A gradually shrinking pool: the unsung loss of biodiversity

Most of us are familiar with one or more plants that have been designated as sensitive or threatened in some way with extirpation or even extinction. Unfortunately, for every plant known to be dwindling, there may be others that decline in silence.

The story of Buddleja sessiliflora, the Rio Grande butterfly bush, began in southern Arizona in 1883 when it was first collected in the Tucson area. By the early 1900s the plant was known to be common in the Santa Cruz River valley and springs in the foothills of the Santa Catalina Mountains, although it was extirpated from the Santa Cruz River after it ran dry in the 1940s. In the Coyote Mountains, it was still recorded as “common” in 1973 but as “uncommon” by 1981. In 2013 a small group of biologists found a single plant struggling at a drying spring where once it had been considered an invasive weed.

Buddleja sessilifora is in no way threatened with extinction in its range that extends far down into the tropical deciduous forests of Sonora, Mexico. There, it is known as a small tree up to five meters in height. In the Sky Island region of Arizona - the very northernmost part of its range - it presents as an herbaceous shrub with pale flowers that emit a scent of vinegar and chocolate and draw a multitude of insect pollinators, including flies, bees, butterflies, and ants. The plants are always found at riparian locations, in moist soils near springs and along streambeds or where water collects.

In 2014 my husband and I, intrigued by the lone specimen from the Catalinas, decided to revisit the sites where Rio Grande butterfly bush had been collected in southern Arizona. It wasn’t a particularly ambitious undertaking as there were only seven known locations, one of which was the dying spring by the Catalinas, an inaccessible site on the Tohono O’odham Nation, and the now-vanished Santa Cruz River population. The remaining four locations were scattered in local mountain ranges and waterways: the Coyote, Tumacacori, and Pajarito mountains, and in the Sonoita Creek basin. An additional two locations were found near the Mexican border at Pena Blanca Lake and in the San Luis Mountains.

The low numbers of mature plants at these locations was striking. Even more striking was the almost complete lack of seedlings or propagules, despite some locations still having ample water. After visiting and searching all these locations, we found a grand total of 29 individual plants. The most surprising part was that we found the species had no mention whatsoever of its rareness in any of our local biodiversity databases, such as the Arizona Game and Fish Department’s Heritage Data Management System or the NatureServe network. Buddleja sessiliflora was quietly slipping out of Arizona and nobody had noticed a thing.

The reasons for the losses are not entirely clear. Groundwater declines are clearly a problem for the Rio Grande butterfly bush, but not all locations were drying. The Catalina foothills site is now part of a golf course. One very wet site is heavily invaded with non-native carrizo grass (Arundo donax), and another by native poison ivy (Rhus toxicodendron). Several other sites had been disturbed by roads and spring boxes, although not recently. Sonoita Creek appeared to our eyes to contain many areas of suitable habitat, yet we found few plants. This area is heavily grazed, though. Temperatures have increased across the region, which could decrease pollen viability and account for the lack of seedlings, yet a local plant nursery easily germinated seeds. Perhaps all of these things together are in some way driving B. sessiliflora out of Arizona, in a death by a thousand cuts.

Despite the depressing results, the upside to our survey is that today the plant is recognized as “critically imperiled” by the state of Arizona. Unfortunately, this status alone can do little to halt the range contraction of the Rio Grande butterfly bush as it leaves the state, nor help us understand the implications of losing this humble plant. Will certain pollinators or herbivores suffer from the loss? Will some of them also quietly decline in obscurity, with unknown consequences for their surrounding biodiversity? And perhaps most importantly, what other peripheral species are slipping away, out of our sight, leaving empty unknown spaces behind?