The role of nurse successional stages on species‐specific facilitation in drylands: Nurse traits and facilitation skills

Plant establishment is a challenge in semiarid environments due to intense and frequent drought periods. The presence of neighboring trees (nurses) can increase the establishment of seedlings (targets) by improving resource availability and microclimate. The nurse effect, however, might vary depending on nurse‐target species combinations but factors that predict this specificity are poorly known. We used a multispecies experiment to investigate the facilitation potential of trees from a range of successional stages, focusing on how nurse functional traits can predict species‐specific interaction outcomes. We conducted a factorial field experiment in a Brazilian semiarid tropical forest during a severe drought period. Sixty pairs of interacting tree species, 20 potential nurses, and three targets were used. Seedlings of all targets were planted both under and far from the nurse canopy, in a randomized block design replicated five times. Target growth and survival were monitored for 275 days from the beginning of the dry season, and interaction outcomes were calculated using the Relative Interaction Intensity (RII) index. Nurse functional traits such as successional stage, height, wood density, and canopy diameter were used as explanatory variables to predict RII values. The average effect of nurse species on target plants was in general positive, that is, seedling survival and growth increased under the nurse canopy. However, for growth pairwise interactions were significantly species specific. Successional stage was the only functional trait explaining RII values, with pioneer tree species being stronger facilitators than later successional trees. However, the explanation power of this variable was low, and positive, negative, or neutral interactions were found among nurse trees of all successional stages. Because seedling mortality during drought in semiarid systems is high, future studies should investigate how nurse traits related to water use could influence nurse facilitation skills.     

Indirect facilitation of an anuran invasion by non-native fishes

Positive interactions among non‐native species could greatly exacerbate the problem of invasions, but are poorly studied and our knowledge of their occurrence is mostly limited to plant‐pollinator and dispersal interactions. We found that invasion of bullfrogs is facilitated by the presence of co‐evolved non‐native fish, which increase tadpole survival by reducing predatory macroinvertebrate densities. Native dragonfly nymphs in Oregon, USA caused zero survival of bullfrog tadpoles in a replicated field experiment unless a non‐native sunfish was present to reduce dragonfly density. This pattern was also evident in pond surveys where the best predictors of bullfrog abundance were the presence of non‐native fish and bathymetry. This is the first experimental evidence of facilitation between two non‐native vertebrates and supports the invasional meltdown hypothesis. Such positive interactions among non‐native species have the potential to disrupt ecosystems by amplifying invasions, and our study shows they can occur via indirect mechanisms.     

High N,dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought

Hotter, longer, and more frequent global change‐type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought‐induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean‐type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water‐use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation‐type conversion.     

How community adaptation affects biodiversity–ecosystem functioning relationships

Evidence is growing that evolutionary dynamics can impact biodiversity–ecosystem functioning (BEF) relationships. However the nature of such impacts remains poorly understood. Here we use a modelling approach to compare random communities, with no trait evolutionary fine‐tuning, and co‐adapted communities, where traits have co‐evolved, in terms of emerging biodiversity–productivity, biodiversity–stability and biodiversity–invasion relationships. Community adaptation impacted most BEF relationships, sometimes inverting the slope of the relationship compared to random communities. Biodiversity–productivity relationships were generally less positive among co‐adapted communities, with reduced contribution of sampling effects. The effect of community‐adaptation, though modest regarding invasion resistance, was striking regarding invasion tolerance: co‐adapted communities could remain very tolerant to invasions even at high diversity. BEF relationships are thus contingent on the history of ecosystems and their degree of community adaptation. Short‐term experiments and observations following recent changes may not be safely extrapolated into the future, once eco‐evolutionary feedbacks have taken place.     

Ecological correlates of the spatial co‐occurrence of sympatric mammalian carnivores worldwide

The composition of local mammalian carnivore communities has far‐reaching effects on terrestrial ecosystems worldwide. To better understand how carnivore communities are structured, we analysed camera trap data for 108 087 trap days across 12 countries spanning five continents. We estimate local probabilities of co‐occurrence among 768 species pairs from the order Carnivora and evaluate how shared ecological traits correlate with probabilities of co‐occurrence. Within individual study areas, species pairs co‐occurred more frequently than expected at random. Co‐occurrence probabilities were greatest for species pairs that shared ecological traits including similar body size, temporal activity pattern and diet. However, co‐occurrence decreased as compared to other species pairs when the pair included a large‐bodied carnivore. Our results suggest that a combination of shared traits and top‐down regulation by large carnivores shape local carnivore communities globally.     

Toward spatio‐temporal delineation of positive interactions in ecology

Given unprecedented rates of biodiversity loss, there is an urgency to better understand the ecological consequences of interactions among organisms that may lost or altered. Positive interactions among organisms of the same or different species that directly or indirectly improve performance of at least one participant can structure populations and communities and control ecosystem process. However, we are still in need of synthetic approaches to better understand how positive interactions scale spatio‐temporally across a range of taxa and ecosystems. Here, we synthesize two complementary approaches to more rigorously describe positive interactions and their consequences among organisms, across taxa, and over spatio‐temporal scales. In the first approach, which we call the mechanistic approach, we make a distinction between two principal mechanisms of facilitation—habitat modification and resource modification. Considering the differences in these two mechanisms is critical because it delineates the potential spatio‐temporal bounds over which a positive interaction can occur. We offer guidance on improved sampling regimes for quantification of these mechanistic interactions and their consequences. Second, we present a trait‐based approach in which traits of facilitators or traits of beneficiaries can modulate their magnitude of effect or how they respond to either of the positive interaction mechanisms, respectively. Therefore, both approaches can be integrated together by quantifying the degree to which a focal facilitator's or beneficiary's traits explain the magnitude of a positive effect in space and time. Furthermore, we demonstrate how field measurements and analytical techniques can be used to collect and analyze data to test the predictions presented herein. We conclude by discussing how these approaches can be applied to contemporary challenges in ecology, such as conservation and restoration and suggest avenues for future research.     

Belowground interactions shift the relative importance of direct and indirect genetic effects

Intraspecific genetic variation can affect decomposition, nutrient cycling, and interactions between plants and their associated belowground communities. However, the effects of genetic variation on ecosystems can also be indirect, meaning that genes in a focal plant may affect ecosystems by altering the phenotype of interacting (i.e., neighboring) individuals. We manipulated genotype identity, species identity, and the possibility of belowground interactions between neighboring Solidago plants. We hypothesized that, because our plants were nitrogen (N) limited, the most important interactions between focal and neighbor plants would occur belowground. More specifically, we hypothesized that the genotypic identity of a plant's neighbor would have a larger effect on belowground biomass than on aboveground biomass, but only when neighboring plants were allowed to interact belowground. We detected species‐ and genotype‐level variation for aboveground biomass and ramet production. We also found that belowground biomass and ramet production depended on the interaction of neighbor genotype identity and the presence or absence of belowground interactions. Additionally, we found that interspecific indirect genetic effects (IIGEs; changes in focal plant traits due to the genotype identity of a heterospecific neighbor) had a greater effect size on belowground biomass than did focal genotype; however, this effect only held in pots that allowed belowground interactions. These results expand the types of natural processes that can be attributed to genotypes by showing that, under certain conditions, a plant's phenotype can be strongly determined by the expression of genes in its neighbor. By showing that IIGEs are dependent upon plants being able to interact belowground, our results also provide a first step for thinking about how genotype‐based, belowground interactions influence the evolutionary outcomes of plant‐neighbor interactions.     

Image spectroscopy and stable isotopes elucidate functional dissimilarity between native and nonnative plant species in the aquatic environment

? Nonnative species may change ecosystem functionality at the expense of native species. Here, we examine the similarity of functional traits of native and nonnative submersed aquatic plants (SAP) in an aquatic ecosystem. ? We used field and airborne imaging spectroscopy and isotope ratios of SAP species in the Sacramento-San Joaquin Delta, California (USA) to assess species identification, chlorophyll (Chl) concentration, and differences in photosynthetic efficiency. ? Spectral separability between species occurs primarily in the visible and near-infrared spectral regions, which is associated with morphological and physiological differences. Nonnatives had significantly higher Chl, carotene, and anthocyanin concentrations than natives and had significantly higher photochemical reflectance index (PRI) and δ(13) C values. ? Results show nonnative SAPs are functionally dissimilar to native SAPs, having wider leaf blades and greater leaf area, dense and evenly distributed vertical canopies, and higher pigment concentrations. Results suggest that nonnatives also use a facultative C(4) -like photosynthetic pathway, allowing efficient photosynthesis in high-light and low-light environments. Differences in plant functionality indicate that nonnative SAPs have a competitive advantage over native SAPs as a result of growth form and greater light-use efficiency that promotes growth under different light conditions, traits affecting system-wide species distributions and community composition.     

Non-trophic interactions in deserts: Facilitation,interference, and an endangered lizard species

Research on plant–animal interactions has been focused on direct consumer interactions (i.e. plants as resources), but non-trophic interactions including providing shelter or interference with movement can also affect the fine-scale distribution of animals. In particular, non-trophic interactions that are positive could support threatened animal populations. Positive interactions have been used in the restoration of plant communities, but have not yet been extended to the management of animal habitat. In this study, we tested the hypothesis that non-trophic interactions influence the occurrence of an endangered lizard species in an arid shrub-annual system. At a location known to have a population of blunt-nosed leopard lizards (Gambelia sila), we geotagged 700 shrubs, measured shrub morphometric traits, collected biomass samples, and surveyed for lizard presence using scat detection dogs over two years. Relative to 2014, in 2013 plant productivity was high and lizard scats were found more frequently in areas with low invasive grass cover (i.e. residual dry matter, RDM). In 2014, plant productivity was low because of an extreme drought year, and lizard scats were more frequently observed under shrub canopies, particularly those with relatively dense cover. These findings support the novel theory that positive non-trophic interactions are a critical form of plant–animal interactions in addition to consumption. Dominant shrubs can act as a foundation species by functioning as a basal node in structuring both plant and animal communities through a network of interactions. Managing dominant plants, in addition to habitat, is therefore important for conserving animal species in arid ecosystems.     

Liana co‐occurrence patterns in a temperate rainforest

Questions: Are liana–host interactions structured at the community level? Do liana–host interactions differ between species, growth form guilds or habitats? Location: Otari‐Wilton's Bush, on the southern tip of North Island, New Zealand. The forest contains 75 ha of mature and regenerating conifer–broadleaf forest. Methods: Nine liana species were quantified among 217 trees to test for negative co‐occurrence patterns. We also conducted additional analyses within and among compartments embedded in the community‐level matrix. Liana and host abundance distributions were assessed across two contrasting habitats. Results: Community‐level analyses revealed negative co‐occurrence patterns. Positive, neutral and negative co‐occurrence patterns were found among compartments within the community‐level matrix. Host species compartments were consistent with randomized expectations, while positive co‐occurrence patterns were found within the host species matrix. Negative co‐occurrence patterns were found inconsistently among lianas that share the same region of host space, and those that do not. Conclusions: Overall, results indicate the liana community is structured non‐randomly. Liana–host interactions appear to follow an opportunistic growth strategy and interactions are due mostly to habitat partitioning.     

The phylogenetic structure of plant–pollinator networks increases with habitat size and isolation

Similarity among species in traits related to ecological interactions is frequently associated with common ancestry. Thus, closely related species usually interact with ecologically similar partners, which can be reinforced by diverse co‐evolutionary processes. The effect of habitat fragmentation on the phylogenetic signal in interspecific interactions and correspondence between plant and animal phylogenies is, however, unknown. Here, we address to what extent phylogenetic signal and co‐phylogenetic congruence of plant–animal interactions depend on habitat size and isolation by analysing the phylogenetic structure of 12 pollination webs from isolated Pampean hills. Phylogenetic signal in interspecific interactions differed among webs, being stronger for flower‐visiting insects than plants. Phylogenetic signal and overall co‐phylogenetic congruence increased independently with hill size and isolation. We propose that habitat fragmentation would erode the phylogenetic structure of interaction webs. A decrease in phylogenetic signal and co‐phylogenetic correspondence in plant–pollinator interactions could be associated with less reliable mutualism and erratic co‐evolutionary change.     

Functional Group Change within and across Scales following Invasions and Extinctions in the Everglades Ecosystem

Cross-scale resilience theory predicts that the combination of functional diversity within scales and functional redundancy across scales is an important attribute of ecosystems because it helps these systems resist minor ecological disruptions and regenerate after major disturbances such as hurricanes and fire. Using the vertebrate fauna of south Florida, we quantified how the loss of native species and invasion by nonnatives may alter functional group richness within and across scales. We found that despite large changes in species composition due to potential extinctions and successful invasions by nonnative species, functional group richness will not change significantly within scales, there will not be any significant loss of overall redundancy of ecology function across scales, and overall body mass pattern will not undergo substantial change. However, the types of functions performed will change, and this change may have profound effects on not only the Everglades ecosystem but on the entire landscape of south Florida. Received 14 November 2000; accepted 20 December 2001.     

Trait matching and phenological overlap increase the spatio‐temporal stability and functionality of plant–pollinator interactions

Morphology and phenology influence plant–pollinator network structure, but whether they generate more stable pairwise interactions with higher pollination success remains unknown. Here we evaluate the importance of morphological trait matching, phenological overlap and specialisation for the spatio‐temporal stability (measured as variability) of plant–pollinator interactions and for pollination success, while controlling for species' abundance. To this end, we combined a 6‐year plant–pollinator interaction dataset, with information on species traits, phenologies, specialisation, abundance and pollination success, into structural equation models. Interactions among abundant plants and pollinators with well‐matched traits and phenologies formed the stable and functional backbone of the pollination network, whereas poorly matched interactions were variable in time and had lower pollination success. We conclude that phenological overlap could be more useful for predicting changes in species interactions than species abundances, and that non‐random extinction of species with well‐matched traits could decrease the stability of interactions within communities and reduce their functioning.     

Independent and interactive effects of two facilitators on their habitat‐providing host plant,Spartina alterniflora

The role of habitat‐providing species in facilitating associated species abundance and diversity is recognized as a key structuring force in many ecosystems. Reciprocal facilitation by associates, often involving multiple species, can be important for the maintenance of the host species. As with other multi‐species interactions (e.g. multiple predator effects), non‐additive relationships may be common among these associates, yet relatively few studies have examined potential interactions among multiple facilitator species. We combined field surveys and a mesocosm experiment to examine the independent and interactive effects of two co‐occurring facilitator species, ribbed mussels Geukensia demissa and fiddler crabs Uca pugilator, on their host salt marsh plant species, cordgrass Spartina alterniflora. We also experimentally examined how these relationships varied across different host plant genotypes. Overall, facilitator effects increased with increasing facilitator density. There was a significant interaction between mussel and fiddler crab presence, indicating that the effects of each species on cordgrass were dependent on the presence of the other facilitator species. In addition, there were strong interactions among mussels, fiddler crabs, and plant genotype, with greater variation in the performance of individual genotypes when fiddler crabs were absent. Our work reinforces the importance of considering multiple responses when assessing the functional redundancy of co‐occurring facilitators, as species are seldom completely redundant across the range of services they provide. It also highlights that the strength and direction of species interactions can vary due to genetic variation within the interacting species.     

Mycorrhizal associations of an invasive tree are enhanced by both genetic and environmental mechanisms

Biotic interactions involving exotic plants in their introduced ranges may differ from those of co‐occurring plant species and from interactions in their native ranges. When interactions are less negative, or more positive compared to native plant species, this may increase invasion success, and differences among ranges may cause changes in exotic plant traits. Here, we investigated arbuscular mycorrhizae (AM) associated with Triadica sebifera seedlings from populations in native (China) and introduced ranges (US) and with seedlings from US and China species within three co‐occurring genera (Liquidambar, Ulmus, Celtis) grown in multiple common gardens in both ranges. No general pattern of higher or lower AM colonization was found in the introduced range for China and US Celtis, Liquidambar, or Ulmus species. However, AM colonization was significantly higher for Triadica than for other genera, particularly in the introduced range, suggesting AM may improve Triadica's invasion success. Triadica AM colonization was higher in US than China gardens, decreased with increasing soil nitrogen in China, but was independent of soil nitrogen in the US. This might reflect a different effect of soil fertility on this mutualism among ranges. Introduced Triadica populations had higher AM colonization than native populations, particularly in US gardens, implying a possible advantage from greater AM association in the introduced range. This is the first field study demonstrating post‐introduction changes in mycorrhizal colonization of an invasive species. It indicates that there are ecological and evolutionary components to the effect of positive interactions on plant invasions.     

Anthropogenic impacts on the contextual morphological diversification and adaptation of Nile tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis>, L. 1758) in East Africa

Nile tilapia occurs naturally in East Africa where it’s an economically important species. Many of the natural populations of Nile tilapia have been affected by anthropogenic activities including translocations, associated with programmes aimed at enhancing capture fisheries and aquaculture productivity. Using geometric morphometric analyses, we tested the hypothesis that such anthropogenic activities have augmented admixture among natural populations of Nile tilapia and influenced the geographical distribution of morphological variation within the species. Our expectation was that Nile tilapia shape divergent might be consistent with reportedly anthropogenic activities in nonnative environments. To test this hypothesis, we analyzed the shapes of 490 individuals from thirteen populations; three farms, six natives and four nonnative natural populations. Our analysis revealed that the most pronounced shape diversification was observed in seven populations; three nonnatives (Victoria, Kyoga and Sindi farm) and four natives (Albert, River Nile, George and Turkana). The features responsible for the observed morphotypes were mainly related to the orientation of the anterior region of the fish and may be due to diversifying selection in response to new environmental pressures (for nonnative populations), admixture or drift. Shape change in the nonnative high-altitude populations was unexpectedly conserved, suggesting recent introductions which may have not resulted in admixture or there was strong selection against change in the traits measured. On the other hand, the recorded morphotypic clusters explained the possible genetic link to their putative ancestral home. Our results were partially consistent with our prediction that the nonnative populations exhibited divergent morphotypes. We recommend further investigations with molecular genetics for follow-up of these findings.     

Shared environmental responses drive co‐occurrence patterns in river bird communities

Positive or negative patterns of co‐occurrence might imply an influence of biotic interactions on community structure. However, species may co‐occur simply because of shared environmental responses. Here, we apply two complementary modelling methodologies – a probabilistic model of significant pairwise associations and a hierarchical multivariate probit regression model – to 1) attribute co‐occurrence patterns in 100 river bird communities to either shared environmental responses or to other ecological mechanisms such as interaction with heterospecifics, and 2) examine the strength of evidence for four alternative models of community structure. Species co‐occurred more often than would be expected by random community assembly and the species composition of bird communities was highly structured. Co‐occurrence patterns were primarily explained by shared environmental responses; species’ responses to the environmental variables were highly divergent, with both strong positive and negative environmental correlations occurring. We found limited evidence for behaviour‐driven assemblage patterns in bird communities at a large spatial scale, although statistically significant positive associations amongst some species suggested the operation of facilitative mechanisms such as heterospecific attraction. This lends support to an environmental filtering model of community assembly as being the principle mechanism shaping river bird community structure. Consequently, species interactions may be reduced to an ancillary role in some avifaunal communities, meaning if shared environmental responses are not quantified studies of co‐occurrence may overestimate the role of species interactions in shaping community structure.     

Multiple stressors,nonlinear effects and the implications of climate change impacts on marine coastal ecosystems

Global climate change will undoubtedly be a pressure on coastal marine ecosystems, affecting not only species distributions and physiology but also ecosystem functioning. In the coastal zone, the environmental variables that may drive ecological responses to climate change include temperature, wave energy, upwelling events and freshwater inputs, and all act and interact at a variety of spatial and temporal scales. To date, we have a poor understanding of how climate‐related environmental changes may affect coastal marine ecosystems or which environmental variables are likely to produce priority effects. Here we use time series data (17 years) of coastal benthic macrofauna to investigate responses to a range of climate‐influenced variables including sea‐surface temperature, southern oscillation indices (SOI, Z4), wind‐wave exposure, freshwater inputs and rainfall. We investigate responses from the abundances of individual species to abundances of functional traits and test whether species that are near the edge of their tolerance to another stressor (in this case sedimentation) may exhibit stronger responses. The responses we observed were all nonlinear and some exhibited thresholds. While temperature was most frequently an important predictor, wave exposure and ENSO‐related variables were also frequently important and most ecological variables responded to interactions between environmental variables. There were also indications that species sensitive to another stressor responded more strongly to weaker climate‐related environmental change at the stressed site than the unstressed site. The observed interactions between climate variables, effects on key species or functional traits, and synergistic effects of additional anthropogenic stressors have important implications for understanding and predicting the ecological consequences of climate change to coastal ecosystems.     

Incorporating phylogenetic metrics to microbial co‐occurrence networks based on amplicon sequences to discern community assembly processes

Co‐occurrence network analysis based on amplicon sequences is increasingly used to study microbial communities. Patterns of co‐existence or mutual exclusion between pairs of taxa are often interpreted as reflecting positive or negative biological interactions. However, other assembly processes can underlie these patterns, including species failure to reach distant areas (dispersal limitation) and tolerate local environmental conditions (habitat filtering). We provide a tool to quantify the relative contribution of community assembly processes to microbial co‐occurrence patterns, which we applied to explore soil bacterial communities in two dry ecosystems. First, we sequenced a bacterial phylogenetic marker in soils collected across multiple plots. Second, we inferred co‐occurrence networks to identify pairs of significantly associated taxa, either co‐existing more (aggregated) or less often (segregated) than expected at random. Third, we assigned assembly processes to each pair: patterns explained based on spatial or environmental distance were ascribed to dispersal limitation (2%–4%) or habitat filtering (55%–77%), and the remaining to biological interactions. Finally, we calculated the phylogenetic distance between taxon pairs to test theoretical expectations on the linkages between phylogenetic patterns and assembly processes. Aggregated pairs were more closely related than segregated pairs. Furthermore, habitat‐filtered aggregated pairs were closer relatives than those assigned to positive interactions, consistent with phylogenetic niche conservatism and cooperativism among distantly related taxa. Negative interactions resulted in equivocal phylogenetic signatures, probably because different competitive processes leave opposing signals. We show that microbial co‐occurrence networks mainly reflect environmental tolerances and propose that incorporating measures of phylogenetic relatedness to networks might help elucidate ecologically meaningful patterns.