Phenotypic adjustments following environmental change are ubiquitous and trait changes arising through phenotypic plasticity often lag behind their environmental stimuli. Evolutionary biologists seeking to understand how adaptive plasticity can evolve have extensively studied this phenomenon. However, the ecological consequences of common features of plastic responses to environmental variability, including gradual phenotypic change (i.e., slower than the pace of environmental change), are underappreciated. We present a framework based on the unifying concept of phenotype  environment performance landscapes that encompasses gradual plasticity. Then, we experimentally investigate the environmental contexts where gradual plasticity is important, using freshwater phytoplankton populations exposed to thermal variation. Finally, based on our conceptual framework, we develop a mathematical model of gradual plasticity that explains population dynamics in variable environments better than common alternative models. Understanding and accounting for the ecological effects of plasticity in variable environments is critical to making vital predictions and advancing ecology.

24Describing the mechanisms that drive variation in species interaction strengths is central 25 to understanding, predicting, and managing community dynamics. Multiple factors have 26 been linked to trophic interaction strength variation, including species densities, species 27 traits, and abiotic factors. Yet most empirical tests of the relative roles of multiple 28 mechanisms that drive variation have been limited to simplified experiments that may 29 diverge from the dynamics of natural food webs. Here, we used a field-based 30 observational approach to quantify the roles of prey density, predator density, predator-31 prey body-mass ratios, prey identity, and abiotic factors in driving variation in feeding 32 rates of reticulate sculpin (Cottus perplexus). We combined data on over 6,000 predator-33 prey observations with prey identification time functions to estimate 289 prey-specific 34 feeding rates at nine stream sites in Oregon. Feeding rates on 57 prey types showed an 35 approximately log-normal distribution, with few strong and many weak interactions. 36Model selection indicated that prey density, followed by prey identity, were the two most 37 important predictors of prey-specific sculpin feeding rates. Feeding rates showed a 38 positive, accelerating relationship with prey density that was inconsistent with predator 39 saturation predicted by current functional response models. Feeding rates also exhibited 40 four orders-of-magnitude in variation across prey taxonomic orders, with the lowest 41 feeding rates observed on prey with significant anti-predator defenses. Body-mass ratios 42were the third most important predictor variable, showing a hump-shaped relationship 43 with the highest feeding rates at intermediate ratios. Sculpin density was negatively 44 correlated with feeding rates, consistent with the presence of intraspecific predator 45 interference. Our results highlight how multiple co-occurring drivers shape trophic 46All rights reserved. No reuse allowed without permission.was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint (which . http://dx

Although parasites are increasingly recognized for their ecosystem roles, it is often assumed that free‐living organisms dominate animal biomass in most ecosystems and therefore provide the primary pathways for energy transfer. To examine the contributions of parasites to ecosystem energetics in freshwater streams, we quantified the standing biomass of trematodes and free‐living organisms at nine sites in three streams in western Oregon, USA. We then compared the rates of biomass flow from snails Juga plicifera into trematode parasites relative to aquatic vertebrate predators (sculpin, cutthroat trout and Pacific giant salamanders). The trematode parasite community had the fifth highest dry biomass density among stream organisms (0.40 g/m2) and exceeded the combined biomass of aquatic insects. Only host snails (3.88 g/m2), sculpin (1.11 g/m2), trout (0.73 g/m2) and crayfish (0.43 g/m2) had a greater biomass. The parasite ‘extended phenotype’, consisting of trematode plus castrated host biomass, exceeded the individual biomass of every taxonomic group other than snails. The substantial parasite biomass stemmed from the high snail density and infection prevalence, and the large proportional mass of infected hosts that consisted of trematode tissue (M = 31% per snail). Estimates of yearly biomass transfer from snails into trematodes were slightly higher than the combined estimate of snail biomass transfer into the three vertebrate predators. Pacific giant salamanders accounted for 90% of the snail biomass consumed by predators. These results demonstrate that trematode parasites play underappreciated roles in the ecosystem energetics of some freshwater streams.

Describing the mechanisms that drive variation in species interaction strengths is central to understanding, predicting, and managing community dynamics. Multiple factors have been linked to trophic interaction strength variation, including species densities, species traits, and abiotic factors. Yet most empirical tests of the relative roles of multiple mechanisms that drive variation have been limited to simplified experiments that may diverge from the dynamics of natural food webs. Here, we used a field-based observational approach to quantify the roles of prey density, predator density, predator-prey body-mass ratios, prey identity, and abiotic factors in driving variation in feeding rates of reticulate sculpin (Cottus perplexus). We combined data on over 6,000 predator-prey observations with prey identification time functions to estimate 289 prey-specific feeding rates at nine stream sites in Oregon. Feeding rates on 57 prey types showed an approximately log-normal distribution, with few strong and many weak interactions. Model selection indicated that prey density, followed by prey identity, were the two most important predictors of prey-specific sculpin feeding rates. Feeding rates showed a positive relationship with prey taxon densities that was inconsistent with predator saturation predicted by current functional response models. Feeding rates also exhibited four orders-of-magnitude in variation across prey taxonomic orders, with the lowest feeding rates observed on prey with significant anti-predator defenses. Body-mass ratios were the third most important predictor variable, showing a hump-shaped relationship with the highest feeding rates at intermediate ratios. Sculpin density was negatively correlated with feeding rates, consistent with the presence of intraspecific predator interference. Our results highlight how multiple co-occurring drivers shape trophic interactions in nature and underscore ways in which simplified experiments or reliance on scaling laws alone may lead to biased inferences about the structure and dynamics of species-rich food webs.

Light fluctuations are ubiquitous, exist across multiple spatial and temporal scales, and directly affect the physiology and ecology of photoautotrophs. However, the indirect effects of light fluctuations on the sensitivity of organisms to other key environmental factors are unclear. Here, we evaluate how photoperiod regime (period of time each day where organisms receive light), a dynamic element of aquatic ecosystems, can influence the interactive effects of temperature and irradiance (intensity of light) on the growth rate of phytoplankton populations. We first completed a literature review and meta-analysis that suggests photoperiod alters the individual effects of temperature -but not irradiance -on algal growth rates and that highlights how few studies experimentally manipulate photoperiod, temperature and irradiance. To address this empirical gap, we conducted a set of laboratory experiments on three freshwater phytoplankton species (Chlamydomonas reinhardtii, Chlorella vulgaris and Cryptomonas ovata). We measured performance surfaces relating growth rate to irradiance and temperature gradients for each species in constant (24:0 h of light:dark) environments. We then evaluated whether analogous surfaces measured under different photoperiods (6:18, 12:12 and 16:8 h of light:dark) and scaled by the duration of light availability could be inferred from results under constant light. For a majority of the combinations of species and photoperiods examined, photoperiod meaningfully altered the intercept and shape of performance surfaces. These differences were most pronounced under the shortest photoperiod (6:18 h light:dark), where populations underperformed expectations. Alterations to performance surfaces were non-linear and mostly structured by temperature with higher temperatures yielding higher than anticipated growth rates. Collectively, these experiments and synthesis reveal the potential for photoperiod regime to influence the effects of temperature, irradiance and their interaction on phytoplankton growth. Beyond the environmental variables and organisms presently considered, this research highlights the capacity for dynamic, abiotic variables to exert direct effects while also influencing relationships among other environmental factors.