May 2019 – ECOL 8990 – Ecology Reading Group (2024)

Historically, ecological theory has been developed on the assumption that the most important processes structuring communities are overwhelmingly negative- namely competition, predation, and environmental stress. This week however, both papers we read argued that the role of positive species interactions has been severely underestimated. Facilitative interactions may play a key roll in increasing biodiversity and shaping communities.

The first paper explored the ways in which facilitation could be incorporated into the theoretical framework of modern ecology (Bruno, Stachowicz, & Bertness, 2003). We found their conclusion that facilitative interactions like habitat amelioration could result in a realized niche that is in fact larger than predicted by the fundamental niche to be particularly insightful. This is counter to the idea taught in most intro ecology courses that always depicts a shrunken realized niche relative to the fundamental niche. Our conversations led us to discuss the role of facilitative interactions in species invasions. Invasive species have been a growing source of concern for scientists and resource managers. Understanding how native species could facilitate invasives and contribute to the success of invaders could help us better predict invasion success.

These two papers complemented each other nicely. Where the first took a more theoretical approach, the second took a targeted approach to highlight how invaluable the role of facilitation may be in biodiversity experiments (Wright, Wardle, Callaway, & Gaxiola, 2017). They outlined three key mechanisms by which facilitative interactions could affect biodiversity and result in species specific overyielding. We focused our discussion on one of these mechanisms, the abiotic microclimate amelioration, which is the result of species reducing the effects of environmental stressors. These facilitative relationships are characteristic of many foundation species including mussels, corals and mangroves, that provide both structure and mitigate abiotic stressors (Jones, Lawton, & Shachak, 1994). Facilitations like these have been shown to be extremely important in harsh environments where heat stress, drought conditions or freezing temperatures may severely limit diversity. The implications of these facilitative interactions for diversity-productivity relationships suggest that incorporating facilitation into our framework could help us better predict outcomes for ecosystem stability.

Reading these papers this week feels like we’ve come full circle from where we started discussion biodiversity, niche theory, and competition. Developing new theories and frameworks that address the key role of facilitation will help advance ecological theory and generate valuable new ideas and understandings of the natural world. The real challenge is going to be changing how we teach the basics to reflect these changes to make sure that we move forward instead of getting stuck in the past.

1. Bruno, J. F., Stachowicz, J. J., & Bertness, M. D. (2003). Inclusion of facilitation into ecological theory. Trends in Ecology and Evolution, 18(3), 119–125. https://doi.org/10.1016/S0169-5347(02)00045-9
2. Jones, C. G., Lawton, J. H., & Shachak, M. (1994). Organisms as Ecosystem Engineers. Oikos. https://doi.org/10.2307/3545850
3. Wright, A. J., Wardle, D. A., Callaway, R., & Gaxiola, A. (2017). The Overlooked Role of Facilitation in Biodiversity Experiments. Trends in Ecology and Evolution, 32(5), 383–390. https://doi.org/10.1016/j.tree.2017.02.011

To finish out the course, we selected two papers on the topic of diversity metrics. First was the 2005 paper by Chao et al., as well as the recent 2017 paper by Hillebrand et al. Both papers proposed improvements to more traditional diversity metrics like the Jaccard index through more comprehensive inclusion of either unseen species or species composition. I think strikingly, these two papers address very different audiences. Chao et al. published in Ecology Letters target ecologists who may adopt this new metric in their own scientific research. Hillebrand et al. rather utilize their new species diversity metric as a way to inform land and biological monitoring program managers on the nuances of stable local richness levels.

I think the adoption of the Chao’s statistical approach to beta diversity has largely been accepted (with >1000 citations). Yet by reading the paper, we began to understand the importance of choosing metrics that best test your questions. Especially with measures as contentious as beta diversity, being clear on the benefits and limitations of certain measures can lead to better selection of complementary metric choices (similar to model stacking). The primary goal of Chao et al. was to draw attention that almost all richness measures will be undersampled, comparisons between sites are rarely equal in sampling sizes, and species occurrence is uneven. The authors compare their revised metric (that incorporates both composition and abundance) to existing frameworks on empirical data and simulations of uneven and incomplete sampling. I think parasite systems fit nicely into datasets that the authors suggest for use of their updated metric.

In many ways the Hillebrand paper seemed to reference key themes we had discussed with island biogeography, though now comparable or connected sites are the islands with various immigration and emmigration rates. While their adoption of species composition as a main reason for mismatches between global and local biodiversity trends, I did not find their results particularly compelling. Also, they fail to fully reach into how the species composition of local sites with stable species richness values may influence the function of these communities. I think by using available datasets that certainly have trait data such as biomass this would have been a simple calculation that could have really hammered home their point. I do think this paper forced me to gain a deeper understanding of immigration credit and extinction debt, and perhaps how quasi-equilibriums from island biogeography may be misleading as to the equilibrium of certain sites. I did appreciate the conceptual diagram given in Figure 1 that shows how both richness and evenness may change in a system, and then the use of their metrics on already established datasets. I’m not sure how much the Dutch phytoplankton added to the analysis, and would have perhaps wanted a more diverse dataset to complement the Iowa phytoplankton or grasslands example.

I think these papers may be best referenced when researching a specific problem with a dataset we are using or when designing a diversity study. I appreciate the recognition of imperfect data but using statistics to overcome imperfect data in logical and repeatable ways. Whenever explaining species richness and its importance, walking through the sampling and analysis approach iteratively, as well as showing alternatives through validated datasets or simulations, strengthens the interpretations and importance of richness measures.

Chao A, Chazdon RL, Colwell RK, Shen TJ. A new statistical approach for assessing similarity of species composition with incidence and abundance data. Ecology letters. 2005 Feb;8(2):148-59.

Hillebrand H, Blasius B, Borer ET, Chase JM, Downing JA, Eriksson BK, Filstrup CT, Harpole WS, Hodapp D, Larsen S, Lewandowska AM. Biodiversity change is uncoupled from species richness trends: Consequences for conservation and monitoring. Journal of Applied Ecology. 2018 Jan;55(1):169-84

Focusing on the theme of island biogeography, we read an experimental paper by Simberloff & Wilson as well as a synthesis paper from Patiño et al. that came out of working group discussions at the 2016 Island Biology Conference. Together, these two works span the wide breadth of topics foundational to island biogeography including dispersal dynamics, colonization patterns, and extinction rates. As my research may deal with thinking of patch dynamics between different habitats of various resources, clarification of island biogeography theory and its translatable application to non-island systems such as fragmented landscapes will be important.

Simberloff & Wilson’s goal is to test theory put forth by McArthur & Wilson’s Monograph in Population in Biology series. By fumigating mangrove islands off the coast of Florida, the authors are able to observe colonization and competition dynamics in action, until the islands reach equilibrium arthropod species richness levels. I was particularly impressed with the speed in which this equilibrium was reached, in less than a year. I think this may be in some ways do the methodology of the eradication, as resources and habitat were maintained unlike many natural experiments of island colonization after volcanic eruptions. However, this method may be appropriate for more targeted disturbances such as an invasive species or an epidemic that wipes out the arthropod community. With large scale manipulative projects like this, it seems inevitable that sampling and replication are issues. Across the six islands, the authors were not able to support that distance from the faunal source was indicative of time to equilibrium, despite clear distance patterns pre-defaunation on species compositions of islands. While species richness equilibriums were achieved, population abundances were not comparable to pre-defaunation. I think this exemplifies the more recent term of “immigration credit”, where colonizers can be counted for richness values before they are functionally fixed.

Patiño et al. ask 50 new questions from the context of island biogeography to drive future work. This genre of a “horizon scan” is an appealing synthesis piece that acts as a concrete product from conference discussions. To coordinate pre-conference surveys, during conference discussions, and post-conference editing certainly was a challenge, yet this paper exemplifies a model the type of planning necessary to optimize conference-based collaborations. I think in contrast to the methods used in the paper that spurred this reading group (Courchamp and Bradshaw 2018), this process seemed more iterative and with strategic inclusion of multiple perspectives. I also appreciated the ten questions that are heavily related to how island biogeography theory can inform conservation and management policies. This field seems to especially be influenced by cross-disciplines such as genetics, paleobiology, climatology and geology. I think similar to other areas of ecology, island biology studies will move to an increasingly ecosystem-level and function studies of multiple trophic level interactions, community dynamics, and global change.

Simberloff DS, Wilson EO. (1969) Experimental zoogeography of islands: the colonization of empty islands. Ecology 50, 278–296.

Patiño J, Whittaker RJ, Borges PA, Fernández‐Palacios JM, Ah‐Peng C, Araújo MB, Ávila SP, Cardoso P, Cornuault J, de Boer EJ, de Nascimento L. (2017) A roadmap for island biology: 50 fundamental questions after 50 years of The Theory of Island Biogeography. Journal of Biogeography 44(5):963-83.

The two papers we picked this week related to food web structuring and the ways we explore complex communities. We delved into our discussions of food web dynamics starting with the classic paper on food webs in Rocky intertidal systems (Paine, 1966). This paper is often credited with demonstrating the ideas of keystone predators, based on the removal experiment of Pisaster sea stars which led to a decline in species diversity. Though this “classic” paper is often recommended reading for new ecologists, we found the concepts and ideas surprisingly underwhelming. Maybe it’s just knowledge we take for granted now that the field of ecology has progressed beyond describing who eats who.

We moved on to discuss a modern approach to mapping food web dynamics (Kéfi et al., 2015). This paper used an interaction network to assess trophic as well as non-trophic links in a food web. This type of network approach relies on a detailed understanding of all of the species interactions including competition and facilitation in addition to predation. Building this network requires in depth knowledge of the system but can highlight key areas of important interactions. For example, by including non-trophic interactions, Kefi et al demonstrated the importance of competition at basal trophic levels.

As a side note, while exploring some of the background on the Kefi et al. paper, we noticed that they went one step further by making an interactive online version of their network that can be found on the Chilean Ecological Network website (http://app.mappr.io/play/chile-marine-intertidal-network). This allows others to manipulate the web and visualize the different sets of interactions. I admit I probably spent 20 minutes just messing with the online app and getting a feel for the different species involved in different types of interactions. I found this online interactive web to be a great example of engaging and informative data visualization that supports their research.

While reading both of these papers, however, I was struck by the ways in which both were focused on direct species interactions through predation, competition, etc. and the distinct lack indirect interactions which may be just as important for structuring communities (Peacor & Werner, 2001).

Lastly, I’ve been reminded time and time again this semester that the major flaw with using food webs to understand a system is that they often exclude vital feedbacks with ecosystem processes. Food webs often ignore detrital pathways entirely and gloss over the vital role of decomposers in nutrient recycling. To that end, I would argue that the contributions of the Paine and Kefi papers are valuable for understanding the basic species interaction structure but a broader approach to food web ecology that expands on non-trophic and indirect interactions and incorporates an ecosystem perspective is vital to progress in this field.

1. Kéfi, S., Berlow, E. L., Wieters, E. A., Joppa, L. N., Wood, S. A., Brose, U., & Navarrete, S. A. (2015). Network structure beyond food webs : mapping non-trophic and trophic interactions on Chilean rocky shores. Ecology, 96(1), 291–303.
2. Paine, R. T. (1966). Food Web Complexity and Species Diversity. The American Naturalist, 100(910), 65–75. https://doi.org/10.1086/282400
3. Peacor, S. D., & Werner, E. E. (2001). The contribution of trait-mediated indirect effects to the net effects of a predator. Proceedings of the National Academy of Sciences, 98(7), 3904–3908. https://doi.org/10.1073/pnas.071061998