Are there real differences among aquatic and terrestrial food webs?

Fundamental constraints on vertebrate life history are shaped by aquatic-terrestrial transitions and reproductive mode

Vertebrate life histories evolve in response to selection imposed by abiotic and biotic environmental conditions while being limited by genetic, developmental, physiological, demographic and phylogenetic processes that constrain adaptation. Despite the well-recognized shifts in selective pressures accompanying transitions among environments, the conditions driving innovation and the consequences for life-history evolution remain outstanding questions. Here we compare the traits of vertebrates that occupy aquatic or terrestrial environments as juveniles to infer shifts in evolutionary constraints that explain differences in their life-history traits and thus their fundamental demographic rates. Our results emphasize the reduced potential for life-history diversification on land, especially that of reproductive strategies, which limits the scope of viable life-history strategies. Moreover, our study reveals differences between the evolution of viviparity in aquatic and terrestrial realms. Transitions from egg laying to live birth represent a major shift across life-history space for aquatic organisms, whereas terrestrial egg-laying organisms evolve live birth without drastic changes in life-history strategy. Whilst trade-offs in the allocation of resources place fundamental constraints on the way life histories can vary, ecological setting influences the position of species within the viable phenotypic space available for adaptive evolution.

Multi-cascade physiologically based kinetic (PBK) matrix model: Simulating chemical bioaccumulation across food webs

In modern ecosystems, many substances that biomagnify within food webs adversely affect organisms and ecological systems. To facilitate high-throughput screening of contemporary chemicals, we developed a multi-cascade physiologically-based kinetic (PBK) matrix model to simulate bioaccumulation and biomagnification along food chains. The model's validity is supported by the mean model bias (MB) values, which fall within the acceptable range when compared with measured biomagnification factors (BMFs) reported in the literature. Analyzing 3,074 organic chemicals, we estimated their biotransfer factors (BTFs)-the steady-state ratio of chemical concentrations in primary consumer tissues to those in their feed-and their BMFs-the steady-state ratio of chemical concentrations in predators to their prey. Our results reveal consistent BTF trends across different tissues within the same species for a given chemical. Chemicals with moderate lipophilicity and low diffusivity tend to be retained longer in organisms, leading to higher BMFs. Notably, mammals appear particularly sensitive to persistent organic pollutants, while birds and ectothermic species do not exhibit clear patterns. Overall, the study highlights that animal physiological parameters and chemical physicochemical properties are more critical in determining bioaccumulation and potential toxicity than an organism's trophic position. Future research should refine key physiological parameters, such as hepatic metabolic rate constants, account for life-stage variations, and evaluate multiple exposure pathways to further enhance model accuracy and real-world applicability.

How often are ecosystems top-down controlled? Experiments in grassland, grasshopper, and bird systems over time and space

Ecosystems are frequently considered to be controlled by predation (top-down). Experiments examined this in four bird/spider/grasshopper/prairie habitats over 34 years, employing in each habitat three 100 m2 bird exclosures and controls (121 habitat/year cases) where plant, grasshopper, and spider abundances were measured. Top-down control (plants decrease and grasshoppers increase with bird exclusion) was observed in only 13.2% of cases, while plants increased and grasshoppers decreased in 33.1% of cases, plants decreased and grasshoppers decreased in 25.6% of cases, and plants increased and grasshoppers increased in 28.1% of cases. Therefore, top-down control was not common and system responses were not constant, but varied among sites, years, and directionally over time with climate change. This diversity of responses is expected given the variety of underlying processes in complex ecosystems. For example, decision tree/discriminant analysis found that plant decreases and increases with bird exclusion were correctly identified in 78.3% of cases by grasshopper hatchling abundance, plant cover, and annual net primary production (ANPP), while grasshopper decreases and increases with bird exclusion were correctly identified in 76.7% of cases by edible plant biomass per grasshopper hatchling, grasshopper hatchling abundance, and large grasshopper abundance. Analysis of other system-wide terrestrial trophic experiments indicates that the variety of responses observed by us over time and space may be common so that system-wide trophic responses may, in general, be more variable than either top-down or bottom-up as often considered.

Emergent competition shapes top-down versus bottom-up control in multi-trophic ecosystems

Ecosystems are commonly organized into trophic levels-organisms that occupy the same level in a food chain (e.g., plants, herbivores, carnivores). A fundamental question in theoretical ecology is how the interplay between trophic structure, diversity, and competition shapes the properties of ecosystems. To address this problem, we analyze a generalized Consumer Resource Model with three trophic levels using the zero-temperature cavity method and numerical simulations. We derive the corresponding mean-field cavity equations and show that intra-trophic diversity gives rise to an effective "emergent competition" term between species within a trophic level due to feedbacks mediated by other trophic levels. This emergent competition gives rise to a crossover from a regime of top-down control (populations are limited by predators) to a regime of bottom-up control (populations are limited by primary producers) and is captured by a simple order parameter related to the ratio of surviving species in different trophic levels. We show that our theoretical results agree with empirical observations, suggesting that the theoretical approach outlined here can be used to understand complex ecosystems with multiple trophic levels.

Differences in the internal PFAS patterns of herbivores, omnivores and carnivores - lessons learned from target screening and the total oxidizable precursor assay

Per- and polyfluorinated alkyl substances (PFAS) are a group of anthropogenic chemicals, which are not (fully) biodegradable and accumulate in different environmental compartments worldwide. A comprehensive, quantitative analysis - consisting of target analysis (66 different analytes, including e. g. ultrashort-chain perfluorinated carboxylic acids (PFCAs), precursor compounds and novel substitutes) and the Total Oxidisable Precursor (TOP) assay (including trifluoroacetic acid (TFA)) - were conducted to analyse the PFAS concentrations and patterns in 12 mammalian and two bird species from different areas of Germany and Denmark. The PFAS contamination was investigated in dependance of the trophic class (herbivores, omnivores, carnivores), ecological habitat (terrestrial, (semi-) aquatic) and body tissue (liver, musculature). PFAS concentrations were highest in carnivores, followed by omnivores and herbivores, with ∑PFAS concentration ranging from 1274 μg/kg (Eurasian otter liver) to 22 μg/kg (roe deer liver). TFA dominated in the herbivorous species, whereas perfluorooctanesulfonic acid (PFOS) and the long-chain PFCAs covered the majority of the PFAS contamination in carnivorous species. Besides trophic class, ecological habitat also affected the PFAS levels in the different species, with terrestrial herbivores and omnivores showing higher PFAS concentration than their aquatic counterparts, whereas for carnivores this relationship was reversed. The TOP assay analysis indicated similar trends, with the PFCA formation pattern differing significantly between the trophic classes. TFA was formed predominantly in herbivorous and omnivorous species, whereas in carnivorous species a broad spectrum of PFCAs (chain-length C2-C14) was formed. Musculature tissue of six species exhibited significantly lower PFAS concentrations than the respective liver tissue, but with similar PFAS patterns. The comprehensive approach applied in the present study showed, that primarily the trophic class is decisive for the PFAS concentration, as herbivores, omnivores and carnivores clearly differed in their PFAS concentrations and patterns. Additionally, the TOP assay gave novel insights in the PFCA formation potential in biota samples.

From headwaters to outlets: Bird species accrual curves are faster downstream with different implications for varying landcovers and ecoregions

Terrestrial and aquatic systems are geographically connected, yet these systems are typically studied independently of each other. This approach omits a large amount of ecological information as landscapes are best described as mosaics in watersheds. Species Accumulation Curves (SACs) that incorporate sampling effort are familiar models of how biodiversity will change when landcovers are lost. In land-based systems, the consistent pattern of increased species richness with increasing number of sites sampled is an ecological norm. In freshwater systems, fish species discharge relationships are analogous to species-area relationships in terrestrial systems, but the relationship between terrestrial species and discharge remains largely unexplored. Although some studies investigate the effect of terrestrial systems on neighboring aquatic species, less work has been done on exploring the effect of aquatic systems on terrestrial species. Additionally, creating statistical models to observe these interactions need to be explored further. Using data from the Ontario Breeding Bird Atlas (2001–2005), we created bird SACs to explore how increases in diversity with sites sampled varies with watershed position on the Canadian side of the Great Lakes Basin (GLB). The mosaic landscape of the GLB was characterized using six majority land cover classes at a 15 m resolution. This work shows that rates of species accrual and potential maximum species richness vary as a function of watershed position, underlying land cover, and the Ecoregion in which sampling was performed. We also found that Urban landcover has the potential to retain relatively high levels of species richness, which is further modified by Ecoregion and watershed position. Through our ‘world building,’ we believe that we can increase knowledge around the importance of land-water interactions and further the goals of viewing landscapes as mosaic watersheds.

Ecosystem specific accumulation of organohalogenated compounds: A comparison between adjacent freshwater and terrestrial avian predators

Insight into processes determining the exposure of organohalogenated contaminants (OHCs) in wildlife might be gained from comparing predators in different ecosystems. This study compared two avian predator species with similar food chain lengths: the goldeneye duck (Bucephala clangula) and the tawny owl (Strix aluco) breeding in adjacent freshwater- and terrestrial ecosystems in central Norway. We measured lipophilic organochlorines (OCs) and protein-bound perfluorinated substances (PFASs) in eggs of the two species over 21 years (1999-2019). Across years, the proportional distribution of OCs (∼90% of the ΣOHC load) relative to PFASs (∼10%) was similar in the two species. Moreover, ΣOC concentrations were similar between the species, but PFAS compounds were 2-12 times higher in the goldeneyes than in tawny owls. OC-pesticides dominated in tawny owls (∼60% of ΣOC), whereas persistent polychlorinated biphenyl (PCBs) congeners were the main OC components in goldeneyes (∼70% of ΣOC). The lipid-normalized concentrations of most OC-pesticides and the less persistent PCB101 declined significantly in both species. Hexachlorobenzene (HCB), p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE), and more persistent PCBs decreased in tawny owls, while they tended to increase in goldeneyes. The increase in HCB was particulary robust. Among the PFASs, contrasted temporal trends were found across the species for four out of 11 compounds: PFOS declined while most perfluorocarboxylic acids (PFCAs) increased in tawny owls. In contrast, most PFASs were stable in goldeneyes. Moreover, there was no annual covariance between the OHC exposure in the two species: i.e., high concentrations in one species in a given year did not translate into high concentrations in the other. Hence, the two avian predators in adjacent ecosystems seem to be subject to different processes determining the OHC exposure, probably related to variation in diet and climate, long-range transport of different contaminants, and emissions of pollution locally.

Release and transformation of phosphorus in sediment following seasonal freezing-thawing cycles

River ice in the upper Yarlung Zangbo River is characterized by seasonal freezing-thawing cycles (SFTC). It is important to explore the effects of SFTC on phosphorus release and transformation from upstream surface sediments to protect the ecosystem of the Yarlung Zangbo River. The process and mechanism of phosphorus release and transformation in sediments following SFTC were investigated in a laboratory simulation experiment. The results showed that after freezing, sediment particles were broken, the specific surface area was increased by 14%, and the particle size was decreased by 43%, which resulted in weakened adsorption of phosphorus by sediments. Moreover, the destruction of organic matter (OM) on the sediment surface will release more ion adsorption sites and promote the release of phosphorus. The bioavailabilities of exchangeable phosphorus (Ex-P), aluminum phosphorus (Al-P) and iron phosphorus (Fe-P) increased by 60.09%, 86.86% and 31.86%, respectively, after freezing. Organic phosphorus (O-P) is used indirectly by organisms, and O-P content showed a significant correlation with OM content. Water affected the oxygen content in sediments during the freezing period, and continuous hypoxia promoted the release and transformation of Fe-P and Al-P.

Species compositions mediate biomass conservation: the case of lake fish communities

Environmental and geographical factors are known to influence the number, distribution and combination of species that coexist within ecological communities. This, in turn, should influence ecosystem functions such as biomass conservation, or the ability of a community to sustain biomass from small to large organisms. We tested this hypothesis by assessing the role of environmental factors in determining how biomass is conserved in over 600 limnetic fish communities spread across a broad geographic gradient in Canada. Comprehensive and accurate information on water conditions and community characteristics such as taxonomy, abundance, biomass and size distributions were used in our assessment. Results showed that species combinations emerge as one of the main predictors of biomass conservation among the effects of individual species and abiotic factors. Our study highlights the strong role that geographic patterns in the distribution of species can play in shaping key ecosystem functions, with consequences for ecosystem services such as the provision of harvestable fish biomass.

Fish size spectra are affected by nutrient concentration and relative abundance of non-native species across streams of the NE Iberian Peninsula

Aquatic ecosystems are strongly body-size structured with a decline of numerical abundance with increasing body size (hereafter, the size spectrum). Marine and lake fish studies have reported consistent variations of size spectra in relation to environmental conditions and biotic composition, but little is known about stream fishes. Accordingly, in this study we test several hypotheses about the effects of local water conditions, biotic introductions and cumulative pressures (measured as the IMPRESS index) on the fish size-spectrum slope (that is, the linear rate of decline of fish abundance as body size increase in a log-log scale) and the size-spectrum intercept (commonly used as proxy for carrying capacity) among 118 local fish assemblages in streams of the NE Iberian Peninsula. To our knowledge, this is the first time that an extensive river fish dataset is used in a dendritic network to cover systematic changes of size-spectrum parameters. We find that the slope and intercept of the fish size spectrum are negatively correlated with nutrient concentration (mainly total phosphorus), with a greater relative abundance of small fishes but a decline of overall carrying capacity. Moreover, fish assemblages with greater relative abundance of non-native species have flatter size-spectrum slopes. In contrast, the IMPRESS index and climate-related variables are poor predictors of the shape of the fish size spectra. This study contributes to better understanding of the main factors structuring fish assemblages in lotic environments of the Iberian Peninsula. We encourage more research on this line to further explore the use of fish size structure to evaluate the ecological health of riverine ecosystems.

A case for associational resistance: Apparent support for the stress gradient hypothesis varies with study system

According to the stress gradient hypothesis (SGH), ecological interactions between organisms shift positively as environmental stress increases. In the case of associational resistance, habitat is modified to ameliorate stress, benefitting other organisms. The SGH is contentious due to conflicting evidence and theoretical perspectives, so we adopted a meta-analytic approach to determine if it is widely supported across a variety of contexts, including different kingdoms, ecosystems, habitats, interactions, stressors, and life history stages. We developed an extensive list of Boolean search criteria to search the published ecological literature and successfully detect studies that both directly tested the hypothesis, and those that were relevant but never mentioned it. We found that the SGH is well supported by studies that feature bacteria, plants, terrestrial ecosystems, interspecific negative interactions, adults, survival instead of growth or reproduction, and drought, fire, and nutrient stress. We conclude that the SGH is indeed a broadly relevant ecological hypothesis that is currently held back by cross-disciplinary communication barriers. More SGH research is needed beyond the scope of interspecific plant competition, and more SGH research should feature multifactor stress. There remains a need to account for positive interactions in scientific pursuits, such as associational resistance in tests of the SGH.

Acoustic and visual stimuli combined promote stronger responses to aerial predation in fish

Bird predation poses a strong selection pressure on fish. Since birds must enter the water to catch fish, a combination of visual and mechano-acoustic cues (multimodal) characterize an immediate attack, while single cues (unimodal) may represent less dangerous disturbances. We investigated whether fish could use this information to distinguish between non-threatening and dangerous events and adjust their antipredator response to the perceived level of risk. To do so, we investigated the antipredator behavior of the sulphur molly (Poecilia sulphuraria), a small freshwater fish which is almost exclusively preyed on by piscivorous birds in its endemic sulfide spring habitat. In a field survey, we confirmed that these fish frequently have to distinguish between disturbances stemming from attacking birds (multimodal) and those which pose no (immediate) threat such as bird overflights (unimodal). In a laboratory experiment, we then exposed fish to artificial visual and/or acoustic stimuli presented separately or combined. Sensitivity was high regardless of stimulus type and number (more than 96% of fish initiated diving), but fish dove deeper, faster, and for longer when both stimuli were available simultaneously. Based on the system's high rates of bird activity, we argue that such an unselective dive initiation with subsequent fine-tuning of diving parameters in accordance to cue modality represents an optimal strategy for these fish to save energy necessary to respond to future attacks. Ultimately, our study shows that fish anticipate the imminent risk posed by disturbances linked to bird predation through integrating information from both visual and acoustic cues.

Similarities, differences and mechanisms of climate impact on terrestrial vs. marine ecosystems

Comparisons between terrestrial and marine ecosystems are generally not in the main stream of scientific literature even though Webb (2012) listed several points for which the transfer of knowledge and concepts related to one or to the other system would benefit our understanding of both. Even sharing this view, the leading hypothesis behind this contribution is that the pelagic system, where the dominant biotic component by number and biomass is microscopic, has specific features which strongly differentiate it from the above-the-surface terrestrial systems. Due to this, climate change, i.e. changes in temperature, precipitation and most importantly in the dynamics of the two fluid media, atmosphere and ocean, act with different mechanisms which prevents proceeding with analogies in many cases. In addition, the non-linearity of most of the processes and responses to perturbations requires, in order to obtain reliable forecasts or hindcasts, a detailed analysis of the path followed by the system which is normally overlooked in the step-change simulations or projections.

A small native predator reduces reproductive success of a large invasive fish as revealed by whole-lake experiments

The extent to which native fish communities might control the success of invasive fish has been of interest to ecologists, but it has been rarely addressed using experiments. We conducted an experiment in six small lakes in the Upper Mississippi Region to test the effects of a small native predator, bluegill sunfish (Lepomis macrochirus) on the recruitment of a large, invasive fish, the common carp (Cyprinus carpio). Bluegills are predominant throughout the region and were previously shown to consume carp eggs and larvae. We stocked both lakes at each of our 3 sites with adult carp (spawners) and one lake at each site with bluegills. We repeated the experiment at two of the three sites for two consecutive years. In each lake we assessed the abundance of post-larval carp one month after spawning (backpack electrofishing surveys) and at the end of the season (mark-recapture). For each site/year combination, catch rate of post-larval carp was typically an order of magnitude higher in control than bluegill lakes, but it often declined quickly over time. The abundance of end-of-seasonal juveniles was significantly higher (no 95% CI overlap) in control lakes than in bluegill lakes, except for one pair of lakes during one year when both the control and bluegill lake had similar, low abundance of end-of-season carp. Overall, our results support the hypothesis that common carp recruitment is substantially reduced in habitats dominated by bluegills. We also suggest our results may be applicable to other species, and that managers should explore how predation on early life stages may control other invasive species.

Pyramids and cascades: a synthesis of food chain functioning and stability

Food chain theory is one of the cornerstones of ecology, providing many of its basic predictions, such as biomass pyramids, trophic cascades and predator-prey oscillations. Yet, ninety years into this theory, the conditions under which these patterns may occur and persist in nature remain subject to debate. Rather than address each pattern in isolation, we propose that they must be understood together, calling for synthesis in a fragmented landscape of theoretical and empirical results. As a first step, we propose a minimal theory that combines the long-standing energetic and dynamical approaches of food chains. We chart theoretical predictions on a concise map, where two main regimes emerge: across various functioning and stability metrics, one regime is characterised by pyramidal patterns and the other by cascade patterns. The axes of this map combine key physiological and ecological variables, such as metabolic rates and self-regulation. A quantitative comparison with data sheds light on conflicting theoretical predictions and empirical puzzles, from size spectra to causes of trophic cascade strength. We conclude that drawing systematic connections between various existing approaches to food chains, and between their predictions on functioning and stability, is a crucial step in confronting this theory to real ecosystems.

Early life-history predator-prey reversal in two cyprinid fishes

Predator-prey relationships are often perceived simply as a situation in which a predator enhances its own fitness while reducing the fitness of its prey. However, this relationship may become reversed when the prey feeds on the juvenile predator stages. We investigated this phenomenon in a model asp (Leuciscus aspius; predator)-bleak (Alburnus alburnus; prey) relationship. The adhesive asp eggs are available for bleak predation after a spawning event for only tens of seconds before they adhere to the stones, where bleak do not forage. Gut content analysis demonstrated that eggs were utilized in high quantities, especially in the spawning peak of the asp reproductive season. Furthermore, using underwater video, we recorded the bleak feeding efficiency on naturally drifting asp eggs as the percentage of eggs eaten. Within the 40 cm egg trajectory captured by our cameras, total egg mortality was 21.2 ± 2.2% on average. The highest survival chances occurred among eggs drifting in aggregations, since the short drifting distance together with their aggregated distribution satiated bleak and part of the egg aggregation could attach to the spawning ground. This study emphasizes the potential efficiency of predator egg utilization by prey, which may have further consequences in predator-prey dynamics.

Global meta-analysis of native and nonindigenous trophic traits in aquatic ecosystems

Ecologists have recently devoted their attention to the study of species traits and their role in the establishment and spread of nonindigenous species (NIS). However, research efforts have mostly focused on studies of terrestrial taxa, with lesser attention being dedicated to aquatic species. Aquatic habitats comprise of interconnected waterways, as well as exclusive introduction vectors that allow unparalleled artificial transport of species and their propagules. Consequently, species traits that commonly facilitate biological invasions in terrestrial systems may not be as represented in aquatic environments. We provide a global meta-analysis of studies conducted in both marine and freshwater habitats. We selected studies that conducted experiments with native and NIS under common environmental conditions to allow detailed comparisons among species traits. In addition, we explored whether different factors such as species relatedness, functional feeding groups, latitude, climate, and experimental conditions could be linked to predictive traits. Our results show that species with traits that enhance consumption and growth have a substantially increased probability of establishing and spreading when entering novel ecosystems. Moreover, traits associated with predatory avoidance were more prevalent in NIS and therefore favour invasive species in aquatic habitats. When we analysed NIS interacting with taxonomically distinctive native taxa, we found that consumption and growth were particularly important traits. This suggests that particular attention should be paid to newly introduced species for which there are no close relatives in the local biota. Finally, we found a bias towards studies conducted in temperate regions, and thus, more studies in other climatic regions are needed. We conclude that studies aiming at predicting future range shifts should consider trophic traits of aquatic NIS as these traits are indicative of multiple interacting mechanisms involved in promoting species invasions.

Relating trophic resources to community structure: a predictive index of food availability

The abundance and the distribution of trophic resources available for consumers influence the productivity and the diversity of natural communities. Nevertheless, assessment of the actual abundance of food items available for individual trophic groups has been constrained by differences in methods and metrics used by various authors. Here we develop an index of food abundance, the framework of which can be adapted for different ecosystems. The relative available food index (RAFI) is computed by considering standard resource conditions of a habitat and the influence of various generalized anthropogenic and natural factors. RAFI was developed using published literature on food abundance and validated by comparison of predictions versus observed trophic resources across various marine sites. RAFI tables here proposed can be applied to a range of marine ecosystems for predictions of the potential abundance of food available for each trophic group, hence permitting exploration of ecological theories by focusing on the deviation from the observed to the expected.

Comparison of PFASs contamination in the freshwater and terrestrial environments by analysis of eggs from osprey (Pandion haliaetus), tawny owl (Strix aluco), and common kestrel (Falco tinnunculus)

The level of PFAS (per- and polyfluorinated alkyl substances) contamination in freshwater and terrestrial Swedish environments in 2013/2014 was assessed by analyzing a range of perfluorinated alkyl acids, fluorotelomer acids, sulfonamides, sulfonamidoethanols and polyfluoralkyl phosphate diesters (diPAPs) in predator bird eggs. Stable isotopes ((13)C and (15)N) were analyzed to elucidate the dietary source. The tawny owl (Strix aluco, n=10) and common kestrel (Falco tinnunculus, n=40), two terrestrial species, and the osprey (Pandion haliaetus, n=30), a freshwater specie were included. In addition, a temporal trend (1997-2001, 2008-2009, 2013) in osprey was studied as well. The PFAS profile was dominated by perfluorooctane sulfonic acid (PFOS) in eggs from osprey and tawny owl, while for common kestrel perfluorinated carboxylic acids (∑PFCA) exceeded the level of PFOS. PFOS concentration in osprey eggs remained at the same level between 1997 and 2001 and 2013. For the long-chained PFCAs, there were a significant increase in concentrations in osprey eggs between 1997 and 2001 and 2008-2009. The levels of PFOS and PFCAs were about 10 and five times higher, respectively, in osprey compared to tawny owl and common kestrel. Evidence of direct exposure from PFCA precursor compounds to birds in both freshwater and terrestrial environment was observed. Low levels of diPAPs were detected in a few samples of osprey (<0.02-2.4ng/g) and common kestrel (<0.02-0.16ng/g) eggs, and 6:2 FTSA was detected in a majority of the osprey eggs (<6.3-52ng/g). One saturated telomer acid (7:3 FTCA), which is a transformation marker from precursor exposure, was detected in all species (<0.24-2.7ng/g). The (15)N data showed higher levels in osprey eggs compared to tawny owl and common kestrel, indicating that they feed on a 2-3 times higher trophic level. We conclude that ospreys are continuously exposed to PFAS at levels where adverse toxic effects have been observed in birds.

Herbivory and Stoichiometric Feedbacks to Primary Production

Established theory addresses the idea that herbivory can have positive feedbacks on nutrient flow to plants. Positive feedbacks likely emerge from a greater availability of organic carbon that primes the soil by supporting nutrient turnover through consumer and especially microbially-mediated metabolism in the detrital pool. We developed an entirely novel stoichiometric model that demonstrates the mechanism of a positive feedback. In particular, we show that sloppy or partial feeding by herbivores increases detrital carbon and nitrogen allowing for greater nitrogen mineralization and nutritive feedback to plants. The model consists of differential equations coupling flows among pools of: plants, herbivores, detrital carbon and nitrogen, and inorganic nitrogen. We test the effects of different levels of herbivore grazing completion and of the stoichiometric quality (carbon to nitrogen ratio, C:N) of the host plant. Our model analyses show that partial feeding and plant C:N interact because when herbivores are sloppy and plant biomass is diverted to the detrital pool, more mineral nitrogen is available to plants because of the stoichiometric difference between the organisms in the detrital pool and the herbivore. This model helps to identify how herbivory may feedback positively on primary production, and it mechanistically connects direct and indirect feedbacks from soil to plant production.

The body size dependence of trophic cascades

Trophic cascades are indirect positive effects of predators on resources via control of intermediate consumers. Larger-bodied predators appear to induce stronger trophic cascades (a greater rebound of resource density toward carrying capacity), but how this happens is unknown because we lack a clear depiction of how the strength of trophic cascades is determined. Using consumer resource models, we first show that the strength of a trophic cascade has an upper limit set by the interaction strength between the basal trophic group and its consumer and that this limit is approached as the interaction strength between the consumer and its predator increases. We then express the strength of a trophic cascade explicitly in terms of predator body size and use two independent parameter sets to calculate how the strength of a trophic cascade depends on predator size. Both parameter sets predict a positive effect of predator size on the strength of a trophic cascade, driven mostly by the body size dependence of the interaction strength between the first two trophic levels. Our results support previous empirical findings and suggest that the loss of larger predators will have greater consequences on trophic control and biomass structure in food webs than the loss of smaller predators.

Widespread and persistent invasions of terrestrial habitats coincident with larval feeding behavior transitions during snail-killing fly evolution (Diptera: Sciomyzidae)

BACKGROUND

Transitions in habitats and feeding behaviors were fundamental to the diversification of life on Earth. There is ongoing debate regarding the typical directionality of transitions between aquatic and terrestrial habitats and the mechanisms responsible for the preponderance of terrestrial to aquatic transitions. Snail-killing flies (Diptera: Sciomyzidae) represent an excellent model system to study such transitions because their larvae display a range of feeding behaviors, being predators, parasitoids or saprophages of a variety of mollusks in freshwater, shoreline and dry terrestrial habitats. The remarkable genus Tetanocera (Tetanocerini) occupies five larval feeding groups and all of the habitat types mentioned above. This study has four principal objectives: (i) construct a robust estimate of phylogeny for Tetanocera and Tetanocerini, (ii) estimate the evolutionary transitions in larval feeding behaviors and habitats, (iii) test the monophyly of feeding groups and (iv) identify mechanisms underlying sciomyzid habitat and feeding behavior evolution.

RESULTS

Bayesian inference and maximum likelihood analyses of molecular data provided strong support that the Sciomyzini, Tetanocerini and Tetanocera are monophyletic. However, the monophyly of many behavioral groupings was rejected via phylogenetic constraint analyses. We determined that (i) the ancestral sciomyzid lineage was terrestrial, (ii) there was a single terrestrial to aquatic habitat transition early in the evolution of the Tetanocerini and (iii) there were at least 10 independent aquatic to terrestrial habitat transitions and at least 15 feeding behavior transitions during tetanocerine phylogenesis. The ancestor of Tetanocera was aquatic with five lineages making independent transitions to terrestrial habitats and seven making independent transitions in feeding behaviors.

CONCLUSIONS

The preponderance of aquatic to terrestrial transitions in sciomyzids goes against the trend generally observed across eukaryotes. Damp shoreline habitats are likely transitional where larvae can change habitat but still have similar prey available. Transitioning from aquatic to terrestrial habitats is likely easier than the reverse for sciomyzids because morphological characters associated with air-breathing while under the water's surface are lost rather than gained, and sciomyzids originated and diversified during a general drying period in Earth's history. Our results imply that any animal lineage having aquatic and terrestrial members, respiring the same way in both habitats and having the same type of food available in both habitats could show a similar pattern of multiple independent habitat transitions coincident with changes in behavioral and morphological traits.

Dimensionality of consumer search space drives trophic interaction strengths

Trophic interactions govern biomass fluxes in ecosystems, and stability in food webs. Knowledge of how trophic interaction strengths are affected by differences among habitats is crucial for understanding variation in ecological systems. Here we show how substantial variation in consumption-rate data, and hence trophic interaction strengths, arises because consumers tend to encounter resources more frequently in three dimensions (3D) (for example, arboreal and pelagic zones) than two dimensions (2D) (for example, terrestrial and benthic zones). By combining new theory with extensive data (376 species, with body masses ranging from 5.24 × 10(-14) kg to 800 kg), we find that consumption rates scale sublinearly with consumer body mass (exponent of approximately 0.85) for 2D interactions, but superlinearly (exponent of approximately 1.06) for 3D interactions. These results contradict the currently widespread assumption of a single exponent (of approximately 0.75) in consumer-resource and food-web research. Further analysis of 2,929 consumer-resource interactions shows that dimensionality of consumer search space is probably a major driver of species coexistence, and the stability and abundance of populations.

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.

Cross-ecosystem differences in stability and the principle of energy flux

Here, we review consumer-resource (C-R) theory to show that the paradox of enrichment is a special case of a more general theoretical result. That is, we show that increased energy flux, relative to the consumer loss rate, makes C-R interactions top heavy (i.e., greater C:R biomass ratio) and less stable. We then review the literature on the attributes of aquatic and terrestrial ecosystems to argue that empirical estimates of parameters governing energy flux find that aquatic ecosystems have higher rates of relative energy flux than terrestrial ecosystems. Consistent with theory, we then review empirical work that shows aquatic ecosystems have greater herbivore:plant biomass ratios while we produce novel data to show that aquatic ecosystems have greater variability in population dynamics than their terrestrial counterparts. We end by arguing that theory, allometric relationships and a significant, negative correlation between body size and population variability suggest that these results may be driven by the smaller average body sizes of aquatic organisms relative to terrestrial organisms.

Food web complexity and allometric scaling relationships in stream mesocosms: implications for experimentation

1. Mesocosms are used extensively by ecologists to gain a mechanistic understanding of ecosystems based on the often untested assumption that these systems can replicate the key attributes of natural assemblages. 2. Previous investigations of stream mesocosm utility have explored community composition, but here for the first time, we extend the approach to consider the replicability and realism of food webs in four outdoor channels (4 m(2)). 3. The four food webs were similarly complex, consisting of diverse assemblages (61-71 taxa) with dense feeding interactions (directed connectance 0.09-0.11). Mesocosm food web structural attributes were within the range reported for 82 well-characterized food webs from natural streams and rivers. When compared with 112 additional food webs from standing freshwater, marine, estuarine and terrestrial environments, stream food webs (including mesocosms) had similar characteristic path lengths, but typically lower mean food chain length and exponents for the species-link relationship. 4. Body size (M) abundance (N) allometric scaling coefficients for trivariate taxonomic mesocosm food webs (-0.53 to -0.49) and individual size distributions (-0.60 to -0.58) were consistent and similar to those from natural systems, suggesting that patterns of energy flux between mesocosm consumers and resources were realistic approximations. 5. These results suggest that stream mesocosms of this scale can support replicate food webs with a degree of biocomplexity that is comparable to 'natural' streams. The findings highlight the potential value of mesocosms as model systems for performing experimental manipulations to test ecological theories, at spatiotemporal scales of relevance to natural ecosystems.

The role of body mass in diet contiguity and food-web structure

1. The idea that species occupy distinct niches is a fundamental concept in ecology. Classically, the niche was described as an n-dimensional hypervolume where each dimension represents a biotic or abiotic characteristic. More recently, it has been hypothesised that a single dimension may be sufficient to explain the system-level organization of trophic interactions observed between species in a community. 2. Here, we test the hypothesis that species body mass is that single dimension. Specifically, we determine how the intervality of food webs ordered by body size compares to that of randomly ordered food webs. We also extend this analysis beyond the community level to the effect of body mass in explaining the diets of individual species. 3. We conclude that body mass significantly explains the ordering of species and the contiguity of diets in empirical communities. 4. At the species-specific level, we find that the degree to which body mass is a significant explanatory variable depends strongly on the phylogenetic history, suggesting that other evolutionarily conserved traits partly account for species' roles in the food web. 5. Our investigation of the role of body mass in food webs thus helps us to better understand the important features of community food-web structure and the evolutionary forces that have led us to the communities we observe.

Investigational piscivory of some juvenile Australian freshwater fishes by the introduced Mozambique tilapia Oreochromis mossambicus

Experimental tanks were used to observe predatory effects in three different size classes of Mozambique tilapia Oreochromis mossambicus (one of the world's most widespread exotic species and generally regarded to be a herbivore or both herbivore and detritivore) when tested against 10 juvenile Australian freshwater fish species, and significant levels of predation against all were recorded. There was a general trend for larger O. mossambicus to kill more prey and this was also reflected in a separate series of experiments using juvenile barramundi Lates calcarifer over a range of size classes. Predatory effects by O. mossambicus broadly reflected the accepted models of predator-prey interactions, being that mortality (and survival) was closely related to relative body size and mouth gape limitation. Experimental evidence for piscivory in O. mossambicus was supported by field sampling that detected prey fish remains in 16% of all fish surveyed (n = 176). The recognition of active piscivory by O. mossambicus in laboratory and field situations is the first such evidence, and suggests a need to re-evaluate the nature of their effects in introduced environments.

Ecology and evolution of host-parasite associations: mycophagous Drosophila and their parasitic nematodes

Associations between mycophagous Drosophila and nematode parasites occur throughout the temperate and boreal regions of North America, Europe, and Asia. The nematode Howardula aoronymphium has substantial adverse effects on host survival and fertility on North American Drosophila. Long-term data show that rainy summers lead to a high prevalence of parasitism in the fall and the following spring, resulting in up to a 1-yr time lag between present rainfall and increased prevalence of H. aoronymphium parasitism. A biogeographic analysis of the relative abundance of different Drosophila species has shown that H. aoronymphium may facilitate the coexistence of different species of Drosophila that compete for larval food resources. The actual host range of parasites in nature is determined by the intrinsic suitability of potential hosts for parasite infection and reproduction and various ecological factors. For H. aoronymphium in eastern North America, intrinsically suitable hosts fall within a restricted clade within the genus Drosophila. However, the temperature sensitivity of H. aoronymphium prevents it from using several host species that occur outside the geographical range of the nematodes. Finally, the host range, virulence, and geographical range of Drosophila-parasitic nematodes appear to be highly dynamic over evolutionary timescales.