In this post Rebecca Barak discusses her recent article ‘Restored tallgrass prairies have reduced phylogenetic diversity compared with remnants‘
Tallgrass prairie is one of the most endangered habitats on earth. In my home state of Illinois, USA, back in 1820, almost 60% of the state was covered by prairie – 8,760,750 hectares or 33,825 square miles! By 1980, less than 0.01% of the state was prairie (931 hectares, 3.6 square miles). Thankfully, prairie restoration can be used to gain back some of this lost prairie acreage. Managers restore prairie by planting seeds of native prairie species, and use the small patches of remnant prairie as a source of inspiration (and sometimes seeds as well). But, restored communities, which are established over very short time periods are often not as biologically diverse as remnants, which grew over millennia.
Biodiversity is a common restoration objective. While we often think about biodiversity as a count of species, biodiversity is multifaceted, and can be measured in many different ways. In this study, we wanted to study tallgrass prairies from a phylogenetic perspective, one based on evolutionary relatedness between species. A plant community with high phylogenetic diversity would have distantly related species living together, for example, species from many different plant families. On the other hand, a community with lower phylogenetic diversity might have species from only a few families. Phylogenetic diversity could be important for ecosystem function in prairies. Plant communities with higher phylogenetic diversity are more productive and stable, support more pollinators and other insects, and are more resistant to invasive weeds than those with lower phylogenetic diversity. These ecosystem functions are also important restoration objectives.
When we compared plant biodiversity of restored prairies to remnants, we found that the number of species didn’t differ. However, when exotic species were excluded, remnant prairies had higher numbers of native species. Remnant prairies also had higher phylogenetic diversity by the two measures of phylogenetic diversity that we used, at both small (plot) and large (site) scales.
If increasing phylogenetic diversity of restored communities to the level of remnant prairies is a restoration goal – we need to determine how it can be achieved. Since all the prairies we studied were established from seed, we wanted to know if diversity of the seed mix could be used to predict diversity of the realized plant community. If so, designing a phylogenetically diverse seed mix is an important step towards building a phylogenetically diverse prairie.
This probably won’t be a surprise to anyone involved with restoration, but not all of the planted species could be found (by us) at the site. Depending on the specific restored prairie, we only found between 25 and 77% of all the species that had been planted. Also, we found that seed mix diversity didn’t predict diversity of the whole plant community, but it did predict the diversity of just the species that had been planted. While fire history is important for maintaining phylogenetic diversity in remnant prairies, we didn’t have fire records for enough of our sites in order to draw conclusions on how fire interacts with seed mixes to influence phylogenetic diversity. But, we hope to continue to work on these questions in the future!
Lastly, we found phylogenetic structure to the species included in restoration seed mixes. That is, the species in the mixes were more closely related to one another than we would expect by chance. We also found lots of “missing branches” – families or groups of plants that were found in remnant prairies, but that weren’t included in restoration seed mixes. Some of these missing branches include the families Liliaceae, Orchidaceae, Convolvulaceae and Polygalaceae. These families include some species that are of special concern in Illinois prairies, and some hemiparasitic species, which have been shown to be beneficial in restoration.
Planting species from missing branches in restored prairies could help meet multiple biodiversity goals for prairie restoration. However, this process could be complicated, because seeds of these species may be difficult to obtain, or grow to establishment. In other words, these species may be missing for a reason. In the short term, these hurdles may be addressed by planting plugs, rather than seeds of these species.
Adding species from missing branches would increase overall phylogenetic diversity of restored prairie communities, because species added would be from clades or families that are distantly related to those already present in the community. Planting species that are missing from restoration seed mixes, but found in remnants would also increase similarity of remnant prairies to restored ones in terms of species richness, phylogenetic diversity and overall community composition. If the species to be added are of special concern, adding them to restored prairies could also help achieve species-specific conservation goals. Identifying (and trying to fill in) these phylogenetic gaps could therefore be a way to meet species conservation goals while also increasing multiple measures of community biodiversity, and building restored prairies that are more similar to remnants.