The way farming changes the land is a major threat to biodiversity. Now researchers suggest varying human land use over time could actually increase biodiversity, especially of rarer species. The findings are detailed in the Proceedings of the National Academy of Sciences.
Past research has shown that variations in a landscape over space can promote biodiversity by creating habitats for many different species to stably coexist. Ample work has also revealed that variations in a landscape over time can similarly boost such coexistence of species.
Scientists knew that human land use erodes biodiversity. However, previous research on these effects charted how these changes impacted biodiversity overall, neglecting the question of how variations in land-use intensity over time might affect biodiversity, says community ecologist Eric Allan at the University of Bern in Switzerland.
A key challenge when studying the effects of land use on biodiversity is that different taxonomic groups of organisms can respond quite differently from one another. This makes it difficult to assess the overall impacts of land use. To address this problem, Allan and his colleagues developed a measure of total ecosystem biodiversity they called “multidiversity.” It computes the average level of species per taxonomic group.
The researchers analyzed the effects of fertilization, grazing and mowing on biodiversity data from 150 plots of grasslands in Germany collected over three years as part of the German Biodiversity Exploratories project. They examined up to 49 taxonomic groups of plants, animals, fungi, and bacteria. They also investigated the duration of grazing and type of grazing animals; the number of times areas were mowed per year; and the amount of nitrogen fertilizer per hectare.
The scientists found that as expected, multidiversity declined strongly as land use intensified, particular for rarer species and above-ground organisms, whereas common species and below-ground organisms were less affected. Common species are often generalists and therefore less sensitive to the impacts of land use; above-ground species are more likely directly influenced by human surface activity.
However, the researchers discovered that changing land-use intensity across years actually increased multidiversity.
“Temporal variation in land-use intensity is probably good for biodiversity because different species do well at different land-use intensities — for instance, some might prefer the short grass of a heavily grazed grassland and others might prefer longer grass in a lightly grazed one,” Allan says. “By switching the intensities between years, a greater number of them can coexist together.”
Varying land-use intensity across years was especially beneficial for rare species. In grasslands where land-use intensity was high overall but varied greatly over time, the diversity of rare species was 31 percent of the maximum diversity seen in all the grasslands. In contrast, in grasslands where land-use intensity was greatest and stayed relatively static over time, the diversity of rare species was 18 percent of the maximum diversity seen in all the grasslands.
“This might be because rare species do well in the years of low intensity and then can hang on in the grassland or in the surroundings during years in which land-use intensity is high,” Allan says.
Allan notes that very large changes in land-use intensity are unlikely to be good for biodiversity. “For instance, dumping a lot of nitrogen fertilizer on an extensively managed grassland would not be a good idea,” he says. “Changing the number of livestock in the grassland or the frequency of mowing seems to be very good for biodiversity, but really drastic changes in land use would almost certainly not be, as this could perturb the ecosystem too much.”
Allan and his colleagues suggest that farmers should not only be encouraged to decrease their overall land use, but also vary land-use intensity across years to help promote biodiversity. Benefiting ecosystems could also benefit people as well — “for example, high plant diversity might be beneficial for services such as carbon storage, which is important for climate regulation,” Allan says. Also, “if changing land-use intensity over time increases pollinator diversity in grasslands, then this could increase pollination rates in surrounding cropland, which is of commercial value as well.”
With longer-term data, “it would be very interesting to look in detail at how the abundances of individual species vary with changes in land-use intensity,” Allan says. “It would also be interesting to see which species benefit most from changing land use intensity — perhaps more mobile species, which can easily colonize the grassland.”