Ecosystem services

What are ecosystem services?

Ecosystem services are the benefits that nature provides to society. Humanity is totally dependent on natural processes for our survival, health, wellbeing and the functioning of our economies. The term ecosystem services is used to encompass the range of these natural processes. Some examples of ecosystem services are:

• Pollination of crops

• Soil nutrient recycling

• Well being from enjoying nature

• Taking in carbon dioxide from the atmosphere

• Providing raw material for art, literature, poetry and other culture

In the past 20 years natural scientists and economists have used the idea of ecosystem services to try to measure and explain the ways that different natural processes contribute to humanity. It has now become an important way that decision makers like governments and intergovernmental organisations talk about managing the natural world. Quantifying some ecosystem services has allowed monetary values to be attached to them. This has then enabled aspects of management of the natural world to be considered with more traditional economic factors in decision making. Not all ecosystem services can be quantified, and not all of those which can be quantified can have a monetary value attached to them. It is relatively easy to put a monetary value on carbon dioxide removal because carbon dioxide is traded on global markets ('carbon trading'). We can measure people's improved mental health from encounters with nature, and we might be able to link that to a reduction in their use of mental health services, or better productivity at work. But it is probably more difficult to measure and understand what the monetary value of nature as art or literature might be. For many ecosystem services it may not be relevant or appropriate to link them to a monetary value.

What ecosystem services do urban trees provide?

Trees in towns and cities provide a wealth of ecosystem services such as:

• Improving air quality by taking pollution from the air

• Providing homes for urban wildlife

• Storing and taking in carbon dioxide

• Reducing flood risk by increasing evaporation of rainfall and reducing water flow from the land surface

• Keeping cities (and citizens) cool in hot weather

• Providing links to our past

• Shaping the character of towns and cities

• Improving individuals' well being

It is important to recognise that urban trees also incur costs or 'disservices'. There are costs of routine tree management, but also costs associated with the risk of subsidence for buildings, disruption of pavements and damage done if trees or branches fall on property. One important reason for calculating monetary values for tree ecosystem services is to allow those in charge of environmental management to balance the costs against benefits when deciding whether to plant, or how to look after, urban trees.

How are ecosystem services calculated in Treezilla?

The Treezilla site is built on an open source software platform called OpenTreeMap2. This uses data from a set of tools for evaluating ecosystem services of trees in the USA called iTree. The iTree tools are themselves built from many years of research on thousands of trees of different species. This has allowed researchers to understand how different ecosystem services relate to the size and species of trees. They have used this information to produce mathematical models of the relationship between tree size (measured by, for example, the circumference of the trunk) and the amount of ecosystem services it provides. Some of these models are based on hard data, some are based on expectations or predictions of other models. The ecosystem services values are then multiplied by a monetary value for each service to give an overall monetary value per tree.

In Treezilla we have taken this US system and translated it to be relevant to the UK, with species information that is relevant for UK urban trees, and monetary values that are relevant to the UK.

Treezilla includes valuations for carbon sequestration and storage, air quality improvement, energy reduction (through cooling and hence less need for air conditioning) and flood risk mitigation.

Can I believe the numbers?!

Yes! But their limitations should be understood. The models that generate ecosystem services values in Treezilla are designed to apply to surveys of large numbers of trees, so the 'real' values for any one tree might be skewed by that tree's individual circumstances. However, for an 'average' tree in an average urban setting, the ecosystem services values and valuations can be considered to be in the right ballpark. It is also the case that any numbers generated from models are only as good as the models that create them, and the models are only as good as the data that they are based on. There is certainly room to improve our understanding of the relationship between urban trees and their ecosystem services through more research.

It is also important to recognise that the numbers in Treezilla relate to typical conditions in UK urban areas. The site will generate ecosystem services values for trees in the countryside, but many of these are not really applicable. For example, an oak tree in a polluted city will be carrying out air pollution removal, but a similar sized tree in the countryside, with less polluted air, will not be removing as much pollution. Treezilla will give the same air quality improvement ecosystem services value for both trees, because the calculations assume that trees are in urban environments.

Is 'putting a price on nature' a good idea?

Many people have criticised the idea of putting a price on nature by quantifying and monetising ecosystem services. There are arguments for and against this approach. Some people argue that, because decision making is overwhelmingly based on economic considerations, quantifying ecosystem services allows us to put nature on a level with other things like job creation, productivity and GDP (gross domestic product). Others argue that talking about nature in monetary terms devalues it when we should protect nature for its own sake, and appreciate the benefits. There is undoubtedly truth in both these positions and the many different opinions that people hold. It is certainly useful for environmental managers to speak to those who hold budgets about the benefits of managing nature, alongside its costs. But it may well be that winning people's hearts and minds to support protecting and caring for the natural world is best done without talking simply about money.