Two of the most important components of biodiversity are species diversity (how many species are out there) and genetic diversity (how wide a variety of genes are present in one species). Islands are often considered to be “hot spots” of biodiversity, because they provide a usually predator-free environment with many ecological niches where species from the mainland are often forced to evolve quickly in order to survive intense competition.
But what about “islands” created by historical land use? When forests are cleared for agriculture, but then revert back to forest, they become islands of secondary forest in a sea of primary (not virgin, but never cleared for agriculture) forest, younger secondary forest, or open fields.
One scientist, Mark Vellund at Cornell University, wanted to find out exactly how land-use history affected the species and genetic diversity of herbaceous plants; he analyzed communities and genes of large-flowered trillium (Trillium grandiflorum) to find out, and his results were published in the November issue of the journal Ecology.
First, Vellund wanted to find out if one of the same trends found on islands – that if you find more species (higher species diversity), you’ll also find more genetic diversity in any single species – held true for landlocked “islands” of habitat. He also sought to determine whether factors besides habitat influence species and genetic diversity. He used large-flowered trillium first because it represents forest herbs in general, which make up most of plant species diversity in northeastern forests, and because of its poor ability to colonize, which would make it especially sensitive to past land use.
When land is cleared for agriculture, large-flowered trillium populations are extirpated or severely curtailed. If that land is taken out of agricultural production, as has happened to much of the land across the region, it becomes secondary forest. Earlier studies have shown that these secondary forests have reduced forest-herb species diversity compared to primary forests, mainly because herbs have a hard time dispersing back into secondary forests. Vellund looked at large-flowered trillium populations in primary and secondary forests in central New York to assess differences in species and genetic diversity and thereby draw conclusions about forest-herb biodiversity in human-altered landscapes.
He chose 27 forest stands, some primary and others secondary, and determined population sizes and genetic variation of the trilliums in each stand, as well as the diversity of all forest herbs between and across stands. Vellund found that species diversity, or the total number of forest-herb types found, and genetic diversity of large-flowered trilliums could be closely predicted by looking at the past history of the land, the size of forest stands, and the number of trilliums in each population.
Diversity was highest in primary forests, larger forest stands, and larger trillium populations, and Vellund’s study also confirmed that with higher numbers of herb species across all forest stands, the genetic diversity of large-flowered trillium also rose, just as on islands. This study confirms previous reports that species diversity in forests that were formerly agricultural fields is lower than in forests that were never totally cleared, and also shows a clear link between secondary forests and significantly lower genetic diversity in large-flowered trilliums and, by association, many other forest herbs.