Absence Of Herbivores Research Articles

Intraspecific allometries reveal hyper‐slender stems in forest gaps and the impact on tree growth from insect herbivores

AbstractConspicuously absent from plant allometric models are insect herbivores, despite their ubiquity in pantropical forests as well as the importance of gauging their impact on plant growth for understanding tree regeneration dynamics. Conceivably, the scaling relationships of plant populations with and without exposure to insects could be compared to gain insight, but this has yet to be tried. Here, I examined the size allometry of three canopy‐statured species (Fabaceae) by fitting standard major axis regressions of stem height, number of leaves, and maximum area per leaf against stem diameter (root collar) of their juveniles (seedlings/saplings) in 41 canopy gaps after ca. 2 years of herbivore exclusion (using mesh cages with mesh‐rooftop controls), in a central African forest. Herbivores did not change the slope or intercept, nor shift the location of Tetraberlinia bifoliolata and T. korupensis populations, whose seedlings tolerate shade well. In stark contrast, all three size allometries were altered in the faster‐growing Microberlinia bisulcata, whose seedlings cannot tolerate shade indefinitely. More pronounced than either an altered slope (scaling exponent) or intercept was the strong rightward shift in location (to larger sizes) of protected M. bisulcata, whereas its normal (control) population was left stunted, with fewer and smaller leaves. Nevertheless, all species had higher than theoretically predicted scaling exponents, even in the absence of herbivores, suggesting a convergent response early in tree ontogeny across the shade‐tolerance spectrum prioritizing height gain over mechanical stability in forest gaps. But insect herbivores were able to significantly strengthen this deviation in M. bisulcata, and perhaps in T. korupensis. These results demonstrate an allometric approach may prove useful for revealing how insects differentially impact trees' growth and form (slenderness) when they are most vulnerable to herbivory.

Sedimentation and overfishing drive changes in early succession and coral recruitment.

Sedimentation and overfishing are important local stressors on coral reefs that can independently result in declines in coral recruitment and shifts to algal-dominated states. However, the role of herbivory in driving recovery across environmental gradients is often unclear. Here we investigate early successional benthic communities and coral recruitment across a sediment gradient in Palau, Micronesia over a 12-month period. Total sedimentation rates measured by 'TurfPods' varied from 0.03 ± 0.1 SE mg cm-2 d-1 at offshore sites to 1.32 ± 0.2 mg cm-2 d-1 at inshore sites. To assess benthic succession, three-dimensional settlement tiles were deployed at sites with experimental cages used to exclude tile access to larger herbivorous fish. Benthic assemblages exhibited rapid transitions across the sediment gradient within three months of deployment. At low levels of sedimentation (less than 0.6 mg cm-2 d-1), herbivory resulted in communities dominated by coral recruitment inducers (short turf algae and crustose coralline algae), whereas exclusion of herbivores resulted in the overgrowth of coral inhibitors (encrusting and upright foliose macroalgae). An 'inducer threshold' was found under increasing levels of sedimentation (greater than 0.6 mg cm-2 d-1), with coral inducers having limited to no presence in communities, and herbivore access to tiles resulted in sediment-laden turf algal assemblages, while exclusion of herbivores resulted in invertebrates (sponges, ascidians) and terrestrial sediment accumulation. A 'coral recruitment threshold' was found at 0.8 mg cm-2 d-1, below which net coral recruitment was reduced by 50% in the absence of herbivores, while recruitment was minimal above the threshold. Our results highlight nonlinear trajectories of benthic succession across sediment gradients and identify strong interactions between sediment and herbivory that have cascading effects on coral recruitment. Local management strategies that aim to reduce sedimentation and turbidity and manage herbivore fisheries can have measurable effects on benthic community succession and coral recruitment, enhancing reef resilience and driving coral recovery.

Growth and functional traits of Julbernardia globiflora (Benth) resprouts and seedlings in response to fire frequency and herbivory in miombo woodlands

Understanding the effects of fire and herbivory on vegetation is crucial for biodiversity conservation in miombo woodlands. Changes in fire regimes in miombo alter plant growth and functional traits, such as carbon and nitrogen content, specific leaf area (SLA), and the concentrations of polyphenols, which can also change herbivory patterns in these systems. We compared the relative growth rate in height and basal diameter of seedlings and resprouts of Julbernardia globiflora in areas of high and low fire frequency and in the presence and absence of herbivores in Gorongosa National Park. We also measured functional traits (SLA, phenolics) from resprouts and seedlings in high and low fire frequency areas. Overall, resprouts were more responsive to fire frequency than seedlings, perhaps because they may have already survived at least one fire. Resprout RGR in diameter was greatest in areas with a low frequency of fire and where herbivores were excluded. SLA was higher in areas with lower fire frequency, where canopy cover is likely greater. Higher concentrations of polyphenols were measured in seedlings from the high fire frequency area, indicating a greater investment in defenses in vulnerable seedlings. In sum, when fires occur every two years, plant growth was limited, leaves were thicker, and plants invested more in C-rich defenses. Lastly, we observed that Combretum adenogonium, a species favored by fire and not characteristic of true miombo, was more abundant in the frequently burned site, and J. globiflora regenerates more from resprouts than from seed in areas with a short fire return interval. This study suggests that high frequency fires affect miombo conservation, and efforts to prevent frequent fires are important for the integrity of the ecosystem.

Nutrient-mediated silica uptake from agricultural runoff in invasive floating macrophytes: implications for biological control

Silica (Si) plays a significant role in alleviating the effects of biotic and abiotic stressors in many plants, especially in an agricultural context. With increased use of Si-based fertilisers, understanding plant responses to the addition of Si to their environment, particularly aquatic environments, is important. We investigated how two invasive macrophytes, Eichhornia crassipes and Pistia stratiotes, responded to different nutrient and Si concentrations, in the presence and absence of herbivory. Both species incorporated Si into their foliage, but uptake of Si did not increase under high nutrient availability but rather decreased, especially in P. stratiotes. Plant quality (i.e. C:N) for both weed species was affected more by nutrient concentrations than Si content, and the addition of Si had a negative effect on plant growth. Eichhornia crassipes increased daughter plant production under high Si conditions, while P. stratiotes plants showed no reproductive response to increased Si except in low nutrient conditions where reproduction was reduced. The addition of Si resulted in increased biomass of E. crassipes, while P. stratiotes was unaffected. These results highlight that runoff of Si from fertiliser alter aquatic plant–insect interactions, which has consequences for biological control.

Transgenerational impacts of herbivory and inbreeding on reproductive output in Solanum carolinense.

PremisePlant maternal effects on offspring phenotypes are well documented. However, little is known about how herbivory on maternal plants affects offspring fitness. Furthermore, while inbreeding is known to reduce plant reproductive output, previous studies have not explored whether and how such effects may extend across generations. Here, we addressed the transgenerational consequences of herbivory and maternal plant inbreeding on the reproduction of Solanum carolinense offspring.Methods Manduca sexta caterpillars were used to inflict weekly damage on inbred and outbred S. carolinense maternal plants. Cross‐pollinations were performed by hand to produce seed from herbivore‐damaged outbred plants, herbivore‐damaged inbred plants, undamaged outbred plants, and undamaged inbred plants. The resulting seeds were grown in the greenhouse to assess emergence rate and flower production in the absence of herbivores. We also grew offspring in the field to examine reproductive output under natural conditions.ResultsWe found transgenerational effects of herbivory and maternal plant inbreeding on seedling emergence and reproductive output. Offspring of herbivore‐damaged plants had greater emergence, flowered earlier, and produced more flowers and seeds than offspring of undamaged plants. Offspring of outbred maternal plants also had greater seedling emergence and reproductive output than offspring of inbred maternal plants, even though all offspring were outbred. Moreover, the effects of maternal plant inbreeding were more severe when plant offspring were grown in field conditions.ConclusionsThis study demonstrates that both herbivory and inbreeding have fitness consequences that extend across generations even in outbred progeny.

Trifolium polymorphum: an amphicarpic species with an interesting reproductive biology

An interesting aspect of the biology of Trifolium polymorphum, an endemic species of the natural pastures of Rio Grande do Sul State in Brazil, is that it is amphicarpic. Amphicarpy is a fascinating reproductive strategy in which aerial and subterranean seeds are produced by the same plant. We evaluated the number, size and weight of aerial and subterranean seeds of two populations (Bage and Santana do Livramento) of T. polymorphum which have grown from the regrowth of storage roots and in the absence of herbivory. Aerial flowers produced more seeds than subterranean flowers, about 14 times in the Santana do Livramento population and nine times in the Bage population. However subterranean seeds were heavier than aerial seeds. We suggest that in the natural pastures of Rio Grande do Sul, where intense grazing and trampling can periodically destroy the aerial part of the plants, the occasional formation of subterranean seeds in T. polymorphum provides an alternative safety system in terms of population persistence. Although amphicarpic, T. polymorphum invests early in asexual reproduction through the production of storage roots, which favors the persistence in unfavorable habitats for sexual reproduction, and thus plants may persist vegetatively year after year without the need of regeneration by seeds.

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.

Herbivory may promote a non-native plant invasion at low but not high latitudes.

The strengths of biotic interactions such as herbivory are expected to decrease with increasing latitude for native species. To what extent this applies to invasive species and what the consequences of this variation are for competition among native and invasive species remain unexplored. Here, herbivore impacts on the invasive plant Alternanthera philoxeroides and its competition with the native congener A. sessilis were estimated across latitudes in China. An common garden experiment spanning ten latitudinal degrees was conducted to test how herbivore impacts on A. philoxeroides and A. sessilis, and competition between them change with latitude. In addition, a field survey was conducted from 21°N to 36.8°N to test whether A. philoxeroides invasiveness changes with latitude in nature as a result of variations in herbivory. In the experiment, A. sessilis cover was significantly higher than A. philoxeroides cover when they competed in the absence of herbivores, but otherwise their cover was comparable at low latitude. However, A. philoxeroides cover was always higher on average than A. sessilis cover at middle latitude. At high latitude, only A. sessilis emerged in the second year. Herbivore abundance decreased with latitude and A. philoxeroides emerged earlier than A. sessilis at middle latitude. In the field survey, the ratio of A. philoxeroides to A. sessilis cover was hump shaped with latitude. These results indicate that herbivory may promote A. philoxeroides invasion only at low latitude by altering the outcome of competition in favour of the invader and point to the importance of other factors, such as earlier emergence, in A. philoxeroides invasion at higher latitudes. These results suggest that the key factors promoting plant invasions might change with latitude, highlighting the importance of teasing apart the roles of multiple factors in plant invasions within a biogeographic framework.

The relative importance of plant-soil feedbacks for plant-species performance increases with decreasing intensity of herbivory.

Under natural conditions, aboveground herbivory and plant-soil feedbacks (PSFs) are omnipresent interactions strongly affecting individual plant performance. While recent research revealed that aboveground insect herbivory generally impacts the outcome of PSFs, no study tested to what extent the intensity of herbivory affects the outcome. This, however, is essential to estimate the contribution of PSFs to plant performance under natural conditions in the field. Here, we tested PSF effects both with and without exposure to aboveground herbivory for four common grass species in nine grasslands that formed a gradient of aboveground invertebrate herbivory. Without aboveground herbivores, PSFs for each of the four grass species were similar in each of the nine grasslands-both in direction and in magnitude. In the presence of herbivores, however, the PSFs differed from those measured under herbivory exclusion, and depended on the intensity of herbivory. At low levels of herbivory, PSFs were similar in the presence and absence of herbivores, but differed at high herbivory levels. While PSFs without herbivores remained similar along the gradient of herbivory intensity, increasing herbivory intensity mostly resulted in neutral PSFs in the presence of herbivores. This suggests that the relative importance of PSFs for plant-species performance in grassland communities decreases with increasing intensity of herbivory. Hence, PSFs might be more important for plant performance in ecosystems with low herbivore pressure than in ecosystems with large impacts of insect herbivores.

Biotic Interactions Contribute to the Geographic Range Limit of an Annual Plant: Herbivory and Phenology Mediate Fitness beyond a Range Margin.

Species' geographic distributions have already shifted during the Anthropocene. However, we often do not know what aspects of the environment drive range dynamics, much less which traits mediate organisms' responses to these environmental gradients. Most studies focus on possible climatic limits to species' distributions and have ignored the role of biotic interactions, despite theoretical support for their importance in setting distributional limits. We used field experiments and simulations to estimate contributions of mammalian herbivory to a range boundary in the Californian annual plant Clarkia xantiana ssp. xantiana. A steep gradient of increasing probability of herbivory occurred across the boundary, and a reanalysis of prior transplant experiments revealed that herbivory drove severalfold declines in lifetime fitness at and beyond the boundary. Simulations showed that populations could potentially persist beyond the range margin in the absence of herbivory. Using data from a narrowly sympatric subspecies, Clarkia xantiana parviflora, we also showed that delayed phenology is strongly associated with C. xantiana ssp. xantiana's susceptibility to herbivory and low fitness beyond its border. Overall, our results provide some of the most comprehensive evidence to date of how the interplay of demography, traits, and spatial gradients in species interactions can produce a geographic range limit, and they lend empirical support to recent developments in range limits theory.

Macroalgae and nutrients promote algal turf growth in the absence of herbivores

Closely cropped algal turfs are characteristic of healthy coral reefs, but unchecked growth can cause transitions into long sediment-laden turfs, which may be an alternative degraded state. While control by herbivores is well established, potential interactions between nutrients and macroalgae have not been evaluated. We varied macroalgal presence and nutrient addition and measured turf height and sediment accumulation over 18 d on an algal-dominated fringing reef. We found a novel facilitative effect of macroalgal presence on turf height as macroalgal presence increased growth by 34% over its absence. Nutrient addition also significantly, but independently, increased turf height by 127% compared to no nutrient addition. Macroalgal presence reduced sediment accumulation, possibly by trapping the sediment and/or by a whiplash effect. Thus, overfished coral reefs experiencing macroalgal blooms and/or nutrient additions may be at risk of developing long algal turfs, which could maintain a persistent shift to a degraded state.

Sexual dimorphism in response to herbivory and competition in the dioecious herb Spinacia oleracea

Sexual dimorphism is common in dioecious plant species and is usually attributed to different cost of reproduction associated with male and female functions. Differences in growth and performance between male and female plants may be accentuated under stress, potentially leading to sex-ratio biases and affecting population growth. Environmental stress involves multiple factors that often occur simultaneously. Among different stress combinations that occur in field conditions, competition and herbivory and their interaction are key biotic factors that can affect plant growth and performance. Here, we conducted a glasshouse experiment in Cardiff, UK, using the cultivated spinach, Spinacia oleracea, as a model system to study sexual dimorphism in intra- and inter-specific competitive ability and in response to herbivory by a generalist herbivore Helix aspersa. We found stronger inter- than intra-specific competition: growth (above-ground biomass) and chlorophyll content of male and female plants was reduced when growing with Brassica oleracea, but not when growing with conspecifics. In the absence of herbivory, females growing with same-sex neighbour had greater root biomass than males; whilst herbivory reduced root biomass significantly only in females competing with same sex neighbours. Plant damage caused by herbivores was similar when growing with male or female conspecifics but greater when growing with B. oleracea. Finally, plant damage caused by herbivores did not differ between male and female plants; however, males increased their allocation to roots and reduced their chlorophyll content after damage. Our results showed that sexual dimorphism occurs in S. oleracea, despite being a worldwide crop, selectively bred for its edible leaves. In particular, our results suggest stronger same-sex competition for females and greater tolerance to herbivory in males than in females of S. oleracea..

Reduced Defenses and Increased Herbivore Preference of Island Chaparral Shrubs Compared to Mainland Relatives

The absence of herbivores from certain environments may select for plants that are less defended and more palatable compared to plants in similar environments that are exposed to herbivory. This potential loss of defenses becomes especially important when exotic herbivores are introduced to islands that historically lack such organisms, potentially resulting in negative effects on native island ecosystems. To test the hypothesis that island plants have reduced defensive traits, we measured structural defenses and palatability of 10 taxonomic pairs from contrasting island and mainland environments. Our study compares plants from Santa Catalina Island, California—which has a long history without large native herbivores followed by more recent human-caused introductions—to the adjacent southern California mainland where ungulate herbivores are native and common. Our results suggest that island plants have reduced structural defenses and are more preferred by herbivores compared to their mainland plant relatives. These patterns are likely driven by selection on plant traits in the unique and insular island environment. Reduced defenses and increased palatability of island plant species should be considered when developing management strategies for island ecosystems that historically lack native ungulate herbivores.

Integration of two herbivore-induced plant volatiles results in synergistic effects on plant defence and resistance.

Plants can use induced volatiles to detect herbivore‐ and pathogen‐attacked neighbors and prime their defenses. Several individual volatile priming cues have been identified, but whether plants are able to integrate multiple cues from stress‐related volatile blends remains poorly understood. Here, we investigated how maize plants respond to two herbivore‐induced volatile priming cues with complementary information content, the green leaf volatile (Z)‐3‐hexenyl acetate (HAC) and the aromatic volatile indole. In the absence of herbivory, HAC directly induced defence gene expression, whereas indole had no effect. Upon induction by simulated herbivory, both volatiles increased jasmonate signalling, defence gene expression, and defensive secondary metabolite production and increased plant resistance. Plant resistance to caterpillars was more strongly induced in dual volatile‐exposed plants than plants exposed to single volatiles.. Induced defence levels in dual volatile‐exposed plants were significantly higher than predicted from the added effects of the individual volatiles, with the exception of induced plant volatile production, which showed no increase upon dual‐exposure relative to single exposure. Thus, plants can integrate different volatile cues into strong and specific responses that promote herbivore defence induction and resistance. Integrating multiple volatiles may be beneficial, as volatile blends are more reliable indicators of future stress than single cues.

Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

Abstract Savanna ecosystems span a diverse range of climates, edaphic conditions, and disturbance regimes, the complexity of which has stimulated long‐standing interest in the mechanisms that maintain tree–grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the tree life cycle. A major impediment to testing this hypothesis is the lack of data on the relative strengths of different bottlenecks across key environmental gradients. To identify demographic bottlenecks that limit early tree establishment (0–18 months), we conducted a series of transplant experiments with two savanna trees species (Acacia robusta and Acacia tortilis) across a natural rainfall and soil fertility gradient in the Serengeti ecosystem, Tanzania. We tested the interactive effects of precipitation, herbivory, seed scarification, grass competition, water limitation, and tree species identity on two key life stages: germination and early seedling survival (0–2 months), and juvenile seedling survival (2–18 months). Germination and early seedling survival increased as a function of rainfall, in the absence of herbivores and when seeds were scarified. Juvenile seedling survival, in contrast, decreased with rainfall but increased in the absence of herbivores. Grass removal had the single strongest (positive) effect on juvenile seedling survival of any treatment. Soil moisture monitoring and grass‐addition treatments revealed that grasses negatively affected seedlings in ways that were not necessarily linked to soil moisture. A demographic model combining all effects across early life stages showed that the strength of grass competition on juvenile seedling survival was the key factor limiting early tree establishment. While rainfall had an unexpected opposing effect on the two life stages, the net effect of mean annual precipitation on early tree establishment was positive. Synthesis. Successful tree establishment in Serengeti is maximized by a seemingly unlikely sequence of events: (a) scarification of seeds by browsers, (b) heavy rainfall to promote germination, (c) intensive grazing (but absence of browsers), and (d) dry conditions during juvenile seedling growth (>2 months) to reduce competition with grasses. By considering a wide suite of conditions and their interactions, our experimental results are relevant to ongoing debates about savanna vegetation dynamics and structural shifts in tree:grass ratios.

Defense with benefits? Ducking plants outperformed erect plants in the goldenrod Solidago gigantea in the absence of herbivory.

Despite the fact that herbivores can be highly detrimental to their host plants' fitness, plant populations often maintain genetic variation for resistance to their natural enemies. Investigating the various costs (e.g., allocation tradeoffs, autotoxicity, and ecological costs) that may prevent plants from evolving to their fullest potential resistance has been a productive strategy for shedding insight into the eco-evolutionary dynamics of plant-herbivore communities. Recent studies have shown that some individuals of goldenrod (Solidago spp.) evade apex-attacking herbivores by a temporary nodding of their stem (i.e., resistance-by-ducking). Although ducking provides an obvious fitness benefit to these individuals, nonducking (erect) morphs persist in goldenrod populations. In this study, I investigated potential costs of ducking in Solidago gigantea in terms of tradeoffs involving growth and reproduction in a common garden experiment using field-collected seeds. The S. gigantea population contained substantial genetic variation for stem morph, with 28% erect and 72% ducking stems. In the absence of herbivory, ducking plants were taller, had thicker stems, and produced an average of 20% more seeds than erect plants. This study suggests that resistance-by-ducking, instead of being costly, actually comes with additional, nondefense-related benefits. These results support the conclusion that the factors that constrain the evolution of resistance in plant populations are likely to be more subtle and complex than simple tradeoffs in resource allocation.

Competitive ability, not tolerance, may explain success of invasive plants over natives

When entering a new community, introduced species leave behind members of their native community while simultaneously forming novel biotic interactions. Escape from enemies during the process of introduction has long been hypothesized to drive the increased performance of invasive species. However, recent studies and quantitative syntheses find that invaders often receive similar, or even more, damage from enemies than do native species. Therefore, invasives may be those more tolerant to enemy damage, or those able to maintain competitive ability in light of enemy damage. Here, we investigate whether tolerance and competitive ability could contribute to invasive plant success. We determined whether invasive plants were more competitive than native or noninvasive exotic species in both the presence and absence of simulated herbivory. We found competition and herbivory additively reduced individual performance, and affected the performance of native, invasive, and noninvasive exotic species’ to the same degree. However, invasives exerted stronger competitive effects on an abundant native species (Elymus canadensis) in both the presence and absence of herbivory. Therefore, while invasive species responded similarly to competition and simulated herbivory, their competitive effects on natives may contribute to their success in their introduced range.

Simultaneous synergist, antagonistic and additive interactions between multiple local stressors all degrade algal turf communities on coral reefs

Abstract Ecological communities are subjected to multiple anthropogenic stressors at both global and local scales that are increasing in number and magnitude. Stressors can interact in complex ways and are classified as additive, synergistic or antagonistic; the nature of the interaction is key to predicting changes and understanding community resilience. Coral reefs are among the most impacted communities and have shifted from coral‐ to algal‐dominated states, and overfishing, nutrient enrichment and sedimentation are local stressors that often co‐occur and may support degraded algal states. Short algal turfs are abundant benthic space holders on healthy reefs that may be pushed by local stressors to long algal turfs, a more degraded state that may prevent recovery to coral dominance. We conducted a fully crossed three‐factor field experiment on short algal turf communities manipulating herbivory pressure (+/−cages), nutrients (+/−fertilizer) and sediments (natural accumulation/removal). We applied stressors for 16 days, removed them and monitored turf height during and after manipulations. We found that significant pair‐wise interactions between all stressors pushed the community towards a degraded state with longer algal turfs. All three types of interactions (additive, synergistic and antagonistic) were common and occurred in equal frequency, suggesting more investigations into all types are needed to accurately predict community responses to multiple stressors. For example, when herbivores were present, nutrients and sediments interacted additively, while in the absence of herbivores, nutrients and sediments interacted synergistically. All interactions broke down following termination of experimental manipulations and all effects were undetectable after 49 days, indicating that this reef may be resilient, at least when stressors are applied on a short time‐scale. Synthesis. Because management of local stressors is often more tractable than global stressors, local management has been proposed as a means to offset global stressors. However, ecological communities often experience multiple local stressors simultaneously, and interactions between stressors, including synergisms and antagonisms, may be the source of nonlinear shifts in communities or “ecological surprises.” The majority of interactions in our study were both strong and nonlinear, and we suggest that, if pervasive across systems, nonlinear interactions may drive the recent global increase in “ecological surprises.”

Dealing with mutualists and antagonists: Specificity of plant‐mediated interactions between herbivores and flower visitors, and consequences for plant fitness

Abstract Plants need to deal with antagonists, such as herbivores, while maintaining interactions with mutualists, such as pollinators that help plants to maximize their reproductive output. Although many plant species have inducible defences to save metabolic costs of defence in the absence of herbivores, plant responses induced by herbivore attack can have ecological costs. For example, herbivore‐induced responses can affect flower traits and alter interactions with flower visitors. Such plant‐mediated interactions between herbivores and flower visitors can affect plant reproductive output. Current knowledge on the generality and specificity of plant‐mediated herbivore–flower–visitor interactions and its consequences for plant fitness is limited. In this study, we investigated whether a broad range of herbivores feeding on the annual plant Brassica nigra affect interactions with flower visitors, whether the direction of interactions is predicted by the feeding modes (chewing and sap‐feeding) and sites (above‐ and belowground) of the herbivores, and whether it results in fitness consequences for the plant. Our results show that attack of B. nigra by a range of different herbivores influenced plant interactions with mutualist pollinators and an antagonist florivore, the pollen beetle Meligethes aeneus. Pollinator community composition was affected by herbivory, whereas overall pollinator attraction was maintained. Pollinator community composition of uninfested plants differed from that of chewing and root herbivore‐infested plants. Main responders in the pollinator communities to changes induced by herbivory were syrphid flies, bumblebees, and solitary bees. Although the preference of pollen beetle adults was not affected by herbivory, beetle larvae performed best on plants infested with the nematode Heterodera schachtii. The changes in pollinator community composition and syrphid fly visitation can explain the observed increase in seed set of root herbivore‐infested plants. Interactions of flowering B. nigra plants with mutualist and antagonist insects are well integrated and conflicting interactions do not reduce reproductive output. Our results suggest some degree of specificity in herbivore–flower–visitor interactions with consequences for plant fitness. Specificity of plant responses were determined at the species level as well as the herbivore functional group level, and differed depending on the flower visitor. Because plant reproduction was affected by indirect plant‐mediated interactions, these can potentially result in selection on plant strategies to optimize growth, defence and reproduction. A plain language summary is available for this article.

Livestock exclusion alters plant species composition in fen meadows

AbstractQuestionsOur study evaluated how species composition and plant traits that indicate functioning condition in fens responded to grazing cessation over time in an arid ecosystem of the western US. The specific questions addressed were: (i) how does livestock exclusion influence species composition in fens; (ii) is grazing cessation associated with shifts in species functional traits that indicate fen condition; and (iii) what is the pattern of response to livestock exclusion over time?LocationPlumas National Forest, California, US.MethodsWe studied paired fenced and unfenced study sites in two fens to examine the effects of livestock exclusion. Parallel transects were established at each site, and plant species and ground cover were repeatedly surveyed, once prior to and multiple times following treatment, using 0.01 m2 frequency frames. We used NMDS to analyse species composition, RLQ and fourth‐corner analysis to evaluate species functional traits and environmental variables, and linear mixed effects models to examine differences in responses between fenced and unfenced study sites over time.ResultsAfter fencing, we observed unexpected shifts in species composition and plant functional traits. Grazed sites were associated with peat‐forming obligate wetland, moss and sedge species, while fenced sites were characterized by non‐peat‐forming facultative upland, and upland forb, grass and early seral species. Species composition also varied between sites and sample years.ConclusionsWe found that livestock exclusion strongly affects plant species composition in fens, including promoting species with functional traits that indicate a loss of functioning condition, such as ruderal and upland species. Possible explanations for these observed shifts include: (1) biomass accumulation in the absence of herbivory, (2) competitive exclusion in fenced sites, (3) succession, (4) the abiotic conditions of our study sites, particularly hydrology and nutrient status, and (5) interactions among these factors. We conclude that degradation of fen wetlands caused by livestock grazing in the arid western US may not be reversed by excluding livestock alone.