human–induced species losses: beyond nature’s ability to regenerate

By mei 15, 2016 Algemeen

The general trend during the more than 3.5 billion years that life has been evolving on Earth, has been toward ever greater complexity and diversity, with more new species evolving into existence than there were species going extinct.  Extinction is common, but normally it is balanced by speciation. The balance wavers such that at several times in life’s history extinction rates appear somewhat elevated, but only five times qualify for ‘mass extinction’ status. Different causes are thought to have precipitated these extinctions and they all stand out in having extinction rates spiking higher than in any other geological interval. These extinctions altered ecosystem processes and disturbance regimes at continental scales, triggering cascades of extinction thought to still reverberate today.

We live amid a global wave of anthropogenically driven biodiversity loss: species and population extirpations and, critically, declines in local species abundance. Particularly, human impacts on animal biodiversity are an under-recognized form of global environmental change. Increasingly, we are recognizing modern extinctions of species and populations. Documented numbers are likely to be serious underestimates, because most species have not yet been formally described. Such species declines will cascade onto ecosystem functioning and human well-being. Much remains unknown about this ‘Anthropocene defaunation’; these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet’s sixth mass extinction and also a major driver of global ecological change.

Humans are now causing this sixth mass extinction, through co-opting resources, fragmenting habitats, introducing non-native species, spreading pathogens, killing species directly, and changing global climate. If so, recovery of biodiversity will not occur on any timeframe meaningful to people: evolution of new species typically takes at least hundreds of thousands of years, and recovery from mass extinction episodes probably occurs on timescales encompassing millions of years.

Analyses of the impacts of global biodiversity loss typically base their conclusions on data derived from species extinctions, and typically, evaluations of the effects of biodiversity loss draw heavily from small-scale manipulations of plants and small sedentary consumers. Both of these approaches likely underestimate the full impacts of biodiversity loss. Although species extinctions are of great evolutionary importance, declines in the number of individuals in local populations and changes in the composition of species in a community will generally cause greater immediate impacts on ecosystem function. Moreover, whereas the extinction of a species often proceeds slowly, abundance declines within populations to functionally extinct levels can occur rapidly. Although we are beginning to understand the patterns of species loss, we still have a limited understanding of how compositional changes in communities after defaunation and associated disturbance will affect phylogenetic community structure and phylogenetic diversity.

Around the world, we are increasingly recognizing ecosystems as natural capital assets that supply life-support services of tremendous value. The challenge is to turn this recognition into incentives and institutions that will guide wise investments in natural capital, on a large scale. Advances are required on three key fronts: the science of ecosystem production functions and service mapping; the design of appropriate finance, policy, and governance systems; and the art of implementing these in diverse biophysical and social contexts. Scientific understanding of ecosystem production functions is improving rapidly but remains  a limiting factor in incorporating natural capital into decisions, via systems of national accounting and other mechanisms. Novel institutional structures are being established for a broad array of services and places, creating a need and opportunity for systematic assessment of their scope and limitations. Finally, it is clear that formal sharing of experience, and defining of priorities for future work, could greatly accelerate the rate of innovation and uptake of new approaches.

There remain many highly nuanced scientific challenges for ecologists, economists, and other social scientists to understand how human actions affect ecosystems, the provision of ecosystem services, and the value of those services. But the evidence is incontrovertible that recent extinction rates are unprecedented in human history and highly unusual in Earth’s history. As mentioned, analysis, research emphasizes that our global society has started to destroy species of other organisms at an accelerating rate, initiating a mass extinction episode unparalleled for 65 million years.

If the currently elevated extinction pace is allowed to continue, humans will soon (in as little as three human lifetimes) be deprived of many biodiversity benefits. On human time scales, this loss would be effectively permanent because in the aftermath of past mass extinctions, the living world took hundreds of thousands to millions of years to rediversify. Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species and to alleviate pressures on their populations—notably habitat loss, overexploitation for economic gain, and climate change. All of these are related to human population size and growth, which increases consumption, and economic inequity.

At least as demanding are the social and political challenges associated with incorporating this understanding into effective and enduring institutions, to manage, monitor, and provide incentives that accurately reflect the social values of ecosystem services to society. The candid analyses presented here help light the way.

However, the window of opportunity is rapidly closing.