A key driver of the biodiversity crisis is poorly evaluated: population size

By Patrícia Dérer

We are entering the sixth mass extinction of life on Earth, the first to be caused by humans1. Extinction rates are several orders of magnitude above normal. Humanity has managed to exterminate more than 300 vertebrate animals in modern times, the IUCN estimates that half the globe’s 5,491 known mammals are declining in population and a fifth are clearly at risk of disappearing forever. Invertebrates and reptiles are in even worse shape, and there is no group of animals which has a higher rate of endangerment than amphibians.2

The key global mechanism focused on ending the biodiversity crisis is the Convention on Biological Diversity (CBD). Its Strategic Plan for Biodiversity 2011-2020 includes the 20 “Aichi targets,” which aim to stem biodiversiy loss. In order to evaluate progress in meeting these targets, CBD adopted indicators to measure human actions and the aspects of biodiversity that should improve. Unfortunately, these indicators show so far that the biodiversity crisis is escalating. What is going wrong? Are we missing something important?

A recent article in Nature Ecology & Evolution raises a very important problem: currently we lack indicators that would adequately draw attention to and measure all the drivers of the biodiversity crisis, particularly the underlying, systemic drivers. In “A biodiversity-crisis hierarchy to evaluate and refine conservation indicators3 Driscoll et al. shows that many key drivers of biodiversity loss are either poorly evaluated or entirely lacking in indicators. They use a biodiversity crisis hierarchy model to evaluate the scope of the current indicators and find them seriously wanting.

At the bottom of the model are the pressures directly causing biodiversity loss and species extinction. These immediate threats include agriculture, energy production, urbanization, climate change, pollution, invasive species, roads, hunting, wars, and fires. They have direct drivers, the so-called “threat-industries,” that aim to meet the increasing demands of a growing and increasingly affluent human population. In doing so, they use natural resources, release pollutants, and transport invasive species. Above these are the fundamental drivers of the biodiversity crisis: human population size and per capita resource consumption (Fig.1.a). Two “instigators” are ultimately responsible for trends in population growth and consumption, society and government, while four modifiers, representing actions of government and society, can alter the impact of the threat-industries. The model thus explicitly describes the links between fundamental and direct drivers, and highlights the instigating role of governments and societies in ratcheting these drivers up or down (Fig.1.a).

Fig. 1. The biodiversity crisis hierarchy and  evaluation of the coverage and adequacy of indicators for the Aichi biodiversity targets (Driscoll et al. 2018)

Do existing indicators under the Convention for Biological Diversity represent all the main components of and threats to biodiversity?

Examining the Aichi indicators, the authors found alarming gaps. One of the two fundamental drivers, human population size, is represented by only one indicator, and that very imperfectly. This indicator is “ecological footprint,” which confounds population size with consumption of a range of resources. In contrast, the second fundamental driver, per capita consumption, is well represented by eight indicators covering primary productivity, water, land and material consumption (Fig. 1.b,c).

Equally striking is the fact that even though government efficiency, corruption levels, road building and mining all have a huge influence on biodiversity, they have no indicators at all (Fig 1.b,c). In this way, important aspects of governance, political leadership and systematic human economic activity are not reported or measured among the Aichi indicators.

Driscoll et al. conclude that achieving the UN’s Sustainable Development Goals, including biodiversity conservation, is unlikely, given the failure to measure and address the main impediments to success, particularly continued population growth. They suggest three main areas for action to reduce population growth: increase education, alter economic incentives so that large families are not necessary, and provide the means to reduce fertility while reducing child mortality. The gaps identified by the authors show that solving the biodiversity crisis must involve developing new indicators and new targets that cover all the components in the biodiversity crisis hierarchy. A revised version of the Aichi targets should include indicators measuring the drivers of fertility decline, and governments should work to move these indicators in the right direction.

Watch the biodiversity crisis hierarchy video!


  1. Mckee, J., Chambers, E. & Guseman, J. Human Population Density and Growth Validated as Extinction Threats to Mammal and Bird Species. doi:10.1007/s10745-013-9586-8
  2. Ceballos, G., Ehrlich, P. R. & Dirzo, R. Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines. Proc. Natl. Acad. Sci. U. S. A. 114, E6089–E6096 (2017).
  3. Driscoll, D. A. et al. A biodiversity-crisis hierarchy to evaluate and refine conservation indicators. Nat. Ecol. Evol. (2018). doi:10.1038/s41559-018-0504-8
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