Negative Interspecific Interactions Research Articles

Post-mortem examination on a striped dolphin (Stenella coeruleoalba) reveals a potential fatal interaction with bottlenose dolphins (Tursiops truncatus) in Italian waters

Abstract Despite attacks of bottlenose dolphins towards conspecifics and other species of dolphins being reported worldwide, inside the Mediterranean Sea these behaviours are still considered sporadic and have been, to date, only recorded in Spain and France, mostly directed to striped dolphins. In this work, we reported the necropsy outcomes of a dead striped dolphin that suggest a fatal interaction with bottlenose dolphins inside the Italian side of the Pelagos Sanctuary, Western Ligurian Sea. Results from the post-mortem examination showed multiple, multifocal external rake marks spaced 1–1.2 cm and several internal injuries, consisting of subcutaneous and muscular haematomas and haemorrhages, multiple vertebral and rib fractures, haemothorax and lung laceration. The inter-tooth distance and all the internal findings were consistent with a fatal traumatic interaction with bottlenose dolphins as reported in similar cases in other parts of the world where the same results were found. No other significant gross and microscopic findings and concurrent significant pathogens were detected. This case highlights the existence of negative interspecific interactions in an area where they have never been reported before and adds new important information to the literature for understanding their occurrence, expanding the geographical range of observation in the western Mediterranean Sea.

Synthetic model ecosystem of 12 cryopreservable microbial species allowing for a noninvasive approach

Simultaneous understanding of both population and ecosystem dynamics is crucial in an era marked by the degradation of ecosystem services. Experimental ecosystems are a powerful tool for understanding these dynamics; however, they often face technical challenges, typically falling into two categories: “complex but with limited replicability microcosms” and “highly replicable but overly simplistic microcosms.” Herein, we present a high-throughput synthetic microcosm system comprising 12 functionally and phylogenetically diverse microbial species. These species are axenically culturable, cryopreservable, and can be measured noninvasively via microscopy, aided by machine learning. This system includes prokaryotic and eukaryotic producers and decomposers, and eukaryotic consumers to ensure functional redundancy. Our model system exhibited key features of a complex ecosystem: (i) various positive and negative interspecific interactions, (ii) higher-order interactions beyond two-species dynamics, (iii) probabilistic dynamics leading to divergent outcomes, and (iv) stable nonlinear transitions. We identified several conditions under which at least one species from each of the three functional groups—producers, consumers, and decomposers—and one functionally redundant species, persisted for over six months. These conditions set the stage for detailed investigations in the future. Given its designability and experimental replicability, our model ecosystem offers a promising platform for deeper insights integrating both population and ecosystem dynamics.

Frequency‐dependent community dynamics driven by sexual interactions

AbstractResearch in community ecology has tended to focus on trophic interactions (e.g., predation, resource competition) as driving forces of community dynamics, and sexual interactions have often been overlooked. Here we discuss how sexual interactions can affect community dynamics, especially focusing on frequency‐dependent dynamics of horizontal communities (i.e., communities of competing species in a single ecological guild). By combining mechanistic and phenomenological models of competition, we place sexual reproduction into the framework of modern coexistence theory. First, we review how population dynamics of two species competing for two resources can be represented by the Lotka–Volterra competition model as well as frequency dynamics, and how niche differentiation and overlap produce negative and positive frequency‐dependence (i.e., stable coexistence and priority effect), respectively. Then, we explore two situations where sexual interactions change the frequency‐dependence in community dynamics: (1) reproductive interference, that is, negative interspecific interactions due to incomplete species recognition in mating trials, can promote positive frequency‐dependence and (2) density‐dependent intraspecific adaptation load, that is, reduced population growth rates due to adaptation to intraspecific sexual (or social) interactions, produces negative frequency‐dependence. We show how reproductive interference and density‐dependent intraspecific adaptation load can decrease and increase niche differences in the framework of modern coexistence theory, respectively. Finally, we discuss future empirical and theoretical approaches for studying how sexual interactions and related phenomena (e.g., reproductive interference, intraspecific adaptation load, and sexual dimorphism) driven by sexual selection and conflict can affect community dynamics.

Seagrass is protected from ragworm pressure by a newly discovered grazer–ragworm interaction; implications for restoration

Abstract Coastal ecosystems are generally controlled by the combination of bottom‐up (resource‐driven) and top‐down (consumer‐driven) trophic, and non‐trophic interactions. Anthropogenic disruption of these interactions, for example, through eutrophication or overfishing, leads to loss of the foundation species composing these ecosystems. Within degraded ecosystems, new interactions may become dominant, hampering restoration of the foundation species. We demonstrate this concept in saltwater Lake Grevelingen, where seagrass restoration was seriously hampered by a newly discovered non‐trophic interaction between the seagrass Zostera marina and the ragworm Platynereis dumerilii. Platynereis constructs dwelling tubes between seagrass leaves by glueing and entangling them, leading to loss of biomass and increased epiphyte cover. To test the roles of epiphyte grazing (top‐down) and eutrophication (bottom‐up) on this new‐found interaction, we executed a field experiment introducing the snail Littorina littorea under natural nutrient concentrations and Platynereis densities and performed a full factorial mesocosm experiment applying Littorina, Platynereis and nutrient treatments. Our field results show that survival of reintroduced Zostera in Lake Grevelingen is seriously hampered by Platynereis, which can be suppressed by grazer addition. In our mesocosm experiment, we demonstrate that top‐down control by Littorina protects seagrass through two pathways: (1) mitigating bottom‐up regulated epiphyte growth by trophic top‐down forcing and (2) hampering the non‐trophic interaction between Zostera and Platynereis by grazing epiphytes and dwelling tubes. Synthesis and applications. With coastal ecosystems globally under pressure, our study emphasises the importance of understanding ecosystem‐specific trophic and non‐trophic key interactions when considering restoration. We show how negative interspecific interactions can become dominant in degraded systems with absent key facilitating interactions. Next, we show that these negative interactions (leaf glueing), inhibiting foundation species restoration, can be overruled by active inclusion of a key facilitating interactions (epiphyte grazing by mesograzers). Finally, we provide a three‐step co‐introduction approach for restoration management.

Adjustment in the point-following behaviour of free-ranging dogs - roles of social petting and informative-deceptive nature of cues.

Animals of different taxa can read and respond to various human communicative signals. Such a mechanism facilitates animals to acquire social information and helps them react in a context-dependent manner. Dogs have garnered extensive attention owing to their socio-cognitive skills and remarkable sensitivity to human social cues. For example, dogs readily respond to different human pointing gestures to locate hidden food rewards. However, a general inclination towards testing highly socialized pet dogs has resulted in a dearth of information on other sub-populations of dogs. Free-ranging dogs are one of the least socialized dog populations yet exhibit point-following behaviour flexibly. As a consequence of frequent negative interspecific interactions, they are typically wary of unfamiliar humans; thus, contextual recognition of human actions is paramount for these dogs to avoid potential conflict. However, the mechanisms influencing their point-following behaviour remain unidentified. We asked to what extent the informative-deceptive nature of cues and positive human interactions influence the interspecific communicative behaviour of these minimally socialized dogs. Using a point-following experiment with a 2 × 2 design, we focused on adult free-ranging dogs' behavioural adjustments. Dogs were randomly divided into two groups, with only one receiving brief social petting. Further, informative and deceptive cues were given to separate subsets within each group. Our findings suggest that brief social petting strongly affects the likelihood of free-ranging dogs' point-following tendencies. Dogs who received petting followed the pointing cues regardless of their informative or deceptive nature, whereas dogs who did not receive petting discriminated between informative and deceptive pointing. This study highlights the contribution of positive human interaction and informative-deceptive quality of cues in modulating the behavioural responses of free-ranging dogs in an interspecific communicative context.

Mixed-species groups in bats: non-random roost associations and roost selection in neotropical understory bats

BackgroundMixed-species groups in animals have been shown to confer antipredator, foraging and other benefits to their members that may provide selective advantages. In most cases, however, it is unclear whether functional benefits are a principal driver of heterospecific groups, or whether groups simply result from simultaneous exploitation of common resources. Mixed-species groups that form independently of environmental conditions may, however, evidence direct benefits of species associations. Bats are among the most gregarious mammals, with sometimes thousands of individuals of various species roosting communally. Despite numerous potential functional benefits of such mixed-species roosting groups, interspecific attraction has never been shown. To explore alternative explanations for mixed-species roosting, we studied roost selection in a speciose neotropical understory bat community in lowland rainforest in Costa Rica. Long term roost data were recorded over 10 years in a total of 133 roosts comprising both natural roosts and structurally uniform artificial roosts. We modelled bat roost occupancy and abundance in each roost type and in forest and pasture habitats to quantify the effects of roost- and environmental variability.ResultsWe found that bat species presence in natural roosts is predictable from habitat and structural roost parameters, but that the presence and abundance of other bat species further modifies roost choice. One third of the 12 study species were found to actively associate with selected other bat species in roosts (e.g. Glossophaga commissarisi with Carollia sowelli). Other species did not engage in communal roosting, which in some cases indicates a role for negative interspecific interactions, such as roost competition.ConclusionsMixed-species roosting may provide thermoregulatory benefits, reduce intraspecific competition and promote interspecific information transfer, and hence some heterospecific associations may be selected for in bats. Overall, our study contributes to an improved understanding of the array of factors that shape diverse tropical bat communities and drive the dynamics of heterospecific grouping in mammals more generally.

Species associations in arthropod ectoparasite infracommunities are spatially and temporally variable and affected by environmental factors

1. We applied a Markov random fields/conditional random fields (MRF/CRF) modelling to understand whether and how species associations in ectoparasite infracommunities of small mammals vary along environmental gradients across space and time.2. We analysed the variation in species associations in flea and mite infracommunities of seven host species across localities (spatial scale) and time periods (in the same locality; temporal scale).3. The performance of the CRF models differed between ectoparasite taxa, scales, and host species. The model performance for flea and mite infracommunities in the majority of host species was satisfying although the proportions of correctly predicted positive occurrences and model sensitivity were moderate.4. The probability of occurrence of many flea and mite species depended mostly on the occurrence of another flea and mite species (respectively), with many negative interspecific interactions, although occurrences of some species were mainly affected by environmental factors. The patterns of pairwise associations between some ectoparasite species were affected by environmental factors.5. Although many ectoparasite pairs demonstrated stability in the strength or sign of association across space or time, interactions between other species pairs were spatially or temporally variable, with the association between the same pair of species being stable in one host species and variable in another host species.6. We conclude that ectoparasite associations are mediated by the off‐host environment and may depend on the life histories of both host and ectoparasite species.

Let's move out together: a framework for the intersections between movement and mutualism.

Movement is a widespread behavior across organisms and is driven in part by interspecific interactions. Generally, negative interspecific interactions (such as competition and natural enemies) are more often studied in the context of movement than positive interactions (mutualism). Mutualistic relationships are incredibly common, yet only a subset are studied in the context of movement (transportation mutualisms). Overall, the costs and benefits that an individual experiences are shaped both by their movement behavior and their mutualistic relationships, as well as the intersection between these. Here we argue that the intersection between movement behavior and mutualistic relationships is understudied, and we present a conceptual framework to synthesize the links between movement and mutualisms and give examples of species that exhibit each. Our framework serves both to highlight the ways that mutualism can shape movement (and vice versa) and to draw parallels across different organisms (enabling a more abstract perspective of these biological systems, complementing the system-focused perspective). Finally, we show how considering movement in light of mutualisms (and vice versa) presents a number of new research questions to be answered by each empirical and theoretical approach going forward.

Effects of Interspecific Interactions on Infracommunities of Helminths Parasitizing Creek Chub (Semotilus atromaculatus)

Natural infections of Allocreadium lobatum Wallin, 1909 (Trematoda), Proteocephalus sp. (Cestoda), Rhabdochona canadensis Moravec and Arai, 1971 (Nematoda), Paulisentis missouriensis Keppner, 1974 (Acanthocephala), and Schyzocotyle acheilognathi (Yamaguti, 1934) (Cestoda) in the intestine of creek chub (Semotilus atromaculatus) were analyzed to determine whether patterns of interspecific competition were evident. Data from nearly 2 decades of sampling from the same system were analyzed for patterns of co-occurrence and abundance consistent with negative interactions. Co-occurrence analyses (null models and logistic regression) did not detect negative associations among parasite species. Positive associations among species were more common in null model comparisons. Correlation analyses demonstrated some significant negative correlations between the abundances of pairs of parasite species in some samples, but the overwhelming majority of correlations were insignificant or positive. Previous work did not detect negative microhabitat interactions among the most common helminths studied herein, and together, these results suggest the parasite communities of creek chub in the study area are absent any substantial effect of antagonistic or other negative interspecific interactions. Differences in transmission rate, mode, and spatiotemporal variation are likely dominant processes explaining the patterns observed in the present investigation.

Environmental connectivity controls diversity in soil microbial communities

Interspecific interactions are thought to govern the stability and functioning of microbial communities, but the influence of the spatial environment and its structural connectivity on the potential of such interactions to unfold remain largely unknown. Here we studied the effects on community growth and microbial diversity as a function of environmental connectivity, where we define environmental connectivity as the degree of habitat fragmentation preventing microbial cells from living together. We quantitatively compared growth of a naturally-derived high microbial diversity community from soil in a completely mixed liquid suspension (high connectivity) to growth in a massively fragmented and poorly connected environment (low connectivity). The low connectivity environment consisted of homogenously-sized miniature agarose beads containing random single or paired founder cells. We found that overall community growth was the same in both environments, but the low connectivity environment dramatically reduced global community-level diversity compared to the high connectivity environment. Experimental observations were supported by community growth modeling. The model predicts a loss of diversity in the low connectivity environment as a result of negative interspecific interactions becoming more dominant at small founder species numbers. Counterintuitively for the low connectivity environment, growth of isolated single genotypes was less productive than that of random founder genotype cell pairs, suggesting that the community as a whole profited from emerging positive interspecific interactions. Our work demonstrates the importance of environmental connectivity for growth of natural soil microbial communities, which aids future efforts to intervene in or restore community composition to achieve engineering and biotechnological objectives.

Reproductive interference hampers species coexistence despite conspecific sperm precedence.

Negative interspecific mating interactions, known as reproductive interference, can hamper species coexistence in a local patch and promote niche partitioning or geographical segregation of closely related species. Conspecific sperm precedence (CSP), which occurs when females that have mated with both conspecific and heterospecific males preferentially use conspecific sperm for fertilization, might contribute to species coexistence by mitigating the costs of interspecific mating and hybridization. We discussed whether two species exhibiting CSP can coexist in a local environment in the presence of reproductive interference. First, using a behaviorally explicit mathematical model, we demonstrated that two species characterized by negative mating interactions are unlikely to coexist because the costs of reproductive interference, such as loss of mating opportunity with conspecific partners, are inevitably incurred when individuals of both species are present. Second, we experimentally examined differences in mating activity and preference in two Harmonia ladybird species known to exhibit CSP. These behavioral differences may lead to local extinction of H. yedoensis because of reproductive interference by H. axyridis. This prediction is consistent with field observations that H. axyridis uses various food sources and habitats whereas H. yedoensis is confined to a less preferred prey item and a pine tree habitat. Finally, by a comparative approach, we observed that niche partitioning or parapatric distribution, but not sympatric coexistence in the same habitat, is maintained between species with CSP belonging to a wide range of taxa, including vertebrates and invertebrates living in aquatic or terrestrial environments. Taken together, it is possible that reproductive interference may destabilize local coexistence even in closely related species that exhibit CSP.

Light can modify density‐dependent seedling mortality in a temperate forest

AbstractQuestionsThere is growing evidence on how density‐dependent drivers of plant performance are influenced by habitat heterogeneity such as variation in understorey light availability. How these processes operate in temperate forests still remains an open question, however. We examined the interactive effects of density dependence and light availability on woody seedling mortality across multiple species, two size stages and growth forms.LocationAlluvial old‐growth temperate forest, southeastern Czech Republic.MethodsWe mapped seedlings of woody species (≥10 cm tall) and recorded their mortality over a two‐year period on 21 square seedling plots (25 m2) within one large square plot with all trees mapped (≥1 cm diameter at breast height). Seedling mortality was studied using a combined approach of generalised linear mixed models and spatial point pattern analyses.ResultsDensity‐dependent effects of tree and seedling neighbours influenced seedling mortality at different spatial scales, depending on their size class, growth form and by taxon. Conspecific inhibition raised the mortality of seedlings other than Acer campestre at low light levels, indicating that host‐specific enemies and/or intraspecific competition may be important in unfavourable habitats. While small seedlings experienced conspecific inhibition at low light levels, tall seedlings were additively affected by low‐light conditions and competition with trees. Both conspecific and heterospecific neighbours tended to affect tree seedlings at low light levels, whereas shrub seedlings experienced light‐ and density‐independent mortality. Some negative interspecific interactions indicated ongoing expansion of species like Acer campestre in currently flood‐free habitats.ConclusionsHow density‐dependent effects translate into demographic outcomes for woody seedling populations may be largely determined by variation in understorey light availability. Given that seedlings differ in size, growth form and taxa, we also demonstrate that both seedling and tree neighbours may contribute to the strength and spatial variation in density‐dependent effects at seedling neighbourhood scales.

Positive and negative interspecific interactions between coexisting rice planthoppers neutralise the effects of elevated temperatures.

Global warming is often predicted to increase damage to plants through direct effects on insect herbivores. However, the indirect impacts of rising temperatures on herbivores, mediated through interactions with their biotic environment, could dampen these effects.Using a series of reciprocal density experiments with gravid females and developing nymphs, we examined interspecific competition between two coexisting phloem feeders Nilaparvata lugens (BPH) and Sogatella furcifera (WBPH), on rice at 25 and 30°C.WBPH performed better (i.e. adults survived longer, nymphs developed faster and grew larger) at 25°C and BPH (i.e. nymphs developed faster) at 30°C. However, contrary to predictions, WBPH had a greater effect in reducing oviposition and nymph performance in BPH at 30°C.A decoupling of resource use by WBPH and its antagonistic effects on BPH at the higher temperature suggests that WBPH feeding induces host defences that reduce BPH fitness (i.e. interference competition). Meanwhile, BPH facilitated WBPH oviposition at 30°C and facilitated WBPH nymph performance at 25 and 30°C. Greater facilitation of feeding in WBPH nymphs by BPH at high densities suggests that mechanical damage and host responses to damage increased the fitness of the heterospecific nymphs.Although BPH also facilitated egg‐laying by WBPH, intra‐ and interspecific crowding countered this facilitation at both temperatures. Simulated life tables for planthoppers at 25 and 30°C depicted significantly lower offspring numbers on rice infested by WBPH alone and from mixed BPH‐WBPH infestations than from infestations by BPH alone.Our results indicate how interference competition—mediated through host plant defences—can increase ecosystem resilience to the warmer temperatures predicted under global climate change. A free Plain Language Summary can be found within the Supporting Information of this article.

Habitat partitioning of two closely related pond frogs, Pelophylax nigromaculatus and Pelophylax porosus brevipodus, during their breeding season

Parapatric distributions of closely related species are common in many taxonomic groups. However, habitat overlap in sympatric areas within both species’ ranges is complex and driven by the nature of the interactions between species as well as each species’ ecological requirements. We therefore investigated the distribution of breeding habitats for two congeneric frogs, Pelophylax nigromaculatus and Pelophylax porosus brevipodus, in a region of Japan where these species co-occur, based on observation of mating calls as an index of the abundance of males engaging in reproductive activities. Among a total of 381 surveyed sites, P. nigromaculatus and P. porosus brevipodus were found at 171 and 229 sites, respectively. Pelophylax porosus brevipodus was widely distributed throughout the study area and was more abundant than P. nigromaculatus in sympatric sites. Analysis using cumulative link mixed models revealed that the abundance of calling males was negatively affected by the presence of calling males of the other species and by landscape factors. Interspecific differences in this negative effect appear to favor P. porosus brevipodus over P. nigromaculatus in our study area. Due to differences in the primary habitat preferences of the two species during the non-breeding season, habitat partitioning during the breeding season is likely driven by negative interspecific interactions during the reproductive process (i.e., reproductive interference).

Co-occurrence of Ectoparasites (Insecta and Arachnida) on Bats (Chiroptera) in an Atlantic Forest Remnant, Southeastern Brazil.

Neotropical bats are infested by multiple ectoparasites (like bat fly and mite species) and investigations on these invertebrates on their hosts are crucial to better understand the ectoparasite-ectoparasite and ectoparasite-host associations. The goal of this study was to report ectoparasites species (bat flies and mites) on bats, emphasizing ectoparasite co-occurrences and host-ectoparasite associations. We also test if there is relationship between bat flies and mites on their hosts. This study occurred twice a month from September 2011 to September 2012 in an Atlantic Forest remnant in Rio de Janeiro State, Brazil. Spearman correlation was used to test relationship between bat flies and mites. We found 27 species of ectoparasites on 13 species of bats. Ectoparasites belonged to the bat fly families Streblidae and Nycteribiidae, and the mite families Spinturnicidae, Macronyssidae, Sarcoptidae, and Trombiculidae. Streblid-streblid association was more frequent, but we also recorded streblid-spinturnicid, streblid-sarcoptid and spinturnicid-spinturnicid associations. The abundance of spinturnicid species was negatively related to the abundance of streblid species. We record the first occurrence of Strebla chrotopteri associated with Chrotopterus auritus, Periglischrus paracutisternus associated with Trachops cirrhosus and Basilia juquiensis associated with Myotis riparius for Rio de Janeiro State. There were several co-occurrences between different taxa and between species of the same family. These results show the importance of the integrated taxonomic record. The negative interspecific interactions between spinturnicid and streblid may affect distributions, structuring ectoparasite communities on hosts.

Effects of Interspecific Interactions on Microhabitat Use of Three Helminths Parasitizing Creek Chub (Semotilus atromaculatus) in Southeastern Nebraska

Natural infections of Paulisentis missouriensis (Acanthocephala), Rhabdochona canadensis (Nematoda), and Allocreadium lobatum (Trematoda) in the intestine of creek chub (Semotilus atromaculatus) were studied to determine whether microhabitat use of these helminths was affected by interspecific interactions. Comparison of mean relative positions and intestinal segment use of all 3 helminths revealed no negative interactions among the 3 species. Paulisentis missouriensis appeared entirely unaffected by the presence of either R. canadensis or A. lobatum. Each of the latter 2 species, however, appeared to exhibit some shift toward, rather than away from, the location of P. missouriensis in co-infected hosts. However, the effect size in both cases was small and therefore could be spurious. At higher intensities of co-infecting P. missouriensis, worms of A. lobatum were either distributed far to the anterior or far to the posterior of the intestinal locations occupied by P. missouriensis. This constituted the only evidence in the present investigation for negative interspecific interactions among these 3 frequently co-occurring helminths in this system.

Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem.

Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.

Niche dimensionality and herbivory control stream algal biomass via shifts in guild composition, richness, and evenness.

We developed a framework for the hierarchical pathways of bottom-up (niche dimensionality) and top-down control (herbivory) on biomass of stream algae via changes in guild composition (relative abundance of low profile, high profile, and motile guilds), species richness, and evenness. We further tested (1) the contrasting predictions of resource competition theory vs. the benthic model of coexistence on how the number of added nutrients constrains species richness, (2) the relationship between species richness and evenness, and (3) the biodiversity-ecosystem-function paradigm. Implementing a combination of field and lab experiments that manipulated for the first time in benthic algae herbivory and/or niche dimensionality, i.e., the number of added nutrients (NAN), including nitrogen, phosphorus, iron, and manganese, we made the following discoveries. First, important predictors of guild composition were herbivory (field) and NAN (lab); of richness, NAN (field) and NAN and guild composition (lab); of evenness, guild composition (field and lab) and herbivory (field); and of biomass, guild composition, NAN, and richness + evenness (field and lab). Herbivory increased the proportions of the low profile and motile guilds but decreased the proportion of the high profile guild. In the absence of grazing, greater proportions of the high profile guild resulted in elevated richness and biomass but diminished evenness, whereas in the presence of grazing, these relationships generally disappeared. Second, both experiments confirmed the prediction of the benthic model that species richness increases with NAN, a pattern inconsistent with resource competition theory. Third, supplementation with manganese and/or iron increased algal richness, indicating that micronutrients, which have generally been overlooked in stream ecology, added dimensions to the algal niche. Fourth, the richness-evenness relationship, observed only in the absence of herbivory, depended on the size of the species pool. It was positive at richness lower than 49 species (lab), implying complementarity and facilitation, while at higher richness (field and lab), this relationship was negative, consistent with negative interspecific interactions. Finally, the greater dependence of biomass production on guild composition and NAN than on richness and evenness suggests that more comprehensive, environmentally explicit, and trait-based approaches are necessary for the study of the biodiversity-ecosystem-function paradigm.

Habitat structure influences the seasonality of nekton in seagrass

The presence of a seasonally variable biogenic habitat (eelgrass, Zostera marina) increased the spring–summer variability of associated nekton relative to unvegetated bare tidal flat. This spatio-temporal pattern emerged because most eelgrass-associated taxa tracked the decline in eelgrass biomass from summer to spring, but in one case reached greater density in spring when predation intensity was low. Among 21 taxa (26,884 individuals) captured, a strong correlation arose between structure association and summer dominance, and certain functional traits, in particular morphology and on- vs. off-bottom position, were strong predictors of eelgrass association. Structure-associated taxa were slender-bodied and pelagic schooling fishes, while habitat generalists or bare-associated taxa were more consistent seasonally, primarily benthic, and cryptically colored with sand. Estuarine use (transient, reproducing, or estuarine resident) was not a strong predictor of structure association or seasonality. Because an identical sampling design was used in five regions of Washington State, USA, coarse-scale (> 100 km) differentiation in nekton assemblages was identified, representing less of the total variation than across seasons but more than across different habitats. While regional nekton differences were attributable in part to geographic distance and eelgrass morphotypes, the most seasonally variable nekton were at sites with morphotypes adding the least vertical structure but highest density. These results support two mechanisms that increase seasonal variability of taxa using structured habitats, including both bottom-up provision of habitat and resources, and subsequent possibilities for negative interspecific interactions and top-down control.

Spatial structure of a multi-species guild: the dominant biofilm-grazing microgastropods of seagrass

Although all-too-often ignored ecologically, gastropods < 5 mm in largest dimension may be dominant aquatic assemblage components. In Moreton Bay, Australia, for example, intertidal seagrass supports a guild of 24 such biofilm-grazing species at mean density one-third that of the whole macrobenthic seagrass assemblage. Detailed spatial patterns of the 14 guild members at one c. 1 ha locality were investigated via a 256-station lattice. Relative importance of the guild within the macrobenthic assemblage as a whole was spatially uniform but its abundance was significantly patchy at all spatial scales—a few stations supporting up to 170 times background levels of some species—and guild patchiness showed a relatively constant magnitude across those scales. Patches of individual species were independently distributed, showing no evidence of negative inter-specific interactions, the few significant spatial correlations between species being very weak and positive. Levels of syntopy (up to six species within a 0.0054 m2 area) did not differ from those expected under null models of independent assortment. Although diverse, overall guild abundance was low, appearing well below potential carrying capacity, and dominated by few species. Power–law relationships suggested temporal stability of these patterns. Possible causes of such guild structure are discussed.