I pay £1.50 at the door and a long-haired woman wearing colourful clothes stamps "Outlawed" on my hand. But this is no clandestine rave – I am in a big town hall, the type you might go to for a classical music concert. As I get closer to one of the tables at the centre of the hall, I see plenty of people busy browsing material and talking to the staff at the desks.

A few steps away is a desk stacked with boxes full of little envelopes. Some of these are of brownish recycled paper, others are more colourful, illustrated with images of plants. They carry names like "Marina di Chioggia pumpkin", "aubergine Black Beauty", "Saint-Esprit à Oeil Rouge Bush Bean", followed by the year of collection.

This friendly and diverse crowd was participating in a community seed swap fair known as "Seedy Sunday", which has been taking place in Brighton every year since 2001. The idea is that growers (farmers, allotment holders and garden holders) can exchange seeds of different varieties to enrich their gardens with more diversity. But why are these people bothering to exchange seeds instead of buying them from retailers – and why is this important or even interesting?

Farmers in pre-industrial societies around the world have selected, bred and swapped varieties adapted to different ecological situations and cultural needs and in doing so have produced an immense wealth of agricultural biodiversity – agrobiodiversity. Agrobiodiversity is the raw material that agriculture needs to be able to adapt to a changing environment.

The industrialisation of agriculture has caused an erosion of the diversity of crop varieties. Agrobiodiversity is declining at an alarming rate because growers are increasingly relying on purchased seeds, and the dynamic process that produces and conserves agrobiodiversity has been suddenly interrupted. EU seed marketing regulations have also contributed to this decline by imposing criteria for the commercialisation of seed varieties that are rarely met by locally adapted varieties or landraces. Indeed, seed swappers refer to the seed varieties that are not admitted in the national official lists, which list the varieties that can be sold, as "outlawed".

Many international organisations, recognising the value of agrobiodiversity for the future of humankind, are promoting the conservation of local varieties of crops. Seed banks – huge freezing facilities – have been created to conserve seeds outside their natural habitat (ex situ). Kew Millennium Seed Bank, based in Wakehurst, is the largest ex situ conservation project and an incredibly valuable effort in the preservation of biodiversity on Earth.

However, to conserve agrobiodiversity it is not enough to simply conserve the seeds, it is also necessary to conserve the local knowledge concerning their use and the process of exchange between growers. If we conserve only the seeds but not the process that has in the past created them, we will end up relying only on the breeding of new varieties that occurs in the laboratories of universities and companies. The conservation community is now realising the importance of conserving local seed varieties in their habitat and an increasing number of projects around the world are dealing with this issue.

While Seedy Sundays started just a few years ago, they replicate and continue a tradition of exchanging plant material and knowledge that is at least as old as agriculture itself. The event also highlights the need for regulations and policies that foster the exchange of plant material instead of restricting it. In this sense, events such as the seed swap fair in Brighton are important grassroots initiatives to foster the conservation of the genetic diversity of crops within their habitat – without just locking them in a big freezer.