Emergent patterns in nature and society

Another idea on cascading effects

One neat cascading effect occurs in regime shifts in Salt Marshes, it has to do with fragmentation of natural habitat for migratory species. Bertness et al. (1) reported in Conservation Biology (2008) that land use change reduced the natural habitat of wild migratory species and they end up over using their alternative grounds inducing regime shifts in salt marshes due to strong top-down control. For non-ecologist, this means that the salt marshes ecosystems all of the sudden have too many predator populations (snow geese in Bertness case), who grazed heavily on the key species that form the salt marshes, driving them towards tidal flats. Here is the quote from the paper:

The first serious challenge to the Odum model came from sub-Arctic marshes (Jefferies 1997). Robert Jefferies studied these systems in the early 1970s, initially focusing on positive effects of geese grazing on marsh primary production mediated through soil disturbance and nitrogen cycling. But by the 1980s the Snow Geese (Anser caerulescens) that annually migrate to Hudson Bay switched from feeding in temperate zone wetlands, which were being lost to human activity, to agricultural fields and golf courses, which were receiving nitrogen fertilizer subsides. Consequently, populations of Snow Geese nearly tripled during the 1980s, leading to runaway consumption and the denuding of extensive areas of Arctic marshes (currently > 37,000 ha in southern Hudson Bay alone) that serve as their summer breeding and feeding grounds (Fig. 1). This collapse was driven by intensive goose grubbing of plant roots and rhizomes, which then led directly to low plant cover, quickly followed by hypersaline and anoxic soil conditions. This grazer-generated soil stress created an unstoppable feedback loop in which the remaining, adjacent vegetation died, soil salinity increased even further, and plants that recruited into the newly denuded areas died almost im- mediately from osmotic stress, preventing ecosystem reestablishment. Essentially, at high densities, geese foraging turned off habitat ameliorating positive feedbacks that had historically allowed plants to establish, domi- nate, and support arctic marsh ecosystems. These precipitated and cascading events that led to wholesale system collapse were begun almost entirely by the luxuriant use of artificial nitrogen fertilizer in the temperate zone. Unfortunately, this scenario of seemingly unrelated human activities indirectly triggering top–down forces is not restricted to arctic marshes.

Now the interesting part comes when trying to figure out similar mechanism in social systems: when the usual “habitat” of a group of people, say having the same livelihood, is significantly reduced inducing over pressure over another source of living and perhaps creating a regime shift in livelihoods and potentially on ecosystems if the livelihoods are dependent on them. Since I’ve been reading a lot lately about reindeer husbandry, that would be a nice match example. At least in Finnmark, Norway, reindeer herders have been losing access to areas either because of climate, political pressure or conflicting interest (2). For instance, as infrastructure is develop or oil mining projects go up north, Saamis seen their territory invaded and altering their traditional migratory patterns, limiting them to certain areas and perhaps over harvesting some of them.

What other situations would match such syndrome?


Some references:

1.Bertness MD, Silliman BR (2008) Consumer Control of Salt Marshes Driven by Human Disturbance. Conservation Biology 22:618–623.

2.Tyler NJC et al. (2007) Saami reindeer pastoralism under climate change: Applying a generalized framework for vulnerability studies to a sub-arctic social–ecological system. Global Environmental Change 17:191–206.

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