Endophyte-grass-herbivore interactions: the case of Neotyphodium endophytes in Arizona fescue populations

Neotyphodium endophytes in introduced agronomic grasses are well known to increase resistance to herbivores, but little is known of interactions between Neotyphodium endophytes and herbivores in native grass populations. We investigated whether endophytes mediate plant-herbivore interactions in a native grass species, Festuca arizonica in the southwestern United States, in two ways. First, to test the prediction that the presence and frequency of endophyte-infected (E+) plants should increase with increasing herbivory, we determined endophyte frequencies over a 4-year period in six natural Arizona fescue populations. We compared Neotyphodium frequency among plants growing inside and outside long-term vertebrate grazing exclosures. Second, we experimentally tested the effects of Neotyphodium infection, plant clone, and soil nutrients on plant resistance to the native grasshopper Xanthippus corallipes. Contrary to predictions based upon the hypothesis that endophytes increase herbivore resistance, levels of infection did not increase in plants subjected to grazing outside of exclosures relative to ungrazed plants within exclosures. Instead, endophyte frequencies tended to be greater inside the exclosures, where long-term vertebrate grazing was reduced. The grasshopper bioassay experiment corroborated these long-term patterns. Survival of grasshoppers did not differ between infected (E+) and uninfected (E–) plants. Instead, mean relative growth rate of grasshoppers was higher on E+ grasses than on E– ones. Growth performance of newly hatched grasshopper nymphs varied among host plant clones, although two of six clones accounted for most of this variation. Our results suggest that Neotyphodium-grass-herbivore interactions may be much more variable in natural communities than predicted by studies of agronomically important Neotyphodium-grass associations, and herbivory is not always the driving selective force in endophyte-grass ecology and evolution. Thus, alternative hypotheses are necessary to explain the wide distribution and variable frequencies of endophytes in natural plant populations. Received: 15 February 1999 / Accepted: 19 July 1999     

Resource partitioning along multiple niche dimensions in differently sized African savanna grazers

Resource partitioning among mammalian savanna herbivores is thought to be predominantly driven by differences in body size. In general, large herbivore species utilize abundant low quality forage while small herbivores focus on scarcer high quality food items. However, in a natural system other factors such as digestive strategy, season and the presence of megaherbivores (body size > 1000 kg) are likely to complicate allometric predictions. Non‐ruminants are probably better able to cope with abundant low quality food than ruminants of the same size causing a non‐ruminant to act ‘larger’ than allometrically predicted. Also, the effect of alternating seasons with high and low food availability on diet choice and hence the competitive interactions between co‐occurring herbivores is still poorly understood. Lastly, how megaherbivores deviate from allometric predictions (based on smaller species) is still not well quantified. In this study we examine resource partitioning among three ruminant and three non‐ruminant grazers: impala, wildebeest, buffalo, warthog, zebra and white rhinoceros (megaherbivore) in the savanna of Hluhluwe iMfolozi Park, South Africa. We analysed habitat and diet overlap, specifically grass species (something not commonly investigated) and grass height eaten, in both the wet and the dry seasons. We found that habitat utilization differences among the species were generally small and did not vary between seasons. Diets within feeding patches overlapped during the wet season but highly diverged during the dry season. Body mass differences among species explained their dry season resource partitioning for all species except for comparisons with the megaherbivore (white rhino), while differences in digestive strategy were not related to niche overlap in either season. We conclude that savanna herbivores in this system coexist mostly through body size‐driven resource partitioning in the dry‐season, with the exception of the white rhino (megaherbivore).     

放牧干扰下的蝗虫－植物相互作用关系

本文研究了内蒙古典型草原植物和蝗虫群落在不同放牧强度影响下的多样性、均匀性和丰盛度变化。比较了蝗虫群落与植物群落在放牧梯度上的相互作用关系。研究发现,放牧干扰活动会明显地影响植物和蝗虫群落结构,但蝗虫群落结构的变化趋势并非与植物完全同步。蝗虫与植物间的联系更多地表现在植物起着蝗虫栖息地条件的作用,而并不完全是食料植物的作用。文中对放牧实践与多样性保护的关系进行了讨论。     

Resource partitioning by grass height among grazing ungulates does not follow body size relation

We compared the grass height grazed by white rhino, wildebeest, zebra and impala through the dry season months in the Hluhluwe–iMfolozi Park in South Africa. We expected that the grass height grazed would increase with the body size of the herbivore species, as suggested from past studies of resource partitioning among large mammalian herbivores. Instead we found that the largest of these species, white rhino, concentrated on the shortest grass, while the smallest species, impala, grazed heights intermediate between those grazed by wildebeest and zebra. Results suggest that the scaling of mouth width relative to body size, and hence to metabolic demands, may be the primary factor governing grass height selection, rather than body size alone. This calls into question the widespread assumption that smaller herbivores are superior competitors through being able to persist on sparser vegetation. Furthermore, there was considerable overlap in grass height grazed among these four species, indicating that niche separation by grass height is inadequate alone to explain their coexistence.     

Weather variation and trophic interaction strength: sorting the signal from the noise

Weather can have important consequences for the structure and function of ecological communities by substantially altering the nature and strength of species interactions. We examined the role of intra- and inter-annual weather variability on species interactions in a seasonal old-field community consisting of spider predators, grasshopper herbivores, and grass and herb plants. We experimentally varied the number of trophic levels for 2 consecutive years and tested for inter-annual variation in trophic abundances. Grasshopper emergence varied between years to the extent that the second growing season was 20% shorter than the first one. However, the damage grasshoppers inflicted on plants was greater in the second, shorter growing season. This inter-annual variation in plant abundance could be explained using the foraging-predation risk trade-off displayed by grasshoppers combined with their survival trajectory. Decreased grasshopper survival not only reduced the damage inflicted on plants, it weakened the strength of indirect effects of spiders on grass and herb plants. The most influential abiotic factor affecting grasshopper survival was precipitation. We found a negative association between grasshopper survival and the total yearly precipitation. A finer scale analysis, however, showed that different precipitation modalities, namely, number of rainy days and average precipitation per day, had opposing effects on grasshopper survival, which were inconsistent between years. Furthermore, our results suggest that small changes in these factors should result in changes of up to several orders of magnitude in the mortality rate of grasshoppers. We thus conclude that in this system the foraging-predation risk trade-off displayed by grasshoppers combined with their survival trajectory and relevant weather variability should be incorporated in analytical theory, whose goal is to predict community dynamics.     

Insect herbivory and vertebrate grazing impact food limitation and grasshopper populations during a severe outbreak

1. Competition between herbivores often plays an important role in population ecology and appears strongest when densities are high or plant production is low. Phytophagous insects are often highly abundant, but relatively few experiments have examined competition between vertebrates and phytophagous insects. 2. In grassland systems worldwide, grasshoppers are often the dominant phytophagous insect, and livestock grazing is a dominant land use. For this study, a novel experiment was conducted examining competition between vertebrates and invertebrates, where both grasshopper densities and sheep grazing were manipulated inside 10‐m² caged mesocosms during a grasshopper outbreak. We examined how grasshopper densities and the timing of vertebrate herbivory affected grasshopper densities, if the effects of vertebrates on survival and reproduction changed with grasshopper density, and how a naturally occurring grasshopper outbreak affected grasshopper populations in the following year. 3. Densities of grasshoppers at the site peaked at 130 m^–2. Food‐limited competition was stronger in treatments with higher grasshopper densities and repeated or late livestock herbivory, leading to reduced survival, femur length, and functional ovarioles, a measure of future reproduction. Strong food‐limited density‐dependent reproduction and survival led to reduced hatching densities in 2001. 4. As competition was typically stronger with high grasshopper densities than with livestock grazing, competition from vertebrates could be relatively less important for phytophagous insect population dynamics during outbreaks. The experiment provides insights into how competition between insect and vertebrate herbivores influences insect population dynamics, and indicates that severe outbreaks can rapidly subside with strong competition from vertebrate and insect herbivores.     

Effects of traditional pastoralism on grasshopper (Caelifera) assemblages in East Africa

In East Africa, traditional pastoralists increase landscape heterogeneity by creating traditional livestock corrals (bomas). When these bomas are abandoned, they serve as long‐term hot spots of increased nutrients and unique vegetation. However, the effect of bomas on insect populations is unclear. Grasshopper (Suborder Caelifera) assemblages are thought to reflect vegetation, and within Caelifera, the subfamily Oedipodinae is known to be associated with degraded areas. We sampled vegetation and collected grasshoppers inside of abandoned bomas, 50 m from abandoned bomas and 250 m from abandoned bomas. Bomas had significantly lower grass abundance than background vegetation. Total grasshopper abundance was positively correlated with grass and forb abundance, while the proportion of grasshoppers in the subfamily Oedipodinae was negatively correlated with grass abundance. Grasshopper abundance was significantly lower inside of bomas than outside of them, but the proportion of grasshoppers in the subfamily Oedipodinae was significantly higher inside of bomas than outside of them. This suggests that the decreased vegetation in abandoned bomas supports fewer grasshoppers, but a higher fraction of the grasshopper assemblage in abandoned bomas is composed of Oedipodinae. Thus, traditional pastoralists can have long‐term effects on the size and composition of grasshopper assemblages through the use of bomas.     

Effects of large herbivore grazing on grasshopper behaviour and abundance in a meadow steppe

1. Adaptive phenotypic plasticity has been a major subject in evolutionary ecology, but how a species' behaviour may respond to certain environmental change is still not clear. In grasslands worldwide, large herbivores are increasingly used as a tool for grazing management, and occur to interact with grasshoppers that dominate grassland insect communities. Previous studies have been well-documented about grazing effects on diversity and abundance of grasshoppers. Yet, how grazing may alter grasshopper behaviour, and potential effects on their abundance remains elusive. 2. We conducted a field experiment by manipulating grazing using sheep, cattle, and their mix to examine the behavioural responses and abundance of the grasshoppers (Euchorthippus unicolor) to grazing in a Leymus chinensis-dominated grassland. 3. Results showed that the grasshoppers spent less time on feeding and resting on grasses, but more time on switching and resting on forbs under cattle grazing and mixed grazing with cattle and sheep. In contrast, the grasshoppers spent more time on feeding but less time on switching and resting on forbs under sheep grazing. The behavioural changes were also potentially linked to grasshopper abundance in the context of grazing management. 4. The responses of grasshopper behaviour and abundance to grazing may be largely triggered by altered vegetation and microclimates. Such behavioural flexibility of grasshoppers must be considered when large herbivores are recognised as a management tool for influencing grasshopper abundance, and grazer species should be paid more attention both individually and jointly for better grassland conservation.     

An experimental study of the impacts of understorey forest vegetation and herbivory by red deer and rodents on seedling establishment and species composition in Waitutu Forest, New Zealand

Introduced mammalian herbivores are changing the structure and composition of New Zealand’s forest ecosystems and may modify forest succession after natural disturbances. We studied how introduced ungulates (red deer and feral pigs) and rodents (rats and house mice) affected the rate of recovery (i.e. the engineering resilience) of the forest understorey following artificial disturbance. We imposed disturbances by clearing understorey vegetation dominated by Blechnum ferns in forests on relatively fertile alluvium and elevated infertile marine terraces, and recorded recovery of vegetation (seedling establishment, species composition, cover and volume) in herbivore exclosures and controls. Seedlings quickly established on cleared plots: after 2 years, numbers of woody seedlings and ground cover of vascular plants relative to initial values were similar on cleared and uncleared treatments. Volume of plant biomass <2 m remained low on cleared subplots. Ungulates significantly reduced the re-establishment of woody seedlings ≥ 10 cm tall: only one seedling reached this height outside exclosures, compared with 29 seedlings inside. The number of seedlings <10 cm tall, expressed relative to numbers present pre-clearing, was not significantly affected by ungulates. The species composition of regenerating vegetation was more similar (Jaccard index) to pre-clearing understorey vegetation inside ungulate exclosures than outside. No consistent effect of rodents (primarily house mice) on seedling establishment or species composition was detected after 2 years, and rodent exclosures did not significantly affect survival of seedlings (Griselinia littoralis and Aristotelia serrata) planted as an index of rodent herbivory pressure. No significant differences in vegetation recovery were apparent between forest types. Rapid seedling recruitment in the absence of understorey vegetation and the presence of herbivores provided evidence that understorey vegetation competes with seedlings for light. Ungulate effects were consistent with other experiments that showed herbivores reduced the rate and altered the trajectory of vegetation regrowth after disturbance.     

Range restricted grasshoppers better conserved in a terrestrial zone than in a riparian zone

Riparian zones (RZs) functionally connect aquatic and terrestrial ecosystems, and have azonal and geographically widespread plant communities that differ from those of the neighboring terrestrial zone (TZ). Although well studied botanically, RZs are not well understood in terms of their terrestrial insect diversity, including grasshoppers. The Cape Floristic Region (CFR) is a global biodiversity hotspot with small rocky rivers running through highly diverse sclerophyllous vegetation. It has high levels of endemism among many taxa, including grasshoppers, making it ideal for testing the effect of azonal vegetation on grasshopper assemblages of the RZ, and determining whether conservation efforts should be focused on the RZ as well as the TZ. We determine grasshopper dispersion patterns along the RZ of an important CFR river, and compare these patterns with those of the TZ to understand the habitat occupancy relative to 27 environmental variables of the zones and geographical distribution of the grasshoppers. Forty percent of individuals we collected were CFR endemics. We found only weak differences in the grasshopper assemblages between the RZ and TZ, apparently driven by deep history, complex geomorphology, stressful environmental conditions, a diverse vegetation and land mosaic, and probable high predator pressure. There were two groups: large-sized, well-flighted, geographically widespread generalists that were overall more abundant in the RZ than TZ, and small, flightless or poorly-flighted, vegetation-specialists which are narrow-range endemics adapted to both RZ and TZ, but still more abundant in the TZ. We conclude that although the vegetation of this riparian zone may require some special conservation attention, this is not so for the grasshoppers which overall are best conserved in the TZ.     

Dynamics of grazing lawn formation: an experimental test of the role of scale‐dependent processes

Grazing lawns are characteristic for African savanna grasslands, standing out as intensely grazed patches of stoloniferous grazing‐tolerant grass species. Grazing lawn development has been associated with grazing and increased nutrient input by large migratory herds. However, we argue that in systems without mass migrations disturbances, other than direct grazing, drive lawn development. Such disturbances, e.g. termite activity or megaherbivore middens, also increase nutrient input and keep the bunch vegetation down for a prolonged time period. However, field observations show that not all such disturbances lead to grazing lawns. We hypothesize that the initial disturbance has to be of a minimal threshold spatial scale, for grazing intensity to be high enough to induce lawn formation. We experimentally tested this idea in natural tall savanna grassland. We mowed different‐sized plots to simulate initial disturbances of different scales (six times during one year) and applied fertilizer to half of the plots during two years to simulate increased nutrient input by herbivores or termite activity. Allowing grazing by naturally occurring herbivores, we followed the vegetation development over more than three years. Grazing kept bunch grass short in coarser, fertilized plots, while grasses grew out toward their initial height in fine‐scale and unfertilized plots. Moreover, lawn grasses strongly increased in cover in plots with an increased nutrient input but only after coarser scale disturbance. These results support our hypothesis that an increased nutrient input in combination with grazing indeed induces grazing lawn formation, but only above a threshold scale of the initial disturbance. Our results provide an alternative mechanism for the development of grazing lawns in systems that lack mass migrating herds. Moreover, it gives a new spatial dimension to the processes behind grazing lawn development, and hence help to understand how herbivores might create and maintain spatial heterogeneity in grassland systems.     

Plant functional traits reveal the relative contribution of habitat and food preferences to the diet of grasshoppers

Food preferences and food availability are two major determinants of the diet of generalist herbivores and of their spatial distribution. How do these factors interact and eventually lead to diet differentiation in co-occurring herbivores? We quantified the diet of four grasshopper species co-occurring in subalpine grasslands using DNA barcoding of the plants contained in the faeces of individuals sampled in the field. The food preferences of each grasshopper species were assessed by a choice (cafeteria) experiment from among 24 plant species common in five grassland plots, in which the four grasshoppers were collected, while the habitat was described by the relative abundance of plant species in the grassland plots. Plant species were characterised by their leaf economics spectrum (LES), quantifying their nutrient vs. structural tissue content. The grasshoppers’ diet, described by the mean LES of the plants eaten, could be explained by their plant preferences but not by the available plants in their habitat. The diet differed significantly across four grasshopper species pairs out of six, which validates food preferences assessed in standardised conditions as indicators for diet partitioning in nature. In contrast, variation of the functional diversity (FD) for LES in the diet was mostly correlated to the FD of the available plants in the habitat, suggesting that diet mixing depends on the environment and is not an intrinsic property of the grasshopper species. This study sheds light on the mechanisms determining the feeding niche of herbivores, showing that food preferences influence niche position whereas habitat diversity affects niche breadth.     

Predictions of species interactions from consumer-resource theory: experimental tests with grasshoppers and plants

We tested the ability of consumer-resource theory to predict direct and indirect interactions among species, using an experimental system of insect herbivores and herbaceous plants. Specifically, we examined interactions among three species of grasshoppers (Melanoplus femur-rubrum, Spharagemon collare, andPhoetaliotes nebrascensis; Orthoptera, Acrididae) and herbaceous plants in experimental field cages placed over existing fertilized or unfertilized vegetation in a Minnesota old field. For the conditions inside these cages, we addressed whether (1) grasshopper diet predicted the presence of competition among grasshopper species, and (2) direct effects of grasshoppers on plants produced indirect interactions among plants, grasshoppers and soil nitrogen. Overall,M. femur-rubrum ate a greater proportion of forbs in cages, while the other two species ate primarily grasses. As expected, a pair of grasshopper species competed if they had similar diets. However, there were important exceptions that could be explained from observed indirect effects, although alternative explanations were also possible. First, all three grasshopper species significantly shifted their diets in the presence of other species, and these shifts occurred most often when competition was expected or occurred. Second, the two grassfeeding species reduced the biomass of the dominant grass (Schizachyrium scoparium) and increased available soil nitrogen and biomass of forbs. This effect may explain why the grass-feedingP. nebrascensis had a positive effect on the forb-feedingM. femur-rubrum on unfertilized plots. Overall, we show that direct effects of consumers on resources can predict competition and other important indirect interactions within a community.     

Ungulates increase forest plant species richness to the benefit of non‐forest specialists

Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant‐plant interactions and through soil disturbance. In forest ecosystems, researchers’ attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over 10 years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation‐wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analyzed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light‐demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non‐forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to the peculiarity of forest plant communities and contributes to a landscape‐level biotic homogenization. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition.     

Grasshopper response to a 40-year experimental burning and mowing regime, with recommendations for invertebrate conservation management

Grasshopper assemblages were sampled in 44 plots in each of three adjacent sites in a 40-year-old southern tall grassland experimental area in South Africa. Specific plots received particular mowing and/or burning treatments over the 40-year period. Grasshopper responses to vegetation type, and to different burning and mowing practices, were site-specific, despite the close proximity of sites. This suggests that grasshopper assemblage composition is not entirely deterministic and depends on the trajectory of plant succession. Grasshopper species richness and abundance decreased from annually to triennially burnt plots, and increased in plots mown once per year to plots mown three times per year. Burning in the first week of August (winter) was more favourable for grasshopper assemblages than burning in autumn or after the first spring rains. Mean grasshopper species richness was highest in plots mown after the first spring rains, and the mean number of individuals was highest i n plots mown early in summer. When annually burnt plots were compared with annually mown plots, grasshopper abundance and species richness were highest in the burnt plots. A rotational winter burning programme, which is practical under African conditions, is recommended for the conservation of grasshoppers and other invertebrates.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

The Response of Soil CO2 Fluxes to Progressively Excluding Vertebrate and Invertebrate Herbivores Depends on Ecosystem Type

Grasslands support large populations of herbivores and store up to 30% of the world’s soil carbon (C). Thus, herbivores likely play an important role in the global C cycle. However, most studies on how herbivory impacts the largest source of C released from grassland soils—soil carbon dioxide (CO₂) emissions—only considered the role of large ungulates. This ignores all other vertebrate and invertebrate herbivores and their collective effects on ecosystem properties. We progressively excluded large, medium, and small vertebrates and invertebrates from two subalpine grasslands (productive, heavily grazed short-grass; less productive, lightly grazed tall-grass) using size-selective fences, assessed the impact on soil CO₂ emissions and related biotic and abiotic variables. Exclusion resulted in significant changes in soil CO₂ emissions in both vegetation types. Short-grass soil CO₂ emissions progressively increased when large and medium mammals were excluded. However, no difference was detected among plots were all or no herbivores grazed. In contrast, tall-grass soil CO₂ emissions were not affected by mammal exclusion, but excluding all herbivores lead to reduced emissions. Soil micro-climatic parameters best predicted the patterns of soil CO₂ emissions in short-grass vegetation, whereas root biomass was the best predictor of CO₂ release in tall-grass vegetation. Our results showed that diverse herbivore communities affect soil respiration differently than assumed from previous studies that only excluded large ungulates. Such information is important if we are to understand how changes in herbivore species composition—as could happen through altered management practices, extinction or invasion—impact grassland C storage and release.     

Indirect Short- and Long-Term Effects of Aboveground Invertebrate and Vertebrate Herbivores on Soil Microarthropod Communities

Recognition is growing that besides ungulates, small vertebrate and invertebrate herbivores are important drivers of grassland functioning. Even though soil microarthropods play key roles in several soil processes, effects of herbivores—especially those of smaller body size—on their communities are not well understood. Therefore, we progressively excluded large, medium and small vertebrate and invertebrate herbivores for three growing seasons using size-selective fences in two vegetation types in subalpine grasslands; short-grass and tall-grass vegetation generated by high and low historical levels of ungulate grazing. Herbivore exclusions generally had few effects on microarthropod communities, but exclusion of all herbivore groups resulted in decreased total springtail and Poduromorpha richness compared with exclusion of only ungulates and medium-sized mammals, regardless of vegetation type. The tall-grass vegetation had a higher total springtail richness and mesostigmatid mite abundance than the short-grass vegetation and a different oribatid mite community composition. Although several biotic and abiotic variables differed between the exclusion treatments and vegetation types, effects on soil microarthropods were best explained by differences in nutrient and fibre content of the previous year’s vegetation, a proxy for litter quality, and to a lesser extent soil temperature. After three growing seasons, smaller herbivores had a stronger impact on these functionally important soil microarthropod communities than large herbivores. Over longer time-scales, however, large grazers created two different vegetation types and thereby influenced microarthropod communities bottom-up, e.g. by altering resource quality. Hence, both short- and long-term consequences of herbivory affected the structure of the soil microarthropod community.     

Importance of large and small mammalian herbivores for the plant community structure in the forest tundra ecotone

Both theoretical arguments and empirical evidence suggests that herbivory in general and mammalian winter herbivory in particular is important in arctic–alpine ecosystems. Although knowledge of the effect of herbivores on specific plants and communities is quite extensive, little is known about the relative impact of large and small vertebrate herbivores and how it might vary among different habitats. To address this key issue, we established exclosures with two different mesh sizes in forest and nearby tundra at three different sites in four contrasting locations in the forest–tundra ecotone in northernmost Sweden and Norway. Plant community composition was recorded annually in three permanent plots within each exclosure and an unfenced control. Local densities of vertebrate herbivores were estimated in spring and autumn from 1998 to 2002. Reindeer (Rangifer tarandus) were the most abundant large vertebrate while Norwegian lemmings (Lemmus lemmus) and grey‐sided voles (Clethrionomys rufocanus) were the most common small vertebrates. The study reveals that voles and lemmings have larger effects on the vegetation than reindeer in both habitats in all four locations, even though densities of reindeer differ between locations and only two locations experienced lemming peaks during the period of the experiment. The relative abundance of five of the fifteen most common species was significantly influenced by voles and lemmings whereas only a single species was significantly influenced by reindeer. Different analyses give contrasting results on the importance of herbivory in forest versus open heathlands. A principal component analyses revealed that herbivory influenced the vegetation more in open heathlands than in forests. However, an importance index of herbivores did not differ between forest and open heathlands. Moreover, none of the plant species responded differently in the two habitats, when herbivores were removed. Our results suggest that intense and localised selective foraging by small mammals may have a more marked effect on vegetation than transient feeding by reindeer.     

Grasshopper Herbivory Affects Native Plant Diversity and Abundance in a Grassland Dominated by the Exotic Grass Agropyron cristatum

The indirect effects of native generalist insect herbivores on interactions between exotic and native grassland plants have received limited attention. Crested wheatgrass ( Agropyron cristatum ) is the most common exotic rangeland grass in western North America. Crested wheatgrass communities are resistant to colonization by native plant species and have strong competitive effects on native species, imposing problems for the restoration of native grasslands. Grasshoppers are generalist herbivores that are often abundant in Crested wheatgrass–dominated sites in the northern Great Plains. We conducted two experiments in a Crested wheatgrass–dominated grassland in western North Dakota to test the hypothesis that grasshopper herbivory influences local Crested wheatgrass community composition by impeding native seedlings. Grasshopper herbivory negatively affected the species richness, abundance, and Shannon diversity of native plants in 3 of 4 years. Although additional research is needed to determine if grasshoppers actively select native plants, the effects of grasshopper herbivory may be an important consideration in the restoration of Crested wheatgrass areas. Our findings illustrate the importance of understanding the impact of native generalist invertebrate herbivores on the relationships between exotic and native plants.