Large herbivore impact on plant biomass along multiple resource gradients in the Serengeti

Herbivores form an important link in the transfer of energy within a food web and are strongly influenced by bottom-up trophic cascades. Current hypotheses suggest that herbivore consumption and impact on plants should scale positively with plant resource availability. However, depending on the effect of resources on plant quantity and quality, herbivore impact may vary with different types of resources. We test four alternative hypotheses for the relationship between plant biomass, herbivore impact on plant biomass, and plant resource gradients, each based on how resources might affect plant abundance and quality to herbivores. We measured plant biomass for four non-consecutive years in a long-term grazing exclosure experiment in the Serengeti National Park that includes seven sites that vary substantially in rainfall and soil and plant nitrogen (N) and phosphorus (P). Our data supported the hypothesis that herbivore impact is controlled by plant quality, in this case driven by plant P, as herbivore effects on biomass decreased with higher rainfall but increased with greater plant P, but not N content. To our knowledge, this is the first experimental study to indicate that wild mammalian herbivory is associated with P availability rather than N. Our results suggest that P, in addition to water and N, may play a more important role in driving trophic interactions in terrestrial systems than previously realized.

Nutrient enrichment stimulates herbivory and alters epibiont assemblages at the edge but not inside subtidal macroalgal forests

Nutrient enrichment is a major threat to subtidal macroalgal forests. Several studies have shown that nutrient inputs can enhance the ability of opportunistic algal species to acquire space freed by disturbance, at the expense of architecturally complex species that form forests. However, competition between canopy- and turf-forming macroalgae is not limited to the aftermath of disturbance. Canopy-forming macroalgae can provide suitable substratum for diverse epibiont assemblages, including both algae (epiphytes) and sessile invertebrates (epizoans). Despite evidence of enhanced epiphyte loading under eutrophic conditions, few experimental studies have assessed how nutrient enrichment influences the structure of epibiont assemblages on canopy-forming macroalgae at the edge versus inside forests. In oligotrophic waters of the NW Mediterranean, we experimentally tested the hypothesis that nutrient-driven proliferation of opportunistic epiphytic algae would affect the performance of the fucoid, Carpodesmia brachycarpa, and reduce the richness and abundance of the epizoan species they support. We predicted negative effects of nutrient enrichment to be greater at the edge than inside forests and on thalli that had recovered in cleared areas than on those within undisturbed canopy stands. Nutrient enrichment did not affect the photosynthetic efficiency and reproductive output of C. brachycarpa. By contrast, it enhanced herbivore consumption and decreased the cover and diversity of epizoans at forest edges, likely by stimulating the foraging activity of Arbacia lixula, the most abundant sea urchin in adjacent encrusting coralline barrens. Fertilization of areas inside forests had no effect on either C. brachycarpa or epibiont assemblages. Finally, nutrient enrichment effects did not vary between cleared and undisturbed areas. Our results show that moderate nutrient enrichment of oligotrophic waters does not necessarily cause the proliferation of epiphytes and, hence, a strengthening of their competitive effects on canopy-forming macroalgae. Nevertheless, enhanced herbivory damage to fertilized thalli at forest edges suggests that fragmentation could reduce the resilience of macroalgal forests and associated epibiont assemblages to nutrient enrichment.

Geographic patterns of plant–herbivore interactions are driven by soil fertility

Aims Geographic patterns of the intensity of plant herbivory in relation to climate factors have garnered little general support and appear to be species specific. However, plant–herbivore interactions are also driven by resource availability, such as soil nutrient content, and it remains unclear whether broad-scale variation in soil factors is reflected in herbivore consumption rates across species’ ranges. Additionally, we know little of how intraspecific variation in tissue quality associates with edaphic and climatic factors, and how this variation controls herbivore consumption. The resource availability hypothesis (RAH) predicts that plant individuals growing in low-resource environments will have lower leaf nutritional quality and more constitutive defenses, which will result in lower rates of leaf consumption. Methods We collected leaves from the old-field dominant species, Solidago altissima L., from 20 sites across 10 degrees of latitude in the Eastern USA to determine the percentage leaf area consumed by insect folivores. We obtained soil and climate data for each site, as well as plant functional and defensive traits, including specific leaf area (SLA), leaf carbon:nitrogen (C:N), and trichome density. Important Findings Although we found no significant latitudinal trend of leaf consumption rate, there was strong evidence that leaf herbivory decreased with leaf C:N and trichome density, which themselves decreased with soil N, supporting our hypothesis that the RAH applies for intraspecific variation across spatial gradients. Additionally, high precipitation seasonality and soil nitrogen predicted decreased herbivory. The results suggest that spatial variation in herbivory can be driven by factors other than herbivore communities and climatic gradients, and that bottom-up processes, where plant traits and soil fertility control leaf consumption, must be incorporated into spatial predictions of herbivory.

Feeding the enemy: loss of nectar and nectaries to herbivores reduces tepal damage and increases pollinator attraction in Iris bulleyana

Floral nectar usually functions as a pollinator reward, yet it may also attract herbivores. However, the effects of herbivore consumption of nectar or nectaries on pollination have rarely been tested. We investigated Iris bulleyana , an alpine plant that has showy tepals and abundant nectar, in the Hengduan Mountains of SW China. In this region, flowers are visited mainly by pollen-collecting pollinators and nectarivorous herbivores. We tested the hypothesis that, in I. bulleyana , sacrificing nectar and nectaries to herbivores protects tepals and thus enhances pollinator attraction. We compared rates of pollination and herbivory on different floral tissues in plants with flowers protected from nectar and nectary consumption with rates in unprotected control plants. We found that nectar and nectaries suffered more herbivore damage than did tepals in natural conditions. However, the amount of tepal damage was significantly greater in the flowers with protected nectaries than in the controls; this resulted in significant differences in pollinator visitation rates. These results provide the first evidence that floral nectar and nectaries may be ‘sacrificed’ to herbivores, leading to reduced damage to other floral tissues that are more important for reproduction.

Variable responses of folivorous sawflies to leaf quality of mountain birch

We studied whether the larval performance of four sawfly species feeding on mountain birch (Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti) leaves at different times of the growing season was related to the same chemical and physical leaf traits. In the three phenologically earliest species, larval growth was related to the same set of leaf traits. In contrast, consumption of leaf material was related to different leaf traits in all of the four species. Leaf suitability for herbivore growth and tree resistance to herbivore consumption were thus not always determined by the same leaf traits. Leaf water content and toughness had the strongest associations with larval performance. Phenolics, which are often considered the main resistance compounds in deciduous trees, explained only a small part of the variance in leaf consumption, but some groups of phenolics were negatively related to larval growth. In conclusion, the same mountain birch trees were not resistant to all of the studied sawflies in terms of leaf consumption, but low leaf suitability, which was related to the same set of leaf traits in the case of three sawfly species, may serve birch resistance, e.g., by exposing larvae to natural enemies for a longer period.

I.B.P. studies on montane grassland and moorlands

The aim of the grassland and moorland studies was to measure primary and secondary production and to describe the main pathways of dry matter and nutrients within these ecosystems. The strategy was to make detailed studies on two main sites (Snowdonia and Moor House N.N.R. in the northern Pennines) with a limited number of supporting studies. The examination of the few sites in the U.K. must be seen as part of a series of sites within the International Grassland and Tundra Biomes. They are thus replicates and the series allows examination of trends in productivity related to environmental conditions. The Bi-Polar Botanical Project, with sites in Greenland and South Georgia, supported by the U.K., is part of the international series. The Snowdonia project covered a range of sites but concentrated on a sheep-grazed Agrostis-Festuca sward at 460 m. At Moor House attention focused on blanket bog sites dominated by Calluna, Eriophorum and Sphagnum at about 600 m with supporting studies on dwarf shrub communities ranging from 1100 m in the Cairngorm Mountains to 60 m in Dorset. Comparisons are made of three estimates of primary productivity, of herbivore consumption with production, and decomposer populations with process rates. These results are briefly reviewed in the context of the international range of sites; they allow us to distinguish broad patterns of ecosystem functioning.