An unusual trophic subsidy and species dominance in a tropical stream

In classical theory, species are assumed to achieve dominance through competitive exclusion, but if food resources are limiting, cross-habitat trophic subsidies could also underpin dominance. The impact of dominant species on community dynamics may depend on the energy base of population size. We report on an unusual, spatially subsidized population of a tropical, stream-dwelling crab that dominates the benthic fauna of a Kenyan stream. Diet and stable isotope analyses indicated that this crab is a true omnivore, with terrestrial subsidies dominating both plant and animal resources. Unusually, the animal prey included almost no aquatic invertebrates. Instead, a single species of ant constituted approximately 35% of the annual diet (stomach contents analysis) and up to 90% of assimilated nitrogen (estimates from stable isotope analysis). Ants may be pivotal to enabling crab dominance, and this crab may be largely disconnected from the local trophic network for its dietary needs. The paucity of other invertebrates in the stream community suggests that this super-dominant crab is a strong interactor that suppresses aquatic invertebrate populations. Common stabilizing attributes of spatially subsidized food webs (e.g., asynchronous prey availability, wide feeding niche, consumer migration) were absent from this system, and although apparently stable, it may be vulnerable to disturbance in the donor habitat.     

Environmental impact propagated by cross-system subsidy: chronic stream pollution controls riparian spider populations

Resource subsidies between habitats are common and create the potential for the propagation of environmental impacts across system boundaries. However, recent understanding of the potential for subsidy-mediated cross-system impact propagations is limited and primarily based on passive flows of nutrients and detritus or short-term effects. Here, we assess the effects of sustained alterations in aquatic insect emergence (active subsidy pathway), due to chronic stream pollution, for riparian spiders. The sustained reduction in aquatic insect densities at the polluted reaches resulted in a marked decline in web spider population density and a shift in spider community composition. Our results provide the first evidence that stream pollution can control populations and community structure of terrestrial predators via sustained alterations in aquatic subsidies, emphasizing the role of subtle trophic linkages in the transmission of environmental impacts across ecosystem boundaries.     

Meta-analysis: trophic level, habitat, and productivity shape the food web effects of resource subsidies

Studies of the effects of cross-habitat resource subsidies have been a feature of food web ecology over the past decade. To date, most studies have focused on demonstrating the magnitude of a subsidy or documenting its effect in the recipient habitat. Ecologists have yet to develop a satisfactory framework for predicting the magnitude of these effects. We used 115 data sets from 32 studies to compare consumer responses to resource subsidies across recipient habitat type, trophic level, and functional group. Changes in consumer density or biomass in response to subsidies were inconsistent across habitats, trophic, and functional groups. Responses in stream cobble bar and coastline habitats were larger than in other habitats. Contrary to expectation, the magnitude of consumer response was not affected by recipient habitat productivity or the ratio of productivity between donor and recipient habitats. However, consumer response was significantly related to the ratio of subsidy resources to equivalent resources in the recipient habitat. Broad contrasts in productivity are modified by subsidy type, vector, and the physical and biotic characteristics of both donor and recipient habitats. For this reason, the ratio of subsidy to equivalent resources is a more useful tool for predicting the possible effect of a subsidy than coarser contrasts of in situ productivity. The commonness of subsidy effects suggests that many ecosystems need to be studied as open systems.     

Habitat Mosaic, Wildlife Availability, and Hunting in the Tropical Forest of Calakmul, Mexico

Abstract: Habitat loss and subsistence hunting are two of the main activities that affect wildlife in frontier areas. We compared subsistence hunting patterns in four villages with different ethnic composition and degree of habitat disturbance in the vicinity of Calakmul Biosphere Reserve, Campeche, Mexico. We also compared differences between some of these villages in harvest composition and prey availability to determine hunting preferences. We used a Landsat TM satellite image to analyze the degree of disturbance around the villages. We conducted periodic surveys of subsistence hunting and prey availability. Wildlife availability was assessed monthly on nine transects (3000 m) established in the vicinity of three villages. The relative amount of disturbed habitat was smaller in an indigenous Maya village ( IV ) and larger in a mestizo village ( MV ). The two mixed-composition villages ( MCVs) had intermediate levels of disturbance. Ten species, four large and six small, of birds and mammals accounted for 97% of the hunting records. Hunting was more intense in IV and less intense in MCV1. The three village types had different hunting preferences. The habitat-mosaic composition in the vicinity of the villages influenced prey availability and subsistence-hunting preferences. Changes in the habitat mosaic were caused by the size of the holding and by ethnic composition. In spite of longer settlement time, the habitat mosaic in the vicinity of IV was less transformed than that of the other sites. Their larger holding size and greater diversity of economic activities may explain why the Mayas at IV have transformed the landscape less than the other groups and can hunt more and larger prey.     

Multiple techniques confirm elevational differences in insect size that may influence spider sociality

Social and subsocial spiders of the genus Anelosimus exhibit an altitudinal pattern in their geographic distribution at tropical latitudes in the Americas. Social species, which capture prey cooperatively, occur primarily in the lowland rain forest and are absent from higher elevations, whereas subsocial species are common at higher elevations but absent from the lowland rain forest. Previous studies have suggested that differences in the size of potential insect prey along altitudinal gradients may explain this pattern as insects were found to be, on average, larger in lowland rain forests than at higher elevations. These studies, however, may have under-sampled the insect size composition of each habitat because only one sampling technique was used. Using a number of collection methods we sampled the insect size composition in the environments of social and subsocial spiders in this genus. We found that the average insect size in lowland rain forest habitats was indeed larger than at high-elevation cloud forests in eastern Ecuador. We also found that, even though the various techniques differed in the size of the insects they captured (visual searching and blacklighting yielding larger insects than beating, sweeping, or malaise trapping), they all caught, on average, larger insects in the lowlands. Overall, spider colonies in the lowlands caught larger prey than did spider colonies at higher elevations, paralleling differences in insect size distribution obtained by the various techniques in their respective environments.     

Predators, prey, and transient states in the assembly of spatially structured communities

Ecological theory suggests that both dispersal limitation and resource limitation can exert strong effects on community assembly. However, empirical studies of community assembly have focused almost exclusively on communities with a single trophic level. Thus, little is known about the combined effects of dispersal and resource limitation on assembly of communities with multiple trophic levels. We performed a landscape-scale experiment using spatially arranged mesocosms to study effects of dispersal and resource limitation on the assembly dynamics of aquatic invertebrate communities with two trophic levels. We found that interplay between dispersal and resource limitation regulated the assembly of predator and prey trophic levels in these pond communities. Early in assembly, predators and prey were strongly dispersal limited, and resource (i.e., prey) availability did not influence predator colonization. Later in assembly, after predators colonized, resource limitation was the strongest driver of predator abundance, and dispersal limitation played a negligible role. Thus, habitat isolation affected predators directly by reducing predator colonization rate, and indirectly through the effect of distance on prey availability. Dispersal and resource limitation of predators resulted in a transient period in which predators were absent or rare in isolated habitats. This period may be important for understanding population dynamics of vulnerable prey species. Our findings demonstrate that dispersal and resource limitation can jointly regulate assembly dynamics in multi-trophic systems. They also highlight the need to develop a temporal picture of the assembly process in multi-trophic communities because the availability and spatial distribution of limiting resources (i.e., prey) and the distribution of predators can shift radically over time.     

Biogenic disturbance determines invasion success in a subtidal soft-sediment system

Theoretically, disturbance and diversity can influence the success of invasive colonists if (1) resource limitation is a prime determinant of invasion success and (2) disturbance and diversity affect the availability of required resources. However, resource limitation is not of overriding importance in all systems, as exemplified by marine soft sediments, one of Earth's most widespread habitat types. Here, we tested the disturbance-invasion hypothesis in a marine soft-sediment system by altering rates of biogenic disturbance and tracking the natural colonization of plots by invasive species. Levels of sediment disturbance were controlled by manipulating densities of burrowing spatangoid urchins, the dominant biogenic sediment mixers in the system. Colonization success by two invasive species (a gobiid fish and a semelid bivalve) was greatest in plots with sediment disturbance rates < 500 cm(3) x m(-2) x d(-1), at the low end of the experimental disturbance gradient (0 to > 9000 cm(3) x m(-2) x d(-1)). Invasive colonization declined with increasing levels of sediment disturbance, counter to the disturbance-invasion hypothesis. Increased sediment disturbance by the urchins also reduced the richness and diversity of native macrofauna (particularly small, sedentary, surface feeders), though there was no evidence of increased availability of resources with increased disturbance that would have facilitated invasive colonization: sediment food resources (chlorophyll a and organic matter content) did not increase, and space and access to overlying water were not limited (low invertebrate abundance). Thus, our study revealed the importance of biogenic disturbance in promoting invasion resistance in a marine soft-sediment community, providing further evidence of the valuable role of bioturbation in soft-sediment systems (bioturbation also affects carbon processing, nutrient recycling, oxygen dynamics, benthic community structure, and so on.). Bioturbation rates are influenced by the presence and abundance of large burrowing species (like spatangoid urchins). Therefore, mass mortalities of large bioturbators could inflate invasion risk and alter other aspects of ecosystem performance in marine soft-sediment habitats.     

Reciprocal subsidies between freshwater and terrestrial ecosystems structure consumer resource dynamics

Cross-ecosystem movements of material and energy, particularly reciprocal resource fluxes across the freshwater-land interface, have received major attention. Freshwater ecosystems may receive higher amounts of subsidies (i.e., resources produced outside the focal ecosystem) than terrestrial ecosystems, potentially leading to increased secondary production in freshwaters. Here we used a meta-analytic approach to quantify the magnitude and direction of subsidy inputs across the freshwater-land interface and to determine subsequent responses in recipient animals. Terrestrial and freshwater ecosystems differed in the magnitude of subsidies they received, with aquatic ecosystems generally receiving higher subsidies than terrestrial ecosystems. Surprisingly, and despite the large discrepancy in magnitude, the contribution of these subsidies to animal carbon inferred from stable isotope composition did not differ between freshwater and terrestrial ecosystems, likely due to the differences in subsidy quality. The contribution of allochthonous subsidies was highest to primary consumers and predators, suggesting that bottom-up and top-down effects may be affected considerably by the input of allochthonous resources. Future work on subsidies will profit from a food web dynamic approach including indirect trophic interactions and propagating effects.     

Can bottom trawling indirectly diminish carrying capacity in a marine ecosystem?

Bottom trawling is associated with reduced biomass and production in the marine benthic community. Abundance of hard-bodied organisms such as bivalves, crustaceans and echinoderms typically declines in favour of soft-bodied opportunists such as polychaetes. Trawling effects vary with habitat; impact and recovery time are typically greater for more complex substrates/communities and those with lower rates of natural disturbance. Benthic organisms represent the prey base of a large component of the demersal fish assemblage. Hence, trawling-induced change in benthic community structure and function may exert an indirect effect on feeding success and growth of important commercially exploited fishes such as plaice Pleuronectes platessa. We present habitat-specific mixed effects models of plaice length as a function of age, bottom-trawling effort, population density and near-bottom temperature, with sampling year and area, and fish sex incorporated as random effects. Across an observed gradient of trawling effort in the Celtic Sea, plaice on gravel habitat showed significant declines in length at age while plaice on sand habitat showed significant increases in length at age. Contrasting trawling effects likely reflect dietary differences between habitats. Plaice on sand substrates are known to consume predominately polychaetes, which may proliferate at moderate trawling intensity in this habitat. Conversely, plaice on gravel substrates are reported to consume more of the fragile organisms such as echinoderms and bivalves that show marked declines with bottom trawling. An indirect effect of trawling on prey availability and growth of demersal fish has substantial implications for fisheries sustainability via reduced ecosystem carrying capacity and production of commercial fish.     

A predator’s body coloration enhances its foraging profitability by day and night

Few predators forage by both day and night. It remains unknown, however, how the costs and benefits of foraging or signaling are partitioned in animals that forage at all times. The orb-web spider Cyrtophora moluccensis is brightly colored and forages by day and night. We determined the benefits reaped when it forages by both day and night by estimating the biomass of prey caught in their webs. Additionally, we quantified whether the spider’s presence influences the number of prey caught by day and night and whether its colorful body is visible to diurnal and/or nocturnal insects using diurnal and nocturnal insect vision models. We found that approximately five times the biomass of prey was caught in C. moluccensis’ webs by night than by day. Hemipterans, hymenopterans, and dipterans were predominantly caught by day, while lepidopterans (moths) were predominately caught by night. Accordingly, we concluded that foraging by night is more profitable than foraging by day. We predicted that other benefits, for example, energetic advantages or enhanced fecundity, may promote its daytime activity. Foraging success was greater by day and night when the spider was present in the web than when the spider was absent. We also found that parts of the spider’s body were conspicuous to diurnal and nocturnal insects, possibly through different visual channels. The colorful body of C. moluccensis, accordingly, appears to influence its foraging success by attracting prey during both the day and night.     

Biotic habitat complexity controls species diversity and nutrient effects on net biomass production

Canopy-forming plants and algae commonly contribute to spatial variation in habitat complexity for associated organisms and thereby create a biotic patchiness of communities. In this study, we tested for interaction effects between biotic habitat complexity and resource availability on net biomass production and species diversity of understory macroalgae by factorial field manipulations of light, nutrients, and algal canopy cover in a subtidal rocky-shore community. Presence of algal canopy cover and/or artificial shadings limited net biomass production and facilitated species diversity. Artificial shadings reduced light to levels similar to those under canopy cover, and net biomass production was significantly and positively correlated to light availability. Considering the comparable and dependent experimental effects from shadings and canopy cover, the results strongly suggest that canopy cover controlled net biomass production and species diversity by limiting light and thereby limiting resource availability for community production. Canopy cover also controlled experimental nutrient effects by preventing a significant increase in net biomass production from nutrient enrichment recorded in ambient light (no shading). Changes in species diversity were mediated by changes in species dominance patterns and species evenness, where canopy cover and shadings facilitated slow-growing crust-forming species and suppressed spatial dominance by Fucus vesiculosus, which was the main contributor to net production of algal biomass. The demonstrated impacts of biotic habitat complexity on biomass production and local diversity contribute significantly to understanding the importance of functionally important species and biodiversity for ecosystem processes. In particular, this study demonstrates how loss of a dominant species and decreased habitat complexity change the response of the remaining assembly to resource loading. This is of potential significance for marine conservation since resource loading often promotes low habitat complexity and canopy species are among the first groups lost in degraded aquatic systems.     

Multiple predators indirectly alter community assembly across ecological boundaries

Models of habitat selection often assume that organisms choose habitats based on their intrinsic quality, regardless of the position of these habitats relative to low-quality habitats in the landscape. We created a habitat matrix in which high-quality (predator-free) aquatic habitat patches were positioned adjacent to (predator-associated) or isolated from (control) patches with single or two species of caged predators. After 16 days of colonization, larval insect abundance was reduced by 50% on average in both the predator and predator-associated treatments relative to isolated controls. Effects were largely similar among predator treatments despite variation in number of predator species, predator biomass, and whether predators were native or nonnative. Importantly, the strength of effects did not depend on whether predators were physically present. These results demonstrate that predator cues can cascade with equal strength across ecological boundaries, indirectly altering community assembly via habitat selection in intrinsically high-quality habitats.     

Why do nocturnal orb-web spiders (Araneidae) search for light?

The nocturnal orb-web spider Larinioides sclopetarius lives near water and frequently builds webs on bridges. In Vienna, Austria, this species is particularly abundant along the artificially lit handrails of a footbridge. Fewer individuals placed their webs on structurally identical but unlit handrails of the same footbridge. A census of the potential prey available to the spiders and the actual prey captured in the webs revealed that insect activity was significantly greater and consequently webs captured significantly more prey in the lit habitat compared to the unlit habitat. A laboratory experiment showed that adult female spiders actively choose artificially lit sites for web construction. Furthermore, this behaviour appears to be genetically predetermined rather than learned, as laboratory-reared individuals which had previously never foraged in artificial light exhibited the same preference. This orb-web spider seems to have evolved a foraging behaviour that exploits the attraction of insects to artificial lights. Received: 8 June 1998 / Received in revised form: 18 January 1999 / Accepted: 19 January 1999     

Using stable isotopes to reveal shifts in prey consumption by generalist predators.

The effectiveness of generalist predators in biological control may be diminished if increased availability of alternative prey causes individual predators to decrease their consumption of crop pests. Farming practices that enhance densities of microbidetritivores in the detrital food web can lead to increased densities of generalist predators that feed on pest species. The ability to predict the net biocontrol impact of increased predator densities depends upon knowing the extent to which individual predators may shift to detrital prey and feed less on crop pests when prey of the detritus-based food web are more abundant. We addressed this question by comparing ratios of stable isotopes of carbon (delta13C) and nitrogen (delta15N) in generalist ground predators and two types of prey (crop pests and microbidetritivores) in replicated 8 x 8 m cucurbit gardens subjected to one of two treatments: a detrital subsidy or no addition of detritus (control). Small sheet-web spiders (Linyphiidae) and small wolf spiders (Lycosidae) had delta13C values similar to those of Collembola in both the detrital and control treatments, indicating that small spiders belong primarily to the detrital food web. In control plots the larger generalist predators had delta13C values similar to those of the major insect pests, consistent with their known effectiveness as biocontrol agents. Adding detritus may have caused delta13C of one species of large wolf spider to shift toward that of the microbi-detritivores, although evidence is equivocal. In contrast, another large wolf spider displayed no shift in delta13C in the detrital treatment. Thus, stable isotopes revealed which generalist predators will likely continue to feed on pest species in the presence of greater densities of alternative prey.     

Grazing regulates the spatial variability of periphyton biomass

The presence of consumers not only alters the mean biomass of the prey assemblage, but also affects the spatial heterogeneity of biomass distribution. Whereas the mean prey biomass is generally reduced by consumer presence, the effect on spatial heterogeneity is less clear-cut. A meta-analysis of almost 600 field experiments manipulating the presence of benthic invertebrate or vertebrate grazers was conducted to analyze the effect of grazers on both the absolute spatial variability of periphyton biomass and the relative variability, which was standardized to the mean. Effects on absolute variability were measured as the log response ratio of the standard deviation of biomass (LR-SD), whereas effects on relative variability were measured as the log response ratio of the coefficient of variation of biomass (LR-CV). The overall magnitude and range of LR-SD and LR-CV indicated that grazers not only reduced periphyton biomass, but also substantially altered their spatial distribution. However, grazer effects differed strongly for absolute and relative variability. On average, grazers reduced the absolute spatial variability in prey biomass by 50% (average LR-SD = -0.68) but increased the relative variability by 24% (average LR-CV = 0.22). The magnitude of LR-SD strongly depended on the efficiency of grazing, with strong biomass removal leading to strong homogenization. Moreover, LR-CV and LR-SD were significantly affected by habitat type (freshwater vs. coastal) and substrata. Given the importance of spatial heterogeneity for resource uptake, competition and the maintenance of diversity, grazer presence has potentially strong indirect effects on the interactions within prey assemblages.     

Defining ecologically relevant scales for spatial protection with long‐term data on an endangered seabird and local prey availability

Human activities are important drivers of marine ecosystem functioning. However, separating the synergistic effects of fishing and environmental variability on the prey base of nontarget predators is difficult, often because prey availability estimates on appropriate scales are lacking. Understanding how prey abundance at different spatial scales links to population change can help integrate the needs of nontarget predators into fisheries management by defining ecologically relevant areas for spatial protection. We investigated the local population response (number of breeders) of the Bank Cormorant (Phalacrocorax neglectus), a range‐restricted endangered seabird, to the availability of its prey, the heavily fished west coast rock lobster (Jasus lalandii). Using Bayesian state‐space modeled cormorant counts at 3 colonies, 22 years of fisheries‐independent data on local lobster abundance, and generalized additive modeling, we determined the spatial scale pertinent to these relationships in areas with different lobster availability. Cormorant numbers responded positively to lobster availability in the regions with intermediate and high abundance but not where regime shifts and fishing pressure had depleted lobster stocks. The relationships were strongest when lobsters 20–30 km offshore of the colony were considered, a distance greater than the Bank Cormorant's foraging range when breeding, and may have been influenced by prey availability for nonbreeding birds, prey switching, or prey ecology. Our results highlight the importance of considering the scale of ecological relationships in marine spatial planning and suggest that designing spatial protection around focal species can benefit marine predators across their full life cycle. We propose the precautionary implementation of small‐scale marine protected areas, followed by robust assessment and adaptive‐management, to confirm population‐level benefits for the cormorants, their prey, and the wider ecosystem, without negative impacts on local fisheries.     

Temperature-associated population diversity in salmon confers benefits to mobile consumers

Habitat heterogeneity can generate intraspecific diversity through local adaptation of populations. While it is becoming increasingly clear that population diversity can increase stability in species abundance, less is known about how population diversity can benefit consumers that can integrate across population diversity in their prey. Here we demonstrate cascading effects of thermal heterogeneity on trout-salmon interactions in streams where rainbow trout rely heavily on the seasonal availability of anadromous salmon eggs. Water temperature in an Alaskan stream varied spatially from 5 degrees C to 17.5 degrees C, and spawning sockeye salmon showed population differentiation associated with this thermal heterogeneity. Individuals that spawned early in cool regions of the 5 km long stream were genetically differentiated from those spawning in warmer regions later in the season. Sockeye salmon spawning generates a pulsed resource subsidy that supports the majority of seasonal growth in stream-dwelling rainbow trout. The spatial and temporal structuring of sockeye salmon spawn timing in our focal stream extended the duration of the pulsed subsidy compared to a thermally homogeneous stream with a single population of salmon. Further, rainbow trout adopted movement strategies that exploited the multiple pulses of egg subsidies in the thermally heterogeneous stream. Fish that moved to track the resource pulse grew at rates about 2.5 times higher than those that remained stationary or trout in the reference stream with a single seasonal pulse of eggs. Our results demonstrate that habitat heterogeneity can have important effects on the population diversity of dominant species, and in turn, influence their value to species that prey upon them. Therefore, habitat homogenization may have farther-reaching ecological effects than previously considered.     

Effects of long-term physical disturbance by commercial scallop fishing on subtidal epifaunal assemblages and habitats

 This paper examines spatial differences in the distribution of by-catch assemblages from the scallop [Pecten maximus (L.) and Aequipecten opercularis (L.)] fishing grounds in the North Irish Sea, during 1995. The sites examined have been exposed to differing known levels of fishing disturbance by scallop dredging, based on unusually high-resolution data extracted from fishermens' logbooks. Uni- and multi-variate techniques have been used on a production dataset (a value which incorporates both abundance and biomass figures), as well as abundance and biomass data individually. The original species list was reduced to higher taxonomic groupings in line with the theory that the latter is more appropriate for detecting anthropogenic change. Species diversity and richness, total number of species, and total number of individuals all decrease significantly with increasing fishing effort. Species dominance increases with effort. Total abundance, biomass and production, and the production of most of the major individual taxa investigated decrease significantly with increasing effort. Multivariate analysis reveals a significant relationship between fishing effort and by-catch assemblage structure. The taxa most responsible for the differences are the echinoids and cnidarians, but prosobranch molluscs and crustaceans also contribute to the differences. By-catch assemblage structure is more closely related to fishing effort than any other environmental parameter investigated, including depth and sediment type. We observed an approximately linear decrease in diversity with increasing fishing disturbance, and suggest this is primarily due to selective removal of sensitive species and, more importantly, habitat homogenisation. These results were interpreted in the light of ecological theories relating disturbance to community structure. The argument that invertebrate scavenger populations benefit from prolonged exposure to fishing disturbance was also examined, but no supporting evidence was found. Received: 29 January 2000 / Accepted: 31 May 2000     

Coexistence of Fisheries with River Dolphin Conservation

Abstract: Freshwater biodiversity conservation is generally perceived to conflict with human use and extraction (e.g., fisheries). Overexploited fisheries upset the balance between local economic needs and endangered species’ conservation. We investigated resource competition between fisheries and Ganges river dolphins (Platanista gangetica gangetica) in a human‐dominated river system in India to assess the potential for their coexistence. We surveyed a 65‐km stretch of the lower Ganga River to assess habitat use by dolphins (encounter rates) and fishing activity (habitat preferences of fishers, intensity of net and boat use). Dolphin abundance in the main channel increased from 179 (SE 7) (mid dry season) to 270 (SE 8) (peak dry season), probably as a result of immigration from upstream tributaries. Dolphins preferred river channels with muddy, rocky substrates, and deep midchannel waters. These areas overlapped considerably with fishing areas. Sites with 2–6 boats/km (moderately fished) were more preferred by dolphins than sites with 8–55 boats/km (heavily fished). Estimated spatial (85%) and prey–resource overlap (75%) between fisheries and dolphins (chiefly predators of small fish) suggests a high level of competition between the two groups. A decrease in abundance of larger fish, indicated by the fact that small fish comprised 74% of the total caught, may have intensified the present competition. Dolphins seem resilient to changes in fish community structure and may persist in overfished rivers. Regulated fishing in dolphin hotspots and maintenance of adequate dry season flows can sustain dolphins in tributaries and reduce competition in the main river. Fish‐stock restoration and management, effective monitoring, curbing destructive fishing practices, secure tenure rights, and provision of alternative livelihoods for fishers may help reconcile conservation and local needs in overexploited river systems.     

Consumer-resource body-size relationships in natural food webs

It has been suggested that differences in body size between consumer and resource species may have important implications for interaction strengths, population dynamics, and eventually food web structure, function, and evolution. Still, the general distribution of consumer-'resource body-size ratios in real ecosystems, and whether they vary systematically among habitats or broad taxonomic groups, is poorly understood. Using a unique global database on consumer and resource body sizes, we show that the mean body-size ratios of aquatic herbivorous and detritivorous consumers are several orders of magnitude larger than those of carnivorous predators. Carnivorous predator-prey body-size ratios vary across different habitats and predator and prey types (invertebrates, ectotherm, and endotherm vertebrates). Predator-prey body-size ratios are on average significantly higher (1) in freshwater habitats than in marine or terrestrial habitats, (2) for vertebrate than for invertebrate predators, and (3) for invertebrate than for ectotherm vertebrate prey. If recent studies that relate body-size ratios to interaction strengths are general, our results suggest that mean consumer-resource interaction strengths may vary systematically across different habitat categories and consumer types.