by Roberta LC Dayrell & Fernando AO Silveira
There is deep symbolism behind something tiny and apparently inert transforming itself into something allegedly much greater and lively. Beyond their natural role in plant reproduction, every seed carries a promise of fertility and rebirth to us humans. Indeed, this promise is constantly renewed as we have relied on seeds to obtain regular and predictable food supply throughout the course of our history, and especially succeeded with advances in technology.
From the seeds’ point of view, however, the connection to humans is just one chapter of their history that dates back 370 million years. The vast majority of plant species have never been domesticated by humanity, so they have characteristics shaped by natural selection rather than by human-made productivity.
Many plants adapted to extremely nutrient poor soils, for instance, invest little in reproduction through seeds. As a result, these plants often produce very few seeds, or otherwise a large amount of empty or non-viable seeds that will never germinate. In addition, they may also take more than a year to produce seeds, or only rarely do so. After sown, seeds might also require pretreatments – such as scarification of the seed coat, cold stratification period, and hormone addition – to break dormancy before they can germinate. As far as we know, most of the plants in these nutrient-poor areas are able to resprout, but very few of them are able to effectively reproduce vegetatively. So, they really rely on seeds for reproduction. All these characteristics may seem unfavorable for the persistence of plants in nature, but are rather perfectly suitable for them to live in their original undisturbed habitats and cope with nutrient limitation.
The story changes when the seeds are needed for human purposes such as ecological restoration. The wild plants that live in nutrient poor soils just cannot keep up with the demand, and are thus extremely difficult to be cultivated or effectively used in restoration programs in a large-scale. Therefore, seed supply strongly limits restoration of areas that once harbored an extremely high diversity of species, including degraded areas of some of the most emblematic vegetation types in the world, such as the Brazilian campo rupestre, South-African fynbos and the Australian kwongkan.
Despite this huge limitation in sourcing large quantities of seeds, we increasingly rely on them for species conservation. Habitat conversions by humans have already destroyed more than 85% of the original area of the terrestrial hotspots, and at least a third of these (and also other biologically significant sites) occur in nutrient poor soils. This means that a great deal of the extant plant species makes little investments in seeds, and that we face a big challenge in using ecological restoration to conserve them. Restoring environments with exotic species that are able to produce large amounts of seeds commonly results in habitat invasion and further loss of biodiversity and ecosystem services, and thus is not a good option. The invasion of the biodiverse Brazilian campo rupestre by the African grass Melinis minutiflora provides a great example of the danger in restoring environments with alien species.
If we are truly committed to conserve biodiversity, we need to deconstruct symbolic promises of fertility and rebirth carried by domesticated seeds and look at the reality of the seeds that have never been at our service. Restoring species-rich communities implies overcoming major limitations in source, availability, and handling of a high diversity of seeds. Ecological restoration of biodiverse communities is extremely complex, and despite moderate technological progress, there is still a long way to go. We could start by not taking seed supply for granted.
Dayrell, R., Arruda, A., Buisson, E., & Silveira, F. (2016). Overcoming challenges on using native seeds for restoration of megadiverse resource-poor environments: a reply to Madsen et al. Restoration Ecology, 24 (6), 710-713 DOI: 10.1111/rec.12450
Hopper, S., Silveira, F., & Fiedler, P. (2015). Biodiversity hotspots and Ocbil theory Plant and Soil, 403 (1-2), 167-216 DOI: 10.1007/s11104-015-2764-2
Linkies, A., Graeber, K., Knight, C., & Leubner-Metzger, G. (2010). The evolution of seeds New Phytologist, 186 (4), 817-831 DOI: 10.1111/j.1469-8137.2010.03249.x
Merritt, D., & Dixon, K. (2011). Restoration Seed Banks–A Matter of Scale Science, 332 (6028), 424-425 DOI: 10.1126/science.1203083