Nearly one million species are currently at risk of extinction. This startling fact demands that we rethink how we manage urban and agricultural areas that largely cause habitat loss and degradation. Agroforestry researcher Jo Smith discusses how agroforestry prioritizes biodiversity while addressing our agricultural needs.
Biodiversity in crisis
The biodiversity crisis was one of the headline stories of 2019 when the sobering state of our natural world was exposed by the IPBES Global Assessment Report on Biodiversity and Ecosystem Services (IPBES 2019). The report highlighted that up to one million species are at risk of extinction – many within decades – unless action is taken to reduce the intensity of drivers of biodiversity loss. With over three quarters of the world’s land surface severely altered by the conversion of forests, wetlands and other wild landscapes into agricultural land and urban areas, we need to rethink the way we manage these human environments to make space and provide resources for our wildlife. While conservation efforts have previously focused primarily on attempts to conserve ‘pristine’ nature in protected areas such as national parks, with the continued – and even accelerating – loss of biodiversity, it is clear that this approach alone is not enough to stop the loss of our wild species. The importance of the quality of the landscape that surrounds these protected areas is being increasingly recognized. Agroecological approaches such as organic farming, permaculture, regenerative agriculture and agroforestry are seen as sustainable farming systems that can support biodiversity while also producing food and fiber. These systems emphasize the application of ecological principles and practices to the design and management of farmland, integrating the long-term protection of natural resources as an element of food, fuel and fiber production.
Agroforestry supports biodiverse landscapes
Industrial agricultural landscapes are usually viewed as hostile to wildlife, with a low permeability that restricts movement of species between isolated patches – or islands – of natural or semi-natural habitats. The role of agroforestry in improving the quality of the landscape has only recently been investigated although the value of scattered trees as keystone species in agricultural landscapes has been recognized previously. The ability of agroforestry to act as corridors allowing for the movement of species through landscapes will become of increasing importance under predicted climate change scenarios.
As ecosystem engineers and keystone species, trees provide a range of resources to other flora and fauna and when integrated into agricultural land, they can enhance structural diversity and habitat stability of the landscape. Agroforestry systems by their very nature are more diverse than monocultures of crops and livestock; this increase in ‘planned’ biodiversity i.e. the components chosen by the farmer, increases the associated biodiversity i.e. the wild plants and animals present on the farmland.
There has been a lot of research investigating the role of agroforestry in supporting biodiversity. These studies demonstrate that agroforestry systems support communities of plants and animals that can be as species-rich, abundant and diverse as forests. While agroforestry systems are unlikely to provide habitat for specialist forest species that require large tracts of undisturbed forest or woodland, they can support biodiversity in otherwise open landscapes and allow movement of species between habitat remnants, as well as buffer protected areas from the impacts of more intensive systems.
Windbreaks, shelterbelts and hedgerows are often the only woody habitats in an otherwise homogeneous agricultural landscape in some areas, particularly in parts of the United States. As well as providing shelter for wildlife, they also provide food and nesting resources and act as movement corridors through the agricultural landscape. For example, connectivity, habitat availability and local structure of the hedgerow system have been shown to be important predictors of small mammal populations in lowland farmland in Britain; these animals make their homes in hedgerows, rather than just using them for migration, demonstrating the importance of these linear woody features in landscapes otherwise devoid of wooded habitats.
The impact of agroforestry on biodiversity may extend beyond the landscape scale; there is a correlation between decreasing populations of songbirds in eastern USA and the elimination of shade trees from coffee agro-forests in Latin American countries. Those species in decline were migratory species that overwintered in the southern countries and were found in the forest-like habitats of traditional coffee farms with a diversity of shade tree species.
A keystone species is critical to the survival of the other species within the ecosystem; without them the ecosystem could not survive.
By supporting higher levels of biodiversity, agroforestry systems may benefit from the ecosystem services biodiversity provides, for example, pest control, which can lead to the use of fewer pesticides. Reduced pest problems in agroforestry systems can be attributed to a number of mechanisms including scattered distribution of host plants makes it more difficult for pests to find the plants; one plant species acting as a trap-crop or as a repellent which protects the other crop from herbivore attack; one plant species acting as a repellent to the herbivore; higher predator and parasitoid densities due to higher plant diversity and increased interspecific competition between pest and non-pest species. Trees, hedgerows and other permanent non-cropped areas of agro-ecosystems provide shelter for overwintering natural enemies such as beetles, spiders and wasps, as well as alternative food sources when crop pest populations are reduced following harvest. However, some pests such as slugs have been observed in higher numbers in agroforestry systems because of the permanent vegetation under the trees and shifts in relative importance of pest groups may present novel management problems and influence crop choice.
Farmers can increase the value of agroforestry systems for biodiversity through careful design and management, for example by planting trees adjacent to protected areas to increase landscape connectivity and buffer from agricultural impacts, planting a diverse mixture of trees and shrubs, and sowing a flower-rich mixture underneath the trees.
Despite the many benefits of integrating trees into agriculture, agroforestry is not yet commonly practiced or known. Three key areas essential for promoting agroforestry into the mainstream are research, knowledge exchange and policy. Activity is increasing in all three, supported by agroforestry research organizations such as the World Agroforestry Centre and regional associations including the Association for Temperate Agroforestry in North America, the European Agroforestry Federation and its affiliated country associations, and the recently launched International Union of Agroforestry.
For more information and examples of agroforestry across Europe, visit the European Agroforestry Federation website.