Should we cry for you Poa cuspidata? Long-term change in Philadelphia’s flora, part 2

Why would we cry, and why for Poa cuspidata (early bluegrass), of all things?

Sadly, for several reasons, all reflecting the loss of diversity in our native flora (and fauna), and that’s without even mentioning the drop-by-drop loss of beauty in our world, though we can do that too.

First, to put it bluntly, grassy ecosystems, like the ones where P. cuspidata reside, are under threat globally, largely due to human development and “woody encroachment,” the spread of woody plants into formerly grass-dominated habitats, which reduces light levels and causes the loss of locally adapted species. In the case of P. cuspidata, Barton found that this native grass was already relatively rare in 1818 (not a good sign), and observed it mostly on the New Jersey side of the Delaware. And more recently? Nada. No observations of P. cuspidata have been recorded on iNaturalist within 10 miles of Philadelphia in at least the last five years. Now, the usual “observability” caveats apply to citizen science - that is, even as a relatively handsome grass species, P. cuspidata. is certain to be less conspicuous to the average naturalist than dozens or hundreds of showier species. Nevertheless, that not a single observation has been recorded strongly suggests that this species is no longer present in the region, or is here, but at undetectably low levels.

Who cares though? We have plenty of other grass species around, don’t we? Well, no, we really don’t. Barton’s 1818 catalog includes 156 species of graminoids (grasses, and grass-like plants like sedges), of which only 57 are still found today in the Philadelphia area (according to iNaturalist), a 63% loss in species diversity! In Barton’s time, 16% of all Philadelphia area plant species were graminoids. Today, of the 100 most observed newly established species in iNaturalist, only 1% are graminoids. These numbers are not gospel, but they probably do point to a substantial loss of diversity in grasses in Philadelphia, especially of native species. By comparison, tree species made up only 6% of plants in Barton’s time, but of the 100 most common new species in the area (again based on iNatutalist), trees made up 13% of the total. As Wieczorkowski & Lehmann (2022) put it in a recent paper, “(woody) encroachment is accelerating and grassy ecosystems require urgent attention to determine critical woody cover thresholds that facilitate diverse and resilient grassy ecosystems.”

But there’s more! I didn’t choose P. cuspidata randomly as the subject of this post. P. cuspidata, along with P. nemoralis, another species in its genus that’s disappeared locally since 1818, hosted more species of moth and butterfly larvae in Pennsylvania - 38 - than any other graminoids observed by Barton (based on Narango et al., 2020). That means that as it’s become locally extirpated (or rare enough to avoid detection), those 38 species of insects will have to find an alternate host, which may be challenging, or even impossible.

So maybe hold your tears for P. cuspidata, but feel free to shed them for the dozens of other species that relied intimately on this “keystone” plant. When it disappeared locally, a local foodweb likely had a huge hole poked in it. Why do we have 45% fewer insects than we did just 40 years ago? For a lot of reasons, but dozens, hundreds, or thousands of cases like P. cuspidata clearly play a role.

And just to bring it back to beauty, since I am a gardener first and foremost, a future post will focus on the sheer beauty that we’ve lost over the past decades and centuries, and that most of us never even knew existed, like this:

Cynoglossum virginianum (wild comfrey). Photo by Michelle Wong