Showing papers by "Andy Hector published in 2008"

Biodiversity effects and transgressive overyielding

Abstract: Aims The potential for mixtures of plant species to produce more biomass than every one of their constituent species in monoculture is still controversially discussed in the literature. Here we tested how this socalled transgressive overyielding is affected by variation between and within species in monoculture yields in biodiversity experiments. Methods We use basic statistical principles to calculate expected maximum monoculture yield in a species pool used for a biodiversity experiment. Using a real example we show how between- and withinspecies variance components in monoculture yields can be obtained. Combining the two components we estimate the importance of sampling bias in transgressive overyielding analysis. Important Findings The net biodiversity effect (difference between mixture and average monoculture yield) needed to achieve transgressive overyielding increases with the number of species in a mixture and with the variation between constituent species in monoculture yields. If there is no significant variation between species, transgressive overyielding should not be calculated using the best monoculture, because in this case the difference between this species and the other species could exclusively reflect a sampling bias. The sampling bias decreases with increasing variation between species. Tests for transgressive overyielding require replicated species’ monocultures. However, it can be doubted whether such an emphasis on monocultures in biodiversity experiments is justified if an analysis of transgressive overyielding is not the major goal.

Light partitioning in experimental grass communities

Abstract: Through complementary use of canopy space in mixtures, aboveground niche separation has the potential to promote species coexistence and increase productivity of mixtures as compared to monocultures. We set up an experiment with five perennial grass species which differed in height and their ability to compete for light to test whether plants partition light under conditions where it is a limiting resource, and if this resource partitioning leads to increased biomass production in mixtures (using relative yield-based methods). Further, we present the first application of a new model of light competition in plant communities. We show that under conditions where biomass production was high and light a limiting resource, only a minority of mixtures outperformed monocultures and overyielding was slight. The observed overyielding could not be explained by species differences in canopy structure and height in monoculture and was also not related to changes in the canopy traits of species when grown in mixture rather than monoculture. However, where overyielding occurred, it was associated with higher biomass density and light interception. In the new model of competition for light, greater light use complementarity was related to increased total energy absorption. Future work should address whether greater canopy space-filling is a cause or consequence of overyielding.

The Role of Biodiversity

Abstract: Human activities have caused widespread loss of biodiversity raising concern about the potential impact on ecosystem processes (flows of energy and materials). A large body of recent research has shown that as species are lost from ecosystems there is, generally, a minor impact on ecosystem processes, but that this impact increases disproportionately as species diversity declines. Functional complementarity among species, due to variation in the ecological niches they occupy, appears to be the main mechanism driving this pattern. Species diversity is also usually positively related to ecosystem stability, i.e. their variation through time and the resistance and resilience to perturbation. These findings are already powerful arguments for the conservation of biodiversity, though current research aims to increase their relevance to the real world by including a more extensive range of ecosystems and processes, realistic food web structures, realistic (nonrandom) extinction scenarios and larger spatial scales. Keywords: ecosystem functioning; ecosystem stability; species diversity; conservation biology

Bottom-Up Effects and Feedbacks in Simple and Diverse Experimental Grassland Communities

Abstract: Finding a consistent pattern in the effects of plant diversity on higher trophic levels is a major challenge as populations at all trophic levels of an ecosystem may be regulated by a mixture of top-down and bottom-up forces. A starting point to experimentally approach the problem is to measure the effects of changing plant-species diversity on primary productivity, the basis of each food web, and to explore the potential underlying mechanisms. This was done within the European BIODEPTH project (Biodiversity and Ecological Processes in Terrestrial Herbaceous ecosystems). In this project, a common methodology was used at each of eight sites across Europe to experimentally assemble grassland communities of defined plant-species numbers (e.g. 1, 2, 4, 8, 32) from the local species pools. Ecosystem processes were then monitored in these different herbaceous ecosystems. Here, we report findings gathered from the sites in the UK, Switzerland, Germany and Sweden. Our data suggest that trophic levels cannot be treated as homogeneous units since the response to changes in plant diversity of individual groups within trophic levels was correlated with group-specific attributes such as host specificity, mobility or different size classes of consumers. There was also no evidence for a resource concentration effect, i.e. for a disproportionately higher specialist insect density in plots with high host density and biomass such as monocultures. Part of the diversity effects observed at higher trophic levels was indirectly driven by changes in primary productivity with changing plant diversity. However, experimental additions of a generalist insect herbivore and a plant hemiparasitic species showed that some polyphagous groups within higher trophic levels can benefit from increased diversity not only by the higher quantity, but also by the higher variety of resources.