Herbivore Removal Research Articles

Ocean warming undermines the recovery resilience of New England kelp forests following a fishery-induced trophic cascade.

Ecological theory predicts that kelp forests structured by trophic cascades should experience recovery and persistence of their foundation species when herbivores become rare. Yet, climate change may be altering the outcomes of top-down forcing in kelp forests, especially those located in regions that have rapidly warmed in recent decades, such as the Gulf of Maine. Here, using data collected annually from 30+ sites spanning >350 km of coastline, we explored the dynamics of Maine's kelp forests in the ~20 years after a fishery-induced elimination of sea urchin herbivores. Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20th century, we found that forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest apparently because ocean warming has-directly and indirectly-made this area inhospitable to kelp. Indeed, when modeling drivers of change using causal techniques from econometrics, we discovered that unusually high summer seawater temperatures the year prior, unusually high spring seawater temperatures, and high sea urchin densities each negatively impacted kelp abundance. Furthermore, the relative power and absolute impact of these drivers varied geographically. Our findings reveal that ocean warming is redefining the outcomes of top-down forcing in this system, whereby herbivore removal no longer predictably leads to a sustained dominance of foundational kelps but instead has led to a waning dominance (northeast) or the rise of a novel phase state defined by "turf" algae (southwest). Such findings indicate that limiting climate change and managing for low herbivore abundances will be essential for preventing further loss of the vast forests that still exist in northeast Maine. They also more broadly highlight that climate change is "rewriting the rules" of nature, and thus that ecological theory and practice must be revised to account for shifting species and processes.

Dynamic nitrogen reallocation in rice plants upon insect herbivory by a generalist lepidopteran pest Spodoptera litura (Fabricius).

Nitrogen (N) is a limited nutrient for both plants and herbivores. How plants reallocate N upon herbivore attack is vital for plant tolerance to herbivores. Here we investigated N reallocation in rice during a 2-day herbivore attack by a generalist herbivore Spodoptera litura and 2 days after herbivore removal. Labeled15 N was translocated during insect attack from feeding-damaged leaves to roots, particularly to young roots. The amounts of chlorophyll and Rubisco were significantly reduced in the attacked leaves. Both free amino acids and nitrate accumulated in the damaged leaves and young roots, while ammonium content was decreased. Activities of nitrate reductase and glutamine synthetase were enhanced in feeding-damaged leaves but inhibited in young roots. The expression of amino acid transporters OsAAP6, OsAAT15, and jasmonate-responsive genes OsAOS, OsMAPK3, OsMAPK6 was induced in the damaged leaves. However, 2 days after herbivore removal, N uptake was increased and herbivory-induced 15 N transfer to roots was partially reverted back to the damaged leaves, resulting in N levels in the previously damaged leaves were even higher than that in control leaves. Collectively, our results indicate a dynamic Nreallocation in rice responses to insect herbivory.

Long‐term herbivore removal experiments reveal how geese and reindeer shape vegetation and ecosystem CO2‐fluxes in high‐Arctic tundra

Abstract Given the current rates of climate change, with associated shifts in herbivore population densities, understanding the role of different herbivores in ecosystem functioning is critical for predicting ecosystem responses. Here, we examined how migratory geese and resident, non‐migratory reindeer—two dominating yet functionally contrasting herbivores—control vegetation and ecosystem processes in rapidly warming Arctic tundra. We collected vegetation and ecosystem carbon (C) flux data at peak plant growing season in the two longest running, fully replicated herbivore removal experiments found in high‐Arctic Svalbard. Experiments had been set up independently in wet habitat utilised by barnacle geese Branta leucopsis in summer and in moist‐to‐dry habitat utilised by wild reindeer Rangifer tarandus platyrhynchus year‐round. Excluding geese induced vegetation state transitions from heavily grazed, moss‐dominated (only 4 g m−2 of live above‐ground vascular plant biomass) to ungrazed, graminoid‐dominated (60 g m−2 after 4‐year exclusion) and horsetail‐dominated (150 g m−2 after 15‐year exclusion) tundra. This caused large increases in vegetation C and nitrogen (N) pools, dead biomass and moss‐layer depth. Alterations in plant N concentration and CN ratio suggest overall slower plant community nutrient dynamics in the short‐term (4‐year) absence of geese. Long‐term (15‐year) goose removal quadrupled net ecosystem C sequestration (NEE) by increasing ecosystem photosynthesis more than ecosystem respiration (ER). Excluding reindeer for 21 years also produced detectable increases in live above‐ground vascular plant biomass (from 50 to 80 g m−2; without promoting vegetation state shifts), as well as in vegetation C and N pools, dead biomass, moss‐layer depth and ER. Yet, reindeer removal did not alter the chemistry of plants and soil or NEE. Synthesis. Although both herbivores were key drivers of ecosystem structure and function, the control exerted by geese in their main habitat (wet tundra) was much more pronounced than that exerted by reindeer in their main habitat (moist‐to‐dry tundra). Importantly, these herbivore effects are scale dependent, because geese are more spatially concentrated and thereby affect a smaller portion of the tundra landscape compared to reindeer. Our results highlight the substantial heterogeneity in how herbivores shape tundra vegetation and ecosystem processes, with implications for ongoing environmental change.

Going further on herbivore fishing: the removal of smaller fishes from algal-dominated reefs

Overfishing of large herbivorous fishes is connected to the rise of algal-dominated states on coral reefs. The recovery of their populations is challenging, and future herbivore assemblages may be composed of smaller fish. With fisheries now targeting these smaller-sized herbivore populations, coral reef benthic communities may face unknown outcomes. We performed caging experiments in algal-dominated reefs of Northeastern Brazil, that have been depleted of large herbivorous fishes, to appraise the effects of removing small herbivores on benthic community composition and succession. Full cages simulated herbivore removal, and partial cages and open plots functioned as controls. In total, 36 experimental plots were monitored for 1 yr, accounting for the influence of seasonal changes in local conditions of temperature and turbidity. Overall macroalgal cover did not change between experimental treatments, but filamentous algae increased 5-fold inside full cages by the end of the experiment, surpassing articulated coralline forms as the dominant group. Higher temperatures during the dry season promoted filamentous algae when the top-down control of the herbivores was removed, while a reverse pattern was observed when fishes were allowed to feed inside plots. Small herbivores accelerated benthic succession, facilitating the dominance of articulated coralline algae as the climax community. Our findings oppose previous studies performed at sites with high abundances of large-bodied fishes, where herbivory decreased overall macroalgal cover, promoted filamentous and crustose coralline algae and delayed community succession. The further depletion of smaller-bodied herbivores can trigger shifts in benthic community dynamics that interact with water temperature, which may have implications for reef resilience in an ocean-warming scenario.

Foliar Herbivory Reduces Rhizosphere Fungal Diversity and Destabilizes the Co-occurrence Network.

Insect herbivores can adversely impact terrestrial plants throughout ontogeny and across various ecosystems. Simultaneously, the effects of foliar herbivory may extend belowground, to the soil microbial community. However, the responses in terms of the diversity, assembly, and stability of rhizosphere fungi to aboveground herbivory remain understudied. Here, using high-throughput sequencing, the effects of foliar insect herbivory on rhizosphere fungal microbes were investigated in a common garden experiment that manipulated herbivory intensity and time from herbivore removal. The number of observed fungal species was reduced by a greater herbivory intensity, with some species evidently sensitive to herbivory intensity and time since herbivore removal. Rhizofungal assembly processes were altered by both herbivory intensity and time since herbivore removal. Further, we found evidence that both factors strongly influenced fungal community stability: a high intensity of herbivory coupled with a shorter time since herbivore removal resulted in low stability. These results suggest that foliar herbivory can adversely alter fungal diversity and stability, which would in turn be harmful for plant health. Fortunately, the effect seems to gradually diminish with time elapsed after herbivore removal. Our findings provide a fresh, in-depth view into the roles of rhizofungi in enhancing the adaption ability of plants under environmental stress.

Diverse native island flora shows rapid initial passive recovery after exotic herbivore removal on Santa Rosa Island, California

Diverse native island flora shows rapid initial passive recovery after exotic herbivore removal on Santa Rosa Island, California

Airborne laser scanning reveals increased growth and complexity of boreal forest canopies across a network of ungulate exclosures in Norway

AbstractLarge herbivores are often classed as ecosystem engineers, and when they become scarce or overabundant, this can alter ecosystem states and influence climate forcing potentials. This realization has spurred a call to integrate large herbivores in earth system models. However, we lack a good understanding of their net effects on climate forcing, including carbon and energy exchange. A possible solution to this lies in harmonizing data across the myriad of large herbivore exclosure experiments around the world. This is challenging due to differences in experimental designs and field protocols. We used airborne laser scanning (ALS) to describe the effect of herbivore removal across 43 young boreal forest stands in Norway and found that exclusion caused the canopy height to increase from 1.7 ± 0.2 to 2.5 ± 0.2 m (means ± se), and also causing a marked increase in vertical complexity and above‐ground biomass. We then go on to discuss some of the issues with using ALS; we propose ALS as an approach for studying the effects of multiple large herbivore exclosure experiments simultaneously, and producing area‐based estimates on canopy structure and forest biomass in a cheap, efficient, standardized and reproducible way. We suggest that this is a vital next step towards generating biome‐wide predictions for the effects of large herbivores on forest ecosystem structure which can both inform both local management goals and earth system models.

Soil engineering by ants facilitates plant compensation for large herbivore removal of aboveground biomass.

The interplay between top-down and bottom-up processes determines ecosystem productivity. Yet, the factors that mediate the balance between these opposing forces remain poorly understood. Furthering this challenge, complex and often cryptic factors like ecosystem engineering and trait-mediated interactions may play major roles in mediating the outcomes of top-down and bottom-up interactions. In semiarid grasslands of northeastern China, we conducted a large-scale, three-year experiment to evaluate how soil engineering by ants and plasticity in plants independently and jointly influenced the top-down effects of grazing by a ubiquitous herbivore (cattle) on aboveground standing biomass of the dominant perennial grass, Leymus chinensis. Herbivory had strong top-down effects, reducing L. chinensis AB by 25% relative to baseline levels without cattle or ants. In contrast, soil engineering by ants facilitated weak bottom-up effects in the absence of herbivory. However, in the presence of herbivory, soil engineering effects were strong enough to fully offset herbivore removal of aboveground biomass. This outcome was mediated by L. chinensis's plasticity in reallocating growth from below- to aboveground biomass, a result linked to additive effects of engineers and herbivores increasing soil N availability and engineering effects improving soil structure. Soil engineering increased soil N by 12%, promoting aboveground biomass. Herbivores increased soil N by 13% via defecation, but this increase failed to offset their reductions in aboveground biomass in isolation. However, when combined, engineers and herbivores increased soil N by 26% and engineers improved soil bulk density, facilitating L. chinensis to shift resource allocations from below- to aboveground biomass sufficiently to fully offset herbivore suppression of aboveground biomass. Our results demonstrate that soil engineering and trait-mediated effects of plant plasticity can strongly mediate the outcome of top-down and bottom-up interactions. These cryptic but perhaps ubiquitous processes may help to explain the long-debated phenomenon of plant compensatory responses to large grazers.

Microbial community structure across grazing treatments and environmental gradients in the Serengeti

As one of the last remaining naturally grazed ecosystems on Earth, the Serengeti National Park is an ideal location to study the influence of migratory mammals on the structure of microbial communities and the factors that generate biogeography of soil microbes. Furthermore, volcanic inputs generate environmental gradients that may also structure microbial communities. We studied 16S rRNA amplicons in a 13-year herbivore removal experiment to examine the influence of grazing and environmental gradients on the natural distribution of soil microbes. Removal of mammalian herbivores shifted microbial community structure, with 31 taxa that were significant indicator taxa of the ungrazed treatment and three taxa that were indicators of the grazed treatment. The abundance of many taxa were correlated with soil texture, phosphorus, iron, calcium and rainfall, and the evenness of taxa within samples was also correlated with these variables. Bayesian general linear mixed effects models with single predictors of multiple, highly correlated variables of beta diversity were consistent with a significant, but weak (2%), effect of grazing, and stronger effects of phosphorus (14%). Beta diversity of microbial communities was greater in grazed than in ungrazed plots; suggesting that the impacts of grazing on community assembly of microbes results from deterministic environmental filtering caused by the influence of herbivores on plant communities and soil properties rather than stochastic dispersal via herds of large mammals. These herbivore effects are superimposed on deterministic environmental filtering by natural soil and precipitation gradients across the Serengeti.

Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties.

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin ), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net Nmin . Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin . However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin , in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site-specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is an important context-dependent knowledge for grassland management worldwide.

Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna

Although studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remain unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semiarid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C3 vs. C4 ) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8m2 /ha), and woody canopy cover (31%), translating to a ~8.5t/ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5t/ha) in total soil carbon to 30-cm depth, with ~71% of this derived from C4 grasses (vs. ~76% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C4 grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g., Acacia spp.) on grass cover and productivity in this semiarid system.

Moose effects on soil temperatures, tree canopies, and understory vegetation: a path analysis

AbstractLarge vertebrate herbivores are ubiquitous and increasingly numerous in boreal forests where they are known to influence ecosystems in many ways. However, separating the direct effects of herbivores from their indirect effects on plant communities via forest structural changes and microclimate remains unexplored, limiting the predictability of herbivore impacts. We used an exploratory path analysis approach to investigate potential mechanistic pathways between herbivore removal, forest canopy cover, soil temperature, and understory vegetation dynamics. Moose (Alces alces) were excluded from 15 recently clear‐cut boreal forest sites in Norway using 20 × 20 m fences with paired open un‐fenced plots. Soil temperatures and vegetation data were recorded after eight years (total n = 98). Moose exclusion reduced summer soil temperatures but not winter soil temperatures, leading to no net effect over 344 d. Path analysis showed a strong positive effect of moose exclusion on canopy cover and subsequently reduced summer soil temperatures. There was some support for an increase in the dominant grass species Avenella flexuosa with increasing summer soil temperatures, but neither temperature, canopy cover, nor A. flexuosa had clear links to plant species densities. Moose exclusion directly increased herb biomass and shifted understory species composition toward less shrub‐dominated communities, resulting in increased species densities of vascular plants and bryophytes. Our results indicate that in early‐successional boreal forests, direct effects of large herbivores on plant communities are clearly visible, and indirect effects are detectable but much weaker.

Large herbivores maintain a two-phase herbaceous vegetation mosaic in a semi-arid savanna.

Many arid and semi‐arid rangelands exhibit distinct spatial patterning of vegetated and bare soil‐dominated patches. The latter potentially represent a grazing‐induced, degraded ecosystem state, but could also arise via mechanisms related to feedbacks between vegetation cover and soil moisture availability that are unrelated to grazing. The degree to which grazing contributes to the formation or maintenance of degraded patches has been widely discussed and modeled, but empirical studies of the role of grazing in their formation, persistence, and reversibility are limited.We report on a long‐term (17 years) grazing removal experiment in a semi‐arid savanna where vegetated patches composed of perennial grasses were interspersed within large (>10 m2) patches of bare soil.Short‐term (3 years) grazing removal did not allow bare patches to become revegetated, whereas following long‐term (17 years) grazing removal, bare soil patches were revegetated by a combination of stoloniferous grasses and tufted bunchgrasses. In the presence of grazers, stoloniferous grasses partially recolonized bare patches, but this did not lead to full recovery or to the establishment of tufted bunchgrasses.These results show that grazers alter both the balance between bare and vegetated patches, as well as the types of grasses dominating both patch types in this semiarid savanna.Synthesis: Large herbivores fundamentally shaped the composition and spatial pattern of the herbaceous layer by maintaining a two‐phase herbaceous mosaic. However, bare patches within this mosaic can recover given herbivore removal over sufficiently long time scales, and hence do not represent a permanently degraded ecosystem state.

Synthesizing the effects of large, wild herbivore exclusion on ecosystem function

AbstractWild large herbivores are declining worldwide. Despite extensive use of exclosure experiments to investigate herbivore impacts, there is little consensus on the effects of wild large herbivores on ecosystem function.Of the ecosystem functions likely impacted, we reviewed the five most‐studied in exclosure experiments: ecosystem resilience/resistance to disturbance, nutrient cycling, carbon cycling, plant regeneration, and primary productivity.Experimental data on large wild herbivores' effects on ecosystem functions were predominately derived from temperate grasslands (50% grasslands, 75% temperate zones). Additionally, data were from experiments that may not be of adequate size (median size 400 m2despite excluding all experiments below 25 m2) or duration (median duration 6 years) to capture ecosystem‐scale responses to these low‐density and wide‐ranging taxa.Wild herbivore removal frequently impacted ecosystem functions; for example, net carbon uptake increased by three times in some instances. However, the magnitude and direction of effects, even within a single function, were highly variable.A focus on carbon cycling highlighted challenges in interpreting effects on a single function. While the effect of large herbivore exclusion on carbon cycling was slightly positive when its components (e.g. pools vs. fluxes of carbon) were aggregated, effects on individual components were variable and sometimes opposed.Given modern declines in large wild herbivores, it is critical to understand their effects on ecosystem function. However, this synthesis highlights strong variability in direction, magnitude, and modifiers of these effects. Some variation is likely due to disparity in what components are used to describe a given function. For example, for the carbon cycle we identified eight distinctly meaningful components, which are not easily combined yet are potentially misrepresentative of the larger cycle when considered alone. However, much of the observed difference in responses likely reflects real ecological variability across complex systems.To move towards a general predictive framework we must identify where variation in effect is due to methodological differences and where due to ecosystem context. Two critical steps forward are (a) additional quantitative synthetic analyses of large herbivores' effects on individual functions, and (b) improved, increased systematic exclosure research focusing on effects of large herbivores' exclusion on functions.A freePlain Language Summarycan be found within the Supporting Information of this article.

Effects of grazing exclusion on vegetation and seed bank composition in a mesic mountain grassland in Argentina

ABSTRACTBackground: Grazing is a major factor that affects grassland composition and species richness. While the long-term effects of grazing are well documented, information about the short-term effects of grazing is necessary for adaptive conservation management.Aims: Our objective was to ascertain how this held over varying time periods after herbivore removal vs. exclosures of different ages for grassland and soil seed bank composition in the Argentine Pampas.Methods: The composition of the vegetation and the seed bank assessed in plots with 2 and 12 years of grazing exclusion and in nearby grazed areas. Multivariate analyses were used to evaluate treatment effects.Results: Grazing increased bare ground and reduced plant cover. Grazing reduced grass specific richness in the seed bank, while increased the richness and density of shrubs. The composition of the vegetation and the seed bank was not related. There was a trend whereby grazed areas were characterised by shrub species and non-palatable or annual grasses; the 2-year-old exclosures by forb species and the 12-year-old exclosures were dominated by perennial grasses.Conclusions: The removal of herbivores has changed vegetation and seed bank composition. The observed trend in reduction of species and life form diversity may be countered by controlled grazing for conservation purposes.

Limited Genetic Variability in Native Buckwheats (Eriogonum: Polygonaceae) on San Clemente Island

California Channel Island buckwheats are important components of scrub communities on the Channel Islands. A long history of overgrazing by introduced herbivores on San Clemente Island has contributed to scrub vegetation loss and population declines in 2 endemic island taxa, Eriogonum giganteum var. formosum and E. grande var. grande. Since herbivore removal, Eriogonum species, which are early colonizers of disturbed habitats, have been increasing across the island. These increases are thought to facilitate further recovery of the native flora because the shrubs act as nurse plants and improve the soil profile for other native species.

Grazing and resource availability control soil nematode body size and abundance-mass relationship in semi-arid grassland.

Body size is a central functional trait in ecological communities. Despite recognition of the importance of above ground-below ground interactions, effects of above-ground herbivores on size and abundance-size relationships in soil fauna are almost uncharted. Depending on climate and soil properties, herbivores may increase basal resources of soil food webs, or reduce pore space, mechanisms expected to have contrasting effects on soil animal body size. We investigated how body size and shape of soil nematodes responded to mammalian grazers in three semi-arid grassland sites, along a gradient of soil texture and organic matter (OM) in a long-term herbivore removal study. We analysed nematode mass, length, diameter, body size distribution and biomass distribution. We formulated two mechanistic hypotheses to assess whether resource availability or pore space was the dominant abiotic control and modulated the effects of grazing. In ungrazed soils, average and maximum nematode size, as well as abundance and biomass of large nematodes, were greater in the high-OM than in the low-OM soil, and intermediate in the medium-OM soil. Grazing promoted larger sizes in the low-OM soil, where it had been shown to increase OM and microbial biomass, and led to more homogeneous average size and body size distribution across sites. The results support the hypothesis that nematode size was controlled by basal resource availability rather than by pore space. However, body shape might have been constrained by small pores in the fine-texture, high-OM soil, where nematodes were more elongated. Grazing may facilitate larger sizes in soil nematode communities by boosting basal resources where these are limiting, with important implications for estimations of nematode biomass and contribution to carbon and nutrient cycling. These findings contribute to the insofar-limited mechanistic understanding of how herbivores can shape functional traits of soil fauna and demonstrate that animals at one trophic level may control patterns in body size and abundance-size relationships in other trophic levels without a direct predator prey or competitive linkage between them.

Increased grassland arthropod production with mammalian herbivory and eutrophication: a test of mediation pathways.

Increases in nutrient availability and alterations to mammalian herbivore communities are a hallmark of the Anthropocene, with consequences for the primary producer communities in many ecosystems. While progress has advanced understanding of plant community responses to these perturbations, the consequences for energy flow to higher trophic levels in the form of secondary production are less well understood. We quantified arthropod biomass after manipulating soil nutrient availability and wild mammalian herbivory, using identical methods across 13 temperate grasslands. Of experimental increases in nitrogen, phosphorus, and potassium, only treatments including nitrogen resulted in significantly increased arthropod biomass. Wild mammalian herbivore removal had a marginal, negative effect on arthropod biomass, with no interaction with nutrient availability. Path analysis including all sites implicated nutrient content of the primary producers as a driver of increased arthropod mean size, which we confirmed using 10 sites for which we had foliar nutrient data. Plant biomass and physical structure mediated the increase in arthropod abundance, while the nitrogen treatments accounted for additional variation not explained by our measured plant variables. The mean size of arthropod individuals was 2.5 times more influential on the plot-level total arthropod biomass than was the number of individuals. The eutrophication of grasslands through human activity, especially nitrogen deposition, thus may contribute to higher production of arthropod consumers through increases in nutrient availability across trophic levels.

Herbivores rescue diversity in warming tundra by modulating trait-dependent species losses and gains

Climate warming is altering the diversity of plant communities but it remains unknown which species will be lost or gained under warming, especially considering interactions with other factors such as herbivory and nutrient availability. Here, we experimentally test effects of warming, mammalian herbivory and fertilization on tundra species richness and investigate how plant functional traits affect losses and gains. We show that herbivory reverses the impact of warming on diversity: in the presence of herbivores warming increases species richness through higher species gains and lower losses, while in the absence of herbivores warming causes higher species losses and thus decreases species richness. Herbivores promote gains of short-statured species under warming, while herbivore removal and fertilization increase losses of short-statured and resource-conservative species through light limitation. Our results demonstrate that both rarity and traits forecast species losses and gains, and mammalian herbivores are essential for preventing trait-dependent extinctions and mitigate diversity loss under warming and eutrophication.

Herbivory Differentially Affects Plant Fitness in Three Populations of the Perennial Herb Lythrum salicaria along a Latitudinal Gradient.

Herbivory can negatively and selectively affect plant fitness by reducing growth, survival and reproductive output, thereby influencing plant population dynamics and evolution. Latitudinal variation in intensity of herbivory is common, but the extent to which it translates into corresponding variation in effects on plant performance is still poorly known. We tested the hypothesis that variation in the fitness-consequences of herbivory mirror differences in intensity of herbivory among three natural populations of the perennial herb Lythrum salicaria along a latitudinal gradient from southern to northernmost Sweden. We documented intensity of herbivory and examined its effect on survival, growth and reproductive output over two years by experimentally removing herbivores with insecticide. The intensity of herbivory and the effects of herbivory on plant fitness were strongest in the southern population, intermediate in the central population and weakest in the northern population. The mean proportion of the leaf area removed ranged from 11% in the southern to 3% in the northern population. Herbivore removal increased plant height 1.5-fold in the southern and 1.2-fold in the central population, the proportion plants flowering 4-fold in the southern and 2-fold in the central population, and seed production per flower 1.6-fold in the southern and 1.2-fold in the central population, but did not affect plant fitness in the northern population. Herbivore removal thus affected the relative fecundity of plants in the three populations: In the control, seed output per plant was 8.6 times higher in the northern population compared to the southern population, whereas after herbivore removal it was 2.5 times higher in the southern population. The results demonstrate that native herbivores may strongly affect the demographic structure of L. salicaria populations and thereby shape geographic patterns of seed production. They further suggest that the strength of herbivore-mediated selection varies among populations and decreases towards the north.