Saturday, August 27, 2016

Are Douglas-fir Cones As Depauperate In Spiders As They Appear?

Site location map. Click to enlarge.
The other day I tapped cones at two more sites near North Bend, Washington for my Ozyptila praticola project.  No pine cones being available, I tapped 50 Douglas-fir (Pseudotsuga menziesii) cones at each site and found only 1 spider in one set and two spiders in the other.  I've tapped enough Douglas-fir cones over the past year or so to have formed the impression that they seldom hold many spiders.  But is this really true?  I decided to run the numbers and find out.

Since I began my Ozyptila praticola project in May 2015, I've tapped 31 sets of black pine (Pinus nigra) cones and 35 sets of Douglas-fir cones.  These are two of the three species of cones I've tapped most frequently for this project, and they have similarly sized cones.  (Western white pine, P. monticola, is another frequent cone source but has significantly larger cones, so I excluded it from this analysis.)  Most sets consisted of 50 cones, but since not all did I standardized my data by using average number of spiders per cone per set in my calculations.  All cones tapped were from low-elevation (< 300 m) sites in western Washington.

A Douglas-fir cone: boring but worthy of tapping!
My impression about spider density in Douglas-fir cones was correct; there was a significant (P < 0.0001) difference in the average number of spiders per cone in black pine and Douglas-fir cones, a two-tailed student t-test showed (t = 4.0682, df = 64).  Black pine cones had an average of 0.28 spiders per cone, whereas Douglas-fir cones had only 0.10 spiders per cone on average, an almost 3-fold difference.

I've collected about 20 species of spiders each from black pine and Douglas-fir cones.  (A few specimens still await identification, so the exact number of species may change, but not by much.)   There are 11 species in common to the black pine and Douglas-fir cone lists.

My conclusion is that Douglas-fir cones are indeed low in number of spiders relative to black pine cones, but they're equally species-rich.  Their low relative spider frequency makes Douglas-fir cones less exciting to tap, but they're still very much worth tapping!

Oh, for those keeping track, none of the three spiders I tapped from cones near North Bend the other day were O. praticola.
View of Rattlesnake Ledge from Cedar River Watershed Education Center

No comments:

Post a Comment