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Bark Bugs: Saproxylic Invertebrates in the Morgan Arboretum

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.

Click Beetle Larva, one of the many invertebrates found under bark at the Morgan Arboretum

Click Beetle Larva, one of the many invertebrates found under bark at the Morgan Arboretum

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.

Dead beech logs, a perfect hiding spot for invertebrates to overwinter in!

Dead beech logs, a perfect hiding spot for invertebrates to overwinter in!

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.

Fire coloured beetle larva, the most common species in our samples

Fire coloured beetle larva, the most common species in our samples

Millipedes are another example of common invertebrates living in the bark of dead trees

Millipedes are another example of common invertebrates living in the bark of dead trees

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.

Maple (Left) v.s. Beech (Right) bark. They have different living conditions to offer to saproxylic invertebrates.

Maple (Left) v.s. Beech (Right) bark. They have different living conditions to offer to saproxylic invertebrates.

Our project

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.

References:

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/

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6 comments on “Bark Bugs: Saproxylic Invertebrates in the Morgan Arboretum

  1. Interesting blog post! you provide some good rationale for your project, and I liked the arguments about why these inverts are important in our forest ecosystems. I am curious about the overwintering: many of the inverts are indeed ‘freeze intolerant’ and produce compounds that enable them to supercool. However, what do you know about those that might be ‘freeze tolerant’? Do you know if any of the inverts that potentially live under bark at the Arboretum are freeze tolerant?

  2. That is a good question! Fire coloured beetles are definitely freeze tolerant as their larvae produce some antifreeze proteins that allow them to survive during our harsh winters. (http://link.springer.com/article/10.1007%2FBF00688622#page-1) Some click beetles use super cooling or decrease the water content of their bodies to overwinter.( http://link.springer.com/article/10.1134/S0012496613030186#page-1) Some earthworms, centipede and millipedes are freeze tolerant as well. However even if those invertebrates are indeed freeze tolerant, finding a shelter for the winter is often crucial for them. They often find shelter in dead logs, cracks in rocks, the soil itself and even our homes.

  3. I work on centipedes, so the saproxylic microhabitat is very interesting to me! Many centipedes are quite specific in the logs they inhabit, and in some this specificity varies with geography. For example, the diamond-backed Geophilus vittatus is versatile in the north, but found exclusively on pine in the southern part of its range (i.e., Virginia) where it competes with a greater diversity of other Geophilus species. However this statement is based on anecdotal and not experimental evidence.

    I would be very interested in seeing centipedes included in your results — if you like, I would be more than happy to assist by identifying any centipede material you collect.

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