Invertebrates, while small, play an integral role in our ecosystem in the St. Lawrence Lowlands. In leaf litter, they aid greatly in the decomposition of organic matter, as cited by Vasconcelos and Laurance in their article on soil fauna (2005).
Invertebrates are animals that lack a backbone, but this is where most of the obvious commonalities stop. They make up over 90 percent of all the animals on earth, and are a very diverse group. With more than 35 phyla, they can be found all over the planet, with marine, aquatic, and terrestrial representatives (Invertebrate, 2015).
Soil- and litter-dwelling invertebrates, which are the focus of our research, consist mainly of the phyla Annelida, Arthropoda, Mollusca, Nematoda, Rotifera, and Platyhelminthes. These are all relatively small, from less than a millimeter to a few centimeters in length (Franklin & de Morais, 2006).
These little critters play an important role in the cycling of organic matter, which is vital for the forest ecosystem to persist (Berg, 1986). The decomposition of leaf litter is especially important, as these nutrients can make up to 72% of the aboveground nutrients returned to trees (Fogel & Cromack Jr, 1977). The soil- and litter-dwelling invertebrates do this by consuming leaf litter and other dead plant material, and decomposing it into nutrients available for the surrounding biota (Berg, 1986).
Since invertebrates affect and interact with soil, surrounding plants and animals, they are important and effective biological indicators for changes in soil properties and ecosystem health (Franklin & de Morais, 2006).
Research Question: What difference exists in invertebrate diversity between coniferous hemlock forest floor and deciduous maple leaf litter?
To tackle our question, we went to the Morgan Arboretum, which offers a wide variety of forest types with 18 different collections of trees and shrubs from around the world (Arboretum, 2011). There, we chose a pure Sugar Maple (Acer saccharum) forest, and another plot dominated by Hemlock (Tsuga canadensis).
arbomap_trails :Locations of Hemlock and Maple tree stands at the arboretum where 4 sites from each forest type were chosen and sampled.
In both the Maple forest and the Hemlock forest we set up 4 1×1 meter plots, placed randomly within the forest sections. The leaf litter found in these plots was gathered and sifted in situ, using a Winkler sifter. The litter was sifted over white trays, making it easy to spot any invertebrates hiding in the litter. The bare topsoil left in the plot was searched by hand. Any invertebrates found were collected and brought back to the lab for later identification.
We noticed that the Hemlock forest’s sparse forest floor was humid, shaded, and covered with needles. Conifer forests tend to have more acidic soil, with a lower pH due to the persistence and decomposition of coniferous needles (Turk, Schmidt, & Roberts, 2008).
The Maple forest was the opposite; it had a thick (5-10cm) covering of colourful leaves. The soils of deciduous forests, such as this Maple forest, tend to be less acidic than that of the conifer forest (Turk et al., 2008).
Characteristics of the forest floor, including pH, humidity, and the composition of the leaf litter have a strong influence on what invertebrates will live there (Burghouts, Ernsting, Korthals, & De Vries, 1992). We were curious to see if the needle-filled, acidic Hemlock forest floor fostered different invertebrates than the leafy, nutrient rich Maple forest floor, and to what degree.
From observation alone, it appeared as though there was greater biodiversity in the deciduous Maple leaf litter. We noticed that the Hemlock forest consisted mostly of predaceous specimen while the Maple forest contained a wide variety of phytophagous (plant eating) invertebrates and detritivores. Our team concluded that these might be explained by a few related factors. For one, decomposition of needles leads to a lower soil pH, which reduces the ability of many plants to grow. In addition, slow turnover rate means plants have limited access to the nutrients that are present.
Invertebrates living in coniferous forest floors must be adapted to the harsher conditions of soil and vegetation, or find their nutrients from other sources. Selection would favour predators rather than plant or detritus feeding invertebrates.
We also noticed that specimens collected in the Maple forest were consistently larger than those found in the Hemlock forest. Nutrients limit growth, so we deduced that the size gap was also consequence of richness of leaf-litter. In contrast to the Hemlock forest, the Sugar Maple forests’ nutrient abundance allowed for the development of larger invertebrates, like the stink bugs and ground beetles.
Arboretum, M. (January 2011) Retrieved Oct. 29 2015, from http://www.morganarboretum.org/arboretum/the-tree-collections.html
Berg, B. (1986). Nutrient release from litter and humus in coniferous forest soils—a mini review. Scandinavian Journal of Forest Research, 1(1-4), 359-369. doi: 10.1080/02827588609382428
Burghouts, T., Ernsting, G., Korthals, G., & De Vries, T. (1992). Litterfall, leaf litter decomposition and litter invertebrates in primary and selectively logged dipterocarp forest in Sabah, Malaysia. Philosophical Transactions of the Royal Society B: Biological Sciences, 335(1275), 407-416. doi: 10.1098/rstb.1992.0032
Fogel, R., & Cromack Jr, K. (1977). Effect of habitat and substrate quality on Douglas fir litter decomposition in western Oregon. Canadian Journal of Botany, 55(12), 1632-1640. doi: 10.1139/b77-190
Invertebrate. (2015). In Encyclopædia Britannica. Retrieved Oct. 29 2015 from http://www.britannica.com/animal/invertebrate
Franklin, E. & de Morais, J.W. (2006). Soil Mesofauna in Central Amazon. In F. M. S. Moreira, J. O. Siqueira, & L. Brussaard (Eds), Soil Biodiversity in Amazonian and Other Brazilian Ecosystems. (pp. 142-143) Wallingford, Oxfordshire, GBR: CABI Publishing.
Turk, T. D., Schmidt, M. G., & Roberts, N. J. (2008). The influence of bigleaf maple on forest floor and mineral soil properties in a coniferous forest in coastal British Columbia. Forest Ecology and Management, 255(5), 1874-1882. doi: http://dx.doi.org/10.1016/j.foreco.2007.12.016
Vasconcelos, H. L., & Laurance, W. F. (2005). Influence of habitat, litter type, and soil invertebrates on leaf-litter decomposition in a fragmented Amazonian landscape. Oecologia, 144(3), 456-462. doi: 10.1007/s00442-005-0117-1