diverse grassland communities
L. Mommer
Plant Ecology and Nature Conservation Group, Wageningen University, Wageningen, The Netherlands
Biodiversity is increasingly threatened by human actions such as climate change, exploitation and pollution. This causes great concern, not only about the species that are threatened, but also about the functions that ecosystems can pro- vide.
The question how biodiversity loss would affect ecosystem functioning was first raised in the 1990s and let to the emergence of a new field of ecological research that investigated this question experimentally by manipulating plant species rich- ness in experimental grassland communities (Hector et al. 1999; Tilman et al. 2001; van Ruijven and Berendse 2005). After many experiments across the world, there is consensus among ecologists that primary productivity, an important ecosystem func- tion, increases with plant species richness (Cardinale et al. 2012). In other words: monocultures perform worse than plant species mixtures.
Although it is now well established that plant diversity is important for ecosystem functioning, the underlying mechanisms remain debated. Initially, the explanation for the pattern was sought in resource partitioning among plant species. If plant species that differ in the ways they acquire and use resources are grown together, they would acquire and use these resources more effectively than the species can do separately, and consequently, produce more biomass. However, direct experimental evidence for resource partitioning among species (e. g. based on tracer studies, (Bachmann et al. 2015; von Felten et al. 2009) and its contribution to increased biomass production in mixtures is mixed at best, at least below-ground (Mommer et al. 2010; Oram et al. 2018).
The limited evidence for resource partitioning, sparked the development of an alternative hypothesis, centred around the idea that interactions between plant roots and pathogenic soil biota are key to understand the biodiversity-productivity relationship (de Kroon et al. 2012; Maron et al. 2011; Schnitzer et al. 2011). The pathogen hypothesis is built upon two assumptions about plant-pathogen interactions within plant communities. The first is that plant species accumulate species-specific pathogens, referred to as host-specificity. These species-specific pathogens can reduce the performance of their host species, but have little impact on other plant species. The second assumption is negative density-dependence,
which suggests that the accumulation of plant species-specific pathogens and the negative impact on host performance declines with decreasing relative abundances of their host plants. Hence, along a gradient of plant species richness, pathogen pressure, potentially limiting biomass production, is high at low species richness (i. e. in monocultures where the relative abundance of the host is 100 %) and expected to decrease in plant species-rich mixtures, where the relative abundance of host plants will be lower. This ‘pathogen hypothesis’ is analogous to crop yield reductions from pests in agriculture, which typically increase with repeated cultivation of the same crop on the same field, and can be reduced by rotations with different crops (Bullock 1992).
My presentation at the EUCARPIA symposium will introduce the state-of-the-art of biodiversity-ecosystem functioning research (Mommer et al. in press, 2016), with the aim to provide an integrated perspective for breeders on this topic. The insights that ecologists have developed over the last three decennia are ready to be translated into agricultural practices (Mariotte et al. 2018). For example, the use of different genotypes and (mixtures of) cover-crops may help to increase both plant biodiversity and soil biodiversity and therefore develop sustainable agricultural practices.
References
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de Kroon, H., Hendriks, M., van Ruijven, J., Ravenek, J., Padilla, F. M., Jongejans, E., Visser, E. J. W., & Mommer, L. (2012). Root responses to nutrients and soil biota: Drivers of species coexistence and ecosystem productivity.Journal of Ecology,
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