Consumer Guilds Research Articles

Macroinvertebrates food web and trophic relations of a peri urban mangrove system in a semi-arid region, Gulf of California, México

Despite being structurally limited due to their latitudinal distribution and local climatic conditions, mangroves of the southwest Gulf of California present productivity and carbon storage rates similar or higher than that of tropical mangroves. Despite the importance of their contribution to marine and atmospheric dynamics, few studies exist focused on food web structures in such ecosystems. In order to analyze trophic structure and dynamics of a mangrove system in the Gulf of California, the isotopic composition, trophic position, isotopic niche, and trophic pathways were studied using the statistical packages TrophicPosition (TP), SIBER, and Simmr. Isotopic values of C and N were obtained for 244 samples of carbon sources and invertebrate consumers which were classified into six trophic guilds. The δ13C values of carbon sources varied from −30.7 (Rhizophora mangle) to −15.6‰ (Caulerpa sertularioides) and for consumers varied from −22.3‰ (Aratus sp) to −13.2 (Callinectes arcuatus). Herbivores generally had depleted 13C values related to those of the other trophic guilds. The macroinvertebrate communities' mean TP values (2.6: 2.0–3.5) indicate a composition of primary and secondary consumers, exhibiting a clear separation between trophic guilds. Isotopic niche width (TA) of the community (14.1) suggests niche segregation, specially between herbivores and deposit feeders. Those trophic guilds are the first consumers and intermediaries for two distinct trophic pathways, one derived from detritus and mangroves and the other from microphytobenthos (MPB). Three groups of primary sources were identified contributing >70% of carbon, representing the primary base of several trophic pathways that sustain different consumer guilds: (1) mangrove-detritus, herbivorous crabs, (2) MPB-Sedimentary organic carbon, supporting mainly deposit feeders and filter feeders, and (3) Seston-MPB, supporting mainly omnivorous shrimp. Overall, the identified pathways potentially sustain fish communities, which due to their movement export energy from mangroves to coastal food webs.

DNA metabarcoding highlights cyanobacteria as the main source of primary production in a pelagic food web model.

Models that estimate rates of energy flow in complex food webs often fail to account for species-specific prey selectivity of diverse consumer guilds. While DNA metabarcoding is increasingly used for dietary studies, methodological biases have limited its application for food web modeling. Here, we used data from dietary metabarcoding studies of zooplankton to calculate prey selectivity indices and assess energy fluxes in a pelagic resource-consumer network. We show that food web dynamics are influenced by prey selectivity and temporal match-mismatch in growth cycles and that cyanobacteria are the main source of primary production in the investigated coastal pelagic food web. The latter challenges the common assumption that cyanobacteria are not supporting food web productivity, a result that is increasingly relevant as global warming promotes cyanobacteria dominance. While this study provides a method for how DNA metabarcoding can be used to quantify energy fluxes in a marine food web, the approach presented here can easily be extended to other ecosystems.

Functional diversity in herbivorous fishes on Caribbean reefs: The role of macroalgal traits in driving interspecific differences in feeding behavior

Understanding how functional traits drive plant-herbivore interactions often illuminates the mechanisms driving niche partitioning and functional diversity in this important guild of consumers. On coral reefs, a diverse suite of herbivorous fishes is key for controlling algal abundance and impacting benthic community dynamics. Here, we focus on how algal traits shape complementarity in feeding for a diverse suite of large herbivorous fishes on a coral reef in the wider Caribbean. We used feeding assays in the field to document feeding patterns of the different herbivorous fishes across 50 species of macroalgae. Then, we used a suite of macroalgal traits such as dry mass, nutrient content, calcification, and chemical defenses to assess how these traits influenced patterns of consumption. Algal defenses were the major traits determining the overall vulnerability of algae to herbivores, with uncalcified algae from undefended genera being consumed 40% more rapidly than algae that were either calcified or chemically defended. Nutrient content of algae explained little in patterns of overall herbivory. We found significant niche diversification in feeding both among and within genera with the three major genera having unique feeding patterns on different suites of algae (Acanthurus spp. - red macroalgae, Sparisoma spp. - brown and green macroalgae, and Scarus spp. - benthic filamentous algae). With respect to how algal traits influenced patterns of feeding in individual species, we found that: (1) some herbivore species were almost exclusively influenced by avoiding algal defenses, (2) some species showed strong interactions between algal defenses and nutrient content that determined their feeding, and (3) some species (Kyphosus spp., Pomacanthus arcuatus) targeted chemically-defended algae with high nutrient content. Our work highlights the role that algal traits play in influencing patterns in the diversity and redundancy of feeding in herbivorous fishes on coral reefs.

Loss of phylogenetic diversity under landscape change

Habitat alteration and destruction are primary drivers of biodiversity loss. However, the evolutionary dimensions of biodiversity loss remain largely unexplored in many systems. For example, little is known about how habitat alteration/loss can lead to phylogenetic deconstruction of ecological assemblages at the local level. That is, while species loss is evident, are some lineages favored over others? Using a long-term dataset of a globally, ecologically important guild of invertebrate consumers, stream leaf “shredders,” we created a phylogenetic tree of the taxa in the regional species pool, calculated mean phylogenetic distinctiveness for >1000 communities spanning >10 year period, and related species richness, phylogenetic diversity, and distinctiveness to watershed-scale impervious cover. Using a combination of changepoint and compositional analyses, we learned that increasing impervious cover produced marked reductions in all three measures of diversity. These results aid in understanding both phylogenetic diversity and mean assemblage phylogenetic distinctiveness. Our findings indicate that, not only are species lost when there is an increase in watershed urbanization, as other studies have demonstrated, but that those lost are members of more distinct lineages relative to the community as a whole..

Intertidal estimates of sea urchin abundance reveal congruence in spatial structure for a guild of consumers.

We hypothesized congruence in the spatial structure of abundance data sampled across multiple scales for an ecological guild of consumers that exploit similar nutritional and habitat resources. We tested this hypothesis on the spatial organization of abundance of an herbivorous guild of sea urchins. We also examined whether the amount of local along‐shore rocky habitat can explain the observed spatial patterns of abundance. Standardized estimates of abundance of four intertidal sea urchins—Diadema cf. savignyi, Echinometra mathaei, Parechinus angulosus, and Stomopneustes variolaris—were determined by six observers at 105 sites across 2,850 km of coast of South Africa. For each species and observer, wavelet analysis was used on abundance estimates, after controlling for potential biases, to examine their spatial structure. The relationship between local sea urchin abundance and the amount of upstream and downstream rocky habitat, as defined by the prevailing ocean current, was also investigated. All species exhibited robust structure at scales of 75–220 km, despite variability among observers. Less robust structure in the abundances of three species was detected at larger scales of 430–898 km. Abundance estimates of sympatric populations of two species (D. cf. savignyi and E. mathaei) were positively correlated with the amount of rocky habitat upstream of the site, suggesting that upstream populations act as larval sources across a wide range of scales. No relationship between abundance and habitat size was found for P. angulosus or S. variolaris. Within the range of scales examined, we found robust congruence in spatial structure in abundance at the lower, but not the larger, range of scales for all four species. The relationship between abundance and upstream habitat availability in two species suggests that larval supply from upstream populations was probably the mechanism linking habitat size and abundance.

One size fits some

The size of seeds may determine not only how likely they are to be dispersed or consumed and to germinate but also how competitive their seedlings may be. In the case of oaks (Quercus spp), acorns are dispersed by jays and magpies (both in the Corvidae family) if they fit in the bird’s bill. Similarly, rodents select acorns of sizes they can carry to hoarding sites. Yet acorns are more likely to resist desiccation and tend to produce taller seedlings if they are large. Owing to divergent pressures on acorn size across life stages, acorn mass can vary by two orders of magnitude within a species. While each oak parent tree produces acorns of relatively similar sizes, there is remarkable variability among individual trees, as demonstrated in this image by acorns sorted in groups of three “siblings” from each of nine holm oak (Quercus ilex) parents. Offspring may be most competitive under the conditions in which the parent established, and high variability in acorn size across parents may result from, and persist under, heterogeneous conditions. Curiously, inter-parent variability also applies to acorn shapes. What are the dispersal implications of acorns being elongated, spherical, or thick at the bottom? What consequences could reforestation programs, which select for large acorns due to their greater probability of emergence, have on tree development and survival over generations? How is intraspecific diversity of oak populations affected if defaunation removes certain acorn disperser or consumer guilds? Does the ongoing oak colonization of conifer-dominated ecosystems under climate change select for certain acorn characteristics, and what does this imply for acorn-feeding guilds in these new communities?

Large herbivores transform plant-pollinator networks in an African savanna

Pollination by animals is a key ecosystem service1,2 and interactions between plants and their pollinators are a model system for studying ecological networks,3,4 yet plant-pollinator networks are typically studied in isolation from the broader ecosystems in which they are embedded. The plants visited by pollinators also interact with other consumer guilds that eat stems, leaves, fruits, or seeds. One such guild, large mammalian herbivores, are well-known ecosystem engineers5-7 and may have substantial impacts on plant-pollinator networks. Although moderate herbivory can sometimes promote plant diversity,8 potentially benefiting pollinators, large herbivores might alternatively reduce resource availability for pollinators by consuming flowers,9 reducing plant density,10 and promoting somatic regrowth over reproduction.11 The direction and magnitude of such effects may hinge on abiotic context-in particular, rainfall, which modulates the effects of ungulates on vegetation.12 Using a long-term, large-scale experiment replicated across a rainfall gradient in central Kenya, we show that a diverse assemblage of native large herbivores, ranging from 5-kg antelopes to 4,000-kg African elephants, limited resource availability for pollinators by reducing flower abundance and diversity; this in turn resulted in fewer pollinator visits and lower pollinator diversity. Exclusion of large herbivores increased floral-resource abundance and pollinator-assemblage diversity, rendering plant-pollinator networks larger, more functionally redundant, and less vulnerable to pollinator extinction. Our results show that species extrinsic to plant-pollinator interactions can indirectly and strongly alter network structure. Forecasting the effects of environmental change on pollination services and interaction webs more broadly will require accounting for the effects of extrinsic keystone species.

Consumer roles of small mammals within fragmented native tallgrass prairie

AbstractGrassland ecosystems globally are being negatively impacted by changes in climate, disturbance regimes, nutrient flux, and consumer guilds. Changes in the trophic ecology of consumers can substantially influence local resources, contributing to shifting diversity, community turnover, and other processes of ecosystem change. Small mammals are diverse and abundant within grasslands and yet the impact of changing ecosystems on small mammals and the role of these mammals as consumers are still both under‐studied. We assessed small mammal resource use within grassland and woodland vegetation types that have resulted from landscape‐scale experimental disturbance through fire treatments within the tallgrass prairie ecoregion of the North American Great Plains. We predicted that resource use would vary significantly among grassland vs. woodland communities, in turn reducing the role of small mammals in contributing to future maintenance of native prairies. We sampled five dominant species of rodents across three years and multiple habitats. Using stable isotope analysis, we investigated isotopic niche area and overlap to infer variation in diet, both within and among species. Resource use shifted in bivariate isotopic space seasonally but not across years when combining all species and habitats. Inferred spring diet (based on fur samples) was highly diverse and overlapping. Summer isotopic values (based on liver tissue) in woody habitat treatments were narrower and overlapped less than within grassland habitats. Consumers generally shifted from C4 herbivory to C3 herbivory, or greater omnivory, when analyzing grassland, shrubland, and woodland habitats respectively. Within the tallgrass prairie ecosystem, small mammal populations in herbaceous‐dominated habitats use a broader variety of resources than small mammals in proximate woody‐dominated habitats. As native grasslands experience woody encroachment, small mammal assemblages experience turnover of dominant species and associated changes in diet. Ecosystem changes such as cessation of frequent fire resulting in more woody habitats may include reduced roles by native small mammals as consumers/dispersers/propagators of native grassland plants.

Benthic trophic networks of the southern North Sea: contrasting soft-sediment communities share high food web similarity

We examined whether taxonomically distinct benthic communities from contrasting sediments in the German Bight (southern North Sea) also differ in their trophic structure. As a case study, we compared the Amphiura filiformis community (AFC) of silty sands and the Bathyporeia− Tellina community (BTC) of fine sands using a combination of stable isotope analysis and data on trophic interactions. Differences between the food webs were evident in the feeding guild compo� sition of important primary consumers: deposit and interface feeders are the most diverse primary consumer guilds in the AFC, whereas suspension and interface feeders play a major role in the BTC, reflecting differences in physical properties and food availability at the sediment−water interface. While all primary consumer guilds had the same trophic level (TL) in the AFC, deposit feeders of the BTC occupied a trophic position intermediate between other primary and higher� order consumer guilds, likely explained by partially incomplete knowledge of their trophic eco - logy and selective feeding, including the ingestion of meiofauna. Most food web properties, how�ever, were similar between the AFC and BTC: they mainly depend on pelagic primary production, reach TL 4 and are characterized by a prevalence of generalist higher-order consumers. Further� more, both trophic networks had similar linkage densities and high directed connectance, the lat�ter feature suggesting considerable food web robustness. Our findings suggest that although com�munities in the German Bight differ in some aspects of their trophic structure, they share a similar food web topology, indicating a comparable degree of resilience towards natural and anthro�pogenic disturbances.

Nutrient recycling by insect and fish communities in high-elevation tropical streams

High- to mid-elevation streams are often oligotrophic, but harbor diverse groups of aquatic animals that can satisfy a substantial proportion of nutrient demand. Therefore, we tested the proportion of nutrient demand met by two dominant guilds of animal consumers in the Andes to ask: (1) Do excretion rates vary between insects and fish in montane tropical stream ecosystems? (2) What consumer guild dominates areal nutrient regeneration? (3) What is the nutrient demand and what proportion are consumer taxa regenerating? We combined aquatic insect and fish biomass estimates with measured excretion rates of two fish species (one native, one introduced) and six aquatic insects and estimated nutrient demand in streams by conducting nutrient uptake measurements. Insect taxa had higher per-capita excretion rates than fish and had higher excretion N:P. Aquatic insect biomass tended to be higher than fish biomass and consequently total areal excretion rates by insects were higher. Collectively, communities contributed up to 15–24% of NH4–N demand and 1–19% of SRP demand. The additive effect of these groups on nutrient availability is likely an important function in low-nutrient tropical streams. Further work needs to be conducted to examine the interactions within entire communities and consequential impacts on nutrient cycling.

Small mammal herbivores mediate the effects of soil nitrogen and invertebrate herbivores on grassland diversity.

Simultaneous reductions in herbivore abundance and increases in nitrogen deposition have led to radical shifts in plant communities worldwide. While the individual impacts of these human‐caused disturbances are apparent, few studies manipulate both herbivory and N, nor differentiate among herbivore guilds, to understand contingencies in the ability of these drivers to affect producer diversity and productivity. As such, understanding how the main and combined effects of increasing soil N with declining herbivores may influence plant community structure and function is critical to better understand the future of grassland ecosystems under multiple global change drivers.In this study, we asked: (a) What are the main effects of small mammal herbivores, invertebrate herbivores, and soil N on plant community structure and function? and (b) Are the effects of invertebrate herbivores and soil N on plant community structure and function contingent on small mammal herbivory? We used a nested design, with invertebrate and soil N treatments nested within small mammal manipulations in an existing tallgrass prairie. We measured plant community structure by quantifying plant richness, evenness, diversity, and composition across two full growing seasons. We also recorded total aboveground biomass to quantify grassland productivity.We found that small mammal herbivores strongly shaped plant diversity, species composition, and productivity. Small mammal herbivores also mediated the effects of soil N and invertebrate herbivores on grassland community structure, but not composition or productivity. Small mammal reduction lowered plant species richness while increasing aboveground biomass and altering compositional similarity. Invertebrate herbivores, in the presence of small mammals, promoted plant dominance by reducing evenness without altering compositional similarity. Additionally, soil nitrogen addition reduced plant richness, but only when small mammals were reduced, and no effects were detected on compositional similarity or productivity.Our findings provide further evidence that temperate grasslands can be strongly influenced by consumers, and that consumers mediate the effects of resources as well as other consumer guilds on producer evenness and richness. Taken together, we provide evidence of strong contingencies in the drivers of grassland structure, with small mammals directly altering plant diversity as well as mediating the effects of soil nitrogen and invertebrate herbivory on plant richness and evenness. Therefore, we suggest it is imperative to consider how consumer guilds and resource types may interact to shape grassland plant communities.

Megaherbivores Modify Trophic Cascades Triggered by Fear of Predation in an African Savanna Ecosystem

The loss of apex consumers (large mammals at the top of their food chain) is a major driver of global change [1]. Yet, research on the two main apex consumer guilds, large carnivores [2] and megaherbivores [3], has developed independently, overlooking any potential interactions. Large carnivores provoke behavioral responses in prey [1, 4], driving prey to distribute themselves within a "landscape of fear" [5] and intensify their impacts on lower trophic levels in low-risk areas [6], where they may concentrate nutrients through localized dung deposition [7,8]. We suggest, however, that megaherbivores modify carnivore-induced trophic cascades. Megaherbivores (>1,000kg [9]) are largely invulnerable to predation and should respond less to the landscape of fear, thereby counteracting the effects of fear-triggered trophic cascades. By experimentally clearing plots to increase visibility and reduce predation risk, we tested the collective role of both apex consumer guilds in influencing nutrient dynamics in African savanna. We evaluated whether megaherbivores could counteract a behaviorally mediated trophic cascade by redistributing nutrients that accumulate through fear-driven prey aggregations. Our experiment showed that mesoherbivores concentrated fecal nutrients in more open habitat, but that megaherbivores moved nutrients against this fear-driven nutrient accumulation by feeding within the open habitat, yet defecating more evenly across the risk gradient. This work adds to the growing recognition of functional losses that are likely to have accompanied megafaunal extinctions by contributing empirical evidence from one of the last systems with a functionally complete megaherbivore assemblage. Our results suggest that carnivore-induced trophic cascades work differently in a world of giants.

Phylogenetic trophic specialization: a robust comparison of herbivorous guilds.

Resource specialization is a key concept in ecology, but it is unexpectedly difficult to parameterize. Differences in resource availability, sampling effort and abundances preclude comparisons of incompletely sampled biotic interaction webs. Here, we extend the distance-based specialization index (DSI) that measures trophic specialization by taking resource phylogenetic relatedness and availability into account into a rescaled version, DSI*. It is a versatile metric of specialization that expands considerably the scope and applicability, hence the usefulness, of DSI. The new metric also accounts for differences in abundance and sampling effort of consumers, which enables robust comparisons among distinct guilds of consumers. It also provides an abundance threshold for the reliability of the metric for rare species, a very desirable property given the difficulty of assessing any aspect of rare species accurately. We apply DSI* to an extensive dataset on interactions between insect herbivores from four folivorous guilds and their host plants in Papua New Guinean rainforests. We demonstrate that DSI*, contrary to the original DSI, is largely independent of sample size and weakly and non-linearly related with several host specificity measures that do not adjust for plant phylogeny. Thus, DSI* provides further insights into host specificity patterns; moreover, it is robust to the number and phylogenetic diversity of plant species selected to be sampled for herbivores. DSI* can be used for a broad range of comparisons of distinct feeding guilds, geographical locations and ecological conditions. This is a key advance in elucidating the interaction structure and evolution of highly diversified systems.

Contrasting responses of insects and vertebrates as seed consumers of two neotropical oak species: The interactive effects of individual crop size and seed mass

Tree species often exhibit considerable variability among individuals in seed crop size and averaged seed mass within the same year. However, very little is known about the consequences for seed consumers’ preferences of this potentially large between-individual variability. In this study we quantified seed production and seed manipulation rates by animals over three years in two coexisting oak species of southeast Mexico (Quercus germana and Q. xalapensis) with the principal aim of evaluating the influence of two relevant plant traits (individual crop size and seed mass) on the responses of two guilds of acorn consumers with contrasting foraging behaviors and dietary breadths (vertebrate versus granivorous insects). We detected interactive effects of these two plant traits on seed consumers’ preferences, with important differences between the two groups of acorn-feeding animals. In general, high densities of large-sized acorns triggered a negative density-dependent response (i.e. satiating effect) in granivorous insects and a positive response (i.e. attractive effect) in vertebrates, whereas the opposite occurred when considering the fraction of small-sized acorns. The potential consequences of producing bigger seeds will partly depend on the relative abundance of the two guilds of acorn consumers. Thus, in plant populations with overabundance of vertebrates, the higher attraction of large-sized seeds for these generalist consumers could counteract the satiating effect exercised on granivorous insects through multi-infestation. However, in forest sites with less abundance of vertebrates, the risk of seed predation (mostly by insects) could be reduced in those trees producing huge quantities of large-sized seeds. In summary, we found clear evidences that the direction and magnitude of density-dependent seed removal can differ not only between different groups of seed consumers but also among different fractions of seed size, which highlights the importance of considering this plant trait to better understand the complexity of mechanisms operating in these plant-animal interactions.

Evaluating the impact ofHomo-carnivore competition in European human settlements during the early to middle Pleistocene

AbstractFossil remains and the technological complexes recorded in archaeological sites suggest that the human presence in Europe late in the early and middle Pleistocene was discontinuous. Moreover, competition for meat with other secondary consumers could have delayed the human dispersal through Europe. However, evaluation of the extent competition intensity among secondary consumers suggests this influenced the discontinuity of the human settlement of Europe between 1.1 and 0.2 Ma. Using a mathematical model, we estimate the amount of biomass available in a community for secondary consumers. The amount of available biomass is subsequently distributed among the guild of secondary consumers according to their requirements and prey preferences. Indexes that quantify the competition intensity among secondary consumers to compare the conditions in different paleoecosystems show that the competition intensity late in the early Pleistocene, early in the middle Pleistocene, and late in the middle Pleistocene does not support the view that an increase in competition intensity constrained the expansion of human populations early in the middle Pleistocene. Somewhat paradoxically, the lowest competition intensity is estimated to have occurred early in the middle Pleistocene, most likely because of an increase in the number of large herbivore species and a decrease in the number of secondary consumers. The early Pleistocene paleoecosystems supported higher competition intensity than the middle Pleistocene ecosystems, likely because of the different configuration in the food webs of these two periods (the early and middle Pleistocene).

Carnivores and humans during the Early and Middle Pleistocene at Sierra de Atapuerca

It has been repeatedly proposed that the human presence in Europe during the late Early and the early Middle Pleistocene was conditioned, among other factors, by the available trophic resources and by the competition with carnivores for these resources. Competition for meat with other secondary consumers could delay the human dispersal through Europe. Moreover, some research suggests that human settlement was discontinuous in Europe during the late Early and the Middle Pleistocene and that this discontinuity might be related to the presence of other secondary consumers. The competition intensity among secondary consumer species in Europe has been previously analysed at both local and continental scales. At the local scale, two faunal assemblages from the Sierra de Atapuerca sites were analysed, demonstrating that competition intensity was higher in the early Middle Pleistocene than in the late Early Pleistocene. At a continental scale, 34 faunal assemblages were analysed and demonstrated an opposite pattern. In the present study, we extended the local analysis to include more faunal assemblages from the Sierra de Atapuerca sites to track the changes in the competition intensity during a longer time period. The results show that humans were able to successfully exploit the Atapuerca ecosystems at different levels of competition intensity, and that TD6-2 assemblage exhibits an exceptionally low degree of competition. It is proposed that the TD6-2 paleocommunity likely included a large felid species that is not recorded in the fossil assemblage. Lack of human presence in the TD8 assemblage may be related to increased competition intensity inside the guild of secondary consumers, while the technological development from Mode 2 to Mode 3 could be related to an increased degree of competition. There are similarities between the analyses at the continental and local scales from the point of view of the available biomass for secondary consumers, but the competition intensity shows little variation at the local scale, with the exception of the TD6-2 assemblage.

Increased Primary Production from an Exotic Invader Does Not Subsidize Native Rodents.

Invasive plants have tremendous potential to enrich native food webs by subsidizing net primary productivity. Here, we explored how a potential food subsidy, seeds produced by the aggressive invader cheatgrass (Bromus tectorum), is utilized by an important guild of native consumers – granivorous small mammals – in the Great Basin Desert, USA. In a series of field experiments we examined 1) how cheatgrass invasion affects the density and biomass of seed rain at the ecosystem-level; 2) how seed resources from cheatgrass numerically affect granivorous small mammals; and 3) how the food preferences of native granivores might mediate the trophic integration of cheatgrass seeds. Relative to native productivity, cheatgrass invasion increased the density and biomass of seed rain by over 2000% (P < 0.01) and 3500% (P < 0.01), respectively. However, granivorous small mammals in native communities showed no positive response in abundance, richness, or diversity to experimental additions of cheatgrass seeds over one year. This lack of response correlated with a distinct preference for seeds from native grasses over seeds from cheatgrass. Our experiments demonstrate that increased primary productivity associated with exotic plant invasions may not necessarily subsidize consumers at higher trophic levels. In this context, cheatgrass invasion could disrupt native food webs by providing less-preferred resources that fail to enrich higher trophic levels.

Ecological impacts of invading seaweeds: a meta‐analysis of their effects at different trophic levels

AbstractAimBiological invasions are among the main threats to biodiversity. To promote a mechanistic understanding of the ecological impacts of non‐native seaweeds, we assessed how effects on resident organisms vary according to their trophic level.LocationGlobal.MethodsWe performed meta‐analytical comparisons of the effects of non‐native seaweeds on both individual species and communities. We compared the results of analyses performed on the whole dataset with those obtained from experimental data only and, when possible, between rocky and soft bottoms.ResultsMeta‐analyses of data from 100 papers revealed consistent negative effects of non‐native seaweeds across variables describing resident primary producer communities. In contrast, negative effects of seaweeds on consumers emerged only on their biomass and, limited to rocky bottoms, diversity. At the species level, negative effects were consistent across primary producers' response variables, while only the survival of consumers other than herbivores or predators (e.g. deposit/suspension feeders or detritivores) decreased due to invasion. Excluding mensurative data, negative effects of seaweeds persisted only on resident macroalgal communities and consumer species survival, while switched to positive on the diversity of rocky‐bottom consumers. However, negative effects emerged for biomass and, in rocky habitats, density of consumers other than herbivores or predators.Main conclusionsOur results support the hypothesis that seaweeds' effects on resident biodiversity are generally more negative within the same trophic level than on higher trophic guilds. Finer trophic grouping of resident organisms revealed more complex impacts than previously detected. High heterogeneity in the responses of some consumer guilds suggests that impacts of non‐native seaweeds at higher trophic levels may be more invader‐ and species‐specific than competitive effects at the same trophic level. Features of invaded habitats may further increase variability in seaweeds' impacts. More experimental data on consumers' response to invasion are needed to disentangle the effects of non‐native seaweeds from those of other environmental stressors.

Interaction patterns within a multi-herbivore assemblage derived from stable isotopes

The ecological effects of consumer guilds are strongly driven by the relative resource use of included species. Here we use stable isotopes in faecal samples from 6 co-occurring mammalian herbivores in an African savanna to identify species-specific trophic niches and detect patterns of interactions between herbivores and their feeding resources. Species-specific foraging strategies were reflected in the faecal δ13C values, with species aligning along both a browser-grazer gradient and a niche breadth gradient. Within the broad foraging strategies outlined by δ13C, δ15N indicated a seasonal shift in resource use for some herbivores. We expected that 13C isotope niches would overlap in a nested fashion, with the niches of grazers and browsers being included in those of mixed feeders, and that 15N niches would separate into discrete modules reflecting leguminous and protein content of respective diets. However, the observed structure was more complex, with combined modular and nested patterns of interactions between herbivores and 13C isotopes. We identified an isolated module comprising eland and its exclusive δ13C values, and a series of nested structures with a set of specialist herbivores (hartebeest and sable) which δ13C values were nested within those of more generalist herbivores (impala, waterbuck and zebra). Networks based on δ15N, however, reflected a higher level of overlap in resource use with random patterns in herbivore interactions with resources, and only a significant modular interaction pattern during the dry season. We suggest that the combined modular and nested pattern of δ13C interactions reflect the simultaneous mutualistic and antagonistic characteristics of plant–herbivore interactions. We argue that such interaction patterns could stabilize ecosystems by constraining effects of perturbations to specific modules and by increasing functional redundancy through nested interactions.

No evidence for phylogenetic constraint on natural defense evolution among wild tomatoes.

Plant defense traits can be shaped by evolutionary and physiological constraints, as well as local ecological selection. We assessed the relative importance of these factors in shaping defense trait variation across the wild tomato clade (a group of 13 closely related species) using an herbivore bioassay (Manduca sexta). With phylogenetic comparative methods, we evaluated patterns of constitutive and induced defense variation, and the extent of coupling between alternative defense strategies. We detected substantial variation among species and found no evidence for phylogenetic conservatism among defensive traits, unlike for two other ecologically relevant (reproductive) traits. In addition, constitutive and induced defense syndromes were unassociated. These data indicate that, in this group, there is no evidence for either phylogenetic conservatism of shared consumer guilds that shape defense traits, or for constraints on defense trait evolution, including mechanistic trade-offs between defense strategies. Our data suggest that defense trait variation in this clade instead results from rapid responses to local ecological conditions.