Effects of small mammals and vertebrate predators on vegetation in the Chilean semiarid zone

 We monitored the cover and seed bank response of shrubs, perennial herbs, and ephemeral plants to experimental exclusion of both the principal rodent herbivore, Octodon degus, and its vertebrate predators from 1989 to 1994 in a semiarid Chilean mediterranean site. Although both richness and species composition of the plant community at the study site were largely determined by abiotic factors (mainly rainfall and soil nutrients), predator and herbivore exclusion had significant effects on the relative abundance of several plant species. Experimental exclusion of herbivores was associated with increased cover of some shrubs and a perennial grass, and decreased cover and seed densities of several ephemerals, especially those exotic or restricted to areas underneath shrubs. Herbivores apparently reduced shrubs through browsing and indirectly affected herb cover and seed densities by opening up areas under shrubs and/or modifying physical and chemical conditions of the soil. Plant responses to predator exclusion were less clear. Nevertheless, higher cover of some shrubs and ephemerals in the presence of predators suggests tritrophic effects through changes in small mammal densities and/or foraging behavior. Received: 22 April 1996 / Accepted: 14 August 1996     

Plant–herbivore interactions in Alaskan arctic tundra change with soil nutrient availability

Herbivores in nutrient‐limited systems such as arctic tundra have been suggested to play a minor role in controlling plant growth simply because they are relatively few in number. However, theory predicts that as net primary productivity (NPP) increases because of greater inputs of nutrients or energy, herbivores may have greater effects on plant growth. This prediction has not been tested in the context of climate warming in arctic tundra, which may increase soil nutrient availability and thus NPP. We examined a long‐term experiment that excluded small and large mammalian herbivores and increased soil nutrients in two arctic Alaskan tundra communities: dry heath (DH) and moist acidic tussock (MAT). In the ninth year of manipulations, we measured weekly growth of three plant species of three growth forms: tussock‐forming graminoid, rhizomatous graminoid, and dwarf deciduous shrub, in each community. All species grew better when fertilized. In DH, this increase in growth was exaggerated when plants were protected from herbivores, confirming that herbivory had a negative effect on plant growth under increased nutrient conditions, but was unimportant under ambient soil conditions. However, in MAT, the importance of herbivory differed among species with fertilization. The tussock‐forming sedge at MAT, Eriophorum vaginatum, grew better and flowered more when fenced under both ambient and amended nutrients compared to plants exposed to herbivores. This species decreases in abundance in long‐term fertilized plots when mammals are present, and our results suggest that herbivory may be accounting for at least some of that loss, in addition to shifts in competitive relationships. Although we only focused on individual plants here rather than the entire community, our results suggest that under the increased soil nutrient conditions expected with continued climate warming in the Arctic, herbivores may become more important in affecting several abundant tundra plant populations, and should not be ignored.     

Defoliation of a grass is mediated by the positive effect of dung deposition,moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment

Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing‐tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily‐grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long‐lasting effects on plant productivity and ecosystem functioning after grazing has ceased.     

Season mediates herbivore effects on litter and soil microbial abundance and activity in a semi-arid woodland

Herbivores can directly impact ecosystem function by altering litter quality of an ecosystem or indirectly by shifting the composition of microbial communities that mediate nutrient processes. We examined the effects of tree susceptibility and resistance to herbivory on litter microarthropod and soil microbial communities to test the general hypothesis that herbivore driven changes in litter inputs and soil microclimate will feedback to the microbial community. Our study population consisted of individual piñon pine trees that were either susceptible or resistant to the stem-boring moth (Dioryctria albovittella) and susceptible piñon pine trees from which the moth herbivores have been manually removed since 1982. Moth herbivory increased piñon litter nitrogen concentrations (16%) and decreased canopy precipitation interception (28%), both potentially significant factors influencing litter and soil microbial communities. Our research resulted in three major findings: (1) In spite of an apparent increase in litter quality, herbivory did not change litter microarthropod abundance or species richness. (2) However, susceptibility to herbivores strongly influenced bulk soil microbial communities (i.e., 52% greater abundance beneath herbivore-resistant and herbivore-removal trees than susceptible trees) and alkaline phosphatase activity (i.e., 412% increase beneath susceptible trees relative to other groups). (3) Season had a strong influence on microbial communities (i.e., microbial biomass and alkaline phosphatase activity increased after the summer rains), and their response to herbivore inputs, in this semi-arid ecosystem. Thus, during the dry season plant resistance and susceptibility to a common insect herbivore had little or no observable effects on the belowground organisms and processes we studied, but after the rains, some pronounced effects emerged.     

Herbivory and plant tolerance: experimental tests of alternative hypotheses involving non‐substitutable resources

A mechanistic understanding of the highly variable effects of herbivores on plant production in different ecosystems remains a major challenge. To explain these patterns, the compensatory continuum hypothesis (CCH) predicts plants to compensate for defoliation when resources are abundant, whereas the growth rate hypothesis (GRH) makes the opposite claim of high herbivory tolerance under resource‐poor conditions. The limiting resource model (LRM) tries to reconcile this dichotomy by incorporating the indirect effects of herbivores on plant resources and predicts that the potential for plant compensation is dependent upon whether, and how, herbivory influences limiting resources. Although extensively evaluated in laboratory monocultures, it remains uncertain whether these models can also explain the response of heterogeneous and multi‐species natural plant communities to defoliation. Here we investigate community‐wide plant response to defoliation and report data from a field experiment in the arid and primarily water‐limited Trans‐Himalayan grazing ecosystem in northern India involving clipping, irrigation and nutrient‐feedback with herbivore dung. Without nutrient‐feedback, plants compensated for defoliation in absence of irrigation but failed to compensate under irrigation. Whereas, in the presence of nutrient‐feedback plants compensated for defoliation when irrigated. This divergent pattern is not consistent with the CCH and GRH, and is only partially explained by the LRM. Instead, these pluralistic results are consistent with the hypothesis that herbivory may alter the relative strengths of water and nutrient limitation since irrigation increased root:shoot ratio in absence of fertilization in unclipped plots, but not in the corresponding clipped plots. So, herbivory appears to increase relative strength of nutrient‐limitation for plants that otherwise seem to be primarily water‐limited. Extending the LRM framework to include herbivore‐mediated transitions between water and nutrient‐limitation may clarify the underlying mechanisms that modulate herbivory‐tolerance under different environmental conditions.     

Leaf‐cutting ants,seasonal burning and nutrient distribution in Cerrado vegetation

Abstract Fire and herbivory are known to modify plant community structure. Many studies have suggested that fire ashes may increase soil nutrients in dystrophic soils. Herbivores may also change plant community structure through direct effects of herbivory and affecting nutrient cycling. Leaf‐cutting ants were traditionally viewed as herbivores, although their role may be more complex, because their nests affect both chemical and physical soil properties, thus affecting plants indirectly. We investigated the effects of frequent burning and of leaf‐cutting ants on the nutrient status of an herbaceous and a shrub species occurring in the Brazilian Cerrado, a habitat that is characterized by natural burnings. The proximity of ant nests resulted in an increase of nutrients in the leaves of both vegetation strata, whereas burning sometimes resulted in a decrease of nutrients. Our results do not lead to a possible positive effect of fire on plant nutrient content. On the other hand, ant nests may represent an important source of nutrients for plants on the nutrient‐depleted Cerrado soils and may accelerate vegetation recovery after burning.     

Relative importance of drought,soil quality,and plant species in determining the strength of plant–herbivore interactions

1. Although studies on plant–herbivore interactions comparing different plant species are common, little is known about the importance of environmental conditions in determining variation in herbivory within single plant species. 2. This study assessed the effects of experimentally manipulated nutrient and water availability on plant palatability, and compared these differences with differences among species. The extent to which these patterns can be explained by leaf toughness and specific leaf area was also investigated. Six plant species from the subfamily Carduoideae and four free‐living leaf chewing invertebrates were used in the study. 3. Herbivore preferences were significantly affected by soil nutrients and water regime and varied among plant as well as herbivore species. Generally, herbivores preferred watered plants and plants from nutrient‐poor soil. The effects of soil nutrients and water regime differed between the plant and herbivore species. The differences between the plant species were greater than those between the environmental treatments. Differences at both levels could be partly explained by leaf toughness and specific leaf area. Leaf toughness, in particular, turned to be an important predictor indicating that herbivores preferred species with softer leaves, and species from wetter conditions with reduced leaf toughness. 4. The environmental conditions in which plants are growing have significant effects on plant palatability. Between‐species comparisons thus need to pay attention to this variation. Future studies may consider how the effects of current conditions interact with conditions of plant origin to predict possible effects of changes in environmental conditions on the intensity of plant–herbivore interactions.     

Invertebrate,not small vertebrate,herbivory interacts with nutrient availability to impact tallgrass prairie community composition and forb biomass

The effects of herbivores and their interactions with nutrient availability on primary production and plant community composition in grassland systems is expected to vary with herbivore type. We examined the effects of invertebrate and small vertebrate herbivores and their interactions with nutrient availability on grassland plant community composition and aboveground biomass in a tallgrass prairie ecosystem. The abundance of forbs relative to grasses increased with invertebrate herbivore removals. This increase in forb abundance led to a shift in community composition, where invertebrate removals resulted in greater plant species evenness as well as a divergence in composition among plots. In contrast, vertebrate herbivore removals did not affect plant community composition or aboveground biomass. Nutrient additions alone resulted in a decrease in plant species richness and an increase in the abundance of the dominant grass, but the dominant grass species did not greatly increase in abundance when nutrient additions were combined with invertebrate removals. Rather, several subdominant forbs came to dominate the plant community. Additionally, the combined nutrient addition and invertebrate herbivore removal treatment increased forb biomass, suggesting that invertebrate herbivores suppress the responses of forb species to chronic nutrient additions. Overall, the release of forbs from invertebrate herbivore pressure may result in large shifts in species composition, with consequences for aboveground biomass and forage quality due to altered grass:forb ratios in grassland systems.     

Small mammal herbivores mediate the effects of soil nitrogen and invertebrate herbivores on grassland diversity

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Individual and combined effects of disturbance and N addition on understorey vegetation in a subarctic mountain birch forest

Questions: What are the effects of repeated disturbance and N‐fertilization on plant community structure in a mountain birch forest? What is the role of enhanced nutrient availability in recovery of understorey vegetation after repeated disturbance? How are responses of soil micro‐organisms to disturbance and N‐fertilization reflected in nutrient allocation patterns and recovery of understorey vegetation after disturbance? Location: Subarctic mountain birch forest, Finland. Methods: We conducted a fully factorial experiment with annual treatments of disturbance (two levels) and N‐fertilization (four levels) during 1998–2002. We monitored treatment effects on above‐ground plant biomass, plant community structure and plant and soil nutrient concentrations. Results: Both disturbance and N‐fertilization increased the relative biomass of graminoids. The increase of relative biomass of graminoids in the disturbance treatment was over twice that of the highest N‐fertilization level, and N‐fertilization further increased their relative biomass after disturbance. As repeated disturbance broke the dominance of evergreen dwarf shrubs, it resulted in a situation where deciduous species, graminoids and herbs dominated the plant community. Although relative biomass of deciduous dwarf shrubs declined with N‐fertilization, it did not cause a shift in plant community structure, as evergreen dwarf shrubs remained dominant. Both disturbance and N‐fertilization increased the N concentration in vascular plants, whereas microbial biomass N and C were not affected by the treatments. Concentrations of NH₄⁺, dissolved organic N (DON) and dissolved organic C (DOC) increased in the soil after N‐fertilization, whereas concentrations of NH₄⁺ and DON decreased after disturbance. Conclusions: Disturbances caused by e.g. humans or herbivores contribute more to changes in the understorey vegetation structure than increased levels of N in subarctic vegetation. Fertilization accelerated the recovery potential after repeated disturbance in graminoids. Microbial activities did not limit plant growth.     

Introduced ungulate herbivore alters soil processes after fire

Ungulate herbivory can have profound effects on ecosystem processes by altering organic inputs of leaves and roots as well as changing soil physical and chemical properties. These effects may be especially important when the herbivore is an introduced species. Utilizing large mammal exclosures to prevent access by introduced elk at multiple sites along a fire chronosequence, we examined the effects of elk herbivory and fire on soil microbial activity and nutrient availability. Using time since fire as a co-variate and herbivore exclosures, paired with areas outside of the exclosures, we hypothesized that reductions in plant biomass due to herbivory would reduce organic inputs to soils and impact soil microbial activities and nutrient storage. We found three major patterns: (1) when elk were excluded, surface mineral soils had higher soil organic carbon (C), total nitrogen (N), microbial N pools, and increased extra-cellular enzyme activity of a C-acquiring enzyme across a gradient of time since fire. (2) When introduced elk are present, the activity of some extracellular enzymes as well as NO₃ ⁻ availability are enhanced in the soil but the post-fire patterns described above with respect to nutrient accrual over time are delayed. (3) Herbivory by an introduced ungulate upsets the trajectory of ecosystem “recovery” after wildfire and delays soil C and N dynamics by an estimated 14.5–21 years, respectively. These results suggest that introduced, browsing herbivores significantly decelerate ecosystem processes but herbivory by exotics may also result in unpredictability in specific soil responses.     

Influence of herbivores on a perennial plant: variation with life history stage and herbivore species

Herbivores have diverse impacts on their host plants, potentially altering survival, growth, fecundity, and other aspects of plant performance. Especially for longer-lived plant species, the effects of a single herbivore species can vary markedly throughout the life of the host plant. In addition, the effects of herbivory during any given life history stage of a host plant may also vary considerably with different types of herbivores. To investigate the effects of herbivory by black-tailed deer (Odocoileus hemionus columbianus) and snails (Helminthoglypta arrosa and Helix aspersa) on a nitrogen-fixing shrub, Lupinus chamissonis, we established three exclosure experiments in a sand dune system on the coast of northern California. These experiments documented that deer browsing significantly reduced the volume and growth rate of lupines in the seedling and juvenile life stages. Since plant volume was strongly correlated with aboveground dry biomass for lupines, such herbivore-induced reductions in volume should translate into losses of aboveground biomass. Deer browsing also significantly altered the likelihood of attack by and density of a leaf-galling cecidomyid fly (Dasineura lupinorum), suggesting that a vertebrate herbivore indirectly affected an invertebrate herbivore in this system. Although deer did not significantly affect the survival of lupine seedlings and juveniles, individuals protected from deer had consistently greater survival in the two separate experiments. Our results revealed that snails did not have a significant effect on the survival or growth of juvenile plants, despite being common on and around lupines. An exclosure experiment revealed that herbivory by deer significantly reduced the shoot lengths of mature shrubs, but led only to a minimal reduction in growth rates. In addition, we found that browsed shrubs had significantly greater inflorescence production, but also produced individual seeds with significantly reduced mass. Collectively, these data indicate that deer and snails have widely differing effects on their shared host plant; browsing by deer indirectly affects insect herbivores, and the impacts of deer change markedly with the life history stage of their host plant.     

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature – herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local‐scale herbivory, and its interaction with nutrient enrichment and climate, within global‐scale models to better predict land–atmosphere interactions under future climate change.     

Long-term addition of fertilizer,labile carbon,and fungicide alters the biomass of plant functional groups in a subarctic-alpine community

In subarctic ecosystems, plant growth is mostly limited by nutrient availability and harsh climate. Investigating how soil nutrient availability controls the plant community composition may therefore help to understand indirect effects of climate change. The study was conducted in a long-term field experiment on a subarctic-alpine fellfield dominated by woody evergreen shrubs, bryophytes, and lichens. To manipulate nutrient availability additions of NPK fertilizer, labile C, and fungicide (benomyl) were done in a fully factorial design, replicated in six blocks. The treatments were run for 10 years, and the aboveground plant biomass was harvested 4 and 16 years after initiating the experiment. In addition, soil inorganic N and P concentration was analyzed the same years. Increased nutrient availability (NPK fertilizer) largely increased the biomass of graminoids and unexpectedly of bryophytes, but not of other vascular plant groups. Also, limitation of soil nutrient availability caused by labile C addition decreased the relative proportion of green shoots in evergreen shrubs, although these were expected to cope better with the nutrient limitation than the opportunistic graminoids, which, by contrast, were unaffected. Reduced fungal biomass due to benomyl addition was accompanied by increased evergreen shrub and clubmoss biomass. Taken together, the effects of treatments were most pronounced 16 years after initiation of the experiment, but despite changes in biomass the overall plant community composition was resistant to environmental changes.     

Effect of Herbivory and Plant Species Replacement on Primary Production

Grazing optimization occurs when herbivory increases primary production at low grazing intensities. In the case of simple plant-herbivore interactions, such an effect can result from recycling of a limiting nutrient. However, in more complex cases, herbivory can also lead to species replacement in plant communities, which in turn alters how primary production is affected by herbivory. Here we explore this issue using a model of a limiting nutrient cycle in an ecosystem with two plant species. We show that two major plant traits determine primary production at equilibrium: plant recycling efficiency (i.e., the fraction of the plant nutrient stock that stays within the ecosystem until it is returned to the nutrient pool in mineral form) and plant ability to deplete the soil mineral nutrient pool through consumption of this resource. In cases where sufficient time has occurred, grazing optimization requires that herbivory improve nutrient conservation in the system sufficiently. This condition sets a minimum threshold for herbivore nutrient recycling efficiency, the fraction of nutrient consumed by herbivores that is recycled within the ecosystem to the mineral nutrient pool. This threshold changes with plant community composition and herbivore preference and is, therefore, strongly affected by plant species replacement. The quantitative effects of these processes on grazing optimization are determined by both the recycling efficiencies and depletion abilities of the plant species. However, grazing optimization remains qualitatively possible even with plant species replacement.     

Seasonality, optimal foraging, and prey coexistence

Several empirical studies suggest that herbivores may promote coexistence between plants by relaxing the strength of resource competition. In contrast, recent mathematical models predict that food-limited herbivory instead cause exclusion through apparent competition, regardless of whether herbivore selectivity is constant or density dependent. This study extends existing theory to consider a strongly seasonal system. Herbivores with fixed diet preferences have the same effect regardless of seasonality, but there is a marked difference when the diet selectivity of herbivores conforms to a simple optimal-foraging model. An optimally foraging herbivore in a seasonal environment is able to promote plant coexistence among many species. The mechanism involves diet switching, occurring over narrow density intervals. For this to have an effect in a nonseasonal model, equilibrium resource densities must be in this interval, which requires close parameter fitting. In seasonal environments, resource densities change through the year and may frequently move across narrow regions in which diet changes occur. The potential of gray-sided voles to promote coexistence between two arctic dwarf shrubs is evaluated in terms of the model. For this system, it is shown that vole herbivory has the potential to reverse competitive dominance.     

Experimental evidence that the ecosystem effects of aquatic herbivory by moose and beaver may be contingent on water body type

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Depth‐based differentiation in nitrogen uptake between graminoids and shrubs in an Arctic tundra plant community

Questions

The rapid climate warming in tundra ecosystems can increase nutrient availability in the soil, which may initiate shifts in vegetation composition. The direction in which the vegetation shifts will co‐determine whether Arctic warming is mitigated or accelerated, making the understanding of successional trajectories urgent. One of the key factors influencing the competitive relationships between plant species is their access to nutrients, depending on the depth where they take up most nutrients. However, nutrient uptake at different soil depths by tundra plant species that differ in rooting depth is unclear.

Location

Kytalyk Nature Reserve, northeast Siberia, Russia.

Methods

We injected ¹⁵N to 5 cm, 15 cm and the thaw front of the soil in a moist tussock tundra. The absorption of ¹⁵N by grasses, sedges, deciduous shrubs and evergreen shrubs from the three depths was compared.

Results

The results clearly show a vertical differentiation of N uptake by these plant functional types, corresponding to their rooting strategy. Shallow‐rooting dwarf shrubs were more capable of absorbing nutrients from the upper soil than from deeper soil. Deep‐rooting grasses and sedges were more capable of absorbing nutrients from deeper soil than the dwarf shrubs. The natural ¹⁵N abundances in control plants also indicate that graminoids can absorb more nutrients from the deeper soil than dwarf shrubs.

Conclusions

Our results show that graminoids and shrubs in the Arctic differ in their N uptake strategies, with graminoids profiting from nutrients released at the thaw front, while shrubs mainly forage in upper soil layers. Our results suggest that tundra vegetation will become graminoid‐dominated as permafrost thaw progresses and nutrient availability increases in the deep soil.     

Effects of mycorrhizal symbiosis on tallgrass prairie plant–herbivore interactions

Complex relationships occur among plants, mycorrhizal fungi, and herbivores. By altering plant nutrient status, mycorrhizas may alter herbivory or plant tolerance to herbivory via compensatory regrowth. We examined these interactions by assessing grasshopper preference and plant growth and fungal colonization responses to herbivory under mycorrhizal and non‐mycorrhizal conditions within tallgrass prairie microcosms. Mycorrhizal symbiosis increased plant regrowth following defoliation, and some strongly mycotrophic plant species showed overcompensation in response to herbivory when they were mycorrhizal. Although grasshoppers spent more time on mycorrhizal plants, herbivory intensity did not differ between mycorrhizal and non‐mycorrhizal plants. Aboveground herbivory by grasshoppers significantly increased mycorrhizal fungal colonization of plant roots. Thus mycorrhizas may greatly benefit plants subjected to herbivory by stimulating compensatory growth, and herbivores, in turn, may increase the development of the symbiosis. Our results also indicate strong interspecific differences among tallgrass prairie plant species in their responses to the interaction of aboveground herbivores and mycorrhizal symbionts.     

Herbivore‐induced plant responses in Brassica oleracea prevail over effects of constitutive resistance and result in enhanced herbivore attack

1. Plant responses to herbivore attack may have community‐wide effects on the composition of the plant‐associated insect community. Thereby, plant responses to an early‐season herbivore may have profound consequences for the amount and type of future attack. 2. Here we studied the effect of early‐season herbivory by caterpillars of Pieris rapae on the composition of the insect herbivore community on domesticated Brassica oleracea plants. We compared the effect of herbivory on two cultivars that differ in the degree of susceptibility to herbivores to analyse whether induced plant responses supersede differences caused by constitutive resistance. 3. Early‐season herbivory affected the herbivore community, having contrasting effects on different herbivore species, while these effects were similar on the two cultivars. Generalist insect herbivores avoided plants that had been induced, whereas these plants were colonised preferentially by specialist herbivores belonging to both leaf‐chewing and sap‐sucking guilds. 4. Our results show that community‐wide effects of early‐season herbivory may prevail over effects of constitutive plant resistance. Induced responses triggered by prior herbivory may lead to an increase in susceptibility to the dominant specialists in the herbivorous insect community. The outcome of the balance between contrasting responses of herbivorous community members to induced plants therefore determines whether induced plant responses result in enhanced plant resistance.