Invertebrates are a broad category of animals characterized by the lack of a backbone. Surprisingly and most often unknown is the fact that invertebrates amount to a staggering 95%-99% of all animal species (Encyclopedia of Science, 2002). This assorted group includes insects, spiders, crustaceans, and mollusks, all of which are also ectothermic (cold-blooded). Such a diverse group of organisms calls for an indescribable range of adaptations within varying environments (N.W.F, 2000). For example, the adaptations that allow invertebrates in Canada to survive through the seasonal winters, without producing their own body heat.
Terrestrial species of invertebrates unequipped for migration often rely on biochemical processes to survive sub-zero temperatures, as they are unable to function due to a lack of resources and slower metabolic rates during the winter. These invertebrates exhibit freeze intolerance, and avoid freezing through a process known as supercooling. (Block,1991) This ability allows invertebrates to withstand ice formation within their tissues, to a certain extent. To avoid a lethal level of solidification, many invertebrates produce a surplus of sugars and proteins within their bodies, and reduce water levels to lower their internal freezing temperatures. (Aarset, 1982) However, this alone is often not enough for the invertebrate to make it through the winter. Additionally, during the fall they must also locate a relatively sheltered area to spend the winter season, underneath the bark of a dead tree for example.
Saproxylic invertebrates: Roles and Biodiversity
Speaking of dead trees, an organism that is dependent on dead wood at one point in their life cycle, or on other organisms that are dependent themselves on dead wood, is called a “saproxylic” organism.(Speigth 1991) For our project, we focussed on saproxylic invertebrates.
As mentioned before, dead trees offer a good overwintering refuge for saproxylic invertebrates but they also serve as a their main food source and nesting spot. Others are even parasitic of species that nest in dead logs. (Jonsson et al.2012, p.70-76)
Saproxylic invertebrates are a major player in nutrient cycling. They use the dead wood as food source and therefore, not only do they recycle back nutrients into the surrounding environment, the motile ones even disperse the nutrients throughout the forest! They are also the source of food for vertebrates and they create holes in dead wood that are used by mammals and birds to nest. .(Speight, 1989) They might not be the prettiest of inverts but they are definitely a key factor in forest ecology.
The biodiversity of saproxylic invertebrates is influenced by several factors. Species of invertebrates found will be different based on the tree species, with some differences observed between conifers and deciduous trees. (Ehnström, B., Jonsell, M., & Weslien, J.,1998) The stage of decay of the tree, the coarseness of the log, the part of the dead tree (branch or trunk) observed(Ehnström, B. et al.,1998) as well as the amount of sunlight the dead log gets (Ehnström, B. et al.,1998 , Lindelöw, Å, Lindhe, A., & Åsenblad, N., 2005) also all have an impact on biodiversity. Dead wood connectivity is another factor. This term used to express how close dead logs are to each other (speaking in terms of distance here, not emotional bonds). This influences the biodiversity of saproxylic invertebrates because there is more chance of finding specific food sources where there is high connectivity. Also, as we observed in lab, a lot of these invertebrates are not very mobile and will therefore prefer patches of dead wood over scattered individual logs. (Schiegg, K.,2000). To summarize, saproxylic invertebrates can be very picky.
As far as saproxylic invertebrates are concerned, we thought it would be interesting to see whether there were differences in the communities under the first outer layer of bark on two different tree species. Seeing as we were using the Morgan Arboretum as our study area, it made sense to compare maple and beech communities as there are two distinct areas of both sugar maple and beech forest. This means that there would be a higher number of decaying logs to sample, increasing the accuracy of our results, as well as providing a more homogenous environment less prone to edge effects which might influence the invertebrate communities we sampled.
So, just how did we go about getting the answers to our question? Firstly, we found a relatively homogenous area of forest (either maple or beech) and identified dead logs that had a length of at least 1m since that was our minimal distance needed from the log’s edge before we could choose a sampling area. Each log was given a state of decay ranging from one to five, one being little to no decay, and five being heavily decayed and soft. Once a suitable sampling log was found, the top layer of bark was gently prised open with a crowbar, exposing approximately 1600cm2. Three minutes were then allowed to collect all visible invertebrates into jars for later identification and photographing. The removed bark and invertebrates were then returned as found to the sampled area.
Click to see an example of our sampling technique on a maple log, Oct. 19th 2015
An even number of maple and beech trees were sampled at each weekly session to account for temperature fluctuations over the research period.
Aarset, V., Arne. (1982) Freezing Tolerance in Intertidal Invertebrates, 73(4), 576
Block, W. (1991) To Freeze or Not to Freeze, 5(2), 284-190. DOI: 10.2307/2389266
Ehnström, B., Jonsell, M., & Weslien, J. (1998). Substrate requirements of red-listed saproxylic invertebrates in Sweden. Biodiversity & Conservation, 7(6), 749-764. doi:10.1023/A:1008888319031
“Invertebrates.” UXL Encyclopedia of Science. 2002. Encyclopedia.com. (October 30, 2015)
Jonsson, B., Siitonen, J., & Stokland, J. (2012). Other associations with dead woody material. In Biodiversity in dead wood. New York, New York: Cambridge University Press.
Lindelöw, Å, Lindhe, A., & Åsenblad, N. (2005). Saproxylic Beetles in Standing Dead Wood Density in Relation to Substrate Sun-exposure and Diameter. Biodiversity & Conservation, 14(12), 3033-3053. doi:10.1007/s10531-004-0314-y
National Wildlife Federation. (2000) Invertebrates; Philanthropedia
Schiegg, K. (2000). Effects of dead wood volume and connectivity on saproxylic insect species diversity. ECOSCIENCE, 7(3), 290-298. Retrieved October 28, 2015, from http://www.ecoscience.ulaval.ca/en
Speight, M.C.D. (1989). Saproxylic invertebrates and their conservation. Retrieved from http://www.lsuinsects.org/