Trophic Rewilding: A Cure for a sick Planet?

 

The Mammoth Steppe was a huge area that extended over most of Northern Eurasia, including part of Alaska. It existed during the last glacial period, 126,000 YBP–11,700 YBP. Then, it was superseded by modern boreal forests during the thaw that led into the Holocene. The question of what factors led to this huge switch in the dominant biome is far from being clear. If "rewilding" some areas of the Earth is a good idea, should we strive for forests or steppes? After all, they are both "natural" environments?   

You may have already seen this paper just appeared on "Nature" with the title: "Trophic rewilding can expand natural climate solutions." The study is led by Oswald Schmitz of Yale University and is an assessment of the role of natural trophic chains on climate and of the perspectives of using "rewilding" as an important method for the mitigation of global warming. It has been described as a "landmark paper," and in several respects, it is. It is part of a general movement in favor of rewilding, and there is even a "Global Rewilding Alliance." 

The concept makes plenty of sense. It provides an alternative to the multiple bizarre ideas that have been proposed as "solutions" for the climate change problem, including cutting down the Boreal  Forests to increase Earth's albedo. On the other hand, it is still a subject in its infancy. One problem is that the authors do not mention that there is not just carbon sequestration at play in the climate game. They miss the effects of forests on the hydrological cycle (the kind of effects studied by Gorshkov, Makarieva, and others). But I think that it could be possible to merge these concepts together. In both cases, the idea is to restore the ecosystem to its maximum metabolic rate, balancing the disturbing effect of human activities. 

A deeper problem lies in understanding why exactly trophic chains have the effects claimed in the paper. The paper reports several estimates of the amount of carbon stored by various biomes, noting how it increases when a more diverse ecosystem is restored. To give an idea of the approach of the paper, the authors write that: 

The dividend of creating dynamic landscapes and seascapes is illustrated by the 1.2 million Serengeti wildebeest still found in Africa. This population annually migrates throughout the 25,000 km2 savannah– woodland landscape tracking lush vegetation created by seasonally and spatially varying rainfall. During the migration, wildebeest consume large amounts of grassland carbon and return it as dung that is incorporated by insects into soil storage. In the early twentieth century this dynamic was halted when the wildebeest population plummeted to 300,000 animals, decimated by rinderpest disease transmitted from domestic cattle. Consequently, there were too few animals to fully graze the landscape. The increased standing grass fuelled more frequent and intense wildfires that released carbon stored in the biomass across 80% of the landscape, which rendered the Serengeti a net source of atmospheric CO2 (ref. 47). Similar alterations of fire regimes followed the near-prehistoric extinctions of other large herbivores, the legacies of which persist today. Fire is an essential natural process in most of these systems, but the loss of natural grazing increases their frequency and intensity. Restoring the wildebeest population through disease management led to less frequent and intense wildfires, and gradually restored the Serengeti back to being a carbon sink. The Serengeti now stores up to 4.4 MtCO 2 more than when the wildebeest population was at its lowest. 

Which is truly fascinating. But why exactly should more diverse ecosystems store more carbon? One could say that if there were no wildebeest, then the forest would cover the Serengeti Park, and wouldn't a forest store more carbon than a savanna? Not necessarily. Large herbivores can sequester a lot of carbon in the soil, and it seems that the deep, fertile soil that Europeans found in the central area of North America was the result of the work of the huge herds of large ungulates living there. So, in terms of carbon storage, is a forest better than a savanna, or is it the reverse? 

Probably there is no clear-cut answer, and maybe there will never be one. Biomes are always dynamic; they change all the time. Although, in general, the ecosystem strives for stability, it may not be able to reach it except as an average -- it is sensible to even minor triggers such as the Milankovich oscillations that triggered the cycles of ice ages of the past two million years or so. But the Milankovich effects are just that: triggers. For the huge Earth ecosystem to move from a cold to a warm status, and the reverse, it takes enormous forces at play. In any case, the trophic chain remains the crucial factor in the ecosystem, the backbone of holobionts in their extended definition.

h/t John Day and Михаил Войтехов