Structural diversity and ecosystem-resource relationships in tropical savanna and a legume-cereal intercrop

Abstract

While the most productive ecosystems on Earth, correspondingly the most diverse, function within limitations of energy and resource flows, the general approach of high-yielding agriculture, in contrast, has been to simplify and replace key ecosystem functions with non-renewable inputs. Restoring ecological processes in agriculture requires some understanding of the underlying mechanisms. Here, I explore a unifying framework regarding the coupling between ecosystem structure and function. Namely, that the capacity of water-limited ecosystems to optimize water use, or resist stress is a product of their life-form diversity. I apply this hypothesis to infer the response of structurally contrasting ecosystems at a wide range of scales: (i) At the plot scale, I test the role of resource complementarity (“niche-based” mechanisms) in a perennial grain-legume intercrop, using stable isotope analysis. (ii) At the sub-continental scale, I use satellite data to explore the response of one of the most variable terrestrial biomes, tropical savanna, to rainfall variability. I employ different approaches to demonstrate the degree of coupling between ecosystem response and the environmental forcing. The dissertation concludes by summarizing the key findings and outlining further work needed to explore the role of ecosystem structure on function in water-limited systems. I discuss the implications of niche complementarity on resource relationships of biologically diverse agroecosystems, emphasizing the need for a re-evaluation of the conceptual framework used to envision niche complementarity to account for alternative resource acquisition pathways. I explore the role of savanna hydrophenology as a stabilizing mechanism, relating patterns of synchrony to savanna structure and composition. The relevance of this work is directly linked to the loss of ecological function which manifests in agriculture’s growing dependence on fossil energy to mask diminishing returns from extractive use of land and water resources. Enriched knowledge of potential mechanisms, coupling structural diversity with ecosystem function, in both natural and managed ecosystems, will provide insight into the ecological basis for diversity driven processes in water-limited ecosystems.