The spatial distribution of species, habitats or biomes is influenced by a range of environmental descriptors. In most cases the environment is represented by climate data, such as temperature or precipitation, which influence where species can grow and reproduce and are therefore informative in predicting or explaining species' distributions. However, the distribution of species or habitats is also influenced by a range of non-climatic descriptors, such as elevation (terrestrial biomes), depth (marine biomes) or chemical descriptors such as salinity.
In addition to the above factors, which regulate the distribution of species as a result of differences in the physiological tolerances of organisms, the availability of resources (e.g. nutrients) also play an important role due to their influence on the growth and reproductive capacity of species. For example, although warming sea temperatures have been observed 1 and are predicted to lead to poleward shifts in species' distributions, 2 persistence in a new area will also depend on a species ability to find food and reproduce.
Environmental descriptors are usually averaged over a time range and over a spatial region, such as global annual means or regional monthly means. Datasets of descriptors may be made up of measured observations or modelled data. Although environmental data may be obtained across multiple spatial scales, data at a fine (≤1 km2) spatial resolution is frequently necessary to capture environmental variability. This is particularly the case in areas with a steep topography or slope, as well as in areas with frequent seasonal fluctuations. Interpolation and modeling may therefore be required to obtain a full dataset of an environmental descriptor. Climate models, for example, are numerical models based on equations of physical laws describing the dynamics of the atmosphere and ocean. A strong foundation in physical principles and a tested ability to reproduce observed climate has given confidence to the ability of climate models to provide credible quantitative estimates of future climate change. This confidence is higher for some variables such as temperature than for others such as precipitation. 1
Environmental descriptors may be provided at regional or global scales and their resolution will vary depending on the source of the data, number and resolution of observations made, or the type of mathematical model that has been used to create them. Most descriptors change over time as environmental conditions fluctuate, therefore descriptors should be used with reference to a given time frame. These are often averaged over specified time periods to account for inter-annual variability.
1. Sea surface temperature
Sea surface temperature is a main control on biodiversity, as the majority of organisms respond to the temperature of their immediate environment. Sea surface temperature is a climatic descriptor and it is controlled by other climatic variables such as air temperature, ocean and wind currents. It is expressed as a mean annual figure, derived from remotely-sensed images and given at a global scale. The biology of many organisms depends on the temperature of their environment, which makes sea surface temperature a useful environmental descriptor for several organisms. Marine species as diverse as mammals, fish and invertebrates show strong phenological responses to seasonal temperature changes. For example, warm-water corals have a symbiotic relationship with photosynthetic algae called zooxanthellae, which live in the coral tissue and provide corals with photosynthetic products. Small increases in the mean sea surface temperature can cause acute temperature stress on corals, which causes them to expulse zooxanthellae from their tissue. 3 This phenomenon, known as coral bleaching, is one of the major concerns for coral reefs in light of climate change which is expected to raise mean annual sea surface temperature by up to 1.5oC, depending on regions. 1
Data on sea surface temperature and other marine environmental descriptors can be downloaded through Bio-ORACLE. This data portal also includes data for future oceanic climate variables, based on the predictions of the Intergovernmental Panel on Climate Change (IPCC).
2. Sea surface productivity
The concentration of chlorophyll pigments (the photosynthetic pigments of phytoplankton) is often considered as an index of biological productivity and, in an oceanic environment, it has been widely related to the distribution and abundance of a range of marine species including fish and mammals. The availability of global, daily, systematic, high-resolution images obtained from satellites has been a major data source for estimating global chlorophyll a concentrations. This information can be expressed as global annual averages which are used to capture spatial variation in productivity, for example that existing between highly productive upwelling regions and nutrient poor tropical waters. Alternatively, the value can be expressed monthly or across a range of months (e.g. winter vs summer) in order to capture the significant seasonal changes in mean sea surface productivity. Mean global chlorophyll a at the sea surface is dominated by the unicellular phytoplankton, which are microscopic photosynthesising organisms that are abundant in the upper oceanic layers. Other important primary producers in the ocean surface are photosynthetic bacteria, macroalgae such as kelps and plants such as seagrasses. 4
Mean sea surface productivity is a useful environmental descriptor because productive surface waters attract several marine organisms that benefit from phytoplankton blooms. Many organisms feed on primary producers in the surface waters, including invertebrates and fish. In turn, this attracts larger marine animals such as whales, dolphins and sharks which often migrate vast distances to reach the most productive surface waters. 5 Any changes in the location, duration and extent of highly productive surface waters is therefore expected to cause matching changes in the distribution, abundance and migration patterns of marine mammals and large fish.
Common environmental descriptors for the terrestrial biosphere relate to climate, water quality and soil conditions, as these are major controls on vegetation growth and biodiversity.
1) Climate descriptors
Climatic descriptors include all environmental variables that change according to weather conditions, such as air temperature, precipitation and humidity. Climatic variables are one of the main controls on the distribution of biomes, as different types of organisms have different optimal climatic ranges within which they thrive best. Climatic data is available from national meteorological stations across the world which collect daily weather measurements. These can then be averaged to obtain monthly or annual measures of mean air temperature or mean precipitation, among others. Climate records can also be extended into the geological past by using proxy data such as ice cores, tree rings and stratified sediments, or projected into the future by predictive modeling.
An array of climatic descriptors can be accessed through the UNEP Environmental Data Explorer. This includes global datasets of mean annual precipitation, mean annual air temperature, humidity and other environmental descriptors relating to climate. Climatic descriptors are particularly relevant for biodiversity in the face of climate change, as comparing climatic changes with changes in species distributions can provide information on any climate-induced range shifts of species.
2) Soil descriptors
Soil is a crucial feature of crop production. The ‘health’ of soil and its characteristics that affect vegetation growth can be measured and used in agricultural management, or to understand natural vegetation patterns across the world. The Harmonised World Soil Database contains global maps of selected soil parameters which are relevant to agriculture: nutrient availability, nutrient retention capacity, rooting conditions, oxygen availability to roots, excess salts, toxicities and workability. Apart from the relevance to agriculture, soil descriptors are useful in explaining the nitrogen and phosphorus nutrient cycles which underpin most ecosystem processes like plant growth and decomposition. They can also be a useful tool in monitoring pollution, as excess levels of organic chemicals in soils are often an indication of agricultural or industrial pollutants.
3) Water Quality Index (WQI)
The Water Quality Index (WQI) uses data on water temperature, dissolved oxygen, pH, salinity and chemistry to derive a composite index of water quality. It has been developed by the United Nations Environment Programme Global Environment Monitoring System (GEMS) Water Programme, and can be used on a global scale as data is available for most countries of the world. The WQI is used to assess the overall quality of inland surface water resources, given the importance of clean freshwater for biodiversity and for human health.
- Some environmental descriptors can be used as indicators of biodiversity and ecosystem health. This is based on the known effect that some environmental variables have on species or habitats, which implies that an observed trend in the environmental variable can be expected to cause a matching trend in the biodiversity which it affects. The Biodiversity Indicators Partnership (BIP) is a global collaboration by expert organizations, led by the Convention on Biological Diversity (CBD), which has developed a set of biodiversity indicators to be used at a global scale. The aim is to establish a set of indicators which can comprehensively describe trends in biodiversity, and which would help to track any improvements toward meeting the Aichi Targets. Several environmental descriptors are used as biodiversity indicators, such as nitrogen deposition (as a proxy of pollution) and the Water Quality Index described above.
- The Marine Strategy Framework Directive (MSFD) 6 is the first EU legislative instrument related to the protection of marine biodiversity. Its overall aim is to promote sustainable use of the European seas and to conserve marine ecosystems, by implementing measures to reach Good Environmental Status (GES) in Europe’s seas by 2020. The MSFD defines ‘good environmental status’ as “the environmental status of marine waters where these provide ecologically diverse and dynamic oceans and seas which are clean, healthy and productive”. To help Member States interpret what GES means in practice, the MSFD also sets out 11 environmental descriptors which describe what the seas would look like when GES has been achieved. The descriptors are:
- Biodiversity is maintained
- Non-indigenous species do not adversely alter the ecosystem
- The population of commercial fish species is healthy
- Elements of food webs ensure long-term abundance and reproduction
- Eutrophication is minimized
- The sea floor integrity ensures functioning of the ecosystem
- Permanent alteration of hydrographical conditions does not adversely affect the ecosystem
- Concentrations of contaminants give no effects
- Contaminants in seafood are below safe levels
- Marine litter does not cause harm
- Introduction of energy (including underwater noise) does not adversely affect the ecosystem.
As these are broad descriptors, each one has a set of detailed criteria, indicators and targets to help its implementation. 7
- IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp
- Cheung, W. W. L. L. et al. Projecting global marine biodiversity impacts under climate change scenarios. Fish Fish. 10, 235–251 (2009).
- Hoegh-Guldberg, O. & Australia, G. Climate change, coral bleaching and the future of the world’s coral reefs. - (1999).
- Thornton, D. C. O. in Advances in Photosynthesis - Fundamental Aspects (ed. Najafpour, M. M.) 598 pages (InTech, 2012)
- Learmouth, J. A. et al. Potential effects of climate change on marine mammals. Oceanography and Marine Biology: An Annual Review 44, 431–464 (2006)
- European Commission. Directive 2008/56/EC establishing a framework for Community action in the field of marine environmental policy (Marine Strategy Framework Directive). Off. J. Eur. Union L164/19, (2008)
- Commission, E. Environment: Marine Directive. Achieve Good Environmental Status. (2014
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