About a third of the earth’s land surface forests play a major role in the global carbon cycle and contain a substantial proportion of the world’s terrestrial biodiversity. Forests also provide a broad range of other ‘ecosystem services’- the benefits people obtain from ecosystems.
People often think of biodiversity as a list of species without necessarily considering the roles that species perform in ecosystems. However, in recent decades, there has been an improved understanding of important linkages between species and the way that ecosystems function. These ecosystem services include supporting services such as nutrient cycling, soil formation and primary productivity; provisioning services such as food, water, timber and medicine; regulating services such as erosion control, climate regulation, flood mitigation, purification of water and air, pollination and pest and disease control; and cultural services such as recreation, ecotourism, educational and spiritual values. Deforestation and forest degradation in the tropics and sub-tropics have a large negative impact on terrestrial biodiversity, and thus on the provision of those ecosystem services that are most closely linked to biodiversity.
Forests are diverse ecosystems
Forests are the most diverse ecosystems on land because they hold most the world's terrestrial species. Only a fraction of known species has been examined for potential medicinal, agricultural, or industrial value. Nor do we fully understand how biodiversity contributes to the well-being of the larger global environment. And we are only just beginning to learn how biodiversity helps communities around the world satisfy their economic, dietary, health and cultural needs.
Complete forest loss has the clearest impact on biodiversity, with most forest-dwelling species unable to live in habitats that replace forests. However, it is harder to measure the impacts of changes such as fragmentation and loss of microhabitats. Management often simplifies forests, reducing biodiversity and age range; as older and dead trees disappear, so do many associated species.
Forests as carbon sinks
The impacts of changes to forest and land management on both carbon stocks and emissions, and biodiversity are often complex and non-linear. Changes in the management of forest and non-forest land can contribute significantly to reducing emissions and loss of biodiversity from deforestation and forest degradation. Such changes can include both forest management actions - such as improving the protection and restoration of existing forests, introducing ecologically responsible logging practices and regenerating forest on degraded land - and actions aimed at reducing drivers of forest loss and degradation through changes in agricultural practices.
There is the potential for negative biodiversity impacts if naturally regenerating forest or non-forest ecosystems are converted to plantation forestry. Additionally, REDD+ actions are always expected to contribute to reductions of carbon emissions or increases in carbon sinks, the outcomes for biodiversity can vary greatly, depending on the types of activities, the prior ecosystem state and the wider landscape context. Actions aimed at protecting existing forests from clearance and/or further degradation from fire and the overharvesting of timber and non-timber resources are likely to deliver both the greatest and most immediate benefits for the maintenance of carbon stocks and biodiversity. Where forests are already degraded or converted to non-forest uses, restoration and reforestation can generate rapid increases in carbon stocks but with varying impacts on biodiversity.
Tropical rain forests
Tropical rain forests are the biologically richest terrestrial biome on the planet, hosting 50–90 percent of all plant species. Tropical deforestation is, therefore, the single largest threat to maintaining the planet’s flora and fauna diversity. Estimates of species loss whether 10 or 150 species/day—are at best educated guesses, but many scientists believe that the rate of extinction at the end of the twentieth century constitutes a mass extinction on the order of those driven by nature in the geological past. 
High biological diversity of forests is worth preserving for several reasons; On utilitarian grounds, forest products provide direct benefits to humans. A large proportion of agricultural plants originates in forests, and new plants with potentially high commercial value may yet be discovered. High genetic diversity also enhances plant breeding and productivity.
Protect forest areas
One of the best ways to conserve forest biodiversity is to establish protected forest areas. But these areas must be of a certain size, or consist of a well-designed network of forest areas, to allow the local forest ecosystems to continue operating effectively. The forest surrounding the protected area must then be carefully managed so that it serves as a buffer zone. These surrounding forests also allow local communities to earn a livelihood without infringing on the protected forest.
In particular, the differential responses of species to the landscape matrix (i.e., land uses that have replaced original forests), the effects of forest fragmentation and edge effects can now be modelled and predicted. These theoretical considerations formalize the almost ubiquitous observation that large contiguous forest areas contain more biodiversity (especially species) than smaller and isolated stands. Furthermore, information on both the spatial distribution of biodiversity and responses to different forms of management intervention can be used to guide strategic investments that achieve both significant emissions reductions and biodiversity conservation benefits.
- Parrotta, J., C. Wildburger, and S. Mansourian, Understanding relationships between biodiversity, carbon, forests and people: The key to achieving REDD+ objectives. A global assessment report prepared by the Global Forest Expert Panel on Biodiversity, Forest Management, and REDD+. IUFRO World Series, 2012. 31: p. 1-161.
- cifor. Forests and biodiversity. 2020; Available from: https://www.cifor.org/Publications/Corporate/FactSheet/forests_biodiversity.htm.
- Dudley, N., S. Mansourian, and D. Vallauri, Impact of Forest Loss and Degradation on Biodiversity. 2005. p. 17-21.
- CIFOR. Impacts of forest and land management on biodiversity and carbon. 2020; Available from: https://www.cifor.org/knowledge/publication/4008.
- Angelsen, A., Deforestation–Forestation, in International Encyclopedia of the Social & Behavioral Sciences, N.J. Smelser and P.B. Baltes, Editors. 2001, Pergamon: Oxford. p. 3359-3364.
- Sharma, N.P., Managing the world's forests: looking for balance between conservation and development. 1992: Kendall Hunt.