Different Levels Of Herbivory Research Articles

New applications for point intercept methods: Replacing manual sorting of current-year herbaceous biomass

Productivity is a primary indicator when assessing plant communities or their responses to various stimuli, but it can be difficult and costly to determine. Adding to this complication is the retention of standing litter from previous years’ growth intermingled with current-year biomass. The most accurate and commonly applied method for estimating productivity is to clip and manually sort vegetation to separate current-year and previous years’ growth. Because sorting is costly, it is often only done for subsamples or not done at all. Without accounting for previous years’ growth, productivity estimates will be biased upward for conditions that accumulate more standing dead material, which is problematic when comparing treatments such as fire or different levels of herbivory. We tested whether line-point intercept methods could be adapted to determine proportions of current-year and older vegetation and estimate current-year biomass directly or by calculating the product of percent current-year intercepts and total standing crop. Data were collected by seven observers across four years and five experiments in northern mixed prairie using a total of 306 plots. Point-intercept transects with 20-cm spacing between points were used to determine number of intercepts for current-year and older vegetation and compared with data derived by clipping, manually sorting and weighing herbaceous vegetation. Regression analyses were used to test relationships between 1) total standing crop and total point intercepts; 2) current-year biomass and current-year point intercepts; 3) percent current-year biomass based on manual sorting versus point intercept methods; and 4) sorted current-year biomass and point intercept-derived current-year biomass. Number of intercepts per 100 points was positively related to total standing crop and current-year biomass, but r2 values were only 0.49 and 0.53. The relationship between manually-sorted current-year biomass and that derived from the product of point-intercept percentages and total standing crop was positive, explained 96 % of the variation, and had a mean absolute error of 122 kg ha−1. Point-intercept sampling is more efficient than manual sorting and can provide more indicators, including cover and diversity. Data indicate point intercept methods are accurate and can replace manual sorting of current-year herbaceous biomass.

Priority effects in coral-macroalgae interactions can drive alternate community paths in the absence of top-down control.

The outcomes of species interactions can vary greatly in time and space with the outcomes of some interactions determined by priority effects. On coral reefs, benthic algae rapidly colonize disturbed substrate. In the absence of top-down control from herbivorous fishes, these algae can inhibit the recruitment of reef-building corals, leading to a persistent phase shift to a macroalgae-dominated state. Yet, corals may also inhibit colonization by macroalgae, and therefore the effects of herbivores on algal communities may be strongest following disturbances that reduce coral cover. Here, we report the results from experiments conducted on the fore reef of Moorea, French Polynesia, where we: (1) tested the ability of macroalgae to invade coral-dominated and coral-depauperate communities under different levels of herbivory, (2) explored the ability of juvenile corals (Pocillopora spp.) to suppress macroalgae, and (3) quantified the direct and indirect effects of fish herbivores and corallivores on juvenile corals. We found that macroalgae proliferated when herbivory was low but only in recently disturbed communities where coral cover was also low. When coral cover was <10%, macroalgae increased 20-fold within 1 year under reduced herbivory conditions relative to high herbivory controls. Yet, when coral cover was high (50%), macroalgae were suppressed irrespective of the level of herbivory despite ample space for algal colonization. Once established in communities with low herbivory and low coral cover, macroalgae suppressed recruitment of coral larvae, reducing the capacity for coral replenishment. However, when we experimentally established small juvenile corals (2cm diameter) following a disturbance, juvenile corals inhibited macroalgae from invading local neighborhoods, even in the absence of herbivores, indicating a strong priority effect in macroalgae-coral interactions. Surprisingly, fishes that initially facilitated coral recruitment by controlling algae had a net negative effect on juvenile corals via predation. Corallivores reduced the growth rates of corals exposed to fishes by ~30% relative to fish exclosures, despite increased competition with macroalgae within the exclosures. These results highlight that different processes are important for structuring coral reef ecosystems at different successional stages and underscore the need to consider multiple ecological processes and historical contingencies to predict coral community dynamics.

Herbivory modulates soil CO2 fluxes after windthrow: a case study in temperate mountain forests

Ungulate herbivory can alter functional plant communities of early-successional forest ecosystems. The consequences of such vegetation changes on soil carbon cycling are still not fully understood. Here, we used an ungulate exclusion experiment to investigate how different levels of herbivory and associated changes in vegetation succession modulate soil CO2 efflux and its heterotrophic and autotrophic sources following windthrow in temperate mountain forests. Our results indicate that only high levels of ungulate herbivory and associated vegetation shifts from tree to rather grass dominated plant communities affect soil CO2 fluxes. We did not find evidence that a moderate herbivory level and accompanied smaller shifts in the functional plant community affect soil CO2 fluxes. A greater soil CO2 efflux under the influence of high herbivory pressure was primarily attributed to accelerated heterotrophic respiration, likely due to warmer soil conditions. Moreover, autotrophic respiration from grass roots and associated microbial communities is suggested to contribute to higher soil CO2 fluxes. We conclude that intense herbivory and accompanied successional changes in the functional plant community enhance soil carbon losses following forest windthrow. This might have negative consequences for the soil carbon stocks and for the climate system.

Herbivore removal reduces influence ofarbuscular mycorrhizal fungi onplant growth and tolerance in an East African savanna.

The functional relationship between arbuscular mycorrhizal fungi (AMF) and their hosts is variable on small spatial scales. Here, we hypothesized that herbivore exclusion changes the AMF community and alters the ability of AMF to enhance plant tolerance to grazing. We grew the perennial bunchgrass, Themeda triandra Forssk in inoculum from soils collected in the Kenya Long-term Exclosure Experiment where treatments representing different levels of herbivory have been in place since 1995. We assessed AMF diversity in the field, using terminal restriction fragment length polymorphism and compared fungal diversity among treatments. We conducted clipping experiments in the greenhouse and field and assessed regrowth. Plants inoculated with AMF from areas accessed by wild herbivores and cattle had greater biomass than non-inoculated controls, while plants inoculated with AMF from where large herbivores were excluded did not benefit from AMF in terms of biomass production. However, only the inoculation with AMF from areas with wild herbivores and no cattle had a positive effect on regrowth, relative to clipped plants grown without AMF. Similarly, in the field, regrowth of plants after clipping in areas with only native herbivores was higher than other treatments. Functional differences in AMF were evident despite little difference in AMF species richness or community composition. Our findings suggest that differences in large herbivore communities over nearly two decades has resulted in localized, functional changes in AMF communities. Our results add to the accumulating evidence that mycorrhizae are locally adapted and that functional differences can evolve within small geographical areas.

Selection for monoculture and mixture genotypes in a biodiversity experiment

Studies in experimental grasslands have shown variation in plant individual performance in response to neighbourhood diversity. To which extent these responses are due to phenotypic plasticity or genetic variation is largely unknown. We collected seed families of five herbaceous species ( Cirsium oleraceum, Crepis biennis, Plantago lanceolata, Plantago media and Rumex acetosa) in monocultures and 60-species mixtures five years after establishment and replanted or transplanted the offspring into the same monocultures and 60-species mixtures. In all five species the actual environment significantly affected plant survival, growth and performance in terms of shoot biomass and investment into reproduction, indicating stronger competition for light and different levels of herbivory in mixtures as compared with monocultures. Effects of the original environment were smaller and less consistent, but indicated differential selection in monocultures vs. mixtures. The interaction between actual and original environment, corresponding to the “home” vs. “away” comparison, was rarely significant, yet this was providing a first sign of local adaptation. We conclude that, for the investigated plant species, more than five growing seasons in monocultures or mixtures would be needed to better demonstrate the selection of genotypes specifically adapted to monocultures or mixtures. A faster local adaptation may have been prevented by the ability of these species to respond to variation in neighbourhood diversity to a large degree via phenotypic plasticity and other factors.

Lower resistance and higher tolerance of invasive host plants: biocontrol agents reach high densities but exert weak control

Invasive plants often have novel biotic interactions in their introduced ranges. Their defense to herbivory may differ from their native counterparts, potentially influencing the effectiveness of biological control. If invasive plants have decreased resistance but increased tolerance to enemies, insect herbivores may rapidly build up their populations but exert weak control. Moreover, resource availability to plants may affect the efficacy of biological control agents. We tested these predictions using Chinese tallow tree (Triadica sebifera) and two specialist herbivores (Heterapoderopsis bicallosicollis and Gadirtha inexacta) that are candidates for biological control. We performed a pair of field common garden experiments in China in which Triadica seedlings from the native or introduced range were grown in low or high light conditions and subjected to different levels of herbivory by each herbivore in a factorial design. We found that Heterapoderopsis achieved greater densities on tallow trees from the introduced range or when trees were grown in high light conditions. When Gadirtha was raised in the lab on tallow tree foliage we found that it performed better (larger pupal size) when fed foliage from introduced populations. However, introduced populations generally had greater herbivore tolerance such that the impact of each agent on plant performance was lower than on native populations despite higher herbivore loads. Tallow trees grew more slowly and achieved smaller sizes in lower light levels, but the impact of biological control agents was comparable to that found for higher light levels. Plants from introduced populations grew larger than those from native populations in all conditions. Our results suggest that reduced resistance and increased tolerance to herbivory in introduced populations may impede success of biological control programs. Biological control practitioners should include plants from the introduced range in the prerelease evaluation, which will help predict insect impact on target weeds.

Silencing jasmonate signalling and jasmonate‐mediated defences reveals different survival strategies between two Nicotiana attenuata accessions

To determine the impact of genotypic variation in secondary metabolite production on antiherbivore resistance and plant fitness, we genetically silenced biosynthetic genes for nicotine, trypsin proteinase inhibitors (TPI), and jasmonate (JA) production in two accessions of Nicotiana attenuata: one from Utah (UT) which responds to herbivory with JA-induced nicotine and TPI production, and one from Arizona (AZ) which is TPI-deficient but also produces JA-induced nicotine. Transient silencing of JA biosynthesis increased Manduca sexta larval growth on wild type (WT) plants of both accessions, but not on TPI-deficient UT or nicotine-deficient AZ lines, demonstrating that JA-mediated resistance to M. sexta requires TPIs in the UT and nicotine in the naturally TPI-deficient AZ accession. When transplanted into a native UT population, AZ and UT plants, rendered equally able or unable to produce nicotine and TPIs by stable transformation, received significantly different levels of herbivory. Both accessions differed in their resistance depending on the type of herbivores: resistance to rare, voracious herbivores (Saltatoria and Mammalia) was greater in AZ than UT lines, and dependent on nicotine production, while resistance to small, abundant herbivores (Coleoptera and Thysanoptera) was greater in UT lines, and dependent on TPI production. AZ lines produced more flowers and seed capsules than UT lines independently of TPI production costs. This fitness advantage was lost when accessions did not produce nicotine. We conclude that these two accessions have developed different survival strategies and thus differ in the cost-benefit functions of their JA-mediated defences.

Leaf compensatory growth as a tolerance strategy to resist herbivory in Pancratium sickenbergeri

We examined the effects of leaf herbivory by the dorcas gazelle, Gazella dorcas, on the compensatory growth of the geophyte Pancratium sickenbergeri (Amaryllidaceae) in the Negev desert, Israel. In three populations exposed to different levels of herbivory, we removed different amounts of photosynthetic leaf area from plants in five clipping treatments: 0, 25, 50%-dispersed over all leaves, 50%-entire area of half the leaves, 100%. The population with the lowest level of herbivory showed the lowest relative regrowth rate after clipping. In the population with a constantly high level of herbivory, plants in intermediate-clipping treatments overcompensated in leaf area after clipping. For all the populations, clipped plants produce more new leaves than unclipped plants. In the population with the highest level of herbivory, clipping treatments did not have a significant effect on the number of fruits per plant. In addition, we did not find a trade-off between investments in growth and reproduction in this population. Our results indicated that, in the desert lily, herbivores may select for plant mechanisms that compensate after damage as a tolerant strategy to maintain fitness.

Population Differentiation and the Effects of Herbivory and Sand Compaction on the Subterranean Growth of a Desert Lily

Differences in level of herbivory can select for local adaptation and genetic differentiation of plant populations in different environments. Mean bulb depth of the desert lily Pancratium sickenbergeri, differs considerably among populations differing in the level of herbivory by the dorcas gazelle. The gazelle digs in the sand to remove most of the bulb of the lily. Deeper bulbs have less material removed by herbivory than shallow bulbs and have higher fitness. A possible confounding factor is the degree of sand compaction, which may retard the downward growth of the bulb. We conducted a common garden experiment with 2 sand types with seeds from source populations with different levels of herbivory. There was a large genetic difference among populations. Two of 3 analyses indicated that there was an interaction between population and sand type, indicating that there is a heritable component of plasticity.

The effects of herbivory and resource variability on the production of a second inflorescence by the desert lily, Pancratium sickenbergeri

We investigated the effects of herbivory by the dorcas gazelle, Gazella dorcas, on the production of a second inflorescence in a desert lily, Pancratium sickenbergeri (Amaryllidaceae), after the first inflorescence was eaten. In three populations exposed to different levels of herbivory, we conducted an inflorescence-clipping experiment in the flowering seasons of 1997, 1998 and 1999. The experiment was performed on inflorescences at both the emerging stage and the anthesis stage. Plants in each control group were undamaged. Clipped plants had a greater probability of producing a second inflorescence stalk than unclipped plants. In all the populations, the probability of producing a second inflorescence was greater when the first inflorescence was cut in the emerging stage than when it was cut in anthesis, indicating that there was a cost of compensation. In the population with the highest level of herbivory, the production of a second inflorescence stalk was related to resource availability in all 3 years. Plants produced as many flowers or fruits on the second stalk as on the first, indicating that potential reproductive output is double that usually achieved. Although we found no evidence for a trade-off between investment in first and second inflorescence stalks, the lower probability of producing a second inflorescence if heavy investment had been made in the first inflorescence indicates that compensation is costly. These results contradict the mutualism hypothesis that herbivory is beneficial to plants and provide support for the trade-off hypothesis that benefits of herbivory are proximal and are attained at an evolutionary cost.

Geographic Variation in Alkaloid Production in Conium maculatum Populations Experiencing Differential Herbivory by Agonopterix alstroemeriana

Conium maculatum, a Eurasian weed naturalized in North America, contains high concentrations of piperidine alkaloids that act as chemical defenses against herbivores. C. maculatum was largely free from herbivory in the United States, until approximately 30 yr ago, when it was reassociated via accidental introduction with a monophagous European herbivore, the oecophorid caterpillar Agonopterix alstroemeriana. At present, A. alstroemeriana is found in a continuum of reassociation time and intensities with C. maculatum across the continent; in the Pacific Northwest, A. alstroemeriana can cause severe damage, resulting in some cases in complete defoliation. Studies in biological control and invasion biology have yet to determine whether plants reassociated with a significant herbivore from the area of indigeneity increase their chemical defense investment in areas of introduction. In this study, we compared three locations in the United States (New York, Washington, and Illinois) where C. maculatum experiences different levels of herbivory by A. alstroemeriana to determine the association between the intensity of the interaction, as measured by damage, and chemical defense production. Total alkaloid production in C. maculatum was positively correlated with A. alstroemeriana herbivory levels: plants from New York and Washington, with higher herbivory levels, invested two and four times more N to alkaloid synthesis than did plants from Illinois. Individual plants with lower concentrations of alkaloids from a single location in Illinois experienced more damage by A. alstroemeriana, indicative of a preference on the part of the insect for plants with less chemical defense. These results suggest that A. alstroemeriana may act either as a selective agent or inducing agent for C. maculatum and increase its toxicity in its introduced range.

Plant Architecture and Leaf Damage in Bear Oak. I: Physiological Responses

Bear Oak (Quercus ilicifolia Wang.) presents a unique architecture in which there is a distinct outer set of leaves on the perimeter of the bush and another set of leaves on the inside. In this paper we examine the relationship between plant architecture, leaf-level physiology, and leaf damage patterns by insects. We measured net photosynthesis, stomatal conductance, and leaf water potential in response to different levels of herbivory or leaf-rolling in both outer- and inner-canopy leaves. Leaf-rolling occurred naturally and was mimicked by hand-rolling leaves to different degrees. Net photosynthesis, stomatal conductance, and leaf water potential varied depending upon a combination of leaf position in the crown and degree of rolling, but not herbivory. Outer-canopy leaves had significantly higher rates of net photosynthesis and stomatal conductance, but lower leaf water potentials and leaf areas than inner-canopy leaves. Leaf rolling had an effect upon photosynthesis and stomatal conductance, which varied depending on whether projected areas of rolled leaves or projected areas of the leaves after they had been unrolled were considered. In both canopy layers, using unrolled projected leaf areas, stomatal conductance and net photosynthesis decreased with increasing leaf rolling. In the outer canopy, using projected areas of rolled leaves, photosynthesis decreased while stomatal conductance increased with increasing leaf rolling. In the inner canopy, photosynthesis increased while stomatal conductance was unchanged with increasing leaf rolling. In all cases, leaf water potential increased with increasing leaf rolling. These results suggest (in order of likelihood) that either rolling creates an area of closed stomata (stomatal patchiness) proportional to the amount of rolling, or leaf specific hydraulic conductance increases, or stomata are responding to changes in the boundary layer of the leaf.

Calcium oxalate crystals in leaves of Pancratiumsickenbergeri: constitutive or induced defence?

Summary We investigated the genotypic effects of selection by herbivory by the dorcas gazelle, Gazella dorcas Linnaeus 1758, on the production of calcium oxalate crystals in the lily Pancratium sickenbergeri C. et Barbey in the Negev Desert. A completely crossed experimental design was used to assess the effects of soil calcium concentration and herbivory (simulated by clipping) on the production of calcium oxalate crystals in the leaves of plants from four source populations with different levels of herbivory. Soil calcium concentration did not have significant effects on the production of calcium oxalate crystals in the leaf. The number of crystals did not increase significantly with the clipping treatments. There was no significant difference among plants germinated from seed from populations with high or low herbivory in the production of calcium oxalate. There was a trade‐off between investment in defence versus investment in growth, suggesting a cost to defence. Experimentally clipped plants grew more than unclipped plants. Families from high‐herbivory areas had a stronger regrowth capacity in the absence of herbivory. Our results indicate that crystal production by these plants is not a sink for excess calcium uptake from the soil. We conclude that in this species calcium oxalate crystal production is for defence, and that this defence is constitutive rather than inducible. The environmental unpredictability in this desert leaves plants with few cues to the onset of herbivory. Constitutive defence may be a more appropriate strategy than inducible defence in this environment, particularly when herbivory is high, and leaves are ephemeral.

Effects of water-column mixing on bacteria, phytoplankton, and rotifers under different levels of herbivory in a shallow eutrophic lake.

Water-column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, nutrient status and the plankton community structure, and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water-column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water-column mixing that provoked a substantial nutrient input into the euphotic zone led to a strong net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones because of the greater contribution of a few fast-growing species. After the second mixing event, at a high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower than after the first mixing, and diversity remained unchanged because enhanced growth of small fast-growing phytoplankton was prevented by zooplankton grazing. Bacterial abundance did not increase after the second mixing, when cladoceran biomass was high. Changes in rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our results suggest that water-column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the plankton community via enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the basis of the classic and microbial food chain via enhanced phytoplankton and bacterial growth, but the effects on biomass may be damped by high levels of herbivory.