Excerpt of manuscript accepted for publication mid-2014 by Management of Environmental Quality
Planetary boundary science defines key thresholds in the Earth System’s biogeochemical conditions that precede ecosystem collapse and threaten human well-being. Terrestrial ecosystems enter into the nine originally defined planetary boundaries only indirectly, through boundaries such as biodiversity and land use. This study proposes a measurable terrestrial ecosystem boundary to answer the question: what extent of landscapes, bioregions, continents, and the global Earth System must remain as connected and intact core ecological areas and agro-ecological buffers to sustain local and regional ecosystem services as well as the biosphere commons? Two preeminent considerations are connectivity of large ecosystem patches, enabling them to persist as the matrix for the landscape, and critical collapse of the dominant large habitat patch – or “percolating cluster” – into smaller, more isolated habitats, amid a matrix of human development. This transition, found to occur at about 40% habitat loss in landscapes and bioregions, is likely to be similar at continental and global scales.
From Malthus (1798), through Aldo Leopold’s land ethic (1949), to The Limits to Growth (Meadows et al. 1972), the Millennium Ecosystem Assessment (2005), and finally current planetary boundary and global change science (Rockström et al. 2009a, 2009b) runs a strand of concern about human growth’s impacts upon Earth’s biophysical systems – terrestrial ecosystems in particular – and about requirements for global ecological sustainability, while avoiding biosphere collapse. Our biosphere is composed of Earth’s thin mantle of life present at, and just above and below, the Earth’s surface. Some have indicated that human impacts upon the biosphere are analogous to a large, uncontrolled experiment, which threatens its collapse (Trevors et al. 2010). Little is known regarding what collapse of the biosphere would look like, how long it would take, what are its ecosystem and spatial patterns, and whether it is reversible or survivable. But it is becoming more widely recognized that Earth’s ecosystem services depend fundamentally upon holistic, well-functioning natural systems (Cornell 2009).
Accelerating human pressures on the Earth System are exceeding numerous local, regional, and global thresholds, with abrupt and possibly irreversible impacts upon the planet’s life-support functions (UNEP 2012). Planetary boundaries provide a framework to study these phenomena, by defining a “safe operating space for humanity with respect to the Earth System” (Rockström et al. 2009a). Planetary boundary studies seek to set control variable values that are a safe distance from thresholds of key biophysical processes governing the planet’s self-regulation to maintain conditions conducive to life (Rockström et al. 2009b). This builds upon landmark efforts by Meadows et al. (1972) to first define global limits to growth. Their prediction that key resource scarcities would emerge has proven remarkably accurate (Turner 2008), albeit delayed – but not avoided – through the advent of computer technology. Ecological and economic warnings since at least Malthus have called attention to economies’ dependence upon natural resources. The observation that near-exponential growth of human population and economic activity cannot be sustained, far from being disproven, is more valid than ever (Brown et al. 2011). Those who deny limits to growth are unaware of biological realities (Vitousek 1986).
The initial planetary boundary exercise identified nine global-scale processes, including climate change, rate of biodiversity loss (terrestrial and marine), nitrogen and phosphorus cycles, ozone depletion, ocean acidification, freshwater, land use change, chemical pollution, and atmospheric aerosol loading (Figure 1). Preliminary safe planetary thresholds were established for seven of these, and three – rate of biodiversity loss, climate change, and the nitrogen cycle – were found to have already surpassed such a threshold (Rockström et al. 2009a). Many such changes occur in a nonlinear, abrupt manner; others are more incremental and subtle. Yet both types of change threaten the viability of contemporary human societies by diminishing or destroying ecological life-support systems. If one or more of these boundaries are crossed, it could be “deleterious or even catastrophic” as nonlinear, abrupt environmental change occurs at the continental to planetary scale (Rockström et al. 2009b).
Here an ecologically rich revision to the planetary boundary framework is proposed – in the tradition of political ecology, not ignoring politics – to set the threshold of how many intact terrestrial ecosystems are required to sustain the biosphere. It is not possible to carry out controlled experiments upon our one biosphere to know at what point collapse occurs. We are thus left with observational studies and synthesis papers regarding what is known about ecosystem collapse at other scales. This paper first reviews what is known about biodiversity and old-growth forest loss, abrupt climate change, and ecosystem collapse as ecological systems are diminished at lesser scales. Next, the critical phase shift seen as landscapes percolate from nature surrounding humanity, to small reserves surrounded by human works, is presented as analogous to outcomes for the biosphere, whose terrestrial ecosystems are after all simply a large-scale landscape.
The remainder of the paper synthesizes these findings regarding ecosystem loss and thresholds in loss of ecosystem connectivity into a rationale for recognition of a 10th planetary boundary in regard to terrestrial ecosystem loss. It is suggested that some two-thirds of Earth’s land surface should be protected totally (44%) or partially (another 22%) to avoid biosphere collapse. Given current best estimates are that approximately one-half of Earth’s terrestrial ecosystems have already been lost, the discussion centers around biocentric policy measures required to protect and restore terrestrial ecosystem connectivity in order to maintain global ecological sustainability.
Percolation Theory and Landscape Connectivity
Terrestrial Ecosystem Loss as a Planetary Boundary
Biocentric Discussion on Achieving Global Ecological Sustainability
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