For Ozyptila praticola, The Allure Of Tree Trunk Moss May Evaporate With Its Moisture Content

Map of places where Ozyptila praticola has been confirmed
with a mature specimen. Squares, pins, crosses, and circles show
were O. praticola was collected in moss, cones, litter, and
cardboard live traps, respectively.

Tree trunk moss has turned out to be an excellent microhabitat in which to find the introduced crab spider Ozyptila praticola, just as I'd hoped. I began sifting riparian tree trunk moss in early May in my ongoing effort to determine the local distribution of O. praticola in western Washington. In the short time since, I've documented the presence of O. praticola at points along the Puyallup and White rivers, and along entire stretches of the Tolt, Snoqualmie, Cedar, and Green rivers. These new moss-derived records constitute extensions of the known local range of O. praticola in Washington state.

In addition to distribution information, the moss samples also provided a glimpse into the life history of O. praticola. The following observations are based on moss samples I took at sites that I have since confirmed are within O. praticola's local range. In May, I sifted moss at eleven sites and found adult females present at six of them (54% of sites). Adult males were also present in the moss at five of those six sites, but not at any of the other sites. In June, I sampled nine sites and found females at five (55% of sites), but found no males whatsoever. In July I sampled 7 sites and found females at only two of them (29% of sites), and again no males. Significantly, I didn't find any females after July 5th. The number of O. praticola juveniles present also fell dramatically in July to at most a few per site, whereas dozens were often present in May and June samples.

Average monthly temperatures and
rainfall for Seattle, Washington (USA).
Source

The tree trunk moss microhabitat became noticeably drier during the months I sampled. In May, moss was well hydrated and frequently damp to the touch. By mid-July, with the occasional exception of moss low on the trunk and shielded from desiccating conditions by understory vegetation, the moss was often dry enough to pulverize. This trend generally corresponded to the increasing temperatures and decreasing monthly rainfall totals typical for the region during that time of year (see chart, right). Whether seasonal desiccation of the tree trunk moss microhabitat was a contributing factor to the decreasing occurrence of O. praticola that I observed is unknown.

Xysticus pretiosus female with egg sac,
sifted together from tree trunk moss

Presumably, O. praticola was using tree trunk moss as, among other things, a place to mate and deposit egg sacs. However, I never found a female O. praticola with an egg sac in the sifted material, nor did I find any unattended egg sacs. This was in contrast to female Xysticus pretiosis I frequently sifted from the same moss. Female X. pretiosis that I collected in late June and early July usually managed to hang on to their egg sacs through the moss collection and sifting process, and continued guarding them on the sifting cloth. Perhaps any O. praticola egg sacs present were too firmly attached to the moss to be dislodged during sifting, or had been deposited somewhere other than on the moss.

Although I've spent the past few months concentrating my efforts on moss sifting, I still continue to tap fallen cones when the opportunity arises. Where fallen cones are available, tapping them is a quick and convenient way of searching for Ozyptila praticola, I have found. Although the moss and fallen cone microhabitats are quite different and hardly overlap spatially, it is interesting to compare my two data sets from a phenological point of view.

Looking at a chart of average monthly concentrations of male and female O. praticola in cones (data from a total of almost 4,300 cones tapped from 78 sites), I note two things. First, both males and females decrease in concentration from May through July, similar to the occurrence pattern I observed in tree trunk moss. (Where do the surviving O. praticola that leave the fallen cone and tree trunk moss microhabitats go during the summer months?) Second, O. praticola males reach their highest concentrations in fallen cones in April. If the occurrence of O. praticola males also peaks in April in tree trunk moss, then I missed observing that because I didn't start sifting moss until May. I'll need to adjust my moss sampling schedule next year to learn more.

Every method has its limits, and riparian tree trunk moss is no exception. With mature O. praticola apparently abandoning the microhabitat during the dry season, it looks like I may have to wait until autumn rains resume to continue this mode of sampling.