Critters in the Litter

Arthropods are a successful group of invertebrate animals; they are members of the phylum Arthropoda, which is known to be the largest phylum in the animal kingdom. The distinguishing feature of Arthropods is the presence of a jointed external skeleton composed of chitin, a nitrogen-containing sugar (Barnes,2014). Furthermore their body is divided into distinct parts, they have jointed legs and appendages and have bilateral symmetry.

Figure 1: A distinct coloured arthropod found in sugar maple leaf litter.

Arthropods are known to be present in every habitat on earth and therefore show a great variety of adaptations. Some inhabit aquatic environments while others inhabit terrestrials ones (Barnes, 2014). The topic of this project specifically relates to the leaf litter habitat of these arthropods. Leaf litter is dead plant material, which has fallen to the ground. This dead organic matter and its constituent nutrients are added to the top layer of soil.

Arthropods are predated by a wide range of species: birds and bats (Kalka, 2008). Other arthropods such as spiders, mites and centipedes can use leaf litter critters as food. In that sense, arthropods have a diverse nutrition: some directly consume the components of the leaf litter and others predate on other arthropods, and even sometimes on small mammals. Some leaf litter critters can also be parasitic on other organisms, such as the beaver beetle, whose host is the beaver (Peck,2006).

Arthropods contribute to important functions within the forest environment including nutrient cycling, litter decomposition and pollination (Buddle et al., 2006). In this case, arthropods contribute to the consumption of the litter, which result in the breakdown of simple carbon compounds and the release of inorganic ions (Bot, 2005). A succession of arthropod species are involved in the process of breakdown as the leaf litter decomposes (Crossley et al, 1962)

Figure 2: Vials containing arthropods collected during the field work.

Our Project

Research question: How does forest type affect the abundance and diversity of leaf-litter arthropods in the Morgan Arboretum?

Initially, we were interested in looking at the composition of the leaf litter in different forests and how it affected arthropod abundance and diversity. This would include determining the soil pH, temperature of the leaf litter, and the amount of sunlight reaching the forest floor. After considering the amount of time it would take to measure these factors in addition to collecting insects, we decided to look at the effect of forest type instead, since that alone should influence the amount and types of insects that we find.

The four areas we studied were coniferous, maple, beech forests, and grassland as a control. As forest type changes leaf litter characteristics change, for example the thickness of leaf litter, pH, temperature, moisture, light penetration, rate of decomposition and many others. For instance, soils of coniferous forests have a lower pH than deciduous forests. These changes can influence arthropods, notably with the availability of nutrients.

Our expected results are dependent on the forest type. Since the coniferous forest has a more acidic soil and a small amount of leaf litter (Berg & McClaugherty, 2003), the abundance of arthropods is expected to be lower than the Maple and Beech forests, although more abundant than the grassland. The acidity of the soil reduces the available nutrients which will affect the amount of species found. The Maple and Beech forests will have a greater thickness to their leaf litter due to the slow rate of decomposition (Berg & McClaugherty, 2003), allowing the arthropods to have a wider and protected habitat space, therefore having a greater abundance of species. Lastly, in the case of the grasslands, since we are focusing only on leaf litter arthropods (not taking into account anything lower than the leaf litter layer), with an absence of leaf litter we expect our result to have very little to no arthropods being found.

Arthropods have to be able to adapt to the availability of nutrients, as a result we expect that arthropods will be divided among the forest types by their food habits. Hence, there will be variation in arthropods from one forest to another.

Methods:

Our research was conducted in the beautiful Morgan Arboretum, a 245 hectare forested reserve which is situated on McGill University, Macdonald Campus in St-Anne-de-Belleuvue.

Video 1: A view of the research field at the Morgan Arboretum

In order to help answer the research question two separate collection methods were derived. For the first method we set pitfall traps in the forest. The second method involved sifting the leaf the litter. In both cases the same four forest types were examined; Sugar Maple, Beech, Coniferous and finally grasslands as a control.

Figure 3: The four forest types examined as put of the method. Beginning top left and moving clockwise you see coniferous, grassland, sugar maple, beech.

The first data collection method, pitfall traps, involved digging a hole in the soil into which a container with small amount of ethylene glycol is placed. The following week the pitfall traps would be examined and the insects within would be collected. The second method involved sifting a 50cmX50cm square of leaf litter and collecting the arthropods that were present.

Figure 4: A pitfall trap placed in the beech forest.

Figure 5: A 20cmX20cm square of sugar maple leaf litter to be sifted.

Figure 6: Demonstration of the sifting methods.

Three pitfall traps were set and three areas were sampled for sifting, in each of the forest types. This same method was repeated twice. As a result these methods will allow the examination of both the diversity and the abundance of the leaf litter arthropods

References:

Barnes, Robert.D. (2014, July 17^th). Arthropod. (Retrieved from http://www.britannica.com/EBchecked/topic/36943/arthropod on October 7th 2014)

Berg, B., & McClaugherty, C. (2003). Plant litter. Decomposition, humus formation, carbon sequestration. Berlin, DE.© Springer-Verlag Berlin Heidelberg. (pp. 76-77)

Bot, Alexandra (2005). The Importance of Soil Organic Matter. Rome: Food and Agriculture Organizations of the United Nations. pp. Chapter 3. ISBN 92-5-105366-9

Buddle, C. M., et al. (2006). “Arthropod responses to harvesting and wildfire: Implications for emulation of natural disturbance in forest management.” Biological Conservation 128(3): 346-357. (http://www.cfs.nrcan.gc.ca/bookstore_pdfs/25952cannotpostonline.pdf)

Crossley, D. A., Jr. and M. P. Hoglund (1962). “A Litter-Bag Method for the Study of Microarthropods Inhabiting Leaf Litter.” Ecology 43(3): 571-573. (http://www.jstor.org.proxy1.library.mcgill.ca/stable/1933396?seq=3)

Margareta B. Kalka, Adam R. Smith and Elisabeth K.V. Kalko, Bats Limit Arthropods and Herbivory in a Tropical Forest. Science 4 April 2008 (5872), 71. [DOI:10.1126/science.1153352] (Retrieved from http://www.sciencemag.org/content/320/5872/71.full )

Peck, S. B, Distribution and biology of the ectoparasitic beaver beetle Platypsyllus castoris Ritsema in North America (Coleoptera: Leiodidae: Platypsyllinae). Insecta Mundi March–June 2006, 20 (1–2): 85. ISSN 0749-6737 (Retrieved from http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1054&context=insectamundi)

8.Vanessa L. Fox, Charlotte P. Buehler, Chad M. Byers, Summer E. Drake, Forest composition, leaf litter, and songbird communities in oak- vs. maple-dominated forests in the eastern United States, Forest Ecology and Management 259 (2010) 2426–2432 [DOI:10.1016/j.foreco.2010.03.019]