Saturday, June 13, 2015

Odonates in The Pearl: Tanner Springs Park

Over the years I have occasionally driven by Portland’s Tanner Springs Park and its enticing urban wetland, but I never made the effort to stop and spend some time during the odonate season—until recently. The park occupies a single city block in the heart of the Pearl District. If you’re not from around here, the Pearl District was once an area of warehouses, light industry, and rail yards known as the “Northwest Industrial Triangle”. This section of the city has experienced significant urban renewal since the mid-1980s and is now a vibrant area of commerce and urban residences.

Tanner Springs Park is not your textbook city park. It’s a great example of melding “wild” natural, “tamed” natural, and unnatural elements in complementary ways. About two-thirds of the park are what I would describe as unkempt—which is a good thing. It isn’t all manicured lawns, ornamental vegetation, and fountains. Check out the view at Google Maps.

There’s a pond with lots of emergent vegetation along one edge which transitions into an upland landscape of native plants and gravel paths. The pond itself is equipped with a substantial boardwalk so you can get right out on the water and easily see what’s using it. The pond isn’t the only water feature here: there are even tiny streams which flow down from the upper (west) side of the park out of artificial springs (pumped from the pond and treated within the park before emerging from the springs—apparently a first for a Portland park).

An artificial spring and source of one of the tiny streams flowing into the pond downslope.

This park’s name and its water features are a nod to a now subterranean stream, Tanner Creek, that once flowed over this part of the city into Couch Lake—which itself was filled in long ago for development. It was around the turn of the last century that Tanner Creek (the lower section that used to meander through the city) was buried and piped directly to the Willamette River. The history is pretty interesting.

But I’m really here to talk about odonates—which are here, of course, because of the water features. And even though this little man-made wetland is enveloped by an urban landscape, I found a decent variety of odonates largely thanks to the unmanicured state of pond- and streamside vegetation.

The most abundant species were the damselflies: Pacific Forktail (Ischnura cervula), Western Forktail (I. perparva), and Vivid Dancer (Argia vivida). The forktails were at the pond and feeding in the adjacent upland vegetation, while the dancers were almost exclusively at the tiny streams, perching on rocks or adjacent pathways. I was surprised that I didn’t see a single Tule Bluet (Enallagma carunculatum)—a very common and widespread species in this region.

Female Western Forktail (Ischnura perparva).
Male Vivid Dancer (Argia vivida). This species is certainly at Tanner Springs Park only because of the little spring-fed streams.

The dragonflies were represented by at least one individual each of Common Green Darner (Anax junius), Eight-spotted Skimmer (Libellula forensis), Cardinal Meadowhawk (Sympetrum illotum), and Western Pondhawk (Erythemis collocata). I also found a single darner exuvia which appeared to be California Darner (Rhionaeschna californica) based on its size. Particularly because of their knack for dispersal, it can be hard to know which dragonflies simply stop by this little oasis while wandering, and which species actually breed and complete their lifecycle here, so it was interesting to find the exuvia even though I didn’t see any adults of the species during my visit.

Male Eight-spotted Skimmer (Libellula forensis).

Male Cardinal Meadowhawk (Sympetrum illotum).
Female Western Pondhawk (Erythemis collocata).
A darner exuvia, presumably California Darner (Rhionaeschna californica).

The Common Green Darner was actively patrolling for females, so he was clearly interested in breeding, and the Eight-spotted Skimmer and Cardinal Meadowhawk had their lookout posts like they thought they owned the place, so it wouldn’t surprise me at all to learn that a number of species at least make the attempt to breed here given the opportunity (and willing females).

If you ever find yourself in The Pearl on a warm, sunny mid-day during the odonate season, I encourage you to spend a little time at Tanner Springs Park. I’ll do my best to stop by at least a couple more times as the season progresses to see what is flying.

1 comment:

  1. Great place and writeup. There should be so many more such places in our cities. We're fighting in Seattle now to reduce the influence of woody vegetation, the cottonwoods and willows that grow up around all made wetlands until the wetlands themselves are inaccessible and invisible.