Author: JB

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

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

This week we turned our discussion to the topic of ecological niches. That is, we discussed the variety of ways that a species’ ecological niche could be defined from its environmental tolerances, to the species interactions, to an n-dimensional hypervolume. We also discussed the concept of an “empty niche” and its role in speciation and invasion.

For our classic paper we read “Concluding Remarks” by G.E. Hutchinson,¹ which he wrote to summarize the new ideas and thoughts from a recent Symposium on populations and demography. Hutchinson describes his quantitative approach to understanding niche theory, as pulling out the metaphorical vacuum cleaner to synthesize the “irrelevant litter” that has accumulated around niche theory. He starts by arguing that the applicability (or lack of) to human demography is not a reflection of a flaw in the theory, but instead demonstrates that we are not considering it on the right time scale or asking the right questions. He then uses an example of amphipods that don’t exist in seemingly suitable habitat to argue that the “suitable habitat” is based on our understanding and perception of the environment, which differs considerably from that of an amphipod species.  Our discussion of this long-winded paper ultimately led us to consider how variation in space/time can contribute significantly to the maintenance of species diversity.

We turned next to discuss the Sahney et al.² and the concept of the empty niche contributing to the explosion of biodiversity with the expansion of species to terrestrial environments. Sahney used body size, diet and habitat to explore the potential modes of life that species could fill. The modes of life in some ways are similar to Hutchinson’s n-dimensional cube with 3 specific dimensions. In this case, Sahney argued that when species moved from sea to land there were countless niches waiting to be filled, which contributed to the rapid speciation and expansion of tetrapods. They also suggested that 64% of the terrestrial “modes of life” have yet to be filled and that tetrapod diversity may continue to increase as those modes are filled. While this suggestion will ultimately be difficult to verify, it seems plausible that empty niches may still play a vital role in understanding biodiversity.

Understanding the role of empty niche space can also help us better understand the vulnerability of systems to invasion. Recent anthropogenic disturbances to environments have led to major disruptions to ecosystems through both the extirpation of existing species and the introduction of novel players. These disruptions can lead to unexpected consequences like invader meltdown and community collapse. For that reason taking into account the amount of empty niche space in a habitat may help inform management decisions.

References:

1. Hutchinson, G. E. Concluding Remarks. 117, 1937–1938 (1975).
2. Sahney, S., Benton, M. J. & Ferry, P. A. Links between global taxonomic diversity , ecological diversity and the expansion of vertebrates on land. Biol. Lett. 6, 544–547 (2010).

The World is Green hypothesis, which stems from the classic work of Hairston, Smith and Slobodkin (1960), suggests that since herbivores are not limited by their food, they must instead be limited by their predators.  While this is often considered an oversimplification of the drivers of community structuring, this idea has served as a foundation of ecological research and theory for many years. The authors proposed that trophic levels alternate between bottom-up and top-down control, such that plants and carnivores are both limited by resources while herbivores are limited by carnivores. This idea suggests that species interactions play a vital role in determining not only which species will be found but also how many of each species a community can support.

In the years following the HSS hypothesis, many ecologists used targeted experiments to begin teasing apart the relative influence of biotic and abiotic variables in determining community structure. One such example is Lubchenco and Menge (1978) that sought to disentangle the relative importance of disturbance and predation in coastal rocky intertidal systems. Their research suggests that while environmental factors and disturbance can set a baseline for the community composition, species interactions, like predation and competition, also play a vital role in shaping community structure.

Recent research has supported the idea that species interactions are vital drivers of community structuring. A 2015 article (Lima-Mendez et al) took a modern approach to exploring community structuring. Using global data from the Tara Oceans project, they argued that species distributions could not be predicted from environmental factors alone and that biotic interactions play a large role in structuring communities. They constructed interaction networks for plankton species and used experimental methods to validate predicted interactions. Utilizing techniques from microbiology and systematics, the authors were able to demonstrate that the organization of ocean food webs can be predicted from patterns of species co-occurrence.

These three papers took three very different approaches to understanding community structuring (theory, experiments, survey and synthesis), but I found it compelling that all three argued for more than environmental conditions shaping communities and highlighted the primacy of species interactions.