I learned a new ecological term this week. Johnny Randall, the Director of Conservation Programs at the NC Botanical Garden, mentioned it while we were discussing material for an article we are collaborating on for the spring issue of their magazine, Conservation Gardener. Although the term was new to me, the meaning behind it was not unfamiliar.
Three researchers from the University of Tartu in Estonia developed the concept of dark diversity in a paper they published in 2011. Their term was chosen to parallel the notion of dark matter in astrophysics. As with dark matter, dark diversity can be inferred from data, but it cannot be seen or directly measured.
Conservation ecologists, especially those attempting to restore or sustain ecosystems, are using the idea of dark diversity to help them assess the health of the system they are studying. Imagine a healthy ecosystem, one with all its components, where every plant, fungus, insect, bird, etc. that should live there actually does live there. This is almost never true anymore. Humans have fragmented and/or destroyed so much now that nearly every bit of forest, field, stream, coral reef, etc. is missing species that were, until fairly recently, components of those ecosystems.
Thus, these days when conservation ecologists attempt to preserve/restore special examples of ecosystems, often those containing rare plants, they not only must identify the species present on the site; they must also attempt to figure out what species are missing. Species still present can be seen and counted, their viability assessed; this is visible diversity. The absent species, the ones that should be there but aren’t define dark diversity.
I’m oversimplifying the concept a bit, but, basically, ecologists estimate dark diversity by looking at species diversity in the region in which their study site is located. If, for example, a section of forest being restored was missing wood thrushes (one of my favorite summer visitors of our forests), but those birds were known to live in patches of forest in the region, wood thrushes would be identified as part of the dark diversity of the study site – a species that should be there, based on its presence in the region, but is absent.
As I can best understand the concept, by measuring the amount of dark diversity, ecologists can better guess how difficult it would be to restore a given site to full diversity – to bring back all the missing species that should be living on a site but aren’t.
Dark Diversity on Our Five Acres
When Johnny Randall mentioned the concept of dark diversity, it immediately resonated with me, because Wonder Spouse and I have been playing with the dark, as it were, on our five acres for over three decades now. When I first saw this land on a cold January day almost 33 years ago, it was an ecosystem with substantial dark diversity. The previous owner had eliminated almost all the native grasses, wildflowers, shrubs, and understory trees, leaving only towering canopy-level trees and a lawn full of non-native grasses. I could hear birds, but I rarely saw them in our yard. There was nothing for them to eat near ground level, and no good nesting sites.
However, the floodplain forest on the other side of the creek teemed with ferns, wildflowers, and a healthy shrub layer. Bird song echoed across our empty yard from that area. I knew that all of those species should also be living on our side of the creek. Those species not present on our land but living nearby were dark diversity – the missing pieces. I wanted to bring them back.
Through trial and error, luck, and hard work, we have rebuilt much of the species diversity that lived on our property before it was damaged by the previous owner. Fruit-bearing shrubs provide food for an array of species. Dense plantings of shrubs, grasses, ferns, and wildflowers provide food and shelter for insects, reptiles, amphibians, birds, and an array of mammals.
I am certain that our amateur habitat restoration efforts would not pass muster with the professionals, but I think our results speak for themselves. What once was unseen – dark – is now visible. The species that were nearby but absent on our land have returned. I do not have adequate words to describe how deeply gratifying it has been to bring the dark into light, to bring the missing species home.
Contemplating the dark seems an entirely appropriate occupation as we rapidly approach the darkest night – winter solstice. A wise person recently told me that, when navigating dark times, it is important to trust the invisible. It seems wise folks from astrophysicists to ecologists follow this guidance. Darkness – what we cannot see with our eyes – teems with life, with knowledge. It may well be true that we can learn more from the unseen than from our visible world.