Facilitating the recovery of insect communities in restored streams by increasing oviposition habitat.

Recruitment limitation is known to influence species abundances and distributions. Recognition of how and why it occurs both in natural and in designed environments could improve restoration. Aquatic insects, for instance, rarely reestablish in restored streams to levels comparable to reference streams even years after restoration. We experimentally increased oviposition habitat in five out of 10 restored streams in western North Carolina to test whether insect egg-laying habitat was limiting insect populations in restored streams. A main goal was to test whether adding oviposition habitat in the form of rocks that partially protrude above the water surface could be used to increase the abundance and richness of stream insect eggs and larval insects in restored streams. Adding egg-laying habitat enhanced several response variables (e.g., protruding rocks, number of eggs, egg masses, egg morphotype richness, and oviposition habitat stability) to levels similar to those found in reference streams. Following the addition of protruding rocks, egg mass abundance increased by 186% and richness by 77% in restored-treated streams. Densities of larval insects that attached their eggs to protruding rocks showed an overall pattern consistent with treatment effects due to the combination of nonsignificant and significant increases of several taxa and not just one taxon. Our results indicate that these stream insect populations are limited by oviposition habitat and that adding egg-laying habitat alleviated this component of recruitment limitation. However, the weaker larval response indicates that additional post-recruitment factors, such as egg or larval mortality, may still be limiting a full recovery of larval insect abundances in these restored streams. This study shows the importance of integrating information from animal life histories, ecology, and geomorphology into restoration practices to improve the recovery of aquatic insects, which are commonly used to assess water quality and the biological efficacy of stream restoration.

Consequences of climate-induced range expansions on multiple ecosystem functions.

Climate-driven species range shifts and expansions are changing community composition, yet the functional consequences in natural systems are mostly unknown. By combining a 30-year survey of subalpine pond larval caddisfly assemblages with species-specific functional traits (nitrogen and phosphorus excretion, and detritus processing rates), we tested how three upslope range expansions affected species' relative contributions to caddisfly-driven nutrient supply and detritus processing. A subdominant resident species (Ag. deflata) consistently made large relative contributions to caddisfly-driven nitrogen supply throughout all range expansions, thus "regulating" the caddisfly-driven nitrogen supply. Whereas, phosphorus supply and detritus processing were regulated by the dominant resident species (L. externus) until the third range expansion (by N. hostilis). Since the third range expansion, N. hostilis's relative contribution to caddisfly-driven phosphorus supply increased, displacing L. externus's role in regulating caddisfly-driven phosphorus supply. Meanwhile, detritus processing contributions became similar among the dominant resident, subdominant residents, and range expanding species. Total ecosystem process rates did not change throughout any of the range expansions. Thus, shifts in species' relative functional roles may occur before shifts in total ecosystem process rates, and changes in species' functional roles may stabilize processes in ecosystems undergoing change.

Preoperative SARS-CoV-2 infection increases risk of early postoperative cardiovascular complications following noncardiac surgery.

As the coronavirus disease 2019 (COVID-19) pandemic progresses to an endemic phase, a greater number of patients with a history of COVID-19 will undergo surgery. Major adverse cardiovascular and cerebrovascular events (MACE) are the primary contributors to postoperative morbidity and mortality; however, studies assessing the relationship between a previous severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and postoperative MACE outcomes are limited. Here, we analyzed retrospective data from 457,804 patients within the N3C Data Enclave, the largest national, multi-institutional data set on COVID-19 in the United States. However, 7.4% of patients had a history of COVID-19 before surgery. When comorbidities, age, race, and risk of surgery were controlled, patients with preoperative COVID-19 had an increased risk for 30-day postoperative MACE. MACE risk was influenced by an interplay between COVID-19 disease severity and time between surgery and infection; in those with mild disease, MACE risk was not increased even among those undergoing surgery within 4 wk following infection. In those with moderate disease, risk for postoperative MACE was mitigated 8 wk after infection, whereas patients with severe disease continued to have elevated postoperative MACE risk even after waiting for 8 wk. Being fully vaccinated decreased the risk for postoperative MACE in both patients with no history of COVID-19 and in those with breakthrough COVID-19 infection. Together, our results suggest that a thorough assessment of the severity, vaccination status, and timing of SARS-CoV-2 infection must be a mandatory part of perioperative stratification.NEW & NOTEWORTHY With an increasing proportion of patients undergoing surgery with a prior history of COVID-19, it is crucial to understand the impact of SARS-CoV-2 infection on postoperative cardiovascular/cerebrovascular risk. Our work assesses a large, national, multi-institutional cohort of patients to highlight that COVID-19 infection increases risk for postoperative major adverse cardiovascular and cerebrovascular events (MACE). MACE risk is influenced by an interplay between disease severity and time between infection and surgery, and full vaccination reduces the risk for 30-day postoperative MACE. These results highlight the importance of stratifying time-to-surgery guidelines based on disease severity.

Species-specific traits predict whole-assemblage detritus processing by pond invertebrates.

Functional trait diversity determines if ecosystem processes are sensitive to shifts in species abundances or composition. For example, trait variation suggests detritivores process detritus at different rates and make different contributions to whole-assemblage processing, which could be sensitive to compositional shifts. Here, we used a series of microcosm experiments to quantify species-specific coarse and fine particulate organic matter (CPOM and FPOM) processing for ten larval caddisfly species and three non-caddisfly species in high-elevation wetlands. We then compared trait-based models including life history, dietary, and extrinsic traits to determine which traits explained interspecific variation in detritus processing. Finally, we compared processing by mixed caddisfly assemblages in microcosms and natural ponds to additive predictions based on species-specific processing to determine if single-species effects are additive in multi-species assemblages. We found considerable interspecific variation in biomass-specific CPOM (13-fold differences) and FPOM (8-fold differences) processing. Furthermore, on a mass-specific basis, amphipods, chironomids, and caddisflies processed similar amounts of detritus, suggesting non-shredder taxa could process more than previously recognized. Trait models including dietary percent detritus, development rate, body size, and wetland hydroperiod explained 81 and 57% of interspecific variation in CPOM and FPOM processing, respectively. Finally, species-specific additive predictions were strikingly similar to mixed-assemblage processing in microcosms and natural ponds, with the largest difference being a 15% overestimate. Thus, additivity of species-specific processing suggests single-species rates may be useful for understanding functional consequences of shifting assemblages, and a trait-based approach to predicting species-specific processing could support generating additive predictions of whole-assemblage processing.

Understanding the evolution of nutritive taste in animals: Insights from biological stoichiometry and nutritional geometry.

A major conceptual gap in taste biology is the lack of a general framework for understanding the evolution of different taste modalities among animal species. We turn to two complementary nutritional frameworks, biological stoichiometry theory and nutritional geometry, to develop hypotheses for the evolution of different taste modalities in animals. We describe how the attractive tastes of Na-, Ca-, P-, N-, and C-containing compounds are consistent with principles of both frameworks based on their shared focus on nutritional imbalances and consumer homeostasis. Specifically, we suggest that the evolution of multiple nutritive taste modalities can be predicted by identifying individual elements that are typically more concentrated in the tissues of animals than plants. Additionally, we discuss how consumer homeostasis can inform our understanding of why some taste compounds (i.e., Na, Ca, and P salts) can be either attractive or aversive depending on concentration. We also discuss how these complementary frameworks can help to explain the evolutionary history of different taste modalities and improve our understanding of the mechanisms that lead to loss of taste capabilities in some animal lineages. The ideas presented here will stimulate research that bridges the fields of evolutionary biology, sensory biology, and ecology.

Dissolved organic carbon modulates mercury concentrations in insect subsidies from streams to terrestrial consumers.

Mercury (Hg) concentrations in aquatic environments have increased globally, exposing consumers of aquatic organisms to high Hg levels. For both aquatic and terrestrial consumers, exposure to Hg depends on their food sources as well as environmental factors influencing Hg bioavailability. The majority of the research on the transfer of methylmercury (MeHg), a toxic and bioaccumulating form of Hg, between aquatic and terrestrial food webs has focused on terrestrial piscivores. However, a gap exists in our understanding of the factors regulating MeHg bioaccumulation by non-piscivorous terrestrial predators, specifically consumers of adult aquatic insects. Because dissolved organic carbon (DOC) binds tightly to MeHg, affecting its transport and availability in aquatic food webs, we hypothesized that DOC affects MeHg transfer from stream food webs to terrestrial predators feeding on emerging adult insects. We tested this hypothesis by collecting data over 2 years from 10 low-order streams spanning a broad DOC gradient in the Lake Sunapee watershed in New Hampshire, USA. We found that streamwater MeHg concentration increased linearly with DOC concentration. However, streams with the highest DOC concentrations had emerging stream prey and spiders with lower MeHg concentrations than streams with intermediate DOC concentrations; a pattern that is similar to fish and larval aquatic insects. Furthermore, high MeHg concentrations found in spiders show that MeHg transfer in adult aquatic insects is an overlooked but potentially significant pathway of MeHg bioaccumulation in terrestrial food webs. Our results suggest that although MeHg in water increases with DOC, MeHg concentrations in stream and terrestrial consumers did not consistently increase with increases in streamwater MeHg concentrations. In fact, there was a change from a positive to a negative relationship between aqueous exposure and bioaccumulation at streamwater MeHg concentrations associated with DOC above ~5 mg/L. Thus, our study highlights the importance of stream DOC for MeHg dynamics beyond stream boundaries, and shows that factors modulating MeHg bioavailability in aquatic systems can affect the transfer of MeHg to terrestrial predators via aquatic subsidies.

The Origin of Invasive Microorganisms Matters for Science, Policy, and Management: The Case of Didymosphenia geminata.

The value of distinguishing native from nonnative invasive species has recently been questioned. However, this dichotomy is important for understanding whether a species' successful dominance is caused by introductions, changing environmental conditions that facilitate an existing population, or both processes. We highlight the importance of knowing the origin of hard-to-detect invasive microorganisms for scientific research, management, and policy using a case study of recent algal blooms of the stalk-producing diatom Didymosphenia geminata. Nuisance blooms have been reported in rivers worldwide and have been hastily attributed to introductions. However, evidence indicates that blooms are probably not caused by introductions but, rather, by environmental conditions that promote excessive stalk production by this historically rare species. Effective responses to invasive microorganisms depend on knowing whether their proliferation is caused by being nonnative or is the result of changing environmental conditions that promote invasive characteristics of native species.

Using economic instruments to develop effective management of invasive species: insights from a bioeconomic model.

Economic growth is recognized as an important factor associated with species invasions. Consequently, there is increasing need to develop solutions that combine economics and ecology to inform invasive species management. We developed a model combining economic, ecological, and sociological factors to assess the degree to which economic policies can be used to control invasive plants. Because invasive plants often spread across numerous properties, we explored whether property owners should manage invaders cooperatively as a group by incorporating the negative effects of invader spread in management decisions (collective management) or independently, whereby the negative effects of invasive plant spread are ignored (independent management). Our modeling approach used a dynamic optimization framework, and we applied the model to invader spread using Linaria vulgaris. Model simulations allowed us to determine the optimal management strategy based on net benefits for a range of invader densities. We found that optimal management strategies varied as a function of initial plant densities. At low densities, net benefits were high for both collective and independent management to eradicate the invader, suggesting the importance of early detection and eradication. At moderate densities, collective management led to faster and more frequent invader eradication compared to independent management. When we used a financial penalty to ensure that independent properties were managed collectively, we found that the penalty would be most feasible when levied on a property's perimeter boundary to control spread among properties. At the highest densities, the optimal management strategy was "do nothing" because the economic costs of removal were too high relative to the benefits of removal. Spatial variation in L. vulgaris densities resulted in different optimal management strategies for neighboring properties, making a formal economic policy to encourage invasive species removal critical. To accomplish the management and enforcement of these economic policies, we discuss modification of existing agencies and infrastructure. Finally, a sensitivity analysis revealed that lowering the economic cost of invader removal would strongly increase the probability of invader eradication. Taken together, our results provide quantitative insight into management decisions and economic policy instruments that can encourage invasive species removal across a social landscape.

Fish distributions and nutrient cycling in streams: can fish create biogeochemical hotspots?

Rates of biogeochemical processes often vary widely in space and time, and characterizing this variation is critical for understanding ecosystem functioning. In streams, spatial hotspots of nutrient transformations are generally attributed to physical and microbial processes. Here we examine the potential for heterogeneous distributions of fish to generate hotspots of nutrient recycling. We measured nitrogen (N) and phosphorus (P) excretion rates of 47 species of fish in an N-limited Neotropical stream, and we combined these data with population densities in each of 49 stream channel units to estimate unit- and reach-scale nutrient recycling. Species varied widely in rates of N and P excretion as well as excreted N:P ratios (6-176 molar). At the reach scale, fish excretion could meet >75% of ecosystem demand for dissolved inorganic N and turn over the ambient NH4 pool in <0.3 km. Areal N excretion estimates varied 47-fold among channel units, suggesting that fish distributions could influence local N availability. P excretion rates varied 14-fold among units but were low relative to ambient concentrations. Spatial variation in aggregate nutrient excretion by fish reflected the effects of habitat characteristics (depth, water velocity) on community structure (body size, density, species composition), and the preference of large-bodied species for deep runs was particularly important. We conclude that the spatial distribution of fish could indeed create hotspots of nutrient recycling during the dry season in this species-rich tropical stream. The prevalence of patchy distributions of stream fish and invertebrates suggests that hotspots of consumer nutrient recycling may often occur in stream ecosystems.

Predator effects on prey population dynamics in open systems.

Animal population dynamics in open systems are affected not only by agents of mortality and the influence of species interactions on behavior and life histories, but also by dispersal and recruitment. We used an extensive data set to compare natural loss rates of two mayfly species that co-occur in high-elevation streams varying in predation risk, and experience different abiotic conditions during larval development. Our goals were to generate hypotheses relating predation to variation in prey population dynamics and to evaluate alternative mechanisms to explain such variation. While neither loss rates nor abundance of the species that develops during snowmelt (Baetis bicaudatus) varied systematically with fish, loss rates of the species that develops during baseflow (Baetis B) were higher in streams containing brook trout than streams without fish; and surprisingly, larvae of this species were most abundant in trout streams. This counter-intuitive pattern could not be explained by a trophic cascade, because densities of intermediate predators (stoneflies) did not differ between fish and fishless streams and predation by trout on stoneflies was negligible. A statistical model estimated that higher recruitment and accelerated development enables Baetis B to maintain larger populations in trout streams despite higher mortality from predation. Experimental estimates suggested that predation by trout potentially accounts for natural losses of Baetis B, but not Baetis bicaudatus. Predation by stoneflies on Baetis is negligible in fish streams, but could make an important contribution to observed losses of both species in fishless streams. Non-predatory sources of loss were higher for B. bicaudatus in trout streams, and for Baetis B in fishless streams. We conclude that predation alone cannot explain variation in population dynamics of either species; and the relative importance of predation is species- and environment-specific compared to non-predatory losses, such as other agents of mortality and non-consumptive effects of predators.

Loss of a harvested fish species disrupts carbon flow in a diverse tropical river.

Harvesting threatens many vertebrate species, yet few whole-system manipulations have been conducted to predict the consequences of vertebrate losses on ecosystem function. Here, we show that a harvested migratory detrital-feeding fish (Prochilodontidae: Prochilodus mariae) modulates carbon flow and ecosystem metabolism. Natural declines in and experimental removal of Prochilodus decreased downstream transport of organic carbon and increased primary production and respiration. Thus, besides its economic value, Prochilodus is a critical ecological component of South American rivers. Lack of functional redundancy for this species highlights the importance of individual species and, contrary to theory, suggests that losing one species from lower trophic levels can affect ecosystem functioning even in species-rich ecosystems.

Linking economic activities to the distribution of exotic plants.

The human enterprise is flooding Earth's ecosystems with exotic species. Human population size is often correlated with species introductions, whereas more proximate mechanisms, such as economic activities, are frequently overlooked. Here we present a hypothesis that links ecology and economics to provide a causal framework for the distribution of exotic plants in the United States. We test two competing hypotheses (the population-only and population-economic models) using a national data set of exotic plants, employing a statistical framework to simultaneously model direct and indirect effects of human population and ecological and economic variables. The population-only model included direct effects of human population and ecological factors as predictors of exotics. In contrast, the population-economic model included the direct effects of economic and ecological factors and the indirect effects of human population as predictors of exotics. The explicit addition of economic activity in the population-economic model provided a better explanation for the distribution of exotics than did the population-only model. The population-economic model explained 75% of the variation in the number of exotic plants in the 50 states and provided a good description of the observed number of exotic plants in the Canadian provinces and in other nations in 85% of the cases. A specific economic activity, real estate gross state product, had the strongest positive effect on the number of exotics. The strong influence of economics on exotics demonstrates that economics matter for resolving the exotic-species problem because the underlying causes, and some of the solutions, may lie in human-economic behaviors.

Hydrologic and behavioral constraints on oviposition of stream insects: implications for adult dispersal.

The supply of recruits plays an important role in plant and animal population dynamics, and may be governed by environmental and behavioral constraints on animals. Mated females of the mayfly genus Baetis alight on rocks protruding from streams, crawl under water and deposit a single egg mass under a rock. We surveyed oviposition and emergence of a bivoltine population of B. bicaudatus in multiple stream reaches in one high-altitude watershed in western Colorado over 3 years to establish qualitative patterns at a regional scale (entire watershed), and quantitative patterns over six generations at a local scale (one stream reach). We also measured characteristics of preferred oviposition substrates, performed experiments to test hypotheses about cues used by females to select oviposition sites, and measured mortality of egg masses in the field. Our goals were to determine whether: (1) hydrologic variation necessitated dispersal of females to find suitable oviposition sites; (2) the local supply of females could provide the supply of local recruits; and (3) local recruitment determined the local production of adults. The onset of oviposition corresponded with the decline of spring run-off, which differed dramatically among years and among sites within years. However, eggs appeared before any adults had emerged in 8 of 22 site-years, and adults emerged 2-3 weeks before any eggs were oviposited in 3 site-years. Furthermore, the size distribution of egg masses differed from that predicted by the size distribution of females that emerged from seven of nine stream reaches. Protruding rocks and eggs appeared earlier each summer in smaller tributaries than in larger mainstream reaches, suggesting that hydrologic and behavioral constraints on oviposition may force females to disperse away from their natal reach to oviposit, and possibly explain the predominantly upstream flight of Baetis females reported in other studies. Local oviposition rates in one third-order stream-reach increased rapidly as soon as substrates protruded from the water surface, and females preferred large rocks that became available early in the flight season. However, females oviposited on <10% of all available rocks, and <65% of preferred rocks as determined by an empirical model. These data indicated that the timing of appearance of suitable oviposition sites determined the phenology of local recruitment, but that preferred oviposition sites were not saturated. Thus, the magnitude of local recruitment was not limited by the absolute abundance of preferred oviposition sites. Only 22% of egg masses observed in the field suffered mortality during their embryonic development, and per capita Baetis egg mass mortality was significantly lower on rocks with higher densities of egg masses. Thus, we suspect that specialized oviposition behavior may reduce the probability of egg mortality, potentially compensating for the costs of dispersal necessary to locate suitable oviposition sites. Finally, the number of adults that emerged at one stream reach was independent of the number of egg masses oviposited over six generations of Baetis; and local recruitment was not a function of the number of adults of the previous generation that emerged locally. The patterns of oviposition and emergence of Baetis found in this study are consistent with the following hypotheses. Recruitment of eggs in a stream reach is not limited by the local supply of adults, but is a function of the regional supply of dispersing adults, which are constrained by the spatial and temporal distribution of preferred oviposition habitat. Furthermore, subsequent local production of adults is not a function of the supply of recruits, arguing for post-recruitment control of local populations by processes operating in the larval stage (e.g., predation, competition, dispersal, disturbance). Processes affecting larval and adult stages of Baetis act independently and at different scales, thereby decoupling local population dynamics of successive generations.

Rapid size-specific changes in the drift of Baetis bicaudatus (Ephemeroptera) caused by alterations in fish odour concentration.

Chemical cues from fish can alter the behaviour of stream invertebrates in experimental tanks but their effect in natural streams has received little attention. By adding brook trout (Salvelinus fontinalis) odour to a trout stream in the Rocky Mountains of Colorado, USA, we tested whether changes in the concentration of chemical cues from visually feeding predatory fish would alter the drift of mayfly nymphs (Ephemeroptera). Stream water was piped from stream-side tanks with (odour) and without (control) three brook trout to two locations in the stream 3.5 m upstream of drift nets at six replicate sites. Five-minute drift samples were collected downstream from odour and control pipes before, during and after the release of water from the tanks into the stream during both the day and night. Almost all drift occurred at night and consisted predominantly of Baetis bicaudatus nymphs. The odour manipulation had no measurable effect on Baetis drift during the day but statistical power was low. During the night, however, the drift of large (>0.65 mm head capsule width, HCW) Baetis nymphs decreased significantly during the odour addition compared to control drift. In contrast, the drift of small nymphs (≤0.65 mm HCW) increased both during and after the odour addition in comparison to control drift. Since the stream contains brook trout (0.04-0.18 m-2), and water from the stream (presumably containing fish odour) altered the behaviour of fishless-stream Baetis nymphs in another experiment, we conclude that the changes in Baetis drift density were a response to an increase in the concentration of fish odour in the stream. Furthermore, we were able to detect the effect within 5 min. of odour addition, indicating that mayfly behavioural response to trout odour was rapid. These results suggest that mayflies can distinguish different concentrations of trout odour in natural streams and that the response is size-specific, according to the relative risk of predation of large and small Baetis.

Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive success.

Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle.