Influence Community Dynamics Research Articles

Effects of perennial semi-arid bunchgrass spatial patterns on performance of the invasive annual cheatgrass (Bromus tectorum L.)

Plant spatial patterns critically influence community dynamics, including plant interactions, resource distribution, and community invasibility. Research suggests that resistance of western US plant communities to further invasion by the exotic annual grass Bromus tectorum may be linked to the positions of, and spacing between, perennial plants. In particular, gaps between aggregated clusters of perennial plants may facilitate B. tectorum invasion by providing safe sites for seed germination and establishment. We tested the effects of random, regular, and aggregated bunchgrass patterns, manipulated at both community (plot) and neighborhood scales, on B. tectorum biomass and spikelet production after experimental seed addition. We found strong evidence of treatment effects on both biomass and spikelets, which varied between treatments by approximately 2.5-fold. Mean biomass and spikelet counts were lowest in plots in which bunchgrasses were aggregated at both community and neighborhood scales, likely due to the increased competition. Although not statistically distinguishable from most other treatments, B. tectorum biomass and spikelet counts were highest in plots with bunchgrass patterns that were random at the community scale and aggregated at the neighborhood scale. These plots were characterized by relatively large gaps between bunchgrass clusters, suggesting that B. tectorum may exploit gaps between aggregated perennial plants. Our results support the emerging hypothesis that community resistance to B. tectorum invasion could be increased through manipulation of perennial vegetation to reduce basal gap size and connectivity.

Calcium concentration in leaf litter affects the abundance and survival of crustaceans in streams draining warm–temperate forests

Summary Stream ecosystems receive large quantities of nutrients from terrestrial ecosystems that influence community dynamics. However, few studies have investigated the effects of subsidiary calcium, an essential element of animals, on stream ecosystems. We focused on differences in calcium concentrations of leaf litter between tree species. In particular, litter of Japanese cedar (Cryptomeria japonica) has a higher concentration of calcium than other taxa, and the tree is planted in many parts of Japan. Therefore, we predicted that a C. japonica plantation would affect the supply of subsidiary calcium to stream ecosystems. We compared calcium concentration of leaf litters, soils, stream water and stream benthic invertebrates sampled at nine sites in catchments composed of cedar plantations, evergreen broad‐leaved forest and clear‐cut areas. In addition, we placed 10 Gammarus nipponensis in each of 10 nylon cages in the nine streams and after one month surveyed the number of surviving individuals. Calcium concentration in stream water and soil, and the density and survival of the dominant crustacean (Gammarus nipponensis) were significantly higher at sites dominated by C. japonica compared with other forest types. Furthermore, invertebrate community composition was correlated with total calcium and nitrate in stream water, indicating that terrestrial vegetation can affect stream invertebrate communities, particularly crustaceans.

Water‐use efficiency and relative growth rate mediate competitive interactions in Sonoran Desert winter annual plants

A functional approach to investigating competitive interactions can provide a mechanistic understanding of processes driving population dynamics, community assembly, and the maintenance of biodiversity. In Sonoran Desert annual plants, a trade-off between relative growth rate (RGR) and water-use efficiency (WUE) contributes to species differences in population dynamics that promote long-term coexistence. Traits underlying this trade-off explain variation in demographic responses to precipitation as well as life history and phenological patterns. Here, we ask how these traits mediate competitive interactions. • We conducted competition trials for three species occupying different positions along the RGR-WUE trade-off axis and compared the effects of competition at high and low soil moisture. We compared competitive effect (ability to suppress neighbors) and competitive response (ability to withstand competition from neighbors) among species. • The RGR-WUE trade-off predicted shifts in competitive responses at different soil moistures. The high-RGR species was more resistant to competition in high water conditions, while the opposite was true for the high-WUE species. The intermediate RGR species tended to have the strongest impact on all neighbors, so competitive effects did not scale directly with differences in RGR and WUE among competitors. • Our results reveal mechanisms underlying long-term variation in fitness: high-RGR species perform better in years with large, frequent rain events and can better withstand competition under wetter conditions. The opposite is true for high-WUE species. Such resource-dependent responses strongly influence community dynamics and can promote coexistence in variable environments.

Context-Dependent Competition in a Model Gut Bacterial Community

Understanding the ecological processes that generate complex community structures may provide insight into the establishment and maintenance of a normal microbial community in the human gastrointestinal tract, yet very little is known about how biotic interactions influence community dynamics in this system. Here, we use natural strains of Escherichia coli and a simplified model microbiota to demonstrate that the colonization process on the strain level can be context dependent, in the sense that the outcome of intra-specific competition may be determined by the composition of the background community. These results are consistent with previous models for competition between organisms where one competitor has adapted to low resource environments whereas the other is optimized for rapid reproduction when resources are abundant. The genomic profiles of E. coli strains representing these differing ecological strategies provide clues for deciphering the genetic underpinnings of niche adaptation within a single species. Our findings extend the role of ecological theory in understanding microbial systems and the conceptual toolbox for describing microbial community dynamics. There are few, if any, concrete examples of context-dependent competition on a single trophic level. However, this phenomenon can have potentially dramatic effects on which bacteria will successfully establish and persist in the gastrointestinal system, and the principle should be equally applicable to other microbial ecosystems.

In Situ Quantification of Experimental Ice Accretion on Tree Crowns Using Terrestrial Laser Scanning

In the eastern hardwood forests of North America ice storms are an important disturbance event. Ice storms strongly influence community dynamics as well as urban infrastructure via catastrophic branch failure; further, the severity and frequency of ice storms are likely to increase with climate change. However, despite a long-standing interest into the effects of freezing rain on forests, the process of ice accretion and thus ice loading on branches remains poorly understood. This is because a number of challenges have prevented in situ measurements of ice on branches, including: 1) accessing and measuring branches in tall canopies, 2) limitations to travel during and immediately after events, and 3) the unpredictability of ice storms. Here, utilizing a novel combination of outdoor experimental icing, manual measurements and terrestrial laser scanning (TLS), we perform the first in situ measurements of ice accretion on branches at differing heights in a tree crown and with increasing duration of exposure. We found that TLS can reproduce both branch and iced branch diameters with high fidelity, but some TLS instruments do not detect ice. Contrary to the expectations of ice accretion models, radial accretion varied sharply within tree crowns. Initially, radial ice accretion was similar throughout the crown, but after 6.5 hours of irrigation (second scanning) radial ice accretion was much greater on upper branches than on lower (∼factor of 3). The slope of the change in radial ice accretion along branches increased with duration of exposure and was significantly greater at the second scanning compared to the first. We conclude that outdoor icing experiments coupled with the use of TLS provide a robust basis for evaluation of models of ice accretion and breakage in tree crowns, facilitating estimation of the limiting breaking stress of branches by accurate measurements of ice loads.

Effects of early recruits on temperate sessile marine community composition depend on other species recruiting at the time

In many environments recruitment of dispersive propagules (e.g. seeds, spores and larvae) can vary from situations when particular taxa recruit in relative isolation to times when they recruit simultaneously with other, functionally quite different taxa. Differences in the identity and density of recruiting taxa can have important consequences on community structure, but it is still not clear how the effects of individual taxa on communities are modified when they recruit together with other species. Using an experimental approach we compared early development of a temperate marine sessile community after the recruitment of mixtures of botryllid ascidians and barnacles to that when barnacles or botryllid ascidians recruited alone. Communities exposed to recruitment of botryllid ascidians in isolation differed from those that received barnacles, a mixture of botryllids and barnacles or no recruitment in 2-week-old communities. These early differences were driven by higher abundances of the species that were present as initial recruits in experimental treatments. After 2 months communities also differed between barnacle and mixed recruitment treatments but not mixed and botryllid or botryllid and barnacle treatments. These differences were not directly due to differences in the abundances of our manipulated taxa but occurred because of two abundant arborescent bryozoans, Bugula dentata, which occupied more space in communities that initially received mixed recruitment than in those that received barnacle or no recruitment, and Zoobotryon verticillatum, which occupied more space in communities that initially received only barnacle recruitment than those that initially received botryllid or mixed recruitment. These effects did not persist, and communities did not differ after 6 months. These results suggest that, more generally, species may influence community dynamics differently when they recruit alongside other species than when they recruit in relative isolation.

Intra-guild interactions and projected impact of climate and land use changes on North American pochard ducks

The co-occurrence of functionally similar species is very common in nature, and is often put forward as a basis for ecosystem resilience to disturbance. At the same time, competition between similar species is also considered a strong driver of community composition. However, environmental stochasticity can alter this prediction, either because competitive abilities depend on time-varying factors or because covariance in species' responses to environmental conditions masks the effect of competition. Interactions other than competition can also influence community dynamics but have received less attention. We used a simplified community of two sympatric duck species (redhead Aythya americana and canvasback A. valisineria) and a previously published analysis of 50 years of demographic data to parameterize a stochastic, density-dependent, stage-structured model. These ducks interact via nest parasitism (mostly of canvasback by redhead) in addition to competition for food resources, with consequences at the demographic level; these interactions are modulated by habitat availability (number of ponds in the study landscape). We found that if habitat availability decreased there was a high risk of quasi-extinction, and redheads, although initially able to maintain their numerical dominance, quickly became the least abundant species because they perform worse during droughts. If habitat availability increased, we found that the initially more rare canvasback would increase in relative abundance, albeit slowly. We interpret this as a shift from a community influenced by nest parasitism (which is detrimental to canvasback) to a community mostly driven by species-specific dynamics due to relaxation of resource limitation.

Divergent reactions to olfactory foraging cues between two ecologically similar, sympatric hermit crab species

Relatively little is known about how sensory differences between species can influence community dynamics and interspecific competition. For animals that rely on chemical (olfactory) cues to forage, subtle differences in the olfactory capabilities between species could influence the foods that the species consume and thus permit coexistence between ecologically similar species. This study identified olfactory foraging cues used by two ecologically similar, sympatric hermit crab species, Clibanarius digueti Bouvier, 1898 and Paguristes perrieri Bouvier, 1895. Additionally, the behavioral chemosensitivities to olfactory cues were compared between the two species. Results show that the two species use different chemical cues to mediate their foraging behaviors, despite similarities in their olfactory chemosensitivity. This sensory differentiation provides a plausible mechanism by which the species relax interspecific food competition enough to permit coexistence.

Evolutionary indirect effects of biological invasions

Just as ecological indirect effects can have a wide range of consequences for community structure and ecosystem function, theory suggests that evolutionary indirect effects can also influence community dynamics and the outcome of species interactions. There is little empirical evidence documenting such effects, however. Here, I use a multi-generation selection experiment in the field to investigate: (1) how the exotic plant Medicago polymorpha and the exotic insect herbivore Hypera brunneipennis affect the evolution of anti-herbivore resistance traits in the native plant Lotus wrangelianus and (2) how observed Lotus evolutionary responses to Hypera alter interactions between Lotus and other members of the herbivore community. In one of two study populations, I document rapid evolutionary changes in Lotus resistance to Hypera in response to insecticide treatments that experimentally reduced Hypera abundance, and in response to Medicago-removal treatments that also reduced Hypera abundance. These evolutionary changes in response to Hypera result in reduced attack by aphids. Thus, an evolutionary change caused by one herbivore species alters interactions with other herbivore taxa, an example of an eco-evolutionary feedback. Given that many traits mediate interactions with multiple species, the effects of evolutionary changes in response to one key biotic selective agent may often cascade through interaction webs to influence additional community members.

Terrestrial, benthic, and pelagic resource use in lakes: results from a three-isotope Bayesian mixing model

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.

Response of sea-ice microbial communities to environmental disturbance: an in situ transplant experiment in the Antarctic

Sea-ice microbial communities are integral to primary and secondary production in icecovered regions of the Southern Ocean, but few studies have characterised the heterogeneity of microbes within the ice or determined whether habitat variability influences community dynamics. We examined the response of sea-ice microbes to key physicochemical variables by conducting an 18 d reciprocal transplant experiment within Antarctic fast-ice. A series of ice cores were extracted from 2.6 m annual ice and reinserted upside down to expose resident microbial assemblages to significantly different light, temperature and salinity regimes. The abundance and community composition of bacteria, microalgae and protozoa was subsequently determined within 3 sections of each core (top, middle and bottom) and compared with experimental controls. Results demonstrate that iceassociated microbes are finely attuned to discrete microhabitats within the sea-ice matrix. Positive growth and a shift in community composition was observed for microalgae moved from the top to the bottom of the ice, but significant bleaching of photosynthetic pigments resulted in zero net growth for bottom-ice communities exposed to the surface. Although bacteria may have been less vulnerable to initial change in their microenvironment, there was no significant increase in the average abundance of cells at either end of the flipped cores after 18 d, despite a presumed increase in algal-derived dissolved organic matter. This suggests a significant lag in the response time of bacteria to available growth substrates and a temporary ‘malfunction’ of the microbial loop.

Dynamical complexities in a tri-trophic hybrid food chain model with Holling type II and Crowley–Martin functional responses

We study how predator behavior influences community dynamics of predatorprey systems. It turns out that predator behavior plays a dominant role in community dynamics. The hybrid model studied in this paper reveals that period-doubling and period-doubling reversals can generate short-term recurrent chaos (STRC), which mimics chaotic dynamics observed in natural populations. STRC manifests itself when deterministic changes in a system parameter interrupt chaotic behavior at unpredictable intervals. Numerical results reinforce an earlier suggestion that period-doubling reversals could control chaotic dynamics in ecological models. In ecological terms, the prey and intermediate predator populations may go to extinction in the event of a catastrophe. The top predator is always a survivor. In contrast to this, this is not the case when the constituent populations are interacting through Holling type II functional response. Even this top predator can go to extinction in the event of such catastrophes.

Conspecific and heterospecific plant–soil feedbacks influence survivorship and growth of temperate tree seedlings

Summary 1. The Janzen–Connell (J–C) Model proposes that host‐specific enemies could maintain high tree species diversity by reducing seedling performance near conspecific adults. An implicit, but untested assumption of the J–C Model is that negative conspecific feedbacks would promote replacement by heterospecific seedlings. 2. In a glasshouse experiment, we tested plant–soil feedbacks as a J–C mechanism in four temperate tree species. We assessed effects of conspecific‐ relative to heterospecific‐cultured soil extracts on seedling survival, total mass and performance for each focal species. To test the implicit assumption of replacement by heterospecific seedlings, we also compared relative performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. We also tested whether soil microbes caused these plant–soil feedbacks and whether low irradiance increased seedling vulnerability to pathogens. 3. When grown with conspecific versus heterospecific soil extract, Acer rubrum mass decreased, Quercus rubra mass increased and Fraxinus americana increased survival. Conspecific extract reduced Acer saccharum mass in low light but increased it in high light. To integrate survival and growth, we examined seedling performance [(mean total mass × mean survival time)/(days of experiment)] at low and high light. In conspecific versus heterospecific soil extract, seedling performance was lower in two, higher in four and neutral in 18 of 24 cases, suggesting no advantage to dispersing away from conspecifics. Based on relative seedling performance within a soil extract, conspecific seedlings were disadvantaged in two, favoured in three and neutral in 19 of 24 cases relative to heterospecific seedlings. 4. Species pairwise interactions of soil modification and seedling performance were chemically mediated, occurring regardless of sterilization. Microbes lacked host specificity and reduced performance regardless of extract source. Additionally, microbial factors reduced seedling performance for Q. rubra regardless of light availability, and for A. rubrum and F. americana only in high light. 5. Synthesis. These chemical‐mediated plant–soil feedbacks probably influence community dynamics, but are inconsistent with the J–C Model. Even when a species’ seedlings responded more negatively to conspecific than heterospecific soil, heterospecific seedlings were not necessarily favoured in that species’ soil, precluding heterospecific replacement as an explanation for coexistence.

High-quality seed dispersal by fruit-eating fishes in Amazonian floodplain habitats

Seed dispersal is a critical stage in the life history of plants. It determines the initial pattern of juvenile distribution, and can influence community dynamics and the evolutionary trajectories of individual species. Vertebrate frugivores are the primary vector of seed dispersal in tropical forests; however, most studies of seed dispersal focus on birds, bats and monkeys. Nevertheless, South America harbors at least 200 species of frugivorous fishes, which move into temporarily flooded habitats during lengthy flood seasons and consume fruits that fall into the water; and yet, we know remarkably little about the quality of seed dispersal they effect. We investigated the seed dispersal activities of two species of large-bodied, commercially important fishes (Colossoma macropomum and Piaractus brachypomus, Characidae) over 3 years in Pacaya-Samiria National Reserve (Peru). We assessed the diet of these fishes during the flood season, conducted germination trials with seeds collected from digestive tracts, and quantified fruit availability. In the laboratory, we fed fruits to captive Colossoma, quantified the proportion of seeds defecated by adult and juvenile fish, and used these seeds in additional germination experiments. Our results indicate that Colossoma and Piaractus disperse large quantities of seeds from up to 35% of the trees and lianas that fruit during the flood season. Additionally, these seeds can germinate after floodwaters recede. Overexploitation has reduced the abundance of our focal fish species, as well as changed the age structure of populations. Moreover, older fish are more effective seed dispersers than smaller, juvenile fish. Overfishing, therefore, likely selects for the poorest seed dispersers, thus disrupting an ancient interaction between seeds and their dispersal agents.

Impact of ploughing on soil seed bank dynamics in temporary pools

We examined the impact of ploughing on soil seed banks of plant communities living in temporary marshes located in agricultural fields. The quantity, quality and vertical distribution of seeds were quantified under ploughed or un- ploughed treatment at community level. We also focussed on a typical semi-aquatic species, Damasonium alisma, to investigate the impact of ploughing at population level. We used two complementary techniques to study seed banks: hand sorting and seedling emergence. We found that species richness of seeds, seed abundance and germination ability were strongly affected by ploughing at community level. Concerning D. alisma, most of the seeds (56%) were stored in the two deepest soil layers among the four considered in ploughed pools. Moreover, the germination rate was higher for buried seeds (84%) than for seeds collected at the surface (33.6%). These patterns were almost inverted in unploughed pools. Our results agree with the temporal storage effect generally suggested to describe the seed bank property of plant commu- nities. But in addition, we showed that ploughing induces a spatial storage effect in accumulating species and individuals in the seed banks that favourably influence community dynamics. We conclude that, in contrast to what is usually thought, ploughing disturbance can be of benefit for such ephemeral wetland vegetation.

Architectural classes of aquatic food webs based on link distribution

Link distribution is an important architectural feature of ecological networks, since it is thought to influence community dynamics. Several attempts have been made in order to characterize the typical link distribution of food webs, but the number of webs studied thus far is low and their quality is unbalanced. Comparability is a rarely asked methodological question, and as far as we see only two data bases are available which allow reliable comparison of food webs: one for terrestrial, high resolution, host-parasitoid webs and another for highly aggregated, marine trophic networks. We present an analysis of a set of food webs belonging to the latter type, since the host-parasitoid networks are only subgraphs and therefore uninformative on the structure of the entire community. We address the following three questions: (1) how to characterize the link distribution of these small networks which cannot always be fitted statistically to well-known distributions (such as the exponential or the Poisson, etc.), (2) are these distributions of more or less similar shape or they belong to different „architectural classes”, and (3) if there are different classes, then what are their distinctive topological and biological properties. We suggest that link distribution of such small networks can be compared to each other by principal coordinates ordination and clustering. We conclude that (1) the webs can be categorized into two different classes, and (2) one of the classes contains significantly larger and topologically more heterogeneous webs for which net output of material is also of higher variance. We emphasize that link distribution is an interesting and important property not only in case of complex, speciose food webs, but also in highly aggregated, low-resolution webs.

Analysis of adaptive foraging in an intraguild predation system

Abstract. An intraguild predation (IGP) system with adaptive foraging behavior was analyzed using a simple mathematical model. The main aim was to explore how the adaptive behavior affects species interactions as well as how such interactions derived from adaptive behavior affect community stability. The focal system contained top predators, intermediate predators, and basal prey. Intermediate predators exhibit antipredator behavior and balance costs (e.g. perceived predation risk) and benefits (e.g. resource intake) to determine their foraging effort. Density-dependent foraging behavior with the unique connectance of the IGP food web created unusual species interactions. Notably, increased prey density can transmit negative indirect effects to top predators while increased top predator density transmits positive indirect effects to prey population. The nature of these interactions is density-dependent. The results suggest that both IGP (as opposed to linear food chain) and adaptive foraging behaviors may strongly influence community dynamics due to emergent interactions among direct effects and indirect effects. Furthermore, the adaptive foraging of intermediate predators may stabilize the community as a whole.

Effects of natural barriers on the spillover of a marine mollusc: implications for fisheries reserves

Abstract The movement of organisms and dispersal of propagules is fundamental to the maintenance of populations over time. However, the existence of barriers, created through the spatial configuration of habitats, may significantly affect dispersal patterns and thus influence community dynamics and resource sustainability. Within marine environments unstructured or open habitats may form partial or complete ecological barriers due to elevated risk of predation or physical stresses associated with them. The existence and effects of such barriers may be of particular importance when considering the establishment of marine protected areas with a fisheries enhancement focus. In this paper, the spillover of post‐settlement queen conch (Strombus gigas) from a protected area in the Turks and Caicos Islands is investigated. It is hypothesized that the reserve boundaries overlap with a series of shallow, sand habitats that effectively enclose the protected population, reducing the spillover of conch into the adjacent fished areas. To test this, density gradient maps for juvenile and adult conch populations were constructed using underwater visual survey data at 68 sites within and surrounding the protected area. These maps illustrate very low densities coinciding with poor, shallow sand habitats along the two marine boundaries of the reserve where spillover is expected to take place. These sand habitats are thought to create ecological barriers to a slow, sedentary gastropod largely due to their shallowness (physical stresses of solar exposure or anoxia) and lack of food reducing the tendency of individuals to move across these areas, despite the 10‐times higher density of adult queen conch observed in the protected area compared with outside. Copyright © 2003 John Wiley & Sons, Ltd.

Flight activity of adult stoneflies in relation to weather

Abstract. 1. Dispersal of adult aquatic insects between streams may have important consequences for local and regional population dynamics, but little is known about how dispersal is affected by weather conditions.2. The influence of meteorological variables on flight activity of adult stoneflies (Plecoptera: Leuctridae, Nemouridae, and Chloroperlidae) was investigated using Malaise traps adjacent to three upland streams in the Plynlimon area of mid Wales, U.K.3. Numbers of adult stoneflies captured weekly in the traps were related positively to air temperature and related negatively to wind speed. Meteorological conditions during daylight showed stronger relationships with flight activity than did conditions at night.4. There was inter‐site variation in the strength of weather effects on stonefly flight. Wind speed was significant at only one site, which had higher average wind speed than the other sites.5. Annual variation in weather conditions during adult flight periods may result in varying extent of dispersal between sites, influencing community dynamics over a wide area.

Effects of Top Predator Species on Direct and Indirect Interactions in a Food Web

Current theory on trophic interactions in food webs assumes that ecologically similar species can be treated collectively as a single functional unit such as a guild or trophic level. This theory implies that all species within that unit transmit identical direct and indirect effects throughout the community. We evaluated this assumption by conducting experiments to compare the direct and indirect effects of three top-predator species, belonging to the same hunting spider guild, on the same species of grasshopper and on old-field grasses and herbs. Observations under field conditions revealed that each spider species exhibited different hunting behavior (i.e., sit-and-wait, sit-and-pursue, and active hunting) and occupied different locations within the vegetation canopy. These differences resulted in different direct effects on grasshopper prey. Grasshoppers demonstrated significant behavioral (diet) shifts in the presence of sit-and-wait and sit-and-pursue species but not when faced with actively hunting species. Grasshopper density was significantly reduced by spider species that occupied lower parts of the vegetation canopy (sit-and-pursue and actively hunting species), but it was not significantly reduced by the sit-and-wait spider species that occupied the upper parts of the canopy. These direct effects manifested themselves differently in the plant trophic level. The sit-and-wait spider caused indirect effects on plants by changing grasshopper foraging behavior (a trait-mediated effect). The sit-and-pursue spider caused indirect effects by reducing grasshopper density (density-mediated effects); the effects of changes in grasshopper behavior were thus not reflected in the plant trophic level. The actively hunting spiders had strictly density-mediated indirect effects on plants. The study offers mechanistic insight into how predator species within the same guild can have very different trophic effects in food webs. Thus classical modeling approaches that treat all predator species as a single functional unit may not adequately capture biologically relevant details that influence community dynamics.