as an environmental filter, both reducing beta diversity between managed ponds and driving compositional divergence. Our results suggest that zooplankton dispersal from managed or unmanaged source pools was important to explaining divergence in community composition. Furthermore, local management of algae marginally reduced compositional turnover of zooplankton among ponds but did not lead to significant divergence in community composition. Management practices may act as strong environmental filters by reducing beta diversity between ponds. As hypothesized, source pool constraints led to compositional divergence and local management practices resulted in reduced compositional turnover between ponds. The results of this study suggest that sustainability and management plans may have complex effects on biodiversity both within and across spatial scales.

The worldwide impact of urbanisation on avian functional diversity

Urbanisation is driving rapid declines in species richness and abundance worldwide, but the general implications for ecosystem function and services remain poorly understood. Here, we integrate global data on bird communities with comprehensive information on traits associated with ecological processes to show that assemblages in highly urbanised environments have substantially different functional composition and 20% less functional diversity on average than surrounding natural habitats. These changes occur without significant decreases in functional dissimilarity between species; instead, they are caused by a decrease in species richness and abundance evenness, leading to declines in functional redundancy. The reconfiguration and decline of native functional diversity in cities are not compensated by the presence of exotic species but are less severe under moderate levels of urbanisation. Thus, urbanisation has substantial negative impacts on functional diversity, potentially resulting in impaired provision of ecosystem services, but these impacts can be reduced by less intensive urbanisation practices.

Theoretical Perspectives of the Baltimore Ecosystem Study: Conceptual Evolution in a Social-Ecological Research Project

The Earth's population will become more than 80% urban during this century. This threshold is often regarded as sufficient justification for pursuing urban ecology. However, pursuit has primarily focused on building empirical richness, and urban ecology theory is rarely discussed. The Baltimore Ecosystem Study (BES) has been grounded in theory since its inception and its two decades of data collection have stimulated progress toward comprehensive urban theory. Emerging urban ecology theory integrates biology, physical sciences, social sciences, and urban design, probes interdisciplinary frontiers while being founded on textbook disciplinary theories, and accommodates surprising empirical results. Theoretical growth in urban ecology has relied on refined frameworks, increased disciplinary scope, and longevity of interdisciplinary interactions. We describe the theories used by BES initially, and trace ongoing theoretical development that increasingly reflects the hybrid biological-physical-social nature of the Baltimore ecosystem. The specific mix of theories used in Baltimore likely will require modification when applied to other urban areas, but the developmental process, and the key results, will continue to benefit other urban social-ecological research projects.

Biodiversity of leaf litter fungi in streams along a latitudinal gradient

Global patterns of biodiversity have emerged for soil microorganisms, plants and animals, and the extraordinary significance of microbial functions in ecosystems is also well established. Virtually unknown, however, are large-scale patterns of microbial diversity in freshwaters, although these aquatic ecosystems are hotspots of biodiversity and biogeochemical processes. Here we report on the first large-scale study of biodiversity of leaf-litter fungi in streams along a latitudinal gradient unravelled by Illumina sequencing. The study is based on fungal communities colonizing standardized plant litter in 19 globally distributed stream locations between 69°N and 44°S. Fungal richness suggests a hump-shaped distribution along the latitudinal gradient. Strikingly, community composition of fungi was more clearly related to thermal preferences than to biogeography. Our results suggest that identifying differences in key environmental drivers, such as temperature, among taxa and ecosystem types is critical to unravel the global patterns of aquatic fungal diversity.

Global patterns and drivers of ecosystem functioning in rivers and riparian zones

River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth's biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented "next-generation biomonitoring" by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale.

Effects of pond management on biodiversity patterns and community structure of zooplankton in urban environments

As urban areas continue expanding, major cities become connected forming megacities. Urban encroachment into natural areas transforms the landscape into a built environment with heterogeneously distributed patches of novel habitat. Community structure within novel habitats is influenced by anthropogenic factors including fragmentation and species interactions. Alterations in complex biodiversity patterns may be used to assess how urban stressors impact community assemblages which, ultimately, may inform sustainable management decisions. To manage algal blooms, Aquashade® is applied directly to ponds. We investigated the effects of Aquashade®, nutrient loading and dispersal on local species diversity and compositional turnover of zooplankton communities from suburban ponds in Columbia, MD, USA using a mesocosm approach. We found that Aquashade® acted as an environmental filter by increasing local species diversity and decreasing compositional turnover. This ultimately could have an overall homogenizing effect on the regional species pool (or γ-diversity). The same pattern was observed in mesocosms that received simulated dispersal events of zooplankton. Nutrients, overall, increased autotrophic biomass and while Aquashade® had no effect on autotrophic biomass, the interaction of nutrients and Aquashade® similarly caused a homogenization of the zooplankton community. Additionally, there was an overall increase in cladoceran ephippia in mesocosms receiving Aquashade® compared to those not, suggesting there is a 'trigger' switching cladocerans from parthenogenic to sexual reproduction. Taken together, our results show the application of Aquashade®, nutrient loading and dispersal shift biodiversity patterns in urban zooplankton communities. We hypothesize these shifts originate at the resource level through alterations in the phytoplankton community either through composition or nutritive value in ponds receiving Aquashade®. Our study illustrates the importance of investigating multiple-scales of community assemblages when assessing impacts of anthropogenic stressors. Consideration of how to best manage green spaces may include planned un-managed areas to maintain regional biodiversity and heterogenous communities within the urban environment.

Metacommunity theory meets restoration: isolation may mediate how ecological communities respond to stream restoration

An often-cited benefit of river restoration is an increase in biodiversity or shift in composition to more desirable taxa. Yet, hard manipulations of habitat structure often fail to elicit a significant response in terms of biodiversity patterns. In contrast to conventional wisdom, the dispersal of organisms may have as large an influence on biodiversity patterns as environmental conditions. This influence of dispersal may be particularly influential in river networks that are linear branching, or dendritic, and thus constrain most dispersal to the river corridor. As such, some locations in river networks, such as isolated headwaters, are expected to respond less to environmental factors and less by dispersal than more well-connected downstream reaches. We applied this metacommunity framework to study how restoration drives biodiversity patterns in river networks. By comparing assemblage structure in headwater vs. more well-connected mainstem sites, we learned that headwater restoration efforts supported higher biodiversity and exhibited more stable ecological communities compared with adjacent, unrestored reaches. Such differences were not evident in mainstem reaches. Consistent with theory and mounting empirical evidence, we attribute this finding to a relatively higher influence of dispersal-driven factors on assemblage structure in more well-connected, higher order reaches. An implication of this work is that, if biodiversity is to be a goal of restoration activity, such local manipulations of habitat should elicit a more profound response in small, isolated streams than in larger downstream reaches. These results offer another significant finding supporting the notion that restoration activity cannot proceed in isolation of larger-scale, catchment-level degradation.

Global synthesis of the temperature sensitivity of leaf litter breakdown in streams and rivers

Streams and rivers are important conduits of terrestrially derived carbon (C) to atmospheric and marine reservoirs. Leaf litter breakdown rates are expected to increase as water temperatures rise in response to climate change. The magnitude of increase in breakdown rates is uncertain, given differences in litter quality and microbial and detritivore community responses to temperature, factors that can influence the apparent temperature sensitivity of breakdown and the relative proportion of C lost to the atmosphere vs. stored or transported downstream. Here, we synthesized 1025 records of litter breakdown in streams and rivers to quantify its temperature sensitivity, as measured by the activation energy (E_a , in eV). Temperature sensitivity of litter breakdown varied among twelve plant genera for which E_a could be calculated. Higher values of E_a were correlated with lower-quality litter, but these correlations were influenced by a single, N-fixing genus (Alnus). E_a values converged when genera were classified into three breakdown rate categories, potentially due to continual water availability in streams and rivers modulating the influence of leaf chemistry on breakdown. Across all data representing 85 plant genera, the E_a was 0.34 ± 0.04 eV, or approximately half the value (0.65 eV) predicted by metabolic theory. Our results indicate that average breakdown rates may increase by 5-21% with a 1-4 °C rise in water temperature, rather than a 10-45% increase expected, according to metabolic theory. Differential warming of tropical and temperate biomes could result in a similar proportional increase in breakdown rates, despite variation in E_a values for these regions (0.75 ± 0.13 eV and 0.27 ± 0.05 eV, respectively). The relative proportions of gaseous C loss and organic matter transport downstream should not change with rising temperature given that E_a values for breakdown mediated by microbes alone and microbes plus detritivores were similar at the global scale.

Effects of road deicer (NaCl) and amphibian grazers on detritus processing in pond mesocosms

Road deicers have been identified as potential stressors in aquatic habitats throughout the United States, but we know little regarding associated impacts to ecosystem function. A critical component of ecosystem function that has not previously been evaluated with respect to freshwater salinization is the impact on organic matter breakdown. The purpose of this study was to evaluate cumulative effects of road deicers and tadpole grazers on leaf litter breakdown rate (g d(-1) ) and microbial respiration (mg O(2)  g leaf(-1) h(-1) ). To test this interaction, in May 2008 the authors added dry leaf litter (Quercus spp.) to forty 600-L pond mesocosms and inoculated each with algae and zooplankton. In a full-factorial design, they manipulated a realistic level of road salt (ambient or elevated at 645 mg L(-1) Cl(-) ) and tadpole (Hyla versicolor) presence or absence. The elevated chloride treatment reduced microbial respiration by 24% in the presence of tadpoles. The breakdown of leaf litter by tadpoles occurred 9.7% faster under ambient chloride conditions relative to the elevated chloride treatment. Results of the present study suggest that the microbial community is directly impacted by road deicers and heavy tadpole grazing under ambient conditions limits microbial capacity to process detritus. Road salts and tadpoles interact to limit microbial respiration, but to a lesser extent leaf mass loss rate, thereby potentially restricting energy flow from detrital sources in pond ecosystems.

Incubation time, functional litter diversity, and habitat characteristics predict litter-mixing effects on decomposition

Plant diversity influences many fundamental ecosystem functions, including carbon and nutrient dynamics, during litter breakdown. Mixing different litter species causes litter mixtures to lose mass at different rates than expected from component species incubated in isolation. Such nonadditive litter-mixing effects on breakdown processes often occur idiosyncratically because their direction and magnitude change with incubation time, litter species composition, and ecosystem characteristics. Taking advantage of results from 18 litter mixture experiments in streams, we examined whether the direction and magnitude of nonadditive mixing effects are randomly determined. Across 171 tested litter mixtures and 510 incubation time-by-mixture combinations, nonadditive effects on breakdown were common and on average resulted in slightly faster decomposition than expected. In addition, we found that the magnitude of nonadditive effects and the relative balance of positive and negative responses in mixtures change predictably over time, and both were related to an index of functional litter diversity and selected environmental characteristics. Based on these, it should be expected that nonadditive effects are stronger for litter mixtures made of functionally dissimilar species especially in smaller streams. Our findings demonstrate that effects of litter diversity on plant mixture breakdown are more predictable than generally thought. We further argue that the consequences of current worldwide homogenization in the composition of plant traits on carbon and nutrient dynamics could be better inferred from long-duration experiments that manipulate both functional litter diversity and ecosystem characteristics in "hotspots of biodiversity effects," such as small streams.

Exposure and nontarget effects of transgenic Bt corn debris in streams

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.

Interaction between urbanization and climate variability amplifies watershed nitrate export in Maryland

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.

Identification of a localized mucosal immune response in rainbow trout, Oncorhynchus mykiss (Walbaum), following immunization with a protein-hapten antigen

The ability of rainbow trout, Oncorhynchus mykiss (RBT), to produce a localized mucosal immune response was investigated following intraperitoneal (i.p.) or peranal (p.a.) immunization with a protein-hapten carrier, fluorescein isothiocyanate conjugated to keyhole limpet haemocyanin (FITC/KLH). Antibody levels in serum, mucus, tissue culture supernatant from blood and spleen leucocytes, and excised skin, intestine and gill tissues were determined by ELISA. Significantly, elevated antigen-specific antibodies were elicited in both serum and mucus of fish immunized i.p. Mucosal antibody responses, in general, paralleled serum responses over time. Leucocytes isolated from spleen and blood of i.p. immunized fish at week 10 produced significantly elevated antibody levels against FITC when cultured in vitro. Excised skin, intestine and gill tissues from these fish also exhibited significantly elevated antibody responses indicating localized production in the mucosa from tissue-specific B cells. A localized mucosal immune response was elicited only after i.p. and not p.a. immunization, suggesting that systemically stimulated B cells migrate to mucosal tissues where they produce antibodies locally.

Preferential feeding by an aquatic consumer mediates non-additive decomposition of speciose leaf litter

Forest soils and streams receive substantial inputs of detritus from deciduous vegetation. Decay of this material is a critical ecosystem process, recycling nutrients and supporting detrital-based food webs, and has been attributed, in part, to leaf litter species composition. However, research on why speciose leaf litter should degrade differently has relied on a bottom-up approach, embracing interspecific variation in litter chemistry. We hypothesized that preferential feeding by an aquatic detritivore interacts with species-specific leaf palatability and slows decay of speciose leaf litter. We addressed this by offering four single- and mixed-species leaf resources to field densities of a leaf-shredding consumer. Mixing leaf species resulted in slower total leaf decomposition. Decreases in mixed-species decomposition was partly explained by preferential feeding by the consumers in one case, but the lack of preferential feeding in other mixtures suggested an interactive effect of feeding and microbial degradation. Loss of riparian tree biodiversity may have implications for in-stream consumer-resource interactions.

Composition of speciose leaf litter alters stream detritivore growth, feeding activity and leaf breakdown

Leaf litter derived from riparian trees can control secondary production of detritivores in forested streams. Species-rich assemblages of leaf litter reflect riparian plant species richness and represent a heterogeneous resource for stream consumers. Such variation in resource quality may alter consumer growth and thus the feedback on leaf breakdown rate via changes in feeding activity. To assess the consequences of this type of resource heterogeneity on both consumer growth and subsequent litter breakdown, we performed a laboratory experiment where we offered a leaf-shredding stream detritivore (the stonefly Tallaperla maria, Peltoperlidae) ten treatments of either single- or mixed-species leaf litter. We measured consumer growth rate, breakdown rate and feeding activity both with and without consumers for each treatment and showed that all three variables responded to speciose leaf litter. However, the number of leaf species was not responsible for these results, but leaf species composition explained the apparent non-additive effects. T. maria growth responded both positively and negatively to litter composition, and growth on mixed-litter could not always be predicted by averaging estimates of growth in single-species treatments. Furthermore, breakdown and feeding rates in mixed litter treatments could not always be predicted from estimates of single-species rates. Given that species richness and composition of senesced leaves in streams reflects riparian plant species richness, in-stream secondary production of detritivores and organic matter dynamics may be related to species loss of trees in the riparian zone. Loss of key species may be more critical to maintaining such processes than species richness per se.