RESEARCH NEWS & VIEWS called flat-panel X-ray detectors both improved image sharpness and reduced graininess. This increased the diagnostic value of the imaging, while making it possible to reduce the radiation dose compared with that required in the first available digital X-ray imaging procedure, known as computed radiography3. Flat-panel X-ray detectors are currently based either on the direct-conversion method using amorphous selenium (a semiconductor), or on the indirect-conversion method, which uses the luminescent material caesium iodide (CsI). In the direct method, X-ray energy is converted by the semiconductor into many electrons, which are collected on electrodes. The electrodes form part of an integrated circuit that reads out the image. In the indirect method, photons of light are created as an intermediate signal that is subsequently converted into electrons. A drawback of the latter approach is that the light can scatter and blur the image. Amorphous selenium has been used as the semiconductor in direct-conversion flatpanel detectors to obtain mammograms4. This application demonstrated that such detectors can produce much sharper images than can indirect-conversion instruments. But although selenium can absorb the low-energy X-rays used for mammography, it cannot adequately absorb the higher-energy X-rays needed for more-general applications. Moreover, neither amorphous selenium nor CsI can be used at the fundamental quantum-noise level — the dose at which individual quanta of X-rays are used to construct images, and which is lower than doses used by currently available flat-panel X-ray detectors. Kim et al. therefore turned to a material called methylammonium lead triiodide (MAPbI3), which belongs to a remarkable family of semiconductors known as perov skites. The first solar cell made using a perovskite was reported just eight years ago5. Perovskites have since emerged as some of the most promising semiconducting materials for making solar cells, light-emitting diodes and lasers6. Their characteristics make them suitable for applications involving light from the near-infrared to the visible regions of the spectrum. Unlike most other solar-cell materials, perovskites contain elements (lead and iodine) that have a high atomic number7,8, which makes them effective absorbers of the highly penetrating X-rays used in general radi ology. The authors demonstrated that layers of MAPbI3 almost 1 millimetre thick — as needed to absorb X-rays in detectors — can be made easily using a printing method that works at a relatively low temperature. This method is cheaper than the high-temperature processes needed to prepare CsI or amorphous selenium, and should greatly reduce the cost of making imaging systems. The authors then constructed an X-ray detector using 10-cm × 10-cm samples of
MAPbI3 (Fig. 1). As expected, the material has a high sensitivity to X-rays — almost ten times higher than that of amorphous selenium or CsI. Using the same approach, it should be possible to make detectors that have much larger areas, as required for other medical procedures such as chest X-rays4. The sensitivity and processability of this perovskite could “The potential therefore reduce improvements the radiation dose associated with needed for medical using MAPbI3 X-rays to the fundacould make a mental quantum and big difference spatial resolution to medical limits. procedures.” More work on MAPbI3 X-ray detectors is now needed, for three main reasons. First, thermal activation of electrons in the material can produce a spurious signal known as dark current. This could be reduced by altering the material’s electronic structure — by partly replacing iodine atoms with bromine or chlorine atoms6. Second, the sharpness of the images produced should be further increased, by, for example, increasing the applied electric field. And third, studies are needed to establish that the material is stable and long-lived enough to be made into a practical device. Nevertheless, the potential improvements associated with using MAPbI3 could make
a big difference to medical procedures. For example, stents are often used to hold open blocked coronary arteries, the blood vessels that feed the heart muscle. The stents are inserted non-invasively using catheters, guided by fluoroscopy (an X-ray video technique)4. X-ray detectors made using MAPbI3 should greatly lower the radiation dose used in this procedure, and improve the images of the fine wires that make up the stent. More broadly, the advantages of MAPbI3 detectors could lead to the replacement of existing technologies, and could fuel innovation in other X-ray imaging techniques, such as computed tomography. ■ John A. Rowlands is in the Physical Sciences Division, Sunnybrook Research Institute, University of Toronto, Toronto M4N 3M5, Canada. e-mail:
[email protected] 1. Kim, Y. C. et al. Nature 550, 87–91 (2017). 2. Reiner, B. I., Siegel, E. L. & Siddiqui, K. J. Digit. Imaging 16, 324–330 (2003). 3. Rowlands, J. A. Phys. Med. Biol. 47, R123–R166 (2002). 4. Yorkston, J. & Rowlands, J. in Handbook of Medical Imaging Vol. 1 (eds Van Metter, R. L., Beutel, J. & Kundel, H. L.) 223–328 (SPIE Press, 2000). 5. Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. J. Am. Chem. Soc. 131, 6050–6051 (2009). 6. Sutherland, B. R. & Sargent, E. H. Nature Photon. 10, 295–302 (2016). 7. Yakunin, S. et al. Nature Photon. 9, 444–449 (2015). 8. Shrestha, S. et al. Nature Photon. 11, 436–440 (2017).
EC O LO GY
A global plan for nature conservation An international movement is calling for at least half of the Earth to be allocated for conservation. A global study now reveals that, in many ecoregions, enough habitat exists to reach this goal, and ideas are proposed for the next steps needed. J A M E S E . M . WAT S O N & O S C A R V E N T E R
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limate change and biodiversity loss are the two greatest environmental challenges of our time. The 2015 Paris climate agreement states that global warming must be limited to a rise in temperature of less than 2 oC above pre-industrial levels to avoid the greatest impacts of climate change1. This goal has served as a rallying point for global efforts to limit carbon emissions. However, a comparably clear, agreed target for the amount of natural space that should be conserved to address the biodiversity crisis has been much more elusive. Writing in BioScience, Dinerstein et al.2 analyse the current level of ecosystem protection on a global scale, and propose a way to address the problem of biodiversity loss.
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The idea of securing at least half of the Earth for nature conservation has been gathering momentum. Various studies indicate that achieving this goal, often discussed under the name ‘Half-Earth’, coined by the biologist E. O. Wilson3, would help to avoid widespread biodiversity declines, and prevent the collapse of vital services provided by ecosystems, such as carbon sequestration and climate regulation. However, one problem with making this goal a reality is that 50% protection of all terrestrial ecosystems far exceeds current global conservation commitments. For example, the plan4 that is currently accepted by the United Nations Convention on Biological Diversity has a target of protecting 17% of land and freshwater regions and 10% of marine areas by 2020. The much greater scale of conservation
LIANA JOSEPH
NEWS & VIEWS RESEARCH
Figure 1 | Lowland forest ecosystems of the Congo. It has been proposed3 that half of the Earth should be kept for conservation as a way of preventing biodiversity loss. Dinerstein et al.2 assessed the current extent of natural habitat remaining in Earth’s 846 terrestrial ecoregions. Part of the plan for the next steps needed should focus on ensuring that the last remaining intact forests of the world, such as those found in the Congo Basin, are conserved.
needed for Half-Earth protection has left many people questioning whether it is even possible, given that unrelenting habitat conversion is quickly eroding opportunities for large-scale conservation5,6. Dinerstein and colleagues have made a first attempt to answer this key question for land areas, and their findings are both encouraging and disheartening. Achieving an outcome in which half of the Earth is allocated for nature conservation is possible now only in ecoregions that have at least 50% of their natural habitat remaining. Dinerstein and colleagues therefore assessed the level of natural habitat remaining in each of Earth’s 846 terrestrial ecoregions, such as the northeastern Congo lowland forest ecoregion (Fig. 1). For this, they used satellite-derived maps of forest cover, as well as other data to identify patterns of land use and human populations 7. Their analyses reveal that nearly 50% of the world’s eco regions have the remaining habitat necessary to achieve a Half-Earth protection target, and that, of these ecoregions, 12% are now at least 50% protected. The ecoregions with less than 50% of natural habitat remaining would need substantial restoration efforts to achieve this target, and 24% of these ecoregions are described as being in peril given that they have no more than 20% remaining. The authors’ data clearly demonstrate that achieving Half-Earth conservation is indeed still possible in many places, but not everywhere. By assessing the possibility of achieving Half-Earth conservation, Dinerstein and colleagues have helped to take the first step
towards a road map for implementing such a goal. The authors outline the idea of a “global deal for nature”, whereby nations agree to achieving Half-Earth conservation, where possible, by 2050. Such an agreement would offer a biodiversity-focused counterpart to the Paris climate accord. What is needed now is a clear, science-based plan for the next steps to making Half-Earth conservation a reality. We propose that this plan and the science agenda should be based around three fundamental questions. The first question is in many ways the key question in conservation biology: which are the most important places to conserve? The answer must take into account both the scale of the Half-Earth undertaking and the immense diversity of conservation objectives being considered. As a start, to halt imminent biodiversity loss, places that are home to the last remaining populations of a species and the last samples of any ecosystem type should be prioritized. To complement these irreplaceable sites, places that are still large, intact and functioning in ways that are unimpeded by large-scale human activities should also be identified and protected. Such places provide the backbone of planetary and regional-scale ecosystem services, as well as offering the resilience that enables species and ecosystems to self-regenerate and to adapt to stressors that arise from human activity8. Second, what is the scope, severity and trajectory of the threats to local biodiversity, and what are the processes that sustain them? To address this, it will be necessary to understand not only the drivers of habitat loss and
degradation, but also the nebulous effects that other factors, such as the presence of invasive species, over-harvesting, disease, climate change and altered fire regimes9, can have on a habitat. All of these tend to reduce habitat quality without necessarily affecting its spatial extent. All threats are inherently linked to social and economic forces, and an interdisciplinary approach should therefore be adopted to appraise them. Understanding threats can aid assessment of the places where action is most urgently needed to conserve and restore habitats, and can allow conservation efforts to focus on minimizing the number of ecoregions that dip below retention of half of their natural habitat. Finally, for priority areas for intervention, what measures will be needed to ensure that these areas maintain their natural integrity? Although protected areas have been the cornerstone of conservation action so far10, they would probably have a reduced role in a conservation network that scales up to the level of Half-Earth. Instead, the roles of indigenous governance, private conservation tenants, land and sea stewardship arrangements, and payment to land stewards through ecosystem services schemes run by public and private institutions, must all be appraised11. Moreover, in many places, active conservation might not be required to maintain ecological integrity. Completing the research required to answer these three questions should provide an effective framework for reaching Half-Earth by 2050. The analysis produced by Dinerstein and colleagues has shown that this goal is possible in many places, and has helped to embolden this call for action. Half-Earth is about thinking big, and we urge conservation scientists to play their part in this process. ■ James E. M. Watson is at the Wildlife Conservation Society and School of Earth and Environmental Sciences, University of Queensland, Brisbane 4072, Australia. Oscar Venter is in the Natural Resources and Environmental Studies Institute, University of Northern British Columbia, Prince George V2N 2M7, Canada. e-mail:
[email protected] 1. United Nations. Adoption of the Paris Agreement (UN, 2015). 2. Dinerstein, E. et al. BioScience 67, 534–545 (2017). 3. Wilson, E. O. Half-Earth: Our Planet’s Fight for Life (Norton, 2016). 4. Convention on Biological Diversity. COP Decision X/2: Strategic Plan for Biodiversity 2011–2020 (2012). 5. Büscher, B. et al. Oryx 51, 407–410 (2017). 6. Watson, J. E. M. et al. Conserv. Lett. 9, 413–421 (2016). 7. Ellis, E. C. & Ramankutty, N. Front. Ecol. Environ. 6, 439–447 (2008). 8. Martin, T. G. & Watson, J. E. M. Nature Clim. Change 6, 122–124 (2016). 9. Maxwell, S. L., Fuller, R. A., Brooks, T. M. & Watson, J. E. M. Nature 536, 143–145 (2016). 10. Watson, J. E. M., Dudley, N., Segan, D. B. & Hockings, M. Nature 515, 67–73 (2014). 11. Boyd, C. et al. Conserv. Lett. 1, 37–43 (2008). This article was published online on 27 September 2017.
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