Empirical and theoretical challenges in aboveground-belowground ecology.

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground-belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground-belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.

Effects of foliar herbivory by insects on the fitness of Raphanus raphanistrum: damage can increase male fitness.

Generally, effects of herbivory on plant fitness have been measured in terms of female reproductive success (seed production). However, male plant fitness, defined as the number of seeds sired by pollen, contributes half of the genes to the next generation and is therefore crucial to the evolution of natural plant populations. This is the first study to examine effects of insect herbivory on both male and female plant reproductive success. Through controlled field and greenhouse experiments and genetic paternity analysis, we found that foliar damage by insects caused a range of responses by plants. In one environment, damaged plants had greater success as male parents than undamaged plants. Neither effects on pollen competitive ability nor pollinator visitation patterns could explain the greater siring success of these damaged plants. Success of damaged plants as male parents appeared to be due primarily to changes in allocation to flowers versus seeds after damage. Damaged plants produced more flowers early in the season, but not more seeds, than undamaged plants. Based on total seed production, male fitness measures from the first third of the season, and flower production, we estimated that damaged and undamaged plants had equal total reproductive success at the end of the season in this environment. In a second, richer environment, damaged and undamaged plants had equal male and female plant fitness, and no traits differed significantly between the treatments. Equal total reproductive success may not be ecologically or evolutionarily equivalent if it is achieved differentially through male versus female fitness. Genes from damaged plants dispersed through pollen may escape attack from herbivores, if such attack is correlated spatially from year to year.

Levels of herbivory and parasitism in host hybrid zones.

Hybrid zones often are intermediate in their habitat, and have individuals that are intermediate genetically to those of either parental species. Recently, a number of studies has addressed the distribution of herbivores and parasites in hybrid zones of their hosts. Mechanisms that underlie these distributions include behavioral, environmental and genotypic differences between hybrids and their parental species.

Indirect effects in community ecology: Their definition, study and importance.

The diversity of indirect interactions that can occur within communities is large. Recent research on indirect interactions is scattered in the literature under numerous labels. The definition of indirect effects is an important aspect of their study, and clarifies some of the subtle differences among indirect effects found in natural communities. Choosing which species to study, how to manipulate species and for what duration, which attributes to measure and, finally, which analytical techniques to use are all problems facing the community ecologist. Ultimately, we are striving for the best means of determining the relative importance of direct and indirect effects in structuring communities.