Our delicate ecosystem
Given that we are witnessing a massive extinction event right now, there is a lot of speculation about how biodiversity loss, in general, relates to the resilience of human society and our agroecology support system. I recently published a paper in Nature Ecology and Evolution detailing how urbanization is contributing to the extinction process in soil microorganisms; there are some things that were well beyond the scope of that paper, but were related to the resilience of human society, which I’d like to take a brief opportunity to explore and speculate on.
Human land use is central to the discussion of mass extinction in the anthropocene. Land use conversion, alternatively labeled as “habitat loss,” is the only factor driving our current extinction event. There are multiple contexts, i.e. multiple modes by which humans convert land use, are explored extensively in the literature. They all fall under the umbrella of use of the land (and water) by humans. Ellis et al. mapped human impact across the globe as a new series of biomes based on how humans use the land. This map succinctly summarizes the primary modes by which humans alter ecosystem processes on regional scales. These land uses aggregate together to place humans as the dominant biotic control of ecosystem process at the global scale. For example, our extensive land use substantially alters the carbon cycle through our agricultural production, and consumption patterns. We may soon be said to “control” the carbon cycle, as in our behavior materially impacts all the relevant carbon cycling feedback mechanisms, to substantial consequences. Some might argue we are already there.
The study of resilience and sustainability are primarily concerned with two thresholds, which are intricately related. The first is the impact threshold at which we ecosystem function becomes so degraded that it no longer meets our needs in either the short term or the long term. The second, is the impact threshold at which we begin to more significantly reduce the “buffer capacity” of our ecosystems. These two are related by the nature of ecosystems; a fundamental tenet of resilience theory (which is often misunderstood, or overlooked in sustainability) is that there is an inherent variability in the environment. There is more or less rain in any given year. Some years, Maryland gets hit by a hurricane, others it doesn’t. By changing the biotic community through farming and urbanization, we affect the ability of the ecosystem to recover from such disturbances. Often the result is to reduce the threshold level at which a disturbance event will cause a regime shift (or a collapse of the ecosystem from one state to another). The connection between the biological members of an ecosystem, and the ecosystem’s stability has been a subject of investigation for many years. One of the key findings has been that certain members disproportionately control the state of the ecosystem. One such example is the presence of beavers on the landscape drastically alters the ecology, effecting changes to hydrology, plant community etc. These members of the community are often called keystone species, because if they are removed there is a significant cascading collapse of community structure. So the stability of a community structure, or state of an ecosystem, is differentially affected by the response diversity of its members, not the biodiversity per se.
There is nonetheless been a lot of research into the effects of biodiversity on ecosystem stability. This is because there is rarely only a single species responsible for structuring the biotic community. Instead there are often multiple “pillar” species that may have redundant function. The removal of any one might not significantly alter the state of the ecosystem, but progressively removing more and more of them increases the likelihood that the entire structure will collapse. And, significant reductions of biodiversity can signal that there is already a regime shift (ecosystem collapse) underway. So the question of how human land use intensity impacts biodiversity is also implicitly a question of whether we are already causing a collapse of ecosystem function, and similarly, whether we are priming the ecosystem for a future collapse. Recent research published in Nature found that grazing intensity was correlated positively with rate of biodiversity loss across a land use intensity gradient. They also found that the greatest losses were at what are considered relatively low grazing intensities. In other words, even moderate grazing pressure can substantially degrade rangeland diversity. This has huge implications for restoration, and for the sustainability of agriculture. It implies that intensification increases our risk exposure to ecosystem collapse, and that we cross a diversity-loss threshold at a use-intensity that we currently consider to be low-intensity land use.
It is sometimes difficult to put ecosystem collapse into context- how can we visualize what a collapsed ecosystem looks like when we’ve never seen one? Or rather, when we are already surrounded by ecosystems that already could be considered collapsed ecosystems, how can we take the doomsdayers seriously? (we’re still here afterall). To answer this question, let’s consider the Sahara desert. Only 10,000 years ago the Sahara was a verdant landscape, covered in lakes that were filled by a strong monsoon climate. The change from savannah to desert has long been assumed to be caused by changes to the Earth’s orbit that occured sometime between 8,000 and 4,000 ybp. This, however might not be the whole story. Recent research has been published in Frontiers in Earth Science proposed a new theory. This was that changes to the Earth’s orbit created the conditions in which a regime change to a drier climate might happen, but the human pastoralist lifestyle may have been the proximate trigger of the desertification. The mechanism that the authors point to is an alteration of the vegetative cover caused by intensive livestock production. The unique patterns associated with cattle-grazing would have caused a transition in land cover to more scrub-brush type plant cover. This increases the amount of light that is reflected back into the atmosphere, and reduces the amount of water that is effectively cycled back into the atmosphere. The change in plant cover could generate local high pressure systems (hot air bubbles) that could slow or deflect the monsoon winds. Thus a positive feedback loop could generate an accelerating rate of desiccation, as high pressure systems deflected the monsoon rains, reducing water input and eventually leading to desertification. This type of effect, where the anthropogenic removal of vegetative cover leads to irreversible drying, has been documented before in cloud forests. And, similar situations are developing in the Amazon river basin as it is being deforested and converted to agricultural production.
It’s impossible to know for sure whether the Sahara desert would be dry as it is today without the effects of pastoralism. This is certain. But if it is true, or if it’s not but the scenario is realistic, then we have a problem. We live in a world where we have significantly disrupted the hydrological cycle in many areas. We already know certain ecosystems will never recover from this type of disruption. And we live in a world where the climate is changing. Heating, in fact. Mark Lynas’ book Six Degrees predicted that an increase in 2 degrees centigrade could turn the US farmland of Nebraska and Kansas into a sahara-like desert. Maybe there’s a way to preserve its wetter climate, but there’s little evidence that our current mode of operation is doing anything but accelerating this transition towards a drier state. In fact, much of the Kansas Ogallala aquifer is becoming significantly depleted, and it’s expected to get worst. Thus, we are living in a world that may be on the brink of multiple cascading regime shifts (collapsing ecosystems). With a global population moving towards 8 billion, avoiding the geopolitical instability that accompanies famine necessitates that we take a calculated risk. To feed ourselves, we need to engage in intensive agriculture, a behavior that facilitates a shift away from a relatively hospitable planet towards desert conditions. Meanwhile the earth is warming, making such a transition more likely. The only question is whether we can reduce our population pressure and climate change in time. It’s as if we are on a collision course with two trucks that are each about to hit us, one from each side. If we do nothing, they will surely crush us. Our likeliest strategy to avoid a wreck is to accelerate sharply.
Written By Dietrich Epp Schmidt, a microbiologist and student of ecology and society at the University of Maryland.