A Systematic Review of Facilitation in Intertidal Habitats
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A Systematic Review of Facilitation in Intertidal Habitats Samantha Townsend Advisors: Dr. Brian Silliman and Dr. Carter Smith April 30th, 2021 Master’s Project submitted in partial fulfillment of the requirements for the Master of Environmental Management degree in the Nicholas School of the Environment at Duke University EXECUTIVE SUMMARY Recent decades have seen an increase in research on positive species interactions, and it is now known that they are ubiquitous in nature. However, these interactions were never intentionally used in beneficial ways. This changed in 2015 when a study by Silliman et al. revealed that positive species interactions could aid in salt marsh restoration. Since then, the restoration paradigm has shifted from systematically suppressing negative interactions to harnessing nature’s positive interactions, including ecological facilitation. I performed a systematic review in order to investigate the facilitative interactions that have been observed in intertidal habitats, including salt marshes, mangroves, and oyster reefs, with the intention of highlighting the general trends and research gaps in the study of facilitation across these three habitats. The first section of this paper provides background on how research on species interactions has changed over time, what research has specifically been done on ecological facilitation, and what the specific goals of this study were. The second section looks at the methods used to perform the systematic review (including the search string, inclusion criteria, and screening procedures), what data was extracted from the papers included in the database, and what definitions were used during the screening. The third section reveals the quantitative results of the data extraction, and the fourth section discusses potential explanations for the results, what the results mean for the restoration paradigm, and where more research needs to be done. Seventy-eight studies were included in the database for this study, and the earliest was published in 1984 in a salt marsh. Since then, studies have increased exponentially. The majority were located in mid-latitudes but were spread across six continents and 18 countries. The 78 studies revealed 212 unique, facilitative interactions. One hundred and thirty-two of these interactions were in salt marshes, 77 were in mangroves, and only 3 were in oyster reefs. The majority of interactions involved autotrophs and lower trophic level species. In addition, the majority of facilitative interactions were direct, interspecific, non-trophic, and involved a primary foundation species. It was also noted that most interactions did not result from a disturbance and did not address the Stress-Gradient Hypothesis. 1 The results and subsequent analyses revealed multiple conclusions. The main ones are as follows: 1. Facilitation is common in intertidal habitats, and there is significant diversity in the types of facilitative interactions 2. Facilitative interactions are drastically understudied in oyster systems, and trophic facilitation is understudied in all habitats 3. Because facilitation has shown to be such an important type of positive interaction, we can start to implement the restoration paradigm shift to systematically harness nature’s positive interactions and include facilitation in restoration design Overall, the 78 papers in this database revealed some previously unknown trends in intertidal facilitation which can actively be incorporated into restoration projects. However, this study also revealed the major research gaps in the field that need to be filled in order to more thoroughly establish facilitative theory and most effectively include facilitation in intertidal restoration design. 2 TABLE OF CONTENTS INTRODUCTION......................................................................................................................... 4 METHODS .................................................................................................................................... 6 RESULTS ...................................................................................................................................... 8 DISCUSSION ................................................................................................................................ 9 ACKNOWLEDGEMENTS ....................................................................................................... 14 REFERENCES ............................................................................................................................ 15 Literature Referenced in Manuscript ........................................................................................ 15 Literature Included in Database ............................................................................................... 18 APPENDIX .................................................................................................................................. 24 3 INTRODUCTION Species interactions have been observed as far back as the late 1700s, but it was Darwin in the mid-1800s who gave validity and importance to the concept (Futuyma, 2009; Malthus, 1872). In his book, On the origin of species by means of natural selection, originally published in 1859, Darwin claims that “plants and animals…are bound together by a web of complex relations” (Darwin, 1859; Futuyma, 2009). Historically, research and subsequent models and theories have focused on competitive interactions (Bruno et al., 2003). Competition has been observed as far back as 1798, when Malthus noticed increasing conflict over food between species as their population sizes increased (Malthus, 1872). Other interactions, such as predation and herbivory, were studied less frequently because of the belief that predators and herbivores could consume virtually anything, and thus had unrestricted food (Macan, 1963). By the mid- 1900s, theories, such as the Competitive Exclusion Principle, stating that two species competing for the same limited resource cannot coexist, were starting to be formed (Hardin, 1960). The end of the 1900s finally saw a shift to more research on positive species interactions, including facilitation (Bronstein, 2009). Thus, because of its late recognition as an important type of ecological interaction, most ecological theories do not include facilitation (Bruno et al., 2003). However, as discussed by Bruno et al. (2003), this should not be a reflection of the importance of these interactions. The definition of ecological facilitation is still rather fluid (Bronstein, 2009). Bruno et al. (2003) refers to it as interactions that benefit at least one species and cause harm to neither. Others, such as Callaway (2007), refer to facilitation as an interaction that can have any possible outcome (positive or negative) to the facilitator, but the effect on the other species is positive. As research on the concept continues, it is likely that a more concrete definition will be universally agreed upon. For the purposes of this paper, the Bruno et al. (2003) definition will be used. After facilitation was recognized as a prominent type of positive interaction in the late 1990s and early 2000s, researchers began to find ways to harness the benefits of facilitation for restoration. For example, Zhao et al. (2007) studied areas in Mongolia that were suffering from desertification. The coarse, poor quality soil and low land productivity led to a degraded human living environment that impeded socioeconomic development (Zhao et al., 2007). However, 4 they found that replanting a few shrubs actually increased the fertility of the soil which led to plant-plant facilitation that stabilized the sand (Zhao et al., 2007). In this way, facilitation improved the quality of life of those living in the area (Zhao et al., 2007). As another example, Silliman et al. (2015) discovered that simple tweaks in restoration design could elicit facilitative relationships in salt marshes in the United States and the Netherlands. Specifically, Silliman et al. (2015) determined that marsh plants benefited from their neighbors when plugs were planted in a clumped rather than dispersed design. This led to significantly higher plant yields (increased by up to 107%) and survivorship in the restoration sites at no additional cost (Silliman et al., 2015). This study sparked the shift in the restoration paradigm from systematically suppressing negative interactions in restoration design to instead harnessing nature’s positive interactions, such as facilitation. Recently, more systematic reviews looking at facilitation across different ecosystems have been published in order to better summarize the facilitative effects that have been observed and identify where more research needs to be done. A systematic review involves taking a structured and quantitative approach to the traditional literature review in order to determine whether the scientific findings on a topic can be generalized across populations, settings, etc. (Mulrow, 1994; Pickering & Byrne, 2014). A strict set of guidelines is used to select the papers to be analyzed, and the results of these individual papers are pulled and analyzed as a whole. Being systematic, the review can be replicated as more relevant literature is published (Pickering & Byrne, 2014). The quantitative nature of the review allows for easy identification of where research has been done and research gaps that need to be filled (Pickering & Byrne, 2014). The comprehensiveness and transparency of the methods