Regime Shifts in Coupled Social-Ecological Systems: An interplay of human population density, soil health and climate variability

Alex Awiti

Terrestrial and aquatic ecosystems are exposed to biotic exploitation and climate variability that often leads to changes in soil condition or nutrient loading. The state of terrestrial and aquatic ecosystems may respond in a smooth, continuous way to such changes. In some instances ecosystems may be quite inert over certain ranges of conditions, responding more strongly when conditions approach a certain critical level. A significantly different situation arises when the ecosystem response curve exhibits hysteresis or a bifurcation. This implies that, for certain environmental conditions, the ecosystem has two alternative stable states separated by an unstable equilibrium that marks the border between the basins of attraction of the states. Verifying this diagnosis is important because it implies a radically different view on management options, and on the potential effects of global change on ecosystems. Strategies to assess whether alternative stable states exist are now converging in fields as disparate as limnology and terrestrial ecology.

This paper presents evidence for the existence of alternative stable states in aquatic and terrestrial ecosystems based on recent studies on soil and vegetation interactions in the Lake Victoria Basin. A shift between alternative stable states has occurred in the Winam Gulf of Lake Victoria. Increased nutrient loading from agricultural, often exacerbated by El Nino events, industrial and municipal wastes has changed the once pristine state of clear water to high eutrophication, exemplified by high turbidity and massive algal biomass. Increases algal biomass production developed from the 1930s onwards, which parallels human-population growth, agricultural activity and El Nino episodes in the Lake Victoria Basin. Eutrophication-induced loss of deep-water oxygen started in the early 1960s, and may have contributed to the 1980s collapse of indigenous fish stocks by eliminating suitable habitat for half of the 500 + endemic deep-water cichlids.

Near infrared reflectance spectroscopy (NIRS) analysis illustrates that soil from forest-cropland chronosequence exhibit alternative stable states. Stable carbon isotope analysis a show switch in soil organic carbon input sources from tree based C3 to maize based C4 sources in ca. 30 years post forest conversion. These soils are characterized by two “spectral basins of attraction”, a reference spectral state (non-degraded) and a degraded spectral state (degraded) separated by an intermediate or transition spectral class. At an ecosystem function level, changes in infiltration, water retention and plant biomass partitioning patterns across the forest-cropland chronosequence provide further evidence of the existence of alternative soil fertility condition states. Positive feedback control between plants and limiting resources (e.g., water, nutrients) is likely to be the principle underlying soil condition shifts when natural systems are converted to low input subsistence cultivation.

Although diverse events can trigger shifts in ecosystem states, loss of resilience usually paves the way for a switch to an alternative condition state. Hence, building and maintaining resilience of desired ecosystem states is likely be the most pragmatic and effective way to manage ecosystems in the face of increasing environmental change. The challenge is to sustain a large stability domain rather than to control fluctuations. If switches between alternative condition states could be at least partially predicted, we might be able to develop novel ways of preventing catastrophic ecosystem shifts and devise evidence based systems for restoring degraded ecosystems.

Sequential or nearly simultaneous decline in land and aquatic ecosystems has the potential to deepen poverty traps in coupled social-ecological systems. Robust diagnostic surveillance of land degradation using rapid and inexpensive techniques such as NIRS can aid our understanding and enable an evidence-based approach to agricultural and environmental management.