The world's ecosystems are losing biodiversity fast. A satellite mission designed to track changes in plant functional diversity around the globe could deepen our understanding of the pace and consequences of this change, and how to manage it.
The ability to view Earth's vegetation from space is a hallmark of the Space Age. Yet decades of satellite measurements have provided relatively little insight into the immense diversity of form and function in the plant kingdom over space and time. Humans are rapidly impacting biodiversity around the globe1,2, leading to the loss of ecosystem function3 as well as the goods and services they provide4,5. Recognizing the gravity of this threat, the international community has committed to urgent action to halt biodiversity loss6,
We currently lack consistent, repeated, high-resolution global-scale data on the functional biodiversity of the Earth's vegetation2,10,
The data and knowledge gap
Plant functional biodiversity encompasses the vast variation in the chemical, physiological and morphological properties of plants, such as the concentration of metabolites and non-structural carbohydrates in leaves and the ratio of leaf mass to leaf area. These attributes are related functionally to the uptake, allocation and use of resources such as carbon and nutrients within the plant, and to the defence against pests and environmental stresses.
Functional properties vary within and among individuals (for instance, as determined by the position of a leaf on a plant, or a tree in a forest), populations, species and communities, and may be measured at any of these levels of biological organization. With increasing spatial scale (and thus decreasing spatial resolution of measurements), the capture of functional properties may increasingly represent the aggregate properties of many individuals and species, reflecting the functional biodiversity of whole communities. Aggregate ‘functional diversity’ metrics that characterize the breadth of functional properties of a group of organisms are known to be strongly associated with taxonomic13 and phylogenetic14 measures of biodiversity and their potential decrease under habitat loss15. Plant functional biodiversity is also closely linked to ecosystem processes such as carbon, water and energy exchange, which enables a direct integration with Earth system models16,17. Global information on the functional composition and diversity of plant communities thus provides a necessary foundation for monitoring, understanding and predicting the productivity of ecosystems, and for relating productivity and carbon uptake to other critical ecosystem services.
Available global data on plant functional biodiversity are grossly incomplete and non-representative taxonomically, geographically, environmentally, temporally and functionally. Although datasets of traits and their connection to function continue to grow18,19, local observations of plant functional traits are limited along multiple dimensions. On average, only around 2% of currently known vascular plant species have any trait measurements available at the regional scale (here defined as a 110 × 110 km grid cell, n = 11,626); in the species-rich tropical regions, this figure is even smaller (Fig. 1). Data on other biodiversity attributes such as species occurrence, abundance and biomass hold similar biases20,21. These spatial and environmental data gaps and biases are exacerbated by even scarcer information on temporal variation in plant functional biodiversity. Even in areas for which current data are relatively complete, widespread biodiversity change driven by anthropogenic pressures is rapidly outpacing incremental gains in our knowledge of the Earth's biodiversity afforded by in situ biodiversity sampling22. Furthermore, existing ‘global’ datasets have not been collected consistently or systematically, but instead compiled post hoc from thousands of disparate research activities, often not designed to address long-term trends or large-scale patterns23. These severe sampling inhomogeneities and resulting biases cannot be readily overcome statistically, and continue to impose severe limits on inference and application in global biodiversity science21,24,25. An integrated system for rapidly and consistently monitoring plant functional diversity globally is thus urgently needed.
Filling the gap
Remote sensing has already proved to be a pivotal technology for addressing the global biodiversity data gap. Data on plant productivity, phenology, land cover and other environmental parameters from MODIS (moderate resolution imaging spectroradiometer) and Landsat satellites currently serve as reasonably effective covariates for spatiotemporal biodiversity models based on in situ data12,20,26. However, the coarse spectral resolution of current satellite-borne sensors has prevented a more direct capture of biodiversity, and correlative models are limited by the above-mentioned data gaps.
In contrast, imaging spectroscopy is a well-established, continuously advancing technology capable of monitoring terrestrial plant functional biodiversity in a way that is vastly richer and more sensitive than other remote sensing techniques22,27,28. It captures environmental information at extremely fine spectral resolution by simultaneously mapping the reflectance and emission of light from the Earth's surface in hundreds of narrow spectral bands, producing essentially continuous spectra from the visible to infrared wavelengths29. Distinctive features are imprinted in these spectra as light interacts with the chemical bonds and structural composition of plants. Spectra are thus an aggregate signal of the chemical and structural composition of vegetation, and can be directly related to a number of leaf biochemical and morphological functional traits (Table 1)30,
A global biodiversity observatory
Scaling up processes from fine-grained local studies to larger regions (and ultimately the entire globe) is an urgent challenge for all of the Earth sciences. Environmental understanding at larger scales requires observations that capture dimensions of the entire system to place the microscale measurements in context. Plant functional biodiversity observations from space have the potential to provide a global context for biodiversity science, and to link the evolutionary and functional diversity of plants at local scales to ecosystem function around the globe. Such information would link key dimensions of diversity to ecosystem processes including the carbon cycle, the water cycle and the provisioning of ecosystem services. And it would revolutionize large-scale research on the stability and resilience of ecosystems to shocks such as drought, fire and pathogen outbreaks. Several space missions planned for launch within this decade60 — such as EnMAP (German Spaceborne Imaging Spectrometer Mission)61 and HISUI (Japan Aerospace Exploration Agency, JAXA)62 — will have some capability for mapping plant functional diversity over limited geographic areas. However, none of these will provide the spatial coverage, repeat frequency or mission duration needed to monitor ecosystem-relevant changes in global plant functional biodiversity through time. Satellites technology such as that proposed for HyspIRI63, a mission that was called for in the 2007 National Research Council (NRC) Decadal Survey64, would be able to serve the initial remote sensing capabilities of the envisioned global biodiversity observatory, but no satellite development process or launch date has yet been determined.
Predicting how ecosystems and the services they provide will respond to accelerating environmental change requires more comprehensive, globally consistent and repeated data on the patterns and dynamics of functional biodiversity. Advanced observing technology (which is available but not yet deployed at scale) integrated with in situ measurements65 could transform this situation. The envisioned global biodiversity observatory offers vastly more biologically relevant and spatially and temporally highly resolved information about vegetation than any existing or otherwise planned global sampling or observation scheme. Rates of change today are so high that the longer a global spectroscopic mission is delayed, the more biological information is irretrievably lost22. The earliest possible launch of a mission able to spectroscopically monitor key plant functional traits globally is an urgent priority for understanding and managing our changing biosphere.
This study is an output of the ‘Biodiversity from Space’ Working Group of the National Center for Ecological Analysis and Synthesis (NCEAS) and was produced with support from the National Aeronautics and Space Administration (NASA) grant no. NNX14AN31G to NCEAS, University of California, Santa Barbara. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. The work also benefited from National Science Foundation (NSF) grant nos GEO-1408965 (or “Support for the Future Earth Interim Director and Implementation”) and DBI-1262600; NASA grant no. NNX11AP72G to W.J. and R.G.; NSF–NASA Dimensions of Biodiversity grant no. DEB-1342872 to J.C.B.; and the University of Zurich Research Priority Program on ‘Global Change and Biodiversity’ to M.E.S. and F.D.S.