Thursday, March 17, 2011

And back again

Hello all! Now that finals are over, for me at least, I bring you back to the dry weight of arthropod larvae that I did a while back and publish this post on the comparisons of the bug biomass to its length. But before I get started, some graphs that I ran in R to help illustrate what is going to be discussed.
Figure 1: Here is the original graph when simply plotting out the length of the arthropod larvae to its biomass. Though some details escape the plot, there are 3 points on this graph that show a different type of larvae (dark red circles).
Figure 2: After doing a log transform, this is what the data looks like; much data pertaining to body size are non-linear. There is now more of a distinction between the two types of arthropod larvae. 
Figure 3: Voila! These fit lines better help illustrate the relationship of each type of larvae length to biomass.
And so, we get to the meat of this discussion. To start off, I had collected 37 individual arthropod larvae samples, in which they totaled to about 0.2375g in dry weight. A majority of these samples happen to be of a type of Coleoptera larvae, with the 3 exceptions that were Lepidoptera larvae, and they show that there is a significant increase in weight as their length increases. This appears to be influenced by factors such as body shape, feeding habits (herbivorous or predatory), and the metabolic rate of animals etc[1]. The general idea that can be assumed is that the more an arthropod eats (higher herbivory counts on A. californica), the bigger in mass it becomes, until a certain threshold point. Looking at Figure 1, there seems to be a cutoff value at around 12 mm for arthropod larvae length, which I assume is the maximum size larvae grow to before they are either consumed or pupated. This is useful to know because in such cases, species biomass may reflect functionality more accurately than abundance[2].

It might be more presumable to think that this is due to bird predation because the larger these larvae are, the more conspicuous it is on the sagebrush leaves. This leads to the possible conclusion that there could a top-down effect on the arthropod community, but only on the larvae in the earlier months, and not necessarily on the mature arthropods that appear closer to summer. Maybe the earlier hypothesis that I came up with should also include a comparison of temporal variation of herbivory; it would be interesting to see if the data changes at different times of the year.

1. Sage, R.D. 1982. “Wet and Dry-weight Estimates of Insects and Spiders Based on Length.” American Midland Naturalist 108(2): 407-411.
2. Saint-Germain, M; Buddle, C.M.; Larrivee, M; et al. 2007. “Should Biomass be considered more frequently as a currency in terrestrial arthropod community analyses?” Journal of Applied Ecology 44: 330-339.

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