The role of flow velocity combined with habitat complexity as a top–down regulator in seagrass meadows

Large‐scale losses of seagrass areas have been associated with eutrophication events, which have led to an overproduction of photosynthetic organisms including epiphytes. Grazers that feed on epiphytes can exert a significant top–down control in the system, but the effects of physical factors on grazing activity and feeding behaviour have been rarely examined. We addressed the combination of hydrodynamic regime and seagrass shoot density can alter the feeding and foraging behaviours of mesograzers. A full factorial experiment, with flow velocity (high, medium and low) and shoot density (high versus low) as main factors, was conducted in a racetrack flume using artificial seagrass plots. The results showed that when high flow velocity conditions were combined with low shoot density, consumption of epiphytes by mesograzers was strongly reduced. In contrast, when flow velocity was low or shoot density was high, mesograzers exhibited high feeding rates and vigorous swimming behaviour. These results clearly indicate that hydrodynamic stress reduces the time that mesograzers can spend feeding, since it inhibits their swimming behaviour, and thus indirectly affecting to the density of epiphytes. Therefore, the triggering of trophic cascade effects in seagrass communities under these experimental conditions depended on the interrelationship and feedbacks among shoot density, abiotic (flow velocity) and biotic (epiphytes and mesograzers) compartments, with flow velocity exerting a top–down control on seagrass ecosystems.     

Aquatic grazers reduce the establishment and growth of riparian plants along an environmental gradient

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Regrowth capacity in relation to defence strategy in Stipa clarazii and Stipa trichotoma,native to semiarid Argentina

Abstract Plants can defend themselves against herbivores by either avoiding or tolerating herbivory. Since avoidance mechanisms divert resources to other than growth processes, it could be expected a lower regrowth capacity in species that avoid herbivory than in species that tolerate herbivory, particularly under competition for resources. We tested this hypothesis by quantifying the regrowth of a grazing‐avoidant (Stipa trichotoma Nees, synonymous Nasella trichotoma (Nees) Hackel ex Arechav.) and a grazing‐tolerant (Stipa clarazii Ball, synonymous Nasella clarazii (Ball) Barkworth) grass species native to semiarid Argentina, when growing either singly or in pairs (one individual of each species) under repeated defoliation in field conditions. Twenty pairs of plants were selected in an area codominated by both species. Plants were protected by a 60 cm‐diameter exclosure, in which the rest of the vegetation had been removed. The same procedure was followed with 20 single plants of each species. The same plants were clipped four times in 1996 and 1998 and five times in 1997. The response variable was the cumulative regrowth per plant at the end of each year. The regrowth was analysed for neutral detergent fibre and nitrogen content. Cumulative regrowth of S. clarazii was higher, similar, or lower than the cumulative regrowth of S. trichotoma in 1996, 1997, and 1998, respectively. Regrowth was reduced by interspecific competition, although there was no interaction between species and growing conditions (single or in pairs). Neutral detergent fibre content was consistently higher, whereas nitrogen content was consistently lower, in S. trichotoma than in S. clarazii. Collectively, our results did not support the hypothesis of higher regrowth capacity in the grazing‐tolerant species than in the grazing‐avoidant species. Further, our findings suggest herbivore tolerance is a previously unappreciated trait of S. trichotoma.     

Competition and facilitation structure plant communities under nurse tree canopies in extremely stressful environments

Nurse plant facilitation in stressful environments can produce an environment with relatively low stress under its canopy. These nurse plants may produce the conditions promoting intense competition between coexisting species under the canopy, and canopies may establish stress gradients, where stress increases toward the edge of the canopy. Competition and facilitation on these stress gradients may control species distributions in the communities under canopies. We tested the following predictions: (1) interactions between understory species shift from competition to facilitation in habitats experiencing increasing stress from the center to the edge of canopy of a nurse plant, and (2) species distributions in understory communities are controlled by competitive interactions at the center of canopy, and facilitation at the edge of the canopy. We tested these predictions using a neighbor removal experiment under nurse trees growing in arid environments. Established individuals of each of four of the most common herbaceous species in the understory were used in the experiment. Two species were more frequent in the center of the canopy, and two species were more frequent at the edge of the canopy. Established individuals of each species were subjected to neighbor removal or control treatments in both canopy center and edge habitats. We found a shift from competitive to facilitative interactions from the center to the edge of the canopy. The shift in the effect of neighbors on the target species can help to explain species distributions in these canopies. Canopy‐dominant species only perform well in the presence of neighbors in the edge microhabitat. Competition from canopy‐dominant species can also limit the performance of edge‐dominant species in the canopy microhabitat. The shift from competition to facilitation under nurse plant canopies can structure the understory communities in extremely stressful environments.     

Stability of grassland production is robust to changes in the consumer food web

Theory predicts that consumers may stabilise or destabilise plant production depending on model assumptions, and tests in aquatic food webs suggest that trophic interactions are stabilising. We quantified the effects of trophic interactions on temporal variability (standard deviation) and temporal stability (mean/standard deviation) of grassland biomass production and the plant diversity–stability relationship by experimentally removing heterotrophs (large vertebrates, arthropods, foliar and soil fungi) from naturally and experimentally assembled grasslands of varying diversity. In both grassland types, trophic interactions proportionately decreased plant community biomass mean and variability over the course of 6 years, leading to no net change in temporal stability or the plant diversity–stability relationship. Heterotrophs also mediated plant coexistence; their removal reduced diversity in naturally assembled grasslands. Thus, herbivores and fungi reduce biomass production, concurrently reducing the temporal variability of energy and material fluxes. Because of this coupling, grassland stability is robust to large food web perturbations.     

Relatedness with plant species in native community influences ecological consequences of range expansions

Global warming is enabling many plant species to expand their range to higher latitudes and altitudes, where they may suffer less from natural aboveground and belowground enemies. Reduced control by natural enemies can enable climate warming‐induced range expanders to gain an advantage in competition with natives and become disproportionally abundant in their new range. However, so far studies have only examined individual growth of range expanders, which have common congeneric plant species in their new range. Thus it is not known how general is this reduced effect of above‐ and belowground enemies and how it operates in communities, where multiple plant species also interact with each other. Here we show that range‐expanding plant species with and without congenerics in the invaded habitats differ in their ecological interactions in the new range. In a community‐level experiment, range‐expanding plant species, both with and without congenerics, suppressed the growth of a herbivore. However, only range expanders without congenerics reduced biomass production of the native plant species. In the present study, range expanders without congenerics allocated more biomass aboveground compared to native plant species, which can explain their competitive advantage. Competitive interaction and also biomass allocation of native plants and their congeneric range expanders were similar. Our results highlight that information about species phylogenetic relatedness with native flora can be crucial for improving predictions about the consequences of climate warming‐induced range expansions.     

Spatial variation in the demography and population dynamics of a perennial shrub (Atriplex vesicaria) under sheep grazing in semi‐arid Australian rangelands

Atriplex vesicaria Heward ex Benth. (Chenopodiaceae) is a widespread perennial shrub in southern Australia's chenopod rangelands but is sensitive to grazing. A detailed investigation of the demography and population dynamics of A. vesicaria under sheep grazing was conducted over 6.5 years at a range of sites across a typical paddock to assess the long‐term effects of grazing on the species and elucidate the mechanisms of population change under grazing. The effects of rainfall on recruitment and mortality were also examined. Six‐monthly censuses of all A. vesicaria individuals were conducted in permanent grazed and ungrazed plots at sites located across an 1100‐ha paddock. Grazing increased adult shrub mortality close to water and reduced recruitment over a broader area of the paddock, but seedling survival did not appear to be affected by grazing. As a result of these changes, the population declined on grazed plots up to 2200 m from water during the study, but the decline was greatest closer to water. The population was most dynamic at the sites furthest from the water point where it was unaffected by grazing because of the greater recruitment and mortality of young plants, but because these processes balanced out over time, population density was effectively unchanged by the end of the study. Although statistical models indicated that six‐monthly rainfall did not explain temporal variation in recruitment or mortality, rainfall nevertheless has a central role in both processes. In particular, longer periods of favourable rainfall and drought appear to have an important influence on recruitment and mortality, respectively, with heavy grazing during a drought period increasing mortality. Occasional shortages of seed or rains occurring during the warmer months when seed germination is limited possibly explain poor recruitment at sites unaffected by grazing following good rainfall.     

低丘红壤复合农林系统光能竞争与生产力——以花生南酸枣间作为例

复合农林业 ,尤其是农林间作被设想为防治土壤退化 ,增进土壤肥力 ,提高农作物产量的策略[16～ 18] 。如何利用复合农林技术防治红壤退化 ,提高红壤生产力已成为红壤地区的研究热点[1～ 8] 。然而 ,生产者最关心的往往是农林间作中农作物的产量 ,在以往的农林间作技术研究中 ,有增产的报道[13] ,而更多的研究表明复合农林种植会造成农作物减产[3 ,4 ,6,10 ,12 ,15,19,2 0 ] 。农作物减产的原因可能是树木和农作物之间对光能、水分和养分等资源竞争作用所引起 ,更可能是其综合作用的结果[14 ] 。本文以前人推荐的花生 (Ａｒａｃｈｉｓｈｙｐ…     

Root volatiles in plant–plant interactions II: Root volatiles alter root chemistry and plant–herbivore interactions of neighbouring plants

Volatile organic compounds (VOCs) emitted by plant roots can influence the germination and growth of neighbouring plants. However, little is known about the effects of root VOCs on plant–herbivore interactions of neighbouring plants. The spotted knapweed (Centaurea stoebe) constitutively releases high amounts of sesquiterpenes into the rhizosphere. Here, we examine the impact of C. stoebe root VOCs on the primary and secondary metabolites of sympatric Taraxacum officinale plants and the resulting plant‐mediated effects on a generalist root herbivore, the white grub Melolontha melolontha. We show that exposure of T. officinale to C.stoebe root VOCs does not affect the accumulation of defensive secondary metabolites but modulates carbohydrate and total protein levels in T. officinale roots. Furthermore, VOC exposure increases M. melolontha growth on T. officinale plants. Exposure of T. officinale to a major C. stoebe root VOC, the sesquiterpene (E)‐β‐caryophyllene, partially mimics the effect of the full root VOC blend on M. melolontha growth. Thus, releasing root VOCs can modify plant–herbivore interactions of neighbouring plants. The release of VOCs to increase the susceptibility of other plants may be a form of plant offense.     

Competition and coexistence in plant communities: intraspecific competition is stronger than interspecific competition

Theory predicts that intraspecific competition should be stronger than interspecific competition for any pair of stably coexisting species, yet previous literature reviews found little support for this pattern. We screened over 5400 publications and identified 39 studies that quantified phenomenological intraspecific and interspecific interactions in terrestrial plant communities. Of the 67% of species pairs in which both intra‐ and interspecific effects were negative (competitive), intraspecific competition was, on average, four to five‐fold stronger than interspecific competition. Of the remaining pairs, 93% featured intraspecific competition and interspecific facilitation, a situation that stabilises coexistence. The difference between intra‐ and interspecific effects tended to be larger in observational than experimental data sets, in field than greenhouse studies, and in studies that quantified population growth over the full life cycle rather than single fitness components. Our results imply that processes promoting stable coexistence at local scales are common and consequential across terrestrial plant communities.     

Seasonal contrasts in carbon resources and ecological processes on a tropical floodplain

1. Globally, tropical floodplains are highly productive ecosystems. This is largely because of predictable seasonal rains providing replenishing floodwaters that stimulate nutrient turnover which, in turn, substantially boosts both primary and secondary productivity. This is associated with concomitant shifts in the types of primary producers and associated food webs. 2. The Magela Creek floodplain on Kakadu National Park in northern Australia is one of the most studied tropical freshwater ecosystems in Australia and provides an opportunity to collate and examine information on organic carbon sources and pathways through food webs to gain a fundamental understanding of how these systems may function. 3. We reviewed biophysical information published since the early 1980s to construct an assessment of the carbon resources for the channel and floodplain. 4. We conclude that macrophytes, largely in the form of grasses and aquatic plants, produce the greatest above‐ground biomass on the Magela Creek floodplain. Although macrophytes provide suitable substrata for the attachment of epiphytes, they do not appear to be an important carbon source for aquatic consumers themselves. Nevertheless, macrophytes do provide critical seasonal food and habitat structure for other producers and consumers on the floodplain, such as the abundant magpie geese. 5. We developed a generalised conceptual food web and carbon budget contrasting the ‘wet’ and ‘dry’ seasons for the Magela Creek system, as a representative of tropical seasonal floodplain systems. 6. Our conceptual model of tropical floodplains indicates that knowledge of the seasonal and spatial links and exchanges between the floodplain and the river is critical in understanding ecosystem function.     

Mesophyll cell‐sucking herbivores (Cicadellidae: Typhlocybinae) on rainforest trees in Papua New Guinea: local and regional diversity of a taxonomically unexplored guild

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The tri‐trophic niche concept and adaptive radiation of phytophagous insects

A conceptual divide exists between ecological and evolutionary approaches to understanding adaptive radiation, although the phenomenon is inherently both ecological and evolutionary. This divide is evident in studies of phytophagous insects, a highly diverse group that has been frequently investigated with the implicit or explicit goal of understanding its diversity. Whereas ecological studies of phytophagous insects increasingly recognize the importance of tri‐trophic interactions as determinants of niche dimensions such as host‐plant associations, evolutionary studies typically neglect the third trophic level. Here we attempt to reconcile ecological and evolutionary approaches through the concept of the ecological niche. We specifically present a tri‐trophic niche concept as a foil to the traditional bi‐trophic niche concept for phytophagous insects. We argue that these niche concepts have different implications for understanding herbivore community structure, population divergence, and evolutionary diversification. To this end, we offer contrasting empirical predictions of bi‐ and tri‐trophic niche concepts for patterns of community structure, the process of population divergence, and patterns of evolutionary diversification of phytophagous insects.     

Emerging aquatic insects as predators in terrestrial systems across a gradient of stream temperature in North and South America

1. Empirical and theoretical research over the past decade has demonstrated the widespread importance of aquatic subsidies to terrestrial food webs. In particular, adult aquatic insects that emerge from streams and lakes are prey for terrestrial predators. While variation in the magnitude of this subsidy is clearly important, the potential top‐down effects of the predatory adults of some aquatic insects in terrestrial food webs are largely unknown. 2. I used published data on benthic insect density (as a proxy for emergence) in North and South America to explore how the proportion of benthic insects that are predatory as adults varies across a gradient of mean annual stream temperature. 3. The proportion of benthic insects that are predatory as adults varied widely across sites (0–12% by abundance; 0–86% by biomass). There was a positive relationship between mean annual stream temperature and the proportion of predatory adults across all sites, driven largely by the greater abundance/biomass of predatory taxa (e.g. odonates), relative to non‐predators (e.g. midges, mayflies, caddisflies), in tropical than in temperate streams. 4. The ‘trophic structure’ (i.e. the proportion of predators) of emerging adult aquatic insects is an understudied source of variation in aquatic–terrestrial interactions. Incorporation of trophic structure in future studies is needed to understand how future modification of fresh waters may affect adjacent terrestrial food webs through both bottom‐up and top‐down effects.     

Predator diversity and the functioning of ecosystems: the role of intraguild predation in dampening trophic cascades

Single trophic‐level studies of the relationship between biodiversity and ecosystem functioning highlight the importance of mechanisms such as resource partitioning, facilitation, and sampling effect. In a multi‐trophic context, trophic interactions such as intraguild predation may also be an important mediator of this relationship. Using a salt‐marsh food web, we investigated the interactive effects of predator species richness (one to three species) and trophic composition (strict predators, intraguild predators, or a mixture of the two) on ecosystem functions such as prey suppression and primary production via trophic cascades. We found that the trophic composition of the predator assemblage determined the impact of increasing predator species richness on the occurrence of trophic cascades. In addition, increasing the proportion of intraguild predator species present diminished herbivore suppression and reduced primary productivity. Therefore, trophic composition of the predator assemblage can play an important role in determining the nature of the relationship between predator diversity and ecosystem function.