Paper: Judge, J. and Barry, J. P. (2016), Macroinvertebrate Community Assembly on Deep-sea Wood Falls in Monterey Bay is Strongly Influenced by Wood Type. Ecology. Accepted Author Manuscript. doi:10.1002/ecy.1546
Local ecological communities form in the deep sea via various temporary processes – whales die and fall to the bottom, hydrothermal vents ebb and flow in volcanic activity, and sometimes trees will die and eventually find their way into the ocean, a phenomenon called a wood fall. Animals and microbes from the surrounding area colonize these new areas, taking advantage of the newly arrived source of nutrients. The organisms that are successful colonizers had to go through the proverbial abiotic and biotic ringer to utilize this new resource. First, deep sea currents had to be in their favor, delivering them to the wood fall. Second, they have to have the right physiological adaptations to take advantage of the nutrients. Third, they have to fit into the new community that’s created by the wood fall – they have to fit somewhere in the food web: at the top, bottom, or somewhere in between. Biological, physical, and community interactions all work together to determine what species are able to make it and how many individuals can survive.
Wood falls happen every so often in the marine environment. Trees die for a variety of reasons, and get to the ocean often via rivers or large storm events. Those trees then sink to the bottom, and in some cases, sink to deep depths, where the nutrients within the wood help support diverse communities in a nutrient poor environment.
Animals that colonize these falls do so in stages (Figure 1). The first stage involves wood boring clams that create holes in the wood when they consume it. The second stage involves grazers like limpets that eat the biofilm microbes degrading the wood. The third stage is composed of opportunists like the squat lobster that are drawn to the wood fall to eat the animals consuming it. The fourth and final stage involves filter feeders like crinoids that use the wood fall to find a space on the seafloor to live.
The researchers at the Monterey Bay Aquarium Research Institute and University of California at Berkeley wanted to study the dynamics of these pop up communities and how the wood itself influences their development. More complex habitats, regardless of where they are, support more biodiversity – think of the biodiversity in a coral reef vs the biodiversity of the open ocean – so the researchers hypothesized that the more complex wood falls would support a more diverse community. They also hypothesized that the nutritional content of the wood (large tree vs small branch) would be a major factor in both the colonizers present and the longevity of the site: ie, the rate at which the material is consumed.
To test their hypotheses, the researchers intentionally sunk 10 different types of trees native to the California coastline. The samples ranged from the base of a tree, to solid logs, to bundles of branches, to leafy fronds to give the researchers a full range of insight as to what types of wood produce different communities. They weighed and measured each piece of wood before the pieces were bound in mesh for remotely operated vehicles (ROVs) to carry to the bottom. They then left the bundles at the site, undisturbed and unobserved, for two years, then brought them up for processing. All animals within and on top of the wood were brought back to the lab and identified to the lowest taxonomic level possible.
Results and Significance
All 28 bundles of wood were recovered with a total of 7661 organisms on them. Palm trees had the most organisms on them by quantity and redwoods and tree ferns had the least (Figure 2). The organism type that was most abundant on each wood type varied, and was reported as relative abundance (number of that group of animals over the total number of organisms present). Recruitment on redwood trees and tree ferns was likely so low because those trees have strong chemical deterrents against terrestrial grazers.
All pieces of wood that were composed of branch bundles rather than logs attracted more different animals. The complexity of the habitat allowed more animals the space to coexist. Physical characteristics of the wood also had an impact on the colonizing patterns of the animals. For example, wood boring clams colonized logs from the cut ends rather than boring through the rough, thick bark.
The researchers only kept the wood underwater for two years, which is a short time for deep sea studies. Thus, they didn’t see as many opportunists or filter feeders (the organisms that colonize the wood fall in stages three and four) on the wood. That fact alone tells us that these communities, while temporary, take years to fully be developed.
This study also found several species unknown to science among the wood fall communities, an enduring reminder that so little is known about the organisms of the deep sea.
Go out to your own backyards with an insect guide and look at what happens to terrestrial wood falls!