There are almost 3000 species of vascular plants in Missouri. Which ones should we conserve at Shaw Nature Reserve?
One of the most challenging questions in restoration ecology is what species should live in a restored habitat? When we assist ecosystem recovery by removing invasive species or applying prescribed fire, for example, some plant species reappear on their own. They emerge from seeds that were dormant in the soil, or they are transported in from elsewhere by wind, water, or animals. But other species require assistance.
In ecology, the set of species that could live in a given habitat but are currently absent is called “dark diversity”. Quantifying dark diversity can help set restoration targets by highlighting gaps in species composition. A starting point for calculating a site’s dark diversity is to take a regional species list and subtract out the species present at the restoration site.
To take an example from near the top of the alphabet, at Shaw Nature Reserve, we have observed one species of false foxglove (Agalinis tenuifolia; the most common species statewide) out of nine that are known to occur in Missouri. Of the other eight, seven are known from the same county as Shaw Nature Reserve or from adjacent counties, and all occur in habitats like glades, woodland edges, and prairie swales, which we have restored or reconstructed. It may be reasonable to include these seven species in our enumeration of Shaw’s dark diversity and consider them as candidates for (re)introduction.
Why haven’t these seven missing Agalinis species appeared spontaneously? If they were once present at Shaw, were their seed banks depleted by decades of cattle grazing prior to Missouri Botanical Garden’s purchasing the property in 1925? Are they now unable to disperse to Shaw on their own? This seems likely as Shaw is situated in a fragmented landscape, and Agalinis species do not have any obvious mechanisms for long-distance seed dispersal. Another possibly, not mutually exclusive, is that local environmental conditions are unsuitable. Important symbionts in the soil might be absent, or appropriate host plants, since Agalinis species are hemiparasites that latch onto other plants’ roots to steal sugars.
Climate change makes it even more difficult to think about how to restore a site’s dark diversity. For instance, green false foxglove (A. viridis) is currently found far to the south of Shaw Nature Reserve, in southern Missouri, Arkansas, Louisiana, and Texas. But even optimistic climate models suggest that by 2070 Franklin County, MO may feel more like present-day Franklin County, AR – a county where A. viridis has been collected.