Density-mediated Indirect Interactions Research Articles

The common predator, Sheepshead (Archosargus probatocephalus), enhances survival of the Eastern oyster (Crassostrea virginica) in an experimental setting

Oyster reefs are a vital but declining component of nearshore, estuarine ecosystems. Global efforts to restore this important habitat are ongoing but have had varied success. The potential for biotic factors such as predation to influence restoration trajectories is rarely considered; however, there is mounting evidence that a better understanding of trophic relationships could aid in the restoration of the ecologically critical Eastern oyster (Crassostrea virginica). A common predator-prey interaction within the oyster reef community was examined through manipulative experiments to determine what effect predator presence had on early survivorship of oyster spat – a factor known to influence the success of oyster reef restoration. The juvenile stone crab (Menippe spp.) is a known predator of spat (<30 mm) sized Eastern oysters but also serves as prey for mobile predatory fish. Sheepshead (Archosargus probatocephalus), a fish with a diet high in structure-associated invertebrates, are one of many predatory fishes that utilize oyster reefs. Adult Sheepshead presence was found to significantly increase oyster spat survival. Oyster spat survival can be attributed to both trait and density-mediated indirect interactions. These results suggest that changes in the presence of an exploited finfish species can influence oyster spat survivorship.

Trait-mediated indirect effects in a natural tidepool system

We demonstrate an apparent trait-mediated indirect interaction (TMII) of predators on primary producers in a natural community by altering prey behavior over short and long time scales. Small predatory sea stars (Leptasterias spp.) caused herbivorous snails (Tegula funebralis) added to rocky intertidal tidepools to quickly flee into refuge microhabitats outside tidepools within days, and this was associated with a 58% increase in microalgal growth after 2 weeks. Similarly, removing sea stars caused snails to increase use of tidepools for 1–10 months. After adding sea stars to tidepools, snails quickly fled and then consistently increased use of refuges outside tidepools for 10 months. This was associated with average increases of 59% for microalgal growth over 1 month and 254% for macroalgal growth over 8 months inside tidepools. In 63 unmanipulated tidepools, densities of sea stars and snails were negatively correlated. High densities of snails were associated with unpalatable algal species and bare rock, while high densities of sea stars were associated with palatable algal species, suggesting that this apparent TMII may have community-level impacts. Though multiple lines of evidence suggest TMIIs were likely operating in this system, it was not possible to experimentally partition the relative contributions of TMIIs and density-mediated indirect interactions (DMIIs), so further caging experiments are necessary to distinguish their relative strengths. Overall, we suggest that predators can benefit primary producers by changing prey behavior even when predators and prey are unrestrained by cages or mesocosms, embedded in complex communities, and observed over multiple time scales.

Contrasting indirect effects of an ant host on prey–predator interactions of symbiotic arthropods

Indirect interactions occur when a species affects another species by altering the density (density-mediated interactions) or influencing traits (trait-mediated interactions) of a third species. We studied variation in these two types of indirect interactions in a network of red wood ants and symbiotic arthropods living in their nests. We tested whether the ant workers indirectly affected survival of a symbiotic prey species (Cyphoderus albinus) by changing the density and/or traits of three symbiotic predators, i.e., Mastigusa arietina, Thyreosthenius biovatus and Stenus aterrimus, provoking, respectively, low, medium and high ant aggression. An ant nest is highly heterogeneous in ant worker density and the number of aggressive interactions towards symbionts increases with worker density. We, therefore, hypothesized that varying ant density could indirectly impact prey-predator interactions of the associated symbiont community. Ants caused trait-mediated indirect effects in all three prey-predator interactions, by affecting the prey capture rate of the symbiotic predators at different worker densities. Prey capture rate of the highly and moderately aggressed spider predators M. arietina and T. biovatus decreased with ant density, whereas the prey capture rate of the weakly aggressed beetle predator S. aterrimus increased. Ants also induced density-mediated indirect interactions as high worker densities decreased the survival rate of the two predatory spider species. These results demonstrate for the first time that a host can indirectly mediate the trophic interactions between associated symbionts. In addition, we show that a single host can induce opposing indirect effects depending on its degree of aggression towards the symbionts.

Density-mediated indirect effects from active predators and narrow habitat domain prey.

The hunting-mode-habitat-domain-range framework suggests that the mechanism driving trophic cascades (i.e., trait-mediated indirect interactions [TMIIs] vs. density-mediated indirect interactions [DMIIs]) should depend upon the functional traits of predators and prey. For example, trophic cascades containing active, broad habitat domain range (BHDR) predators interacting with narrow habitat domain range (NHDR) prey are predicted to arise primarily via TMIIs, because these prey should reduce their conspicuous activity in the presence of these predators. Unfortunately, this hypothesis is difficult to test given the strong bias against studies assessing trophic cascades containing NHDR prey. Furthermore, this hypothesis ignores evidence that (1) active predators can have high consumption rates on prey, (2) continuously responding to active predators foraging across broad areas is energetically costly for prey, and (3) cues from active, BHDR predators may not influence prey density. We examined the TMIIs and total indirect interaction (TII) produced during interactions between an active, BHDR ladybeetle predator (Naemia seriata) and its NHDR prey (scale insects). We exposed scale insects to nonlethal and lethal ladybeetle predators in laboratory mesocosms for 15 weeks. We measured the growth of the scale insect's host plant (cordgrass) and the population density of scale insects. Contrary to theory, nonlethal ladybeetles did not induce TMIIs. However, lethal ladybeetles increased cordgrass total and root dry biomass by 36% and 44%, respectively, suggesting the presence of strong DMIIs. Additionally, both lethal and nonlethal ladybeetles reduced scale insect population density. Our findings suggest that DMIIs, rather than TMIIs, can result from interactions between active BHDR predators and NHDR prey.

Predation Risk Reverses the Potential Effects of Warming on Plant-Herbivore Interactions by Altering the Relative Strengths of Trait- and Density-Mediated Interactions.

Climate warming will initiate numerous changes in ecological community structure and function, and such high-level impacts derive from temperature-driven changes in individual physiology. Specifically, top-down control of plant biomass is sensitive to rising temperatures, but the direction of change depends on a complex interaction between temperature, predation risk, and predator thermal preference. Here, I developed an individual-based optimal foraging model of three trophic levels (primary producers, herbivores, and predators) to examine how warming affects top-down control of primary producers via both trait- and density-mediated indirect interactions (TMII and DMII). This model also factorially crossed warm- and cold-adapted herbivores and predators to determine how local adaptation modifies the effects of warming on food web interactions. Regardless of predator thermal preference, warming increased herbivore foraging effort and by extension predation rates. As a result, TMII declined in importance at high temperatures regardless of predator thermal adaptation. Finally, predation risk reduced herbivore fitness via both indirect (i.e., reduced herbivore size) and direct (i.e., reduced herbivore survival) pathways. These results suggest that, contrary to previous predictions, warming might stimulate primary productivity by reducing herbivore population sizes, releasing plants from immediate top-down control.

生态系统基于性状调节的物种间接作用:特征、成因及后果

PDF HTML阅读 XML下载 导出引用 引用提醒 生态系统基于性状调节的物种间接作用:特征、成因及后果 DOI: 10.5846/stxb201610011986 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31230012）；国家科技基础性工作专项（2012FY111900）；教育部创新团队发展计划（IRT-16R11）；中国博士后面上基金（2016M590246） Characteristics, causes, and consequences of trait-mediated indirect interactions in ecosystems Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:物种之间的间接作用关系是维持生物群落结构以及生态系统功能的关键因素。目前的理论模型和实验性研究均认为，物种间接作用的传递主要是由物种的密度变化所引起。但大量的实验证据表明，生物个体在形态、生理、行为和生活史性状的适应性变化，是物种间形成间接互作关系的另一个重要机制。生态学家把这种基于物种性状调控的种间关系称为性状调节的间接作用。深入了解性状调节的间接作用类型，发生机制和作用途径，有助于阐释自然界中多物种种间关系的复杂性和多样性形成机制。对性状调节的概念和性状的分类进行系统的总结，指出性状可塑性和物种特异性是物种间性状调节关系形成的两个重要机制。与其他类型的种间互作一样，性状调节的种间作用是驱动物种进化的重要力量。此外，分析了性状调节在自然群落中传递的几个重要途径，并强调这些间接作用在影响和调节生态系统功能和过程中的重要作用。最后，就性状调节种间关系的研究对象、研究尺度、以及研究方法等问题提出若干建议，为今后对这一问题的研究提供相关参考。 Abstract:Indirect species interactions play a critical role in shaping community structure and the functioning of ecosystems. Previous empirical and theoretical studies commonly assumed that indirect interactions were caused in general by changes in population density of interacting species, which is referred to as ‘density-mediated indirect interactions.’ However, there is growing evidence that changes in traits of the individual intermediate species can also be an important mechanism that regulates indirect interactions among species, and ecologists term these species interactions ‘trait-mediated indirect interactions’ (TMⅡs). TMⅡs are prevalent in both aquatic and terrestrial systems, and their effects are often as strong as, or stronger than, those of density-mediated indirect effects. Studies on the characteristics, causes, and consequences of TMⅡs are essential to understand the complexity and diversity of multispecies interactions in natural ecosystems fully. In this paper, we summarize the concept and classification of TMⅡs, and suggest that trait plasticity and species specificity are the two intrinsic mechanisms for the occurrence of TMⅡs. We also review the direct evidence for TMⅡs in various permutations of interaction webs that can have both trophic and non-trophic links, and then highlight the profound effects of TMⅡs on regulation of ecological functions and processes in a wide range of ecological systems. Finally, we propose future developments regarding the effects of TMⅡs to provide a reference framework for related research. 参考文献 相似文献 引证文献

Density‐mediated indirect interactions alter host foraging behaviour of parasitoids without altering foraging efficiency

1. Foraging decisions of parasitoids are influenced by host density via density-mediated indirect interactions. However, in the parasitoid's environment, non-suitable herbivores are also present. These non-hosts also occur in different densities, which can affect a parasitoid's foraging behaviour. 2. The influence of non-host densities can be expressed during the first phase of the foraging process, when parasitoids use plant volatiles to locate plants infested by their host. They may also play a role during the second phase, when parasitoids use infochemicals from the host and plant to locate, recognise and accept the host. 3. By using laboratory and field setups, it was studied whether the density of non-host herbivores influences these two phases of the foraging behaviour of the parasitoid Cotesia glomerata as well as the parasitoid's efficiency to find its host, Pieris brassicae caterpillars. 4. The findings show that a high non-host density, regardless of the species used, negatively affected parasitoid preference for host-infested plants, but that the behaviour on the plant and the total host-finding efficiency of the parasitoids were not influenced by non-host density. 5. These results are discussed in the context of density-mediated indirect interactions.

Indirect effects of invasive species affecting the population structure of an ecosystem engineer

Species invasion is of increasing concern as non‐native species often have negative impacts on ecosystems that they were introduced to. Invaders negatively affect the abundance of native species due to direct interactions like predation and competition. Additionally, invaders may benefit native biota by imposing indirect effects on resident species interactions. Invaders indirectly affect resident species via both density‐mediated indirect interactions (DMIIs) and trait‐mediated indirect interactions (TMIIs). Previous studies on these different indirect interactions have largely examined the effects on structuring ecological systems, with paying little attention to the role of body size. Here, we experimentally demonstrate that an invasive habitat modifier of European coastal waters, the Pacific oyster (Crassostrea gigas), alters the population structure of native mussels (Mytilus edulis) by modifying the size specific predator‐prey interaction between the mussels and the shore crab (Carcinus maenas). In laboratory split‐plot experiments, the presence of Pacific oysters reduced the mortality of unconditioned mussels as well as mussels that were acclimatized in presence of predatory cues, while being exposed to predation by crabs of two different size classes. The reduction in mortality was size‐dependent both in terms of the predators and the prey. The presence of oysters notably reduced mussel mortality in presence of small crabs, while the mortality rate in presence of big crabs was less affected. Mussels that benefited the most by the presence of oysters were those of recruitment stages, smaller than 20 mm in shell length. Our results suggest that oysters cause a strong shift in the population structure of M. edulis, reducing particularly the mortality of smaller sized mussels.

Predators reduce Batrachochytrium dendrobatidis infection loads in their prey

Summary Disease ecologists and wildlife managers are increasingly interested in understanding how predators regulate infection in prey populations. The healthy herds hypothesis suggests that predators may decrease infection prevalence by decreasing overall population size, reducing density‐dependent transmission and culling infected individuals from a population. While this model incorporates density‐mediated indirect interactions (DMIIs), it does not incorporate the potential role of trait‐mediated indirect interactions (TMIIs). Using wood frog tadpoles (Lithobates sylvatica), we examined whether predator cues could alter the prevalence and intensity of infection by the fungal pathogen Batrachochytrium dendrobatidis [Bd] and whether both stressors could alter the host's life history traits. Exposure to predator cues caused tadpoles to have reduced Bd infection loads, potentially as a result of stress‐induced immunoenhancement. Tadpoles exposed to Bd had faster development than tadpoles not exposed to Bd, but they did not differ in survival or growth. This suggests that, in this life stage of this species, Bd infection does not have fitness costs. These data suggest that the trait‐mediated effects of predators on infection may alter epidemiological outcomes of Bd exposure.

Prey density affects strengths of density- and trait-mediated indirect interactions of predators on an algal community

In trophic cascades, predators can indirectly affect resources by reducing prey density (density-mediated indirect interactions; DMIIs) or by changing prey traits, such as their behavior, morphology, physiology, or life history (trait-mediated indirect interactions; TMIIs). Although the importance of predator (e.g., foraging strategy) and resource characteristics (e.g., quantity) in these indirect interactions is well recognized, little attention has been paid to prey characteristics (e.g., density), especially in field studies. We focused on a marine trophic cascade involving the carnivorous snail Thais clavigera, its limpet prey Siphonaria sirius, and the algae Lithoderma sp. and Ulva sp. Using intertidal rocks as natural replicates, we experimentally evaluated the in situ effects of the DMIIs and TMIIs on the algal community under two density ranges of prey. The strengths of consumptive effects (CEs) and non-consumptive effects (NCEs) of the predator on the prey limpet were also monitored to elucidate the mechanisms of the indirect interactions. At high densities, CEs decreased the percentage of individual limpets that disappeared (that likely died); however, neither DMIIs nor TMIIs were detected. At low densities, both CEs and NCEs reduced the per capita feeding rate and CEs reduced the growth rate of limpets. Moreover, Lithoderma sp. was replaced by Ulva sp. through both DMIIs and TMIIs. These results suggest that prey density is a key determinant of the strengths of CEs and NCEs, and notably, of DMIIs and TMIIs.

Changes in algal community structure via density‐ and trait‐mediated indirect interactions in a marine ecosystem

In various terrestrial and aquatic ecosystems, predators affect resources indirectly via intermediate prey. Such indirect interactions involve reducing the density of the prey (density-mediated indirect interactions, DMIIs) or changing the behavioral, morphological, or life history traits of the prey (trait-mediated indirect interactions, TMIIs). Although the importance of TMIIs has been highlighted recently, the strengths of both DMIIs and TMIIs under natural conditions have rarely been evaluated, especially in the context of resource community structure. We studied a three-level marine food chain involving the carnivorous snail Thais clavigera, its limpet prey Siphonaria sirius, and the limpet's food sources, the algae Lithoderma sp. and Ulva sp. We measured the strengths of DMIIs and TMIIs and observed how the algal community changes under the pressure of natural predation by T. clavigera on S. sirius. Neither DMIIs nor TMIIs affected the total algal cover or chlorophyll content per unit area. However, both types of indirect interactions caused similar changes in algal composition by increasing the cover of Ulva and decreasing the cover of Lithoderma. This change in the algal community was caused by a reduction in the limpet's preferential consumption of the competitively dominant Ulva over Lithoderma. These results suggest that both DMIIs and TMIIs have similar effects on the changes in resource community structure under natural conditions.

Predator Diversity Effects in an Exotic Freshwater Food Web

Cascading trophic interactions are often defined as the indirect effects of a predator on primary producers through the effect of the predator on herbivores. These effects can be both direct through removal of herbivores [density-mediated indirect interactions (DMIIs)] or indirect through changes in the behavior of the herbivores [trait-mediated indirect interactions (TMIIs)]. How the relative importance of these two indirect interactions varies with predator diversity remains poorly understood. We tested the effect of predator diversity on both TMIIs and DMIIs on phytoplankton using two competitive invasive dreissenid mussel species (zebra mussel and quagga mussel) as the herbivores and combinations of one, two or all three species of the predators pumpkinseed sunfish, round goby, and rusty crayfish. Predators had either direct access to mussels and induced both TMII and DMII, or no direct access and induced only TMII through the presence of risk cues. In both sets of treatments, the predators induced a trophic cascade which resulted in more phytoplankton remaining with predators present than with only mussels present. The trophic cascade was weaker in three-predator and two-predator treatments than in one-predator treatments when predators had direct access to dreissenids (DMIIs and TMIIs). Crayfish had higher cascading effects on phytoplankton than both pumpkinseed and round goby. Increased predator diversity decreased the strength of DMIIs but had no effect on the strength of TMIIs. The strength of TMIIs was higher with zebra than quagga mussels. Our study suggests that inter-specific interference among predators in multi-species treatments weakens the consumptive cascading effects of predation on lower trophic levels whereas the importance of predator diversity on trait mediated effects depends on predator identity.

Cascading trait‐mediated interactions induced by ant pheromones

Trait-mediated indirect interactions (TMII) can be as important as density-mediated indirect interactions. Here, we provide evidence for a novel trait-mediated cascade (where one TMII affects another TMII) and demonstrate that the mechanism consists of a predator eavesdropping on chemical signaling. Ants protect scale insects from predation by adult coccinellid beetles – the first TMII. However, parasitic phorid flies reduce ant foraging activity by 50% – the second TMII, providing a window of opportunity for female beetles to oviposit in high-quality microsites. Beetle larvae are protected from ant predation and benefit from living in patches with high scale densities. We demonstrate that female beetles can detect pheromones released by the ant when attacked by phorids, and that only females, and especially gravid females, are attracted to the ant pheromone. As ants reduce their movement when under attack by phorids, we conclude that phorids facilitate beetle oviposition, thus producing the TMII cascade.

Reconciling conflict between the direct and indirect effects of marine reserve protection

SUMMARYNo-take marine reserves directly promote the recovery of predatory species, which can have negative indirect effects on prey populations in reserves. When harvesting also occurs on prey species there is potential conflict between the direct and indirect effects of protection, and reserves may not have conservation benefits for prey species. For example, sea urchins are fished in many regions, but may decline in reserves due to increased predation rates. To investigate this potential conflict, this paper compares density, size, biomass and reproductive potential of both a harvested and an unharvested urchin species between a long-term reserve and unprotected sites in California. Consistent with density-mediated indirect interactions, densities of the unharvested species were 3.4-times higher at unprotected sites compared to reserve sites. However, for the harvested species, densities were comparable between reserve and unprotected sites. Both species were consistently larger at reserve sites, and the biomass and reproductive potential of the harvested species was 4.8- and 7.0-times higher, respectively, than at unprotected sites. This is likely due to differences in size-selectivity between harvesting and predators, and potential compensatory effects of predators. While the generality of these effects needs to be tested, these results suggest mechanisms whereby reserves can benefit both predator and prey species.

Marine reserve designation, trophic cascades and altered community dynamics

Marine ecosystems and their associated populations are increasingly at risk from the cumulative impacts of many anthropogenic threats that increase the likelihood of species extinc- tion and altered community dynamics. In response, marine reserves can be used to protect exploited species and conserve biodiversity. The increased abundance of predatory species in marine reserves may cause indirect effects along chains of multi-trophic interactions. These trophic cascades can arise through direct predation, density-mediated indirect interactions (DMIIs), or indirect behavioural effects, termed trait-mediated indirect interactions (TMIIs). The extent of algal cover and the abundance of 4 primary consumers were determined in Lough Hyne, which was designated Europe's first marine nature reserve in 1981. The primary consumers were the sea urchin Paracentrotus lividus, the topshell Gibbula cineraria, the oyster Anomia ephippium, and the scallop Chlamys varia. The abundances of 3 starfish species (Marthasterias glacialis, Aste- rias rubens, and Asterina gibbosa) were also determined, as were 2 potential crustacean preda- tors, Necora puber and Carcinus maenas. These data were compared with historical data from a 1962 (prey) and a 1963 (predator) survey to determine the nature of community interactions over adjacent trophic levels. The present study reveals a breakdown in population structure of the 4 surveyed prey species. Marine reserve designation has led to an increase in predatory crabs and M. glacialis, a subsequent decrease in primary consumers, especially the herbivore P. lividus, and an increase in macroalgal cover which is indicative of a trophic cascade. The study shows that establishing a Marine Reserve does not guarantee that conservation benefits will be distributed equally.

Trait‐ and Density‐Mediated Indirect Interactions Initiated by an Exotic Invasive Plant Autogenic Ecosystem Engineer

Indirect interactions are important for structuring ecological systems. However, research on indirect effects has been heavily biased toward top-down trophic interactions, and less is known about other indirect-interaction pathways. As autogenic ecosystem engineers, plants can serve as initiators of nontrophic indirect interactions that, like top-down pathways, can involve both trait-mediated indirect interactions (TMIIs) and density-mediated indirect interactions (DMIIs). Using microcosms, I examined a plant --> predator --> consumer interaction pathway involving the exotic autogenic ecosystem engineer Centaurea maculosa; native Dictyna spiders (which exhibit density and trait [web-building] responses to C. maculosa); Dictyna's insect prey, Urophora affinis; and Urophora's host plant (a secondary receiver species) to quantify DMIIs and TMIIs in an autogenic engineered pathway. Both DMIIs and TMIIs were strong enough to reduce Urophora populations, but only DMIIs, which were 4.3 times stronger than TMIIs, were strong enough to also reduce Urophora's fecundity and increase the fecundity of its host plant. Prior field studies support these results, suggesting that the differences between DMIIs and TMIIs are even stronger in nature. This study illustrates that autogenic ecosystem engineers can initiate powerful indirect interactions that generally parallel predator-initiated interactions but also differ in important functional ways.

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.

HABITAT COMPLEXITY DISRUPTS PREDATOR–PREY INTERACTIONS BUT NOT THE TROPHIC CASCADE ON OYSTER REEFS

Despite recognition of the significance of both food web interactions and habitat complexity in community dynamics, current ecological theory rarely couples these two processes. Experimental manipulations of the abundance of the two predators in an oyster-reef trophic cascade, and the structural complexity provided by reefs of living oysters, demonstrated that enhanced habitat complexity weakened the strengths of trophic interactions. The system of tri-trophic interactions included oyster toadfish (Opsanus tau) as the top predator that consumed the mud crab (Panopeus herbstii), which preys upon juvenile oysters (Crassostrea virginica). On reefs of low complexity, toadfish controlled mud crab abundances and indirectly determined the level of mortality of juvenile oysters. The indirect effects of toadfish on oysters emerged through their influence on how intensely mud crabs preyed on oysters. Augmentation of habitat complexity by substituting vertically oriented, living oysters for the flat shells of dead oysters disrupted both of the direct trophic linkages but did not alter the magnitude of the indirect effect of toadfish on juvenile oysters. This paradox can be understood by partitioning the mechanisms by which toadfish influence mud crabs and ultimately juvenile oysters. Trait-mediated indirect interactions (TMIIs; i.e., predator-avoidance behavior in mud crabs) accounted for 95.6–98.2% of toadfish indirect benefits to oyster survival and, consequently, were a much greater contributor than density-mediated indirect interactions (DMIIs; i.e., the reduction in crab abundance by toadfish). Avoidance behavior was unaffected by modification in habitat complexity. Complex reefs increased total oyster survival because added habitat complexity reduced mud crab predation on oysters. Additionally, the magnitude of this effect was much greater than the increase in oyster mortality as a result of complex reefs disrupting toadfish predation on mud crabs. This experimental demonstration of how habitat complexity modifies trophic interactions in a temperate reef community has fundamental implications for our understanding of species interaction webs and community structure. The influence of habitat complexity on the strength of a trophic cascade generally may depend upon whether physical complexity provides actual and perceived refuges for component predator–prey pairs.