Insect predators affect plant resistance via density- and trait-mediated indirect interactions

Predators can affect herbivores both through direct consumption (density-mediated interactions) and by changing behavioural, physiological or morphological attributes of the prey (trait-mediated interactions). These effects on the herbivore can in turn affect the plant through density- and trait-mediated indirect interactions (DMIIs and TMIIs). While the effects of DMIIs and TMIIs imposed by predators has been shown to influence plant density and plant communities, we know little about the effects on plant quality. In addition, the DMII and TMII components of the predator may influence each other so that the total effect of the predator on the plant is not simply the sum of the DMII and TMII. We examined DMIIs and TMIIs between a stinkbug predator and a caterpillar, and show how these interactions affect plant quality, as measured by damage, resistance to herbivores, and a defence chemical, peroxidase. We used novel methods to estimate the independent and non-additive contribution of DMIIs and TMIIs to the plant phenotype. Both predator-induced DMIIs and TMIIs caused decreases in the amount of caterpillar herbivory on plants; a strong non-additive effect between the two resulted from redundancy in their effects. TMIIs initiated by the predator were primarily responsible for a decrease in induced plant resistance. However, DMIIs predominated for reducing the production of peroxidase. These data demonstrate how DMIIs and TMIIs initiated by predators cascade through tri-trophic interactions to affect plant damage and induced resistance.     

Predator‐induced morphological defences in two invasive dreissenid mussels: implications for species replacement

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Predation on invasive zebra mussel, Dreissena polymorpha, by pumpkinseed sunfish, rusty crayfish, and round goby

The enemy release hypothesis states that invasive species are successful in their new environment because native species are not adapted to utilize the invasive. If true for predators, native predators should have lower feeding rates on the invasive species than a predator from the native range of the invasive species. We tested this hypothesis for zebra mussel (Dreissena polymorpha) by comparing handling time and predation rate on zebra mussels in the laboratory by two North American species (pumpkinseed, Lepomis gibbosus, and rusty crayfish, Orconectes rusticus) and one predator with a long evolutionary history with zebra mussels (round goby, Neogobius melanostomus). Handling time per mussel (7 mm shell length) ranged from 25 to >70 s for the three predator species. Feeding rates on attached zebra mussels were higher for round goby than the two native predators. Medium and large gobies consumed 50–67 zebra mussels attached to stones in 24 h, whereas pumpkinseed and rusty crayfish consumed <11. This supports the hypothesis that the rapid spread of zebra mussels in North America was facilitated by low predation rates from the existing native predators. At these predation rates and realistic goby abundance estimates, round goby could affect zebra mussel abundance in some lakes.     

The long‐term impacts of predators on prey: inducible defenses,population dynamics,and indirect effects

Despite the amount of research on the inducible defenses of prey against predators, our understanding of the long‐term significance of non‐lethal predators on prey phenotypes, prey population dynamics, and community structure has rarely been explored. Our objectives were to assess the effects of predators on prey defenses, prey population dynamics, and the relative magnitude of density‐ versus trait‐mediated indirect interactions (DMIIs and TMIIs) over multiple prey generations. Using a freshwater snail and three common snail predators, we constructed a series of community treatments with pond mesocosms that manipulated trophic structure, the identity of the top predator, and whether predators were caged or uncaged. We quantified snail phenotypes, snail population size, and resource abundance over multiple snail generations. We found that snails were expressing inducible defenses in our system although the magnitude of the responses varied over time and across predator species. Despite the expression of inducible defenses, caged predators did not reduce snail population size. There also was no evidence of TMIIs throughout the experiment suggesting that TMIIs have a minimal role in the long‐term structure of our communities. The absence of TMIIs was largely driven by the lack of predator‐induced reductions in resource consumption and the lack of consistent reductions in population size with predator cues. In contrast, we detected strong DMIIs associated with lethal predators suggesting that DMIIs are the dominant long‐term mechanism influencing community structure. Our results demonstrate that although predators can have significant effects on prey phenotypes and sometimes cause short‐term TMIIs, there may be few long‐term consequences of these responses on population dynamics and indirect interactions, at least within simple food webs. Research directed towards addressing the long‐term consequences of predator–prey interactions within communities will help to reveal whether the conclusions and predictions generated from short‐term experiments are applicable over ecological and evolutionary timescales.     

Non-lethal predator effects on the feeding rate and prey selection of the exotic zebra mussel Dreissena polymorpha

Predators may induce changes in prey feeding that indirectly influence both the impact of prey on resource abundances and their interactions with other species in their community. We evaluated whether clearance and excretion (faeces plus pseudofaeces) of phytoplankton by zebra mussels were affected by the presence of predatory cues from roach, Rutilus rutilus, and signal crayfish, Pasifastacus leniusculus . We found that non-lethal effects of predators can alter zebra mussel clearance rate and thus the impact of zebra mussels on phytoplankton. Risk cues released by both predators had similar negative effects on clearance rate of zebra mussels and cascading positive indirect effects on phytoplankton resources. Predation risk had a stronger effect on zebra mussels' clearance rate of cyanobacteria and diatoms than cryptophytes and chrysophytes. The presence of predators did not significantly affect the rate at which zebra mussels expelled and excreted phytoplankton, although there was a tendency for more chlorophyll to be expelled and excreted in the presence of predators. Our results contribute to the growing evidence that predators indirectly affect resource dynamics and food web structure through their non-lethal effects on consumers. Our results suggest that exotic species such as zebra mussels can show behavioural responses to both native (e.g. roach) and exotic (e.g. crayfish) predators.     

Habitat effects on the relative importance of trait- and density-mediated indirect interactions

Classical views of trophic cascades emphasize the primacy of consumptive predator effects on prey populations to the transmission of indirect effects [density-mediated indirect interactions (DMIIs)]. However, trophic cascades can also emerge without changes in the density of interacting species because of non-consumptive predator effects on prey traits such as foraging behaviour [trait-mediated indirect interactions (TMIIs)]. Although ecologists appreciate this point, measurements of the relative importance of each indirect predator effect are rare. Experiments with a three-level, rocky shore food chain containing an invasive predatory crab ( Carcinus maenas ), an intermediate consumer (the snail, Nucella lapillus ) and a basal resource (the barnacle, Semibalanus balanoides ) revealed that the strength of TMIIs is comparable with, or exceeds, that of DMIIs. Moreover, the sign and strength of each indirect predator effect depends on whether it is measured in risky or refuge habitats. Because habitat shifts are often responsible for the emergence of TMIIs, attention to the sign and strength of these interactions in both habitats will improve our understanding of the link between individual behaviour and community dynamics.     

Altered trophic pathway and parasitism in a native predator (Lepomis gibbosus) feeding on introduced prey (Dreissena polymorpha)

Populations of invasive species tend to have fewer parasites in their introduced ranges than in their native ranges and are also thought to have fewer parasites than native prey. This ‘release’ from parasites has unstudied implications for native predators feeding on exotic prey. In particular, shifts from native to exotic prey should reduce levels of trophically transmitted parasites. We tested this hypothesis in native populations of pumpkinseed sunfish (Lepomis gibbosus) in Lake Opinicon, where fish stomach contents were studied intensively in the 1970s, prior to the appearance of exotic zebra mussels (Dreissena polymorpha) in the mid-1990s. Zebra mussels were common in stomachs of present-day pumpkinseeds, and stable isotopes of carbon and nitrogen confirmed their importance in long-term diets. Because historical parasite data were not available in Lake Opinicon, we also surveyed stomach contents and parasites in pumpkinseed in both Lake Opinicon and an ecologically similar, neighboring lake where zebra mussels were absent. Stomach contents of pumpkinseed in the companion lake did not differ from those of pre-invasion fish from Lake Opinicon. The companion lake, therefore, served as a surrogate “pre-invasion” reference to assess effects of zebra mussel consumption on parasites in pumpkinseed. Trophically transmitted parasites were less species-rich and abundant in Lake Opinicon, where fish fed on zebra mussels, although factors other than zebra mussel consumption may contribute to these differences. Predation on zebra mussels has clearly contributed to a novel trophic coupling between littoral and pelagic food webs in Lake Opinicon.     

Indirect interactions in a rice ecosystem: density dependence and the interplay between consumptive and non‐consumptive effects of predators

1. Density‐ and trait‐mediated indirect interactions (DMIIs and TMIIs, respectively) in food chains play crucial roles in community structure and processes. However, factors affecting the relative strength of these interactions are poorly understood, including in widespread and important freshwater rice ecosystems. 2. We studied the strength of DMIIs and TMIIs in a food chain involving a predator (the Reeve’s turtle Chinemys reevesii), its herbivorous prey (the apple snail Pomacea canaliculata) and a plant (rice Oryza sativa) in outdoor containers simulating rice fields. We also evaluated consumptive and non‐consumptive effects of the predator on the snail. We removed a fixed proportion of snails every 2 days to simulate prey consumption and introduced a caged turtle that was fed daily with snails to simulate non‐consumptive effects. 3. Direct consumptive effects increased growth of the remaining snails and their per capita feeding rate. Moreover, consumptive and non‐consumptive effects, and their interaction, affected the proportion of snails buried in the soil. This interaction was presumably because increasing food availability per snail induced their self‐burying behaviour. 4. Both DMIIs and TMIIs affected the number of rice plants remaining, whereas their interaction term was not significant. 5. In summary, density dependence and interactions between consumptive and non‐consumptive effects influenced snail growth and behaviour, respectively. However, no cascading effects of these complicated interactions on rice plants were detected.     

Predation risk in tadpole populations shapes behavioural responses of prey but not strength of trait‐mediated indirect interactions

Prey animals often respond to predators by reducing activity levels. This can produce a trait‐mediated indirect interaction (TMII) between predators and prey resources, whereby reduced foraging by prey in the presence of a predator causes an increase in prey resources. TMIIs play important roles in structuring communities, and it is important to understand factors that determine their strength. One such influence may be behavioural variation in the prey species, with indirect effects of predators being stronger within populations that are more responsive to the presence of a predator. We tested 1) whether the behavioural responsiveness of populations of wood frog tadpoles to predator cues was related to the predation risk in their native ponds, and 2) whether more responsive tadpoles yielded stronger TMIIs. To do this, we 1) measured the activity of tadpoles from 18 populations in mesocosms with and without caged predators, and 2) measured changes in the biomass of periphyton (the tadpoles’ diet) between predator treatments for each population. We found that tadpoles from higher predation risk ponds reduced their time outside refuges more in the presence of predators and tended to move less when visible, suggesting possible local adaptation to predation regimes. Though the presence of predators generally resulted in higher periphyton biomass – a TMII – there was no evidence that the strength of this TMII was affected by variation in tadpole behaviour. Foraging activity and general activity may be decoupled to some extent, enabling high predation risk‐adapted tadpoles to limit the fitness costs of reduced foraging when predators are present.     

Simulation of trait- and density-mediated indirect effects induced by piscivorous predators

The study demonstrates how to use a spatially explicit individual-based model (IBM) to tackle the problem of resolving density-mediated (DMII) and trait-mediated indirect interactions (TMII) when manipulative experimentation is difficult on the scale of community-level interactions in the real word. As an example we simulated predator-induced behavioural changes in a roach (Rutilus rutilus (L.)) population before and after stocking a lake with an additional pelagic predator (Sander lucioperca (L.)). By this it was possible to calculate bioenergetic gains and costs of trait changes as well as the relative proportions of DMIIs and TMIIs in roach consumption on littoral and pelagic resources in the lake food web. Despite higher net activity costs of 64%, roach migrating horizontally over the diel cycle had a net benefit (higher energy intake of 75%) compared to roach that were restricted to the littoral as a behavioural response to the new predator. By separating the components of the indirect predator effects, we demonstrate that a predator-induced modification in prey traits (behaviour, feeding activity) could contribute substantially to the net indirect effect of a predator even when there are strong density effects. When considering the predation pressure by roach on pelagic resources, the net TMIIs due to the behavioural adaptation of the consumer in response to the predator were 13–14 times stronger than the DMIIs alone. When comparing the different effects on littoral resources it can be shown that the TMIIs approximately compensate the DMIIs thus resulting in a nearly zero net effect.

Zusammenfassung

In der vorliegenden Arbeit wurde ein räumlich explizites individuenbasiertes Modell (IBM) verwendet, um dichteabhängige (DMII) und eigenschaftsabhängige (TMII) indirekte Interaktionen zu vergleichen. Da sich dies auf der Ebene der Lebensgemeinschaft unter realen experimentellen Bedingungen als besonders schwierig erwiesen hat, wurden räuberinduzierte Verhaltensänderungen einer Plötzenpopulation (Rutilus rutilus (L.)) vor und nach dem Besatz mit einem zusätzlichen pelagischen Räuber, dem Zander (Sander lucioperca (L.)), simuliert. Dadurch war es möglich, sowohl die energetischen Kosten und Nutzen der räuberinduzierten Verhaltensänderung als auch die relativen Anteile von DMIIs and TMIIs an der Plötzenkonsumtion auf litorale und pelagische Ressourcen innerhalb des Nahrungsnetzes zu kalkulieren. Verglichen mit den Plötzen, welche sich aufgrund des neuen Räubers vor allem im Litoral aufhielten, hatten die täglich (horizontal) wandernden Plötzen trotz höherer Nettoaktivitätskosten von 64% eine größere Energieaufnahme von 75%. Durch die Auflösung der verschiedenen indirekten Räubereffekte konnten wir demonstrieren, dass räuberinduzierte Modifikationen von Beuteeigenschaften (Verhalten, Fraßaktivität) erheblich zu den indirekten Nettoeffekten eines Räubers beitragen können, auch wenn starke dichteabhängige Effekte vorhanden sind. Wenn man die Plötzenkonsumtion auf pelagische Ressourcen betrachtet, so ergeben sich 13-14 fach stärkere TMIIs als DMIIs. Wenn man die verschiedenen Effekte auf litorale Ressourcen vergleicht, zeigt es sich, dass TMIIs die DMIIs kompensieren, wodurch der indirekte Nettoeffekt fast null beträgt.     

Zebra mussels affect benthic predator foraging success and habitat choice on soft sediments

The introduction of zebra mussels (Dreissena spp.) to North America has resulted in dramatic changes to the complexity of benthic habitats. Changes in habitat complexity may have profound effects on predator-prey interactions in aquatic communities. Increased habitat complexity may affect prey and predator dynamics by reducing encounter rates and foraging success. Zebra mussels form thick contiguous colonies on both hard and soft substrates. While the colonization of substrata by zebra mussels has generally resulted in an increase in both the abundance and diversity of benthic invertebrate communities, it is not well known how these changes affect the foraging efficiencies of predators that prey on benthic invertebrates. We examined the effect of zebra mussels on the foraging success of four benthic predators with diverse prey-detection modalities that commonly forage in soft substrates: slimy sculpin (Cottus cognatus), brown bullhead (Ameirus nebulosus), log perch (Percina caprodes), and crayfish (Orconectes propinquus). We conducted laboratory experiments to assess the impact of zebra mussels on the foraging success of predators using a variety of prey species. We also examined habitat use by each predator over different time periods. Zebra mussel colonization of soft sediments significantly reduced the foraging efficiencies of all predators. However, the effect was dependent upon prey type. All four predators spent more time in zebra mussel habitat than in either gravel or bare sand. The overall effect of zebra mussels on benthic-feeding fishes is likely to involve a trade-off between the advantages of increased density of some prey types balanced against the reduction in foraging success resulting from potential refugia offered in the complex habitat created by zebra mussels.     

Intraspecific variation in the consumption of exotic prey – a mechanism that increases biotic resistance against invasive species?

1. Hyper-successful exotic species can both displace the native prey that formerly made up a native predator's diet and represent an abundant potential prey resource for native predators. Little is known about how this drastic change affects native predators, or their short- and long term potential to regulate the exotic species.
2. We compared zebra mussel consumption by pumpkinseed sunfish ( Lepomis gibbosus ), redbreast sunfish ( Lepomis auritus ) and rock bass ( Ambloplites rupestris ) from populations that were either previously exposed to zebra mussels or naive to them.
3. Fish from populations with longer exposure to zebra mussels consumed many more zebra mussels than fish from populations with shorter or no previous exposure to zebra mussels.
4. Our experiment does not allow us to identify the mechanisms that underlie the patterns we found, but we discuss several plausible scenarios and their ecological implications.
5. Predator adaptation to exotic prey may be an important but overlooked factor in invasion biology. The initial response to exotic prey by a native predator may be a poor estimate of its ability to present biotic resistance to the invasion over the long term.     

Indirect trophic interactions with an invasive species affect phenotypic divergence in a top consumer

While phenotypic responses to direct species interactions are well studied, we know little about the consequences of indirect interactions for phenotypic divergence. In this study we used lakes with and without the zebra mussel to investigate effects of indirect trophic interactions on phenotypic divergence between littoral and pelagic perch. We found a greater phenotypic divergence between littoral and pelagic individuals in lakes with zebra mussels and propose a mussel-mediated increase in pelagic and benthic resource availability as a major factor underlying this divergence. Lakes with zebra mussels contained higher densities of large plankton taxa and large invertebrates. We suggest that this augmented resource availability improved perch foraging opportunities in both the littoral and pelagic zones. Perch in both habitats could hence express a more specialized foraging morphology, leading to an increased divergence of perch forms in lakes with zebra mussels. As perch do not prey on mussels directly, we conclude that the increased divergence results from indirect interactions with the mussels. Our results hence suggest that species at lower food web levels can indirectly affect phenotypic divergence in species at the top of the food chain.     

Population genetic history of the dreissenid mussel invasions: expansion patterns across North America

This study tests population genetic patterns across the Eurasian dreissenid mussel invasions of North America—encompassing the zebra mussel Dreissena polymorpha (1986 detection) and the quagga mussel D. rostriformis bugensis (detected in 1990, which now has largely displaced the former in the Great Lakes). We evaluate their source-spread relationships and invasion genetics using 9–11 nuclear microsatellite loci for 583 zebra mussels (21 sites) and 269 quagga mussels (12 sites) from Eurasian and North American range locations, with the latter including the Great Lakes, Mississippi River basin, Atlantic coastal waterways, Colorado River system, and California reservoirs. Additionally, mtDNA cytochrome b gene sequences are used to verify species identity. Our results indicate that North American zebra mussels originate from multiple non-native northern European populations, whereas North American quagga mussels trace to native estuaries in the Southern Bug and Dnieper Rivers. Invasive populations of both species show considerable genetic diversity and structure (zebra F _ST = 0.006–0.263, quagga F _ST = 0.008–0.267), without founder effects. Most newer zebra mussel populations have appreciable genetic diversity, whereas quagga mussel populations from the Colorado River and California show some founder effects. The population genetic composition of both species changed over time at given sites; with some adding alleles from adjacent populations, some losing them, and all retaining closest similarity to their original composition. Zebra mussels from Kansas and California appear genetically similar and assign to a possible origin from the St. Lawrence River, whereas quagga mussels from Nevada and California assign to a possible origin from Lake Ontario. These assignments suggest that overland colonization pathways via recreational boats do not necessarily reflect the most proximate connections. In conclusion, our microsatellite results comprise a valuable baseline for resolving present and future dreissenid mussel invasion pathways.     

Testing a typology system of running waters for conservation planning in Hungary

The zebra mussel (Dreissena polymorpha) and its congener the quagga mussel (Dreissena rostriformis bugensis) are both invaders in freshwater, but have very different invasion histories, with zebra mussels attaining substantially faster rates of spread at virtually all spatial scales. However, in waterbodies where they co-occur, D. r. bugensis can displace D. polymorpha. To determine if the mechanisms for this displacement are associated with different survival and growth, we kept mussels in flow-through tanks for 289 days with two temperature regimes that mimicked the natural surface water (littoral zone) and hypolimnion conditions of Lake Erie. For the littoral zone regime, we used water directly from the surface of Lake Erie (range 4–25°C, average 11.9 ± 0.6°C). For the profundal zone treatment, Lake Erie surface water was chilled to about 6°C (range 5–8°C, average 6.2 ± 0.6°C) for the full duration of the experiment. For each of these temperature regimes, we used three replicate tanks with only zebra mussels present and three replicate tanks with only quagga mussels (150 ind./tank each), and three replicate tanks with both species (75 ind./tank of each species). Quagga mussels had higher survivorship and grew more than zebra mussels in all treatments. For both species, the size of the mussel entering the winter was critical for survivorship. Larger mussels had a higher survival over the winter in all treatments. For both species, there was a survivorship and growth tradeoff. In the warmer littoral zone treatment both species had higher growth, but lower survival than in the colder profundal zone treatment. Surprisingly, although quagga mussels outperformed zebra mussels, zebra mussel survivorship was better when they were faced with competition by quagga mussels than with just intraspecific competition. In addition, quagga mussels suffered size-specific mortality during the growing season only when facing interspecific competition with zebra mussels. Further experiments are needed to determine the possible mechanisms for these interspecific effects.     

Predator diversity strengthens trophic cascades in kelp forests by modifying herbivore behaviour

Although human-mediated extinctions disproportionately affect higher trophic levels, the ecosystem consequences of declining diversity are best known for plants and herbivores. We combined field surveys and experimental manipulations to examine the consequences of changing predator diversity for trophic cascades in kelp forests. In field surveys we found that predator diversity was negatively correlated with herbivore abundance and positively correlated with kelp abundance. To assess whether this relationship was causal, we manipulated predator richness in kelp mesocosms, and found that decreasing predator richness increased herbivore grazing, leading to a decrease in the biomass of the giant kelp Macrocystis. The presence of different predators caused different herbivores to alter their behaviour by reducing grazing, such that total grazing was lowest at highest predator diversity. Our results suggest that declining predator diversity can have cascading effects on community structure by reducing the abundance of key habitat-providing species.     

Effects of invasive zebra mussels on phytoplankton,turbidity, and dissolved nutrients in reservoirs

Few experiments have quantified the effects of invasive zebra mussels (Dreissena polymorpha) on man-made reservoirs relative to other aquatic habitats. Reservoirs, however, are the dominate water body type in many of the states that are at the current front of the zebra mussel invasion into the western United States. The objective of this research, therefore, was to determine how zebra mussels affected phytoplankton, turbidity, and dissolved nutrients in water that was collected from three Kansas reservoirs that varied in trophic state (mesotrophic to hypereutrophic), but all experienced frequent cyanobacterial blooms. Laboratory mesocosm experiments were conducted to document the effects of zebra mussels on cyanobacteria and general water quality characteristics in the reservoir water. Zebra mussels significantly reduced algal biomass, and the total biovolume of cyanobacteria (communities were dominated by Anabaena) in each reservoir experiment. The effects of zebra mussels on other major algal groups (diatoms, flagellates, and green algae) and algal diversity were less consistent and varied between the three reservoir experiments. Similarly, the effects of zebra mussels on nutrient concentrations varied between experiments. Zebra mussels increased dissolved phosphorus concentrations in two of the reservoir experiments, but there was no effect of zebra mussels on dissolved phosphorus in the mesotrophic reservoir experiment. Combined, our results strongly suggest that zebra mussels have the potential to significantly impact reservoirs as they continue to expand throughout the western United States. Moreover, the magnitude of these effects may be context dependent and vary depending on the trophic state and/or resident phytoplankton communities of individual reservoirs as has similarly been reported for natural lakes.     

Not worth the risk: apex predators suppress herbivory on coral reefs

Apex predators are known to exert strong ecological effects, either through direct or indirect predator–prey interactions. Indirect interactions have the potential to influence ecological communities more than direct interactions as the effects are propagated throughout the population as opposed to only one individual. Indirect effects of apex predators are well documented in terrestrial environments, however there is a paucity of information for marine environments. Furthermore, manipulative studies, as opposed to correlative observations, isolating apex predator effects are lacking. Coral reefs are one of the most diverse ecosystems, providing a useful model system for investigating the ecological role of apex predators and their influence on lower trophic levels. Using predator models and transplanted macroalgae we examined the indirect effects of predators on herbivore foraging behaviour. We show that the presence of a model reef shark or large coral‐grouper led to a substantial reduction in bite rate and species richness of herbivorous fishes and an almost absolute localized cessation of macroagal removal, due to the perceived risk of predation. A smaller‐sized coral‐grouper also reduced herbivore diversity and activity but to a lesser degree than the larger model predators. These indirect effects of apex predators on the foraging behaviour of herbivores may have flow‐on effects on the biomass and distribution of macroalgae, and the functioning of coral reef ecosystems. This highlights that the ecological interactions and processes that contribute to ecosystem resilience may be more complex than previously assumed.     

Experimental measurements of zebra mussel (Dreissena polymorpha) impacts on phytoplankton community composition

1 . To investigate direct effects of zebra mussel ( Dreissena polymorpha ) feeding activities on phytoplankton community composition, short‐term microcosm experiments were performed in natural water with complex phytoplankton communities. Both gross effects (without resuspension of mussel excretions) and net effects (with resuspension) were studied.
2. Gross clearance rates were not selective; essentially all taxa were removed at similar rates ranging from 24 to 63 mL mussel^–1 h^–1. Net clearance rates were highly selective; different plankton taxa were removed at very different rates, ranging from 12 to 83% of the gross rates, leading to consistent changes in the phytoplankton community composition. Thus, although zebra mussels can cause most phytoplankton to decline, there is considerable variation among taxa in either pre‐digestive selection or post‐digestive survival.
3. The direct, short‐term effects of zebra mussels on phytoplankton community composition are consistent with some of the major changes observed in the Hudson River since establishment of zebra mussels.
4. We show, with simple calculations, how zebra mussel filtration rate, its selective efficiency on various taxa, and phytoplankton growth rates interact to produce changes in the phytoplankton composition.     

Relative strengths of top-down and bottom-up forces in a tropical forest community

We tested integrative bottom-up and top-down trophic cascade hypotheses with manipulative experiments in a tropical wet forest, using the ant-plant Piper cenocladum and its associated arthropod community. We examined enhanced nutrients and light along with predator and herbivore exclusions as sources of variation in the relative biomass of plants, their herbivores (via rates of herbivory), and resident predaceous ants. The combined manipulations of secondary consumers, primary consumers, and plant resources allowed us to examine some of the direct and indirect effects on each trophic level and to determine the relative contributions of bottom-up and top-down cascades to the structure of the community. We found that enhanced plant resources (nutrients and light) had direct positive effects on plant biomass. However, we found no evidence of indirect (cascading through the herbivores) effects of plant biomass on predators or top predators. In contrast, ants had indirect effects on plant biomass by decreasing herbivory on the plants. This top-down cascade occurred whether or not plant resources were enriched, conditions which are expected to modify top-down forces. Received: 9 August 1998 / Accepted: 1 December 1998