Where has all the nodding trillium gone, or: what’s been happening to Philadelphia’s flora for the past two hundred years?
Barton is a little unclear about exactly what area he traversed in his botanical expeditions, describing them as “within a circuit of ten miles around Philadelphia.” But it’s clear from his place descriptions that he meant Philadelphia and a substantial part of the neighboring counties. These days, “citizen science” tools like iNaturalist, allow us to extract observational data like Barton’s from a large number of observers over any size area you like.
Combining these tools with Barton’s work allows us to ask some of the most obvious questions about how Philadelphia’s flora has changed since 1818. Note that tools like iNaturalist come with lots of caveats, like observability. For instance, Barton must often have been down on his hands and knees examining the aquatic plants along the edges of New Jersey swamps; it’s not clear how many modern users would do the same. Nevertheless, it does allow us to make some gross observations about what’s going on out there, especially when using many thousands of observations.
To compare Barton’s census to a modern data set, I downloaded all “research grade” observations of plants within a 10-mile radius of Philadelphia’s city hall over the last five years. I sorted through those 67,954 observations to identify the 1,660 unique taxa that our modern naturalists observed and then compared them to Barton’s compendium.
Four results really struck me:
1. The variety of plants in the Philadelphia region today is vastly higher than it was in Barton’s time: 1,660 vs. 980. Of course that unfairly compares the efforts of one (very dogged) researcher and his compatriots to 5,532 unique, but far less expert (on average), modern observers. It seems very likely that the modern observers covered much more territory than Barton ever could have, and while they are guided by the app to indicate if a species is “cultivated,” i.e. in a garden, that field is almost never marked (its default is “no”), suggesting that some observations of plants may be in folks’ front yards. All that being said, it does seem likely that there has been a real increase in plant variety here, as we’ll see next.
2. We have lost an enormous number of Philadelphia native species since Barton’s time. When Barton created his compendium, almost all documented plants 857 (88%) were native to Philadelphia or adventive here (meaning they were native to North America, but had moved here from their original range; nativity data are based primarily on the Biota of North America Program). But based on the data from iNaturalist, a staggering 389 (45%) of Barton's Philadelphia native species have not been seen here in the last five years! By contrast, only 36 (30%) of the exotic species noted by Barton have not been recently observed here, and may be presumed lost - in other words, native species have seen greater species loss here over the last 200 years than introduced exotics! Even with the caveats above, this suggests that populations of locally native species have been heavily diminished, to the point of undetectibility, or their extirpation (like extinction, but at a local level). A great many of these native species are uncharismatic plants whose loss few would notice, but some are quite spectacular, like the nodding trillium (Trillium cernuum), shown on the previous page.
3. Newly observed species are overwhelmingly exotic. Since Barton’s time, 76% of newly appearing plants in Philadelphia (based on a sample of the 100 most frequently observed new species) have been exotic - species like: lesser celandine, garlic mustard, red deadnettle, common mugwort, and princess tree. These names strike fear in the hearts of gardeners who know all too well how easily these exotic invasives spread and implant themselves in our landscapes. Meanwhile, only roughly 24% of newly observed plants are native here.
4. These data collectively suggest that while we've seen substantial growth in Philadelphia's flora since 1818, that has been driven overwhelmingly by the introduction of new exotic species, and obscures the loss of almost half of our native species. It’s interesting to note that in mapping the spread of these newer generations of invasives, the Philadelphia region has been a beachhead for the establishment of countless exotic invasives, as in the map below.
Ilex crenata (Japanese Holly) is an exotic invasive. In this map from The Biota of North America Program (BONAP) http://www.bonap.org/, light blue counties are those where the species is present. Philadelphia, Montgomery, and Bucks Counties are all indicated, as are several in the New York and D.C. areas. As major points of entry for the nursery trade, and long-standing centers of horticulture, patterns like this tend to repeat themselves.
As for where the nodding trillium went, we’d need some real botanists to answer that, but it seems reasonable that it’s a victim of some of the same pressures that are reducing species diversity everywhere; perhaps most importantly: habitat loss due to sprawl, and a changing climate. Trilliums are also notoriously slow to grow from seed, so poaching of mature plants is often a problem.
And who knows, perhaps T. cernuum’s pollinator is in decline; or the ant species that are attracted to the fleshy part of its seeds. Those seeds, which are rich in nutrients, are carried back to the ant nests to be eaten, and thereby unintentionally distributed far from the parent plant, spreading the local population. Or maybe populations of T. cernuum’s mycorrhizal partners are collapsing, or the plant has a high sensitivity to airborne pollutants, or, or…
While this trillium is globally secure according to Wikipedia, it is vulnerable (or worse) in at least a dozen states and provinces. In particular, it is critically imperiled in Illinois, Indiana, and West Virginia and is thought to be extirpated from Ohio. This species has still recently been observed nearby, mostly west of the city, in the Piedmont, so perhaps it could return at the hands of an intrepid gardener, bringing its complex ecosystem along with it. Now for Barton’s 388 other missing species…