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Only a small proportion of exotic species invasions give rise to hyper‐successful nuisance species, but those that do have dramatic negative impacts on ecosystems, such as the displacement of native species and disruption of native food webs. For a native predator, such changes may mean a major transformation in its resource base and a decline in its fitness. However, native predators may adapt to become more effective at feeding on exotic prey, either rapidly, via existing phenotypic plasticity, or more slowly, via natural selection. Despite a rapidly growing number of publications on the importance of species invasions as a driver of contemporary evolution in both invading and native species, we know little about how the arrival of exotic prey affects native predators. We propose that native predators could be important in regulating the long‐term dynamics of invading species and, consequently, that the overexploitation of predators could facilitate biological invasions.

Ecosystems across the United States are changing in complex and unpredictable ways and analysis of these changes requires coordinated, long‐term research. This paper is a product of a synthesis effort of the U.S. National Science Foundation funded Long‐Term Ecological Research (LTER) network addressing the LTER core research area of “populations and communities.” This analysis revealed that each LTER site had at least one compelling “story” about what their site would look like in 50–100 yr. As the stories were prepared, themes emerged, and the stories were group into papers along five themes: state change, connectivity, resilience, time lags, and cascading effects. This paper addresses the cascading effects theme and includes stories from the Bonanza Creek (boreal), Kellogg Biological Station (agricultural and freshwater), Palmer (Antarctica), and Harvard Forest (temperate forest) LTER sites. We define cascading effects very broadly to include a wide array of unforeseen chains of events that result from a variety of actions or changes in a system. While climate change is having important direct effects on boreal forests, indirect effects mediated by fire activity—severity, size, and return interval—have large cascading effects over the long term. In northeastern temperate forests, legacies of human management and disturbance affect the composition of current forests, which creates a cascade of effects that interact with the climate‐facilitated invasion of an exotic pest. In Antarctica, declining sea ice creates a cascade of effects including declines in Adèlie and increases in Gentoo penguins, changes in phytoplankton, and consequent changes in zooplankton populations. An invasion of an exotic species of lady beetle is likely to have important future effects on pest control and conservation of native species in agricultural landscapes. New studies of zebra mussels, a well‐studied invader, have established links between climate, the heat tolerance of the mussels, and harmful algal blooms. Collectively, these stories highlight the need for long‐term studies to sort out the complexities of different types of ecological cascades. The diversity of sites within the LTER network facilitates the emergence of overarching concepts about trophic interactions as an important driver of ecosystem structure, function, services, and futures.

This paper reports on the response by amphibians, benthic macroinvertebrates, and zooplankton in naturally fishless alpine lakes to fish introductions and subsequent fish disappearance. We assessed resistance (the degree to which a system is altered when the environment changes) by comparing faunal distribution and abundance in lakes that have never been stocked with fish vs. the distribution and abundance in lakes that have been stocked and still contain fish. We assessed resilience (the degree and rate of a system's return to its previous configuration once the perturbation is removed) by comparing faunal distribution and abundance in lakes that were stocked at one time but have since reverted to a fishless condition (stocked-now-fishless lakes) vs. the distribution and abundance in lakes that have never been stocked. We quantified recovery rates and trajectories by comparing faunal assemblages of stocked-now-fishless lakes that had been fishless for 5-10, 11-20, and >20 yr. Faunal assemblages in the study lakes had low resistance to fish introductions, but in general showed high resilience. The mountain yellow-legged frog (Rana muscosa), conspicuous benthic macroinvertebrates, and large crustacean zooplankton (>1 mm) were dramatically reduced in distribution and abundance by fish introductions but generally recovered to predisturbance levels after fish disappearance. Inconspicuous benthic invertebrate taxa, small crustacean zooplankton (<1 mm), and rotiferan zooplankton (<0.2 mm) were either unaffected by fish or increased in the presence of fish. For both the benthic macroinvertebrate community and the zooplankton community as a whole, fish disappearance was followed by a steady change away from the configuration characteristic of fish-containing lakes and toward that of lakes that had never been stocked. Both communities remained markedly different from those in never-stocked lakes 5-10 yr after fish disappearance and converged on the configuration of never-stocked lakes only 11-20 yr after fish disappearance. Recovery was likely facilitated by the winged adult stages of many benthic macroinvertebrates, resting eggs of zooplankton, and nearby source populations of frogs. However, many frog populations have disappeared since the time that lakes in this study reverted to a fishless condition, and the viability of zooplankton egg banks should decline in fish-containing lakes over time. As a result, faunal resilience may be lower in lakes that revert to a fishless condition today than is suggested by the results of our study. These findings have important implications for the restoration of alpine lake ecosystems.

Allometric studies have shown that individual growth rate is inversely related to body size across a broad spectrum of organisms that vary greatly in size. Fewer studies have documented such patterns within species. No data exist directly documenting the influence of colony size on growth rate for microscopic, colonial organisms. To determine if similar negative relationships between growth rate and size hold for colonial organisms, we developed a technique for measuring the growth of individual colonies of a bloom-forming, toxic cyanobacterium, Microcystis aeruginosa using microscopy and digital image analysis. For five out of six genotypes of M. aeruginosa isolated from lakes in Michigan and Alabama, we found significant negative relationships between colony size and growth rate. We found large intraspecific variation in both the slope of these relationships and in the growth rate of colonies at a standard size. In addition, growth rate estimates for individual colonies were generally consistent with population growth rates measured using standard batch culture. Given that colony size varies widely within populations, our results imply that natural populations of colonial phytoplankton exist as a mosaic of individuals with widely varying ecological attributes (since size strongly affects growth rate, grazing mortality, and migration speed). Quantifying the influence of colony size on growth rate will permit development of more accurate, predictive models of ecological interactions (e.g., competition, herbivory) and their role in the proliferation of harmful algal blooms, in addition to increasing our understanding about why these interactions vary in strength within and across environments.

To assess whether Daphnia populations in eutrophic lakes can increase when bloom-forming cyanobacteria dominate the phytoplankton assemblage and whether such an increase can result in strong suppression of phytoplankton biomass, I created contrasting initial conditions (high Daphnia pulicaria, low cyanobacteria vs. low D. pulicaria, high cyanobacteria) via fish manipulation in large enclosures, then removed fish from some enclosures and subsequently monitored zooplankton and phytoplankton abundance for 48 days. After being released from fish predation, D. pulicaria was apparently able to reduce cyanobacteria and total phytoplankton biomass to very low levels despite the fact that the phytoplankton assemblage was initially composed of about 90% Microcystis aeruginosa, a species that has inhibited Daphnia growth and reproduction in many laboratory studies. Thus, it appears possible for Daphnia to graze down an established bloom of cyanobacteria. In contrast, in enclosures where fish were never present, M. aeruginosa was eventually able to increase from low levels despite initially high D. pulicaria biomass. As a result, the apparent effect of D. pulicaria on M. aeruginosa at the end of the experiment was very different across enclosures with different initial conditions.

We synthesized data from 66 published laboratory studies, representing 597 experimental comparisons, examining the effects of cyanobacterial toxicity and morphology on the population growth rate and survivorship of 17 genera (34 species) of freshwater, herbivorous Zooplankton. Two meta-analyses were conducted with these data. The primary analysis compared herbivore population growth rates for grazers fed treatment diets containing cyanobacteria versus control diets comprising phytoplankton that are generally considered to be nutritious for Zooplankton (chlorophytes and/or flagellates). This analysis confirmed that cyanobacteria were poor foods relative to small chlorophytes and flagellates. More importantly, filamentous cyanobacteria were found to be significantly better foods for grazers than single-celled cyanobacteria over all studies. Surprisingly, the presence or absence of commonly-measured toxic compounds (microcystins in 70% of the cases) in the diet had no overall influence on grazer population growth relative to control diets. A secondary analysis compared survival rates for grazers fed cyanobacteria versus no food. In contrast to the primary analysis, grazer survival was more negatively affected by toxic cyanobacteria than non-toxic cyanobacteria, relative to starvation. However, this difference was attributable to the effects of a single Microcystis strain, PCC7820. Thus, though some cyanobacterial strains appear to be toxic to some strains of zooplankton, the overall role of commonly-assayed cyanobacterial toxins as a determinant of food quality may be less than widely assumed. We suggest that more attention be focused on nutritional deficiencies, morphology, and the toxicity of undescribed cyanobacterial compounds as mediators of the poor food quality of cyanobacteria for zooplankton.

The functional response of Daphnia, a common pelagic herbivore in lakes, was assessed with a combination of secondary and meta-analyses of published data and new data from an experiment conducted using very low food levels. Secondary analyses of literature data (28 studies, n = 239—393) revealed a significant positive influence of food concentration on Daphnia clearance rate at low food levels, i.e., evidence of an overall Type III functional response. This result was not an artifact of including data from Daphnia that were exhausted from prolonged food deprivation (more than three hours at very low food). Meta-analysis of Daphnia clearance rate vs. food concentration across a range of low food concentrations (eight studies) showed a significantly positive slope across studies, which also supports the presence of a Type III response. Congruent with these analyses of published data, the feeding experiment showed clear evidence of a Type III functional response for D. pulicaria feeding on Ankistrodesmus falcatus. Food levels at which Daphnia clearance rate declined with decreasing food were near the minimum resource requirement for Daphnia population maintenance at steady state (R*). We suggest that Type III responses are more common than previously believed, perhaps because of the relative paucity of observations at low food levels, and that reduced prey mortality at low phytoplankton densities could be a stabilizing mechanism for Daphnia—phytoplankton systems under resource scarcity.

I derived predictions about how the magnitude of Daphnia effects on total phytoplankton biomass should vary across a gradient of enrichment (expressed as algal carrying capacity) using two, simple predator-prey models. These predictions were then compared with data from a survey of field experiments in temperate lakes. Algal responses to Daphnia manipulation were quantified as an Algal Response Factor (ARF), defined as total algal biomass in the low-Daphnia treatment divided by total algal biomass in the high-Daphnia treatment. Total phosphorus concentration (TP) was used as an index of algal carrying capacity, ranging from 10 to 460 @mg/L over the 22 experiments surveyed. The Algal Response Factor ranged from 1 to 40 and was a positive, linear function of TP: ARF = @o0.14 + 0.08(TP), r^2 = 0.81; log ARF = @o0.81 + 0.83(log TP), r^2 = 0.75. Thus, algal carrying capacity, as quantified by TP, explains much of the variation in Daphnia effects on total algal biomass across lakes. The survey results supported the prediction of the simpler model, a two-species, Lotka-Volterra model of pure exploitation. Incorporating the additional complexity of inedible algae into this model did little to improve its predictive power. The conclusion that inedible algae are not of major importance to the prediction of Daphnia effects across an enrichment gradient was supported by a survey of Daphnia effects on the proportionate biomass of inedible algae. There was no evidence that Daphnia grazing typically favors dominance by inedible algae at equilibrium in highly enriched lakes. My results suggest that the well-established positive relationship between TP and equilibrium algal biomass in lakes is more likely to be a consequence of increases in Daphnia's death rate with enrichment, rather than decreases in Daphnia's feeding and assimilation rates. When zooplanktivorous fish are rare, Daphnia should be able to prevent phytoplankton biomass from responding to nutrient addition at equilibrium. As a consequence, the success of biomanipulation in eutrophic lakes should critically depend upon the effectiveness of strategies aimed at reducing zooplanktivory.

Climate change is driving large changes in the spatial and temporal distributions of species, with significant consequences for individual populations. Community- and ecosystem-level implications of altered species distributions may be complex and challenging to anticipate due to the cascading effects of disrupted interactions among species, which may exhibit threshold responses to extreme climatic events. Toxic, bloom-forming cyanobacteria like Microcystis are expected to increase worldwide with climate change, due in part to their high temperature optima for growth. In addition, invasive zebra mussels (Dreissena polymorpha) have caused an increase in Microcystis aeruginosa, a species typically associated with eutrophication, in low-nutrient lakes. We conducted a 13-yr study of a M. aeruginosa population in a low-nutrient lake invaded by zebra mussels. In 10 of the 13 years, there was a significant positive relationship between M. aeruginosa biomass and accumulated degree days, which are projected to increase with climate change. In contrast, Microcystis biomass was up to an order of magnitude lower than predicted by the above relationship during the other three years, including the warmest in the data set, following repeated heat-induced mass mortality of D. polymorpha. Thus, the positive relationship between Microcystis biomass and temperature was negated when its facilitating species was suppressed during a series of exceptionally warm summers. Predicting the net response of a species to climate change may therefore require, at minimum, quantification of responses of both the focal species and species that strongly interact with it over sufficiently long time periods to encompass the full range of climatic variability. Our results could not have been predicted from existing data on the short-term responses of these two interacting species to increased temperature.

We quantified within-species variation in the tolerance of the large, lake-dwelling daphnid, Daphnia pulicaria, to toxic cyanobacteria in the diet. Juvenile growth rates on diets consisting of 100% Ankistrodesmus falcatus (a nutritious green alga) or 100% Microcystis aeruginosa (toxic) were compared for D. pulicaria clones isolated from lakes expected to have low and high levels of bloom-forming cyanobacteria during summer. Growth rates of clones isolated from high-nutrient lakes (range of total phosphorus, 31-235 μg L-1) were higher, and showed less relative inhibition, on the cyanobacterial diet compared to clones isolated from low-nutrient lakes (range of total phosphorus, 9-13 μg L-1). Our results suggest that D. pulicaria populations exposed to high cyanobacterial levels over long periods of time can adapt to being more tolerant of toxic cyanobacteria in the diet.