“Let’s not jump to conclusions here.”

Isla’s Tutorial Group
Student lead discussion session 1, led by Georgie

This is what I should have said before embarking on the task of leading a discussion addressing relationships between ecosystem services and trophic interactions. The theme of my discussion connects the impacts of trophic levels on ecosystem services. Reading material was selected to be compelling, challenging, and provocative to inspire discussion and debate. The first paper, investigated the relationships between habitat loss, trophic collapse, and ecosystem services through a “phenomenological” model of biodiversity loss (Dobson et al 2006). The second paper, furthers the discussion by challenging a general assumption that one species has similar effects on ecosystem services across habitat types. It does this through an investigation of the effect the presence of gray wolves on trophic patterns and further on carbon cycling in two tundra habitats (Wilmers and Schmitz, 2016). What became clear from the tutorial group, was the difficulty in understanding how the methodologies and the assumptions in each of the papers led to the results. Alas, our discussion hit a critical turning point revealing the importance of having a clear understanding of methodologies before assessing the conclusions.

To whet the appetites of my classmates, I sent around a link to a YouTube video titled How Wolves Change Rivers.  The video illustrated that the reintroduction of wolves to Yellowstone has spurred an increase in biodiversity thereby increasing functions of many ecosystem components such as water cycling. It breaks a very complex issue into bite size pieces which is good for communication to the general public. However, the video over-simplifies many of the underlying processes leading to miscommunication of the scale of which wolves have an impact. This is dangerous as not all the information is present in the video, introducing biased conclusions about the impacts of wolves in reality. So we ask, how are the conclusions made?

In discussing the papers, it was apparent from the group that the methods were not easily understood, notably in Dobson et al (2006). A question asked was “would you be able to replicate these methods?” There was clear use of modelling in Dobson’s paper – agreed upon throughout the group. There are distinct stages of developing the model; however, we found it hard to interpret the model itself from the equation alone. Figure 1 is a simple graphical representation of the modelled relationship. The main conclusion was there is a sequential loss of ecosystem services as there is a hierarchical loss of species from different trophic levels. But, the simplicity in the assumptions ignored important factors such as species interactions between trophic levels and basic community dynamics which undermine the confidence of the conclusions. Overall, we agreed that the paper is useful as it opens a door for further research; but, although the results are intriguing broad assumptions and lack of clarity in the model make it difficult to be confident in the applicability of the findings to real world scenarios.

fig5

From Dobson et al (2006) paper showing functional forms for the relationship between loss of biodiversity and loss of ecosystem service function for each trophic type. Predators, showed the most sensitivity and decomposers the most resilience.

The Wilmers and Schmitz paper challenges the assumption that species interactions don’t matter in tropic collapses and even cite Dobson et al (2006). They used data from various studies to investigate whether there was a significant difference between the indirect effects from trophic interactions of gray wolves on carbon sequestration between two different tundra habitats, grasslands and boreal forests. The findings revealed that through trophic interaction, wolves negatively impact carbon sequestration in grasslands and positively impact the process in boreal forests. Their paper has more defined assumptions relevant to the tundra biome, but was still a bit challenging to fully understand. In review, there was no explicit mathematical model presented in the study, so we discussed whether it really was a modelling paper or rather more of a data synthesis. I learned here it is important understand the type of paper under assessment as a means of deciphering a studies’ conclusions.

cub

by Gabe Ginsberg, National Geographic. Accessed: http://www.nationalgeographic.com/animals/mammals/g/gray-wolf/

All in all, the tutorial group found discussion of the papers a bit challenging as we had some difficulties in understanding of the methodologies. Nonetheless, the findings and conclusions for both papers are compelling – it’s no wonder I jumped. They say, curiosity killed the cat – well, enthusiasm killed my inner critic. Taking a more analytical approach to future readings focusing on methodologies, assumptions in context and ability to replicate these studies will enable a more enlightened conversation of the conclusions and potential for the application of the findings.

by Georgie

References:

Wilmers, C. and Schmitz, O (2016). Effects of gray wolf-induced trophic cascade on ecosystem carbon cycling. Ecosphere, 7(10). http://onlinelibrary.wiley.com/doi/10.1002/ecs2.1501/full

Dobson, A., Lodge, D., Alder, J., Cumming, G. S., Keymer, J., McGlade, J., Mooney, H., Rusak, J. A., Sala, O., Wolters, V., Wall, D., Winfree, R. and Xenopoulos, M. A. (2006). Habitat loss, trophic collapse, and the decline of ecosystem services. Ecology, 87: 1915–1924. http://onlinelibrary.wiley.com/doi/10.1890/0012-9658(2006)87[1915:HLTCAT]2.0.CO;2/full

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