Invertebrate colonization of live and dead oak (Quercus rubra) and pine (Pinus taeda) logs in an Ozark stream

Background/Question/Methods

Large woody debris (LWD) is an important component of stream ecosystems. Logs provide stable habitat (by restricting bedload movement as well as the wood itself) for biota of all types and nutrition for some species. LWD inputs include fresh (live) wood resulting from ice storms, windstorms, floods (undercutting erosion, debris flows), beaver activity, and silviculture in addition to dead wood. Yet few studies report comparative use of live and dead LWD of different species. Fresh wood retains some secondary compounds that repel invertebrate herbivores as well as exuding more inorganic and organic nutrients than dead wood, and these vary among species. We postulated that some invertebrates are repelled by certain species of fresh-cut wood, while others may be attracted. We cut live and standing-dead trees (killed by an ice storm 3.75 years earlier) of oak and pine into logs 50cm. long by 15cm. diameter, tethered six of each type (24 total) to the stream bank in a pool of a first order perennial stream 24 October 2012, carefully removed them 14 February 2013, rinsed off meiofauna (53-500 micrometers) and rinsed and picked macroinvertebrates from each, to subsequently analyze which taxa were on which logs. Quantitative benthic samples were also collected.

Results/Conclusions

Overall, meiofauna exhibited significant differences in density among LWD types as indicated by lettering: dead oak BC, live oak A (mean 1074 m^-2), dead pine B, live pine C (mean 419 m^-2). Permanent and temporary meiofauna had patterns similar to this: A, B, AB, A; and A, B, A, A. Nematodes had the same pattern of abundance as the permanent meiofauna with 642 m^-2 on live oak.  Tardigrades were denser on oak (120 m^-2) than pine (p=0.005), but ostracods were denser on pine (35 m^-2) than oak (p=0.013). Overall, macroinvertebrate densities were not significantly different between wood species or live and dead wood. Chironomids were most abundant (ca. 500-1000 m^-2), and showed the same indifference to species and ages of LWD.  Mayflies and snails were also indifferent, but Perlidae stoneflies preferred live and dead pine (p=0.0123) to oak. Heptagenidae and Ephemerellidae mayflies preferred benthic substrates to LWD, but Gastropoda preferred the logs without significant preference between wood species (P= 0.0023).  Our study revealed that some invertebrate taxa apparently prefer fresh-cut to dead wood, some prefer oak, others pine, and some prefer benthic substrate to the types of wood we presented. Students in an ecology class performed this research.