Ecosystem organization

Sub-Sahara African ecosystems are water-limited, making these ecosystems sensitive to alterations in precipitation events. One of the primary consequences of climate change is a shift in precipitation patterns, which is therefore expected to affect ecosystem structure and functioning. Nevertheless, savanna ecosystem structure is poorly predicted by climate alone, but is also dependent on the disturbance regime, such as regular fire outbreaks which ‘consume’ large parts of vegetation biomass. Besides these fire disturbances, also other consumers like large herbivores and termites strongly affect ecosystem structure and functioning. The distribution of these consumers is strongly dependent on rainfall. Furthermore, there are interactions between vegetation types (tree-grass interactions), between groups of consumers (fire-grazer interactions) and functional groups of macrodetritivores (dung beetle-termite interactions). Additionally, both vegetation and consumers might feedback on local environmental conditions by changing water and nutrient availabilities. Altogether, this results in a very complex structure of interactions with many determinants and feedback mechanism. Not surprisingly, dynamic global vegetation models (DGVM’s) are generally found poor predictors of savanna ecosystem structure and functioning. Therefore, a general concept based on the interactions between species that incorporates the feedback mechanisms is needed to describe the organization of savanna ecosystems to subsequently explain the patterns that arise at the ecosystem level.

Objectives

I aim to contribute to a better general understanding of ecosystem organization, with a special focus on African savanna ecosystems. To this end, I study the following interrelated topics:

1. I have synthesized (Veldhuis et al. 2018) and continue to develop the concept of ecological autocatalysis, that combines information on consumer-resources linkages with additional feedback mechanisms to identify groups of organisms that positively affect each other (autocatalytic loops). Interactions between multiple autocatalytic loops can provide a mechanistic understanding of the patterns and processes found at the ecosystem level. Therefore, the concept of ecological autocatalysis could serve as a framework to understand ecosystem organization in any ecosystem. I also explore how this concept might yield useful insights in the description of ecosystem organization of real ecosystems, using an African savanna as a study system.

2. I explore how biotic feedback mechanisms, that are central to the concept of ecological autocatalysis, can change the local availability of water (Veldhuis et al. 2014) and nutrients (Veldhuis et al. 2016a; Sitters et al 2017; Veldhuis et al. 2018), loosening the connection with large-scale environmental gradients.

3. I investigate how organisms or groups of organisms (autocatalytic loops) affect important ecosystem functions like primary productivity or decomposition of dead plant material. I therefore quantified the rates of primary production and herbivore consumption in distinct vegetation types (Veldhuis et al. 2016b) and decomposition by different functional groups of detritivores across a rainfall gradient (Veldhuis et al. 2017). These ecosystem functions also depend on environmental conditions and therefore the key question I try to unravel is what the relative impact are of abiotic and biotic effects on ecosystem functioning.

4. Last, I investigate how interactions between organisms or groups of organisms (autocatalytic loops) provide a mechanistic explanation for the spatial patterning and heterogeneity of the landscape and how such patterns change across environmental gradients (Veldhuis et al. 2016c).