Cascading effects of predators on algal size structure

The presence of edible and inedible prey species in a food web can influence the strength that nutrients (bottom-up) or herbivores (top-down) have on primary production. In boreal peatlands, wetter more nutrient-rich conditions associated with ongoing climate change are expanding consumer access to aquatic habitat and promoting sources of primary production (i.e., algae) that are susceptible to trophic regulation. Here, we used an in situ mesocosm experiment to evaluate the consequences of enhanced nutrient availability and food-web manipulation (herbivore and predator exclusion) on algal assemblage structure in an Alaskan fen. Owing to the potential for herbivores to selectively consume edible algae (small cells) in favor of more resistant forms, we predicted that the proportion of less-edible algae (large cells) would determine the strength of top-down or bottom-up effects. Consistent with these expectations, we observed an increase in algal-cell size in the presence of herbivores (2-tiered food web) that was absent in the presence of a trophic cascade (3-tiered food web), suggesting that predators indirectly prevented morphological changes in the algal assemblage by limiting herbivory. Increases in algal-cell size with herbivory were driven by a greater proportion of filamentous green algae and nitrogen-fixing cyanobacteria, whose size and morphological characteristics mechanically minimize consumption. While consumer-driven shifts in algal assemblage structure were significant, they did not prevent top-down regulation of biofilm development by herbivores. Our findings show that increasing wet periods in northern peatlands will provide new avenues for trophic regulation of algal production, including directly through consumption and indirectly via a trophic cascade.

Stoichiometric mismatch causes a warming‐induced regime shift in experimental plankton communities

In many ecosystems, consumers respond to warming differently than their resources, sometimes leading to temporal mismatches between seasonal maxima in consumer demand and resource availability. A potentially equally pervasive, but less acknowledged threat to the temporal coherence of consumer‐resource interactions is mismatch in food quality. Many plant and algal communities respond to warming with shifts toward more carbon‐rich species and growth forms, thereby diluting essential elements in their biomass and intensifying the stoichiometric mismatch with herbivore nutrient requirements. Here we report on a mesocosm experiment on the spring succession of an assembled plankton community in which we manipulated temperature (ambient vs. +3.6°C) and presence versus absence of two types of grazers (ciliates and Daphnia), and where warming caused a dramatic regime shift that coincided with extreme stoichiometric mismatch. At ambient temperatures, a typical spring succession developed, where a moderate bloom of nutritionally adequate phytoplankton was grazed down to a clear‐water phase by a developing Daphnia population. While warming accelerated initial Daphnia population growth, it speeded up algal growth rates even more, triggering a massive phytoplankton bloom of poor food quality. Consistent with the predictions of a stoichiometric producer–grazer model, accelerated phytoplankton growth promoted the emergence of an alternative system attractor, where the extremely low phosphorus content of the abundant algal food eventually drove Daphnia to extinction. Where present, ciliates slowed down the phytoplankton bloom and the deterioration of its nutritional value, but this only delayed the regime shift. Eventually, phytoplankton also grew out of grazer control in the presence of ciliates, and the Daphnia population crashed. To our knowledge, the experiment is the first empirical demonstration of the “paradox of energy enrichment” (grazer starvation in an abundance of energy‐rich but nutritionally imbalanced food) in a multispecies phytoplankton community. More generally, our results support the notion that warming can exacerbate the stoichiometric mismatch at the plant–herbivore interface and limit energy transfer to higher trophic levels.

20 Years of Global Change on the Limnology and Plankton of a Tropical, High-Altitude Lake

The present long-term (1993–2013) study was aimed at identifying the effects of global change on the environmental characteristics and the plankton community of the tropical, high-altitude Lake Alchichica, Puebla, Mexico. We found no statistically significant increasing trend in air temperature, but an increase from 2002 to 2013, and interannual variability in the meteorological variables. Accordingly, the water temperature rose from 1993 to 2003 and remained similar until 2013. Nonetheless, on a longer-term (1966-2018), longer than the period considered in the present study, air and water temperatures showed an increasing trend in the range considered indicative of climate change. The lake displayed a characteristic warm monomictic thermal pattern but exhibited interannual variability. The planktonic community composition and structure remained similar. The small chlorophytes (e.g., Monoraphidium minutum) dominated the phytoplankton abundance, while the large diatom Cyclotella alchichicana dominated the biomass. The calanoid copepod Leptodiaptomus garciai dominated both the zooplankton abundance and biomass. However, as the temperature increased the large-size phytoplankton (i.e., C. alchichicana) abundance and biomass increased, but the small-size phytoplankton abundance and biomass, as well as the adult copepod abundance and biomass, decreased. The increase in temperature could have favoured the large-size over the small-size phytoplankton. In addition, the temperature increase could have negatively affected the calanoid copepods in two different ways—direct association with the negative effects of higher temperatures on the physiology of the copepods, or indirect association with the negative effects of higher temperatures on the small-size phytoplankton, which diminish the copepods’ food resource.

The Importance of Eco-evolutionary Potential in the Anthropocene

Humans are dominant global drivers of ecological and evolutionary change, rearranging ecosystems and natural selection. In the present article, we show increasing evidence that human activity also plays a disproportionate role in shaping the eco-evolutionary potential of systems—the likelihood of ecological change generating evolutionary change and vice versa. We suggest that the net outcome of human influences on trait change, ecology, and the feedback loops that link them will often (but not always) be to increase eco-evolutionary potential, with important consequences for stability and resilience of populations, communities, and ecosystems. We also integrate existing ecological and evolutionary metrics to predict and manage the eco-evolutionary dynamics of human-affected systems. To support this framework, we use a simple eco–evo feedback model to show that factors affecting eco-evolutionary potential are major determinants of eco-evolutionary dynamics. Our framework suggests that proper management of anthropogenic effects requires a science of human effects on eco-evolutionary potential.

Prey adaptation along a competition-defense tradeoff cryptically shifts trophic cascades from density- to trait-mediated

Trophic cascades have become a dominant paradigm in ecology, yet considerable debate remains about the relative strength of density- (consumptive) and trait-mediated (non-consumptive) effects in trophic cascades. This debate may, in part, be resolved by considering prey experience, which shapes prey traits (through genetic and plastic change) and influences prey survival (and therefore density). Here, we investigate the cascading role of prey experience through the addition of mosquitofish (Gambusia affinis) from predator-experienced or predator-naïve sources to mesocosms containing piscivorous largemouth bass (Micropterus salmoides), zooplankton, and phytoplankton. These two sources were positioned along a competition-defense tradeoff. Results show that predator-naïve mosquitofish suffered higher depredation rates, which drove a density-mediated cascade, whereas predator-experienced mosquitofish exhibited higher survival but fed less, which drove a trait-mediated cascade. Both cascades were similar in strength, leading to indistinguishable top-down effects on lower trophic levels. Therefore, the accumulation of prey experience with predators can cryptically shift cascade mechanisms from density- to trait-mediated.

Spatial insurance in multi-trophic metacommunities

Metacommunity theory suggests that dispersal is a key driver of diversity and ecosystem functioning in changing environments. The capacity of dispersal to mitigate effects of environmental change might vary among trophic groups, potentially resulting in changes in trophic interactions and food web structure. In a mesocosm experiment, we compared the compositional response of bacteria, phyto‐ and zooplankton to a factorial manipulation of acidification and dispersal. We found that the buffering capacity of dispersal varied among trophic groups: dispersal alleviated the negative effect of acidification on phytoplankton diversity mid‐experiment, but had no effect on the diversity of zooplankton and bacteria. Likewise, trophic groups differed in whether dispersal facilitated compositional change. Dispersal accelerated changes in phytoplankton composition under acidification, possibly mediated by changes in trophic interactions, but had no effect on the composition of zooplankton and bacteria. Overall, our results suggest that the potential for spatial insurance can vary among trophic groups.

Biogeographic context dependence of trophic cascade strength in bromeliad food webs

Ecosystem functions and the biomass of lower trophic levels are frequently controlled by predators. The strength of top-down control in these trophic cascades can be affected by the identity and diversity of predators, prey, and resources, as well as environmental conditions such as temperature, moisture, and nutrient loading, which can all impact interaction strength between trophic levels. Few studies have been able to replicate a complete community over a large geographic area to compare the full trophic cascade in a manipulative experiment. Here, we identify geographic dependency in trophic cascade strength, and the driving factors and specific mechanisms behind it, by combining geographically replicated experiments with a novel approach of community analogues of common garden and transplant experiments. We studied a predator-detritivore-detritus food web in bromeliads in Puerto Rico, Costa Rica, and Brazil. We found that interaction strengths between resources, consumers, and predators were strongly site-specific, but the exact mechanism differed between trophic levels. Large bodied predators created strong interaction strengths between predator and consumer trophic levels, reducing consumer abundance regardless of the geographic location, whereas small-bodied predators created weak interactions with no impact on consumer abundances in any site. In contrast, the interaction strength between consumers and resources varied among sites, depending on the dominant species of leaf detritus. More labile leaf species in Costa Rica created a strong consumer-resource interaction and therefore strong trophic cascade, whereas tougher leaf species in Brazil created a weak consumer-resource interaction, and an overall weaker trophic cascade. Our study highlights the importance of replicating experiments over geographic scales to understand general patterns of ecological processes.

Evolutionary history of Daphnia drives divergence in grazing selectivity and alters temporal community dynamics of producers

Consumers with different seasonal life histories encounter different communities of producers during specific seasonal phases. If consumers evolve to prefer the producers that they encounter, then consumers may reciprocally influence the temporal composition of producer communities. Here, we study the keystone consumer Daphnia ambigua, whose seasonal life history has diverged due to intraspecific predator divergence across lakes of New England. We ask whether grazing preferences of Daphnia have diverged also and test whether any grazing differences influence temporal composition patterns of producers. We reared clonal populations of Daphnia from natural populations representing the two diverged life history types for multiple generations. We conducted short‐term (24 hr) and long‐term (27 days) grazing experiments in equal polycultures consisting of three diatom and two green algae species, treated with no consumer, Daphnia from lakes with anadromous alewife, or from lakes with landlocked alewife. After 24 hr, life history and grazing preference divergence in Daphnia ambigua drove significant differences in producer composition. However, those differences disappeared at the end of the 27‐day experiment. Our results illustrate that, despite potentially more complex long‐term dynamics, a multitrophic cascade of evolutionary divergence from a predator can influence temporal community dynamics at the producer level.

The role of light availability and herbivory on algal responses to nutrient enrichment in a riparian wetland, Alaska

We investigated how the relative availability of solar radiation in the presence or absence of grazing alters the ability of benthic algae to respond to nutrient enrichment in an Alaskan marsh. We used a factorial mesocosm experiment that included nutrient enrichment (enriched or control), grazing (grazed or ungrazed), and light (unshaded or shaded) to simulate shading by macrophytes early and late in the growing season, respectively. We found stronger effects of grazers and nutrients compared to light on benthic algal biomass and taxonomic composition. Algal biomass increased in nutrient-enriched treatments and was reduced by grazing. Shading did not have an effect on algal biomass or taxonomic composition, but the concentration of chl a per algal biovolume increased with shading, demonstrating the ability of algae to compensate for changes in light availability. Algal taxonomic composition was more affected by grazer presence than nutrients or light. Grazer-resistant taxa (basal filaments of Stigeoclonium) were replaced by diatoms (Nitzschia) and filamentous green algae (Ulothrix) when herbivores were removed. The interacting and opposing influences of nutrients and grazing indicate that the algal community is under dual control from the bottom-up (nutrient limitation) and from the top-down (consumption by herbivores), although grazers had a stronger influence on algal biomass and taxonomic composition than nutrient enrichment. Our results suggest that low light availability will not inhibit the algal response to elevated nutrient concentrations expected with ongoing climate change, but grazers rapidly consume algae following enrichment, masking the effects of elevated nutrients on algal production.

The mixotroph Ochromonas tuberculata may invade and suppress specialist phago- and phototroph plankton communities depending on nutrient conditions

Mixotrophic organisms combine light, mineral nutrients, and prey as supplementary resources. Based on theoretical assumptions and field observations, we tested experimentally the hypothesis that mixotrophs may invade established plankton communities depending on the trophic status of the system, and investigated possible effects on food web structure, species diversity, and nutrient dynamics. To test our hypothesis, we inoculated the mixotrophic nanoflagellate Ochromonas tuberculata into established planktonic food webs, consisting of specialist phototrophs, specialist phagotrophs, and bacteria at different supplies of soluble inorganic nutrients and dissolved organic carbon. Oligotrophic systems facilitated the invasion of O. tuberculata in two different ways. First, the combination of photosynthesis and phagotrophy gave mixotrophs a competitive advantage over specialist phototrophs and specialist phagotrophs. Second, low nutrient supplies supported the growth of small plankton organisms that fell into the food size spectrum of mixotrophs. Conversely, high nutrient supplies prevented O. tuberculata from successfully invading the food webs. Two important conclusions were derived from our experiments. First, in contrast to a paradigm of ecology, specialization may not necessarily be the most successful strategy for survival under stable conditions. Indeed, the use of several resources with lower efficiency can be an equally, or even more, successful strategy in nature. Second, when limiting nutrients promote the growth of bacterio- and picophytoplankton, invading mixotrophs may have a habitat-ameliorating effect for higher trophic levels, gauged in terms of food quantity and quality. Using given resources more efficiently, O. tuberculata generated higher biomasses and expressed an increased nutritional value for potential planktivores, due to decreased cellular carbon to phosphorus (C:P) ratios compared to specialized plankton taxa. Our findings may help to explain why energy transfer efficiency between phytoplankton and higher trophic levels is generally higher in oligotrophic systems than in nutrient rich environments.

The effect of temporal scale on the outcome of trophic cascade experiments

The outcome of experimental manipulations in ecology should depend on the duration of the experiment. We tested this proposition by examining the results of published trophic cascade experiments in freshwater communities and the results of our own previously unpublished pond experiment. In particular, we analyze the results of 90 published trophic cascade experiments to test the prediction that the outcome of freshwater trophic cascade experiments, in which zooplanktivorous fish are either present or absent, depends on the duration of the experiment. We present evidence that there are quantitative differences in the strength of trophic cascades among different venue types (enclosure, mesocosm, pond, and lake), but that the strength of trophic cascades does not diminish with increasing experiment duration. Despite the large number of studies, there have been few replicated studies of trophic cascades for longer than a summer field season, and none for the time required to estimate the long-term result of press perturbations. We therefore present the results of a 4-year study of trophic cascades in experimental ponds, to test the predictions that the addition of the top predator results in a sustained increase in the phytoplankton biomass. We found that, as predicted by our literature review, there was no decline in the strength of the trophic cascade as the experiment progressed.