How many species are needed to maintain ecosystem services in agricultural landscapes ?

GLF 2017 Blog Competition
Thuan Sarzynski

About ten thousand years ago, a hunter-gatherer nomad tamed some wild plants. This technological progress allow our ancestors to settle down and build the civilization we live in today. Since this time, agriculture was developed and now croplands and pastures cover about 40 % of the terrestrial global lands. In those lands, humankind exploits domesticated plants and animals to gain food, fibers and fuel. Those goods are called provisioning ecosystem services. Ecosystem services as defined by the Millenium Ecosystem Assessment include provisioning, regulating and cultural services. These services have a high value that was estimated globally by Costanza to worth about 33 trillions dollars, although they are provided by the ecosystems for free. The products provided by the agricultural ecosystems have a value we can see everyday on our shopping bill. Agroecosystems also provide other ecosystem services which exist thanks to the huge diversity of species present around the field. Nonetheless, for practical efficiency in conservation policies, it is interesting to know how many species are really needed to maintain those ecosystem services.

In the agroecosystem some species are useless to provide ecosystem services. In the soil system live many species which participate in the nutrient cycling. They have a large functional redundancy, several species do the same job, so losing most of the species will not change the quality of the nutrient cycling service. The remaining species will take the job of the disappeared ones. More than being useless, some species are ecosystem service antagonists and instead of helping they make the management of the agroecosystem more complicated. These species are usually animal pests, plant pathogens and weeds. They are commonly called pests. Uncontrolled pests reduce the quantity and the quality of the crop yield which has an impact on the income of the farmer. Losing those species will not have any impact on the ecosystem services and can even enhance them. These species are not important to maintaining ecosystem services ; however, others are key species to providing ecosystem services.

The first very important species are the ones directly introduced by humankind in the ecosystem, the plants we get food from for instance. Obviously, these species need to be conserved in our agroecosystem because they provide goods with a direct market value. All the species which help growing edible species should be preserved. The bees pollinating the crops, the microorganisms enhancing soil fertility and the pest natural enemies need to be conserved to reduce the inputs. Despite the knowledge we have about those useful species, it is very complicated to know which exact species provide a given service. Today, we do not know all the species living on Earth and their characteristics particularly for insects and soil dwellers. Moreover, it is a huge task to identify the properties of each species in a given agroecosystem. We are limited by funding which makes us unable to identify and study each species to know if it is worth conserving it.

As defined by the Millenium Ecosystem Assessment, the biodiversity is the difference and variety of living organisms at all scales. In an ecosystem with high biodiversity the probability to include an ecosystem service provider in this pool of species is higher. Preserving the biological diversity also maintains the resiliency of the ecosystem, the latter is better able to reorganize itself after a disturbance. Species usually have different functions and complement themselves to exploit more fully the limiting resources of their environment. This implies that higher is the biodiversity, more efficiently is exploited the medium and higher is the productivity. In the soil, the activity of arthropods and earthworms eating, cutting and digging in detritus increases the surface area reachable by bacteria and fungi. It facilitates the activity of those microorganisms and accelerates nutrient cycling.

Previously, we focused on provisioning and regulating services, nonetheless agricultural landscapes provide other kinds of services. Biking in the countryside and observing butterflies around colorful fields is entertaining. A naturalist might make a collection of those butterflies and try to identify all the local species. This activity and feeling of joy cannot exist without a diversity of butterfly and other related species. Each species which disappears is a part of humanity culture and heritage flying away. The souvenirs, the description and the stories about this species will fade away. Agricultural landscape managers have the duty to conserve biodiversity to conserve all the cultural service it provides to humanity.

In the future, the biodiversity may be useful as well. Considering the loss of genetic diversity within and between cultivated species, the capacity to feed the people may be threaten. Today, most of our diet depends on only 12 plant species. That is why, to ensure nutrition and food security it is important to introduce higher diversity of crops in our agricultural landscape. Of course, genetic diversity can be conserved ex-situ in seed banks, yet ex-situ plants do not have the chance to continue their evolution race against diseases and fast-changing environmental conditions. For this reason, it is better to keep those species and varieties in our agricultural landscape.

From an utilitarian point of view, there is no reason to conserve the whole biodiversity to preserve the ecosystem services. However, I doubt that today we have a sufficient knowledge and time to identify all the species providing ecosystem services. Thus, the influence of the different interactions between species will have to be investigated too. A short-cut to avoid those expensive and time consuming research is simply to conserve the biodiversity as a whole. In addition, who knows what will come in the future… We are witnessing a high speed global change which may threaten humanity. Maybe our best insurance is biodiversity.

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