Andrew Wilby

Functional benefits of predator species diversity depend on prey identity

Abstract.  1. Determining the functional significance of species diversity in natural enemy assemblages is a key step towards prediction of the likely impact of biodiversity loss on natural pest control processes. While the biological control literature contains examples in which increased natural enemy diversity hinders pest control, other studies have highlighted mechanisms where pest suppression is promoted by increased enemy diversity.

Conservation implications of the link between biodiversity and ecosystem functioning

The relationship between biodiversity and individual ecosystem processes is often asymptotic, saturating at relatively low levels, with some species contributing more strongly than others. This has cast doubt on arguments for conservation based on maintenance of the functioning of ecosystems. However, we argue that the link between biodiversity and ecosystem functioning is an important additional argument for conservation for several reasons. (1) Although species differ in importance to ecosystem processes, we do not believe that this argues for preservation of just a few species for two reasons: first, it is nearly impossible to identify all species important to the numerous systems and processes on which humans depend; second, the important species themselves may depend on an unknown number of other species in their communities. (2) Arguments for conservation based on ecosystem functioning are complementary to other utilitarian, ethical and aesthetic justifications. No single reason will convince all people or protect all species, however the combination produces a strong case for conservation of biodiversity. (3) Even if the relationship between biodiversity and ecosystem functioning is asymptotic at local spatial scales and in the short term, effects of biodiversity loss are likely to be important at larger temporal and spatial scales. (4) Initial arguments for the importance of biodiversity for ecosystem functioning were largely based on a precautionary approach (points 1–3). However, we are now moving to a scientific position based on accumulating experimental evidence. The future challenge is the integration of this scientific research with policy.

Harvester ant response to spatial and temporal heterogeneity in seed availability: pattern in the process of granivory

Abstract. The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied in an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availability for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape patch types (shrub patches and inter-shrub patches) in 1997. Shrub patches were shown to have higher plant and seed-rain density than inter-shrub patches. Patch use and seed selection by M. arenarius foragers were monitored through the spring, summer and autumn of 1997. After a pulse of seed production in the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summer, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two patch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increase in the use of shrub patches by individual foragers. In the autumn, a pulse of seed production by the shrub species Atractylis serratuloides and Noaea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in the proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behaviour showed linked changes in seed selection and patch use resulting in important differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productivity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephemeral plants in the landscape.

Herbivory, litter and soil disturbance as determinants of vegetation dynamics during early old-field succession under set-aside

Early-successional old fields are a major component of the European landscape. While a range of factors governing vegetation development in old fields has been identified, empirical and theoretical studies have tended to concentrate on plant competition as the dominant driving force behind succession. We studied the influence of three little researched, yet inter-related, factors on the early stages of an old-field succession: litter cover, soil disturbance and herbivory. Physical and chemical techniques were used to exclude large vertebrates and insects from experimental plots. These treatments had little effect on plant recruitment. A litter-removal experiment, nested within the exclusion treatments, revealed a significant inhibition of forb seedling germination by litter cover. However, the majority of seedlings died during the first month following emergence, whether or not litter was removed. A second experiment, involving the factorial combination of mollusc exclusion and soil disturbance, revealed that the response to disturbance was dependent on life-history characteristics of the plants. However, the dominant factor regulating community composition was seedling herbivory by molluscs. Molluscs caused high rates of forb seedling mortality and promoted the transition from a forb-dominated, to a grass-dominated community. Herbivory is often assumed to influence plant community dynamics through effects on competitive interactions. However, direct effects of herbivory, on the survival of seedlings, may be a significant factor structuring plant communities in ruderal, or other annual dominated systems.

Ecosystem engineering : a trivialized concept?

In a recent review in TREE [1xThe role of pocket gophers as subterranean ecosystem engineers. Reichman, O. and Seabloom, W. Trends Ecol. Evol. 2002; 17: 44–49Abstract | Full Text | Full Text PDF | Scopus (173)See all References][1], Reichman and Seabloom discuss the important impacts of burrowing mammals as keystone ecosystem engineers. The article eloquently explains the importance of one group of ecosystem engineers, but, in restricting the term ‘ecosystem engineer’ to keystone species, I believe that it seriously understates the value of the concept.Reichman and Seablooms definition requires that the ‘change to the physical environment should be distinctive from processes that are strictly abiotic…and large relative to purely physical processes operating in the system’. They also state that ‘discussions of ecosystem engineering have become trivialized by characterizing any influence on the environment as engineering because all plants and animals affect the physical environment in some way’. I have strong reservations about these two statements.I do not believe that the definition of ecosystem engineering should be based purely on the physical effect. As explicitly stated in the original definitions of ecosystem engineering, the physical effects caused by engineers should ‘modulate the availability of resources to other organisms’ [2xOrganisms as ecosystem engineers. Jones, C.G. et al. Oikos. 1994; 69: 373–386CrossrefSee all References][2]. The key point is that the changes in resource flow caused by ecosystem engineering affect other organisms. Thus, the value of the ecosystem-engineering concept lies in its formalization of interactions among organisms that are mediated by the physical environment, and that were hitherto not included in ecological theory. It is an additional class of interactions that should be integrated with trophic ecology to give us a more complete understanding of interspecific interactions [3.xIntegration of ecosystem engineering and trophic effects of herbivores. Wilby, A. et al. Oikos. 2001; 92: 436–444CrossrefSee all References, 4.xPositive and negative effects of organisms as physical ecosystem engineers. Jones, C.G. et al. Ecology. 1997; 78: 1946–1957CrossrefSee all References].Far from trivializing the concept, the fact that all organisms affect the physical environment in some way emphasizes the possible ubiquity of ecosystem engineering. All organisms also have trophic interactions, but does this trivialize trophic ecology? Should we restrict discussions of trophic interactions to those interactions involving keystone species? If most species do turn out to have ecosystem engineering interactions with other species, then the label ‘ecosystem engineer’ will be trivial, but the engineering processes definitely are not. Indeed, in concentrating on the identification of keystone engineers, rather than on the engineering processes that they control and their generality, we risk an ‘accumulation of ‘‘just-so’’ stories’ that was warned against when the concept was introduced [2xOrganisms as ecosystem engineers. Jones, C.G. et al. Oikos. 1994; 69: 373–386CrossrefSee all References][2].In Box 1 of their article, Reichman and Seabloom explain how soil excavation and herbivory by pocket gophers Thomomys bottae interact with competitive interactions among plant species to govern vegetation composition and spatial pattern. Thus, the authors summarize a considerable body of work that describes pocket gophers in terms of the impact of their herbivory and soil disturbance on plant community structure. From this process perspective, ecosystem-engineering effects, alongside trophic effects, are described in such a way that allows comparison of species in similar or dissimilar ecosystems. I suggest that such an integrative approach is the way forward for the study of ecosystem engineering. Our goal should be to elucidate how widespread and important ecosystem engineering interactions are, rather than restricting ourselves to the most apparent examples from the outset.

DENSITY-DEPENDENT EFFECTS OF PREDATOR SPECIES-RICHNESS IN DIVERSITY–FUNCTION STUDIES

Ecological systems often exhibit a positive but saturating diversity-function curve. Variation in the mechanisms generating this relationship can alter the slope and variance of the curve, with implications for the optimal management of biodiversity for ecosystem services. In biological control, prevalence of selection effects supports augmentation of the most effective natural enemy, but complementarity effects support augmentation of natural enemy diversity. Optimization of biological control strategies from the results of diversity-function studies is limited because few consider changes in function with relative or absolute changes in abundance, and many confound the relative importance of richness and density through experimental designs (additive and substitutive). By manipulating species richness across an abundance gradient we show that effects of species richness are density dependent and indicate how this may be incorporated into experimental designs or models predicting resource consumption in diverse communities. Furthermore, the underlying mechanisms causing an observed diversity-function response, and its associated variation, changed across the richness-abundance gradient. Finally, species-rich assemblages provided higher levels of minimum function than species-poor assemblages, without any compromise on the maximum function possible.

Arthropod Diversity and Community Structure in Relation to Land Use in the Mekong Delta, Vietnam

Declining biodiversity in agro-ecosystems, caused by intensification of production or expansion of monocultures, is associated with the emergence of agricultural pests. Understanding how land-use and management control crop-associated biodiversity is, therefore, one of the key steps towards the prediction and maintenance of natural pest-control. Here we report on relationships between land-use variables and arthropod community attributes (for example, species diversity, abundance and guild structure) across a diversification gradient in a rice-dominated landscape in the Mekong delta, Vietnam. We show that rice habitats contained the most diverse arthropod communities, compared with other uncultivated and cultivated land-use types. In addition, arthropod species density and Simpson’s diversity in flower, vegetable and fruit habitats was positively related to rice cover in the local landscape. However, across the landscape as a whole, reduction in heterogeneity and the amount of uncultivated cover was associated, generally, with a loss of diversity. Furthermore, arthropod species density in tillering and flowering stages of rice was positively related to crop and vegetation richness, respectively, in the local landscape. Differential effects on feeding guilds were also observed in rice-associated communities with the proportional abundance of predators increasing and the proportional abundance of detritivores decreasing with increased landscape rice cover. Thus, we identify a range of rather complex, sometimes contradictory patterns concerning the impact of rice cover and landscape heterogeneity on arthropod community attributes. Importantly, we conclude that that land-use change associated with expansion of monoculture rice need not automatically impact diversity and functioning of the arthropod community.

Managing outbreaks of invasive species: Eradication versus suppression

Abstract A framework is developed to evaluate eradication as one of three alternative management responses to an outbreak of an invasive species: eradication, suppression or no action. This framework can be used to establish under what conditions an eradication option could provide an expected net economic benefit, and whether this net benefit exceeds that of the other two options. The eradication option is more likely to be preferred in situations where there is an immediate export benefit that is derived from eradication of the outbreak, and also the uncertainty associated with the likely success of eradication is low.

Effects of species evenness and dominant species identity on multiple ecosystem functions in model grassland communities

Ecosystems provide multiple services upon which humans depend. Understanding the drivers of the ecosystem functions that support these services is therefore important. Much research has investigated how species richness influences functioning, but we lack knowledge of how other community attributes affect ecosystem functioning. Species evenness, species spatial arrangement, and the identity of dominant species are three attributes that could affect ecosystem functioning, by altering the relative abundance of functional traits and the probability of synergistic species interactions such as facilitation and complementary resource use. We tested the effect of these three community attributes and their interactions on ecosystem functions over a growing season, using model grassland communities consisting of three plant species from three functional groups: a grass (Anthoxanthum odoratum), a forb (Plantago lanceolata), and a N-fixing forb (Lotus corniculatus). We measured multiple ecosystem functions that support ecosystem services, including ecosystem gas exchange, water retention, C and N loss in leachates, and plant biomass production. Species evenness and dominant species identity strongly influenced the ecosystem functions measured, but spatial arrangement had few effects. By the end of the growing season, evenness consistently enhanced ecosystem functioning and this effect occurred regardless of dominant species identity. The identity of the dominant species under which the highest level of functioning was attained varied across the growing season. Spatial arrangement had the weakest effect on functioning, but interacted with dominant species identity to affect some functions. Our results highlight the importance of understanding the role of multiple community attributes in driving ecosystem functioning.

Herbivore species richness, composition and community structure mediate predator richness effects and top-down control of herbivore biomass

Changes in predator species richness can have important consequences for ecosystem functioning at multiple trophic levels, but these effects are variable and depend on the ecological context in addition to the properties of predators themselves. Here, we report an experimental study to test how species identity, community attributes, and community structure at the herbivore level moderate the effects of predator richness on ecosystem functioning. Using mesocosms containing predatory insects and aphid prey, we independently manipulated species richness at both predator and herbivore trophic levels. Community structure was also manipulated by changing the distribution of herbivore species across two plant species. Predator species richness and herbivore species richness were found to negatively interact to influence predator biomass accumulation, an effect which is hypothesised to be due to the breakdown of functional complementarity among predators in species-rich herbivore assemblages. The strength of predator suppression of herbivore biomass decreased as herbivore species richness and distribution across host plants increased, and positive predator richness effects on herbivore biomass suppression were only observed in herbivore assemblages of relatively low productivity. In summary, the study shows that the species richness, productivity and host plant distribution of prey communities can all moderate the general influence of predators and the emergence of predator species richness effects on ecosystem functioning.