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.

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.

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

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 helps to minimize the bias that can occur in traditional literature reviews (Needleman, 2002). Finally, the comprehensiveness of extracted data allows for analysis of various combinations of variables to identify trends that would probably go unnoticed in a traditional literature review (Pickering & Byrne, 2014). Overall, performing a systematic review allows for a fair evaluation of the existing research done on a specific topic (Kitchenham, 2004).

Researchers have begun to conduct systematic reviews and meta-analyses on facilitation in various ecosystems. For example, Gomez-Aparicio et al. (2004) performed a review on facilitation in Mediterranean forests and found that pioneer shrubs are strong facilitators and can

5 positively impact restoration in these degraded forests. Maestre et al. (2005) conducted a review on facilitative plant interactions in arid environments and found that this harsher climate actually revealed unique, facilitative interactions, indicating that theories involving facilitation needed to be further researched and updated. de Toledo Castanho et al. (2015) specifically focused their review on facilitation in coastal dune systems and determined that, because so many factors go into facilitative interactions (such as geographic region and drivers of community structure), scientists have to be careful about making generalizations about the topic until a lot more research is done. However, to the best of my knowledge, a review has not yet been performed across multiple intertidal, marine habitats simultaneously. A better understanding of the species interactions within these communities may help inform our understanding of ecosystem functioning and, ultimately, the health and well-being of the habitats and surrounding areas.

To better understand the role of facilitative interactions in intertidal, marine habitats, I conducted a systematic review of peer-reviewed literature investigating facilitative interactions in salt marshes, mangroves, and oyster reefs in order to summarize general trends and identify research gaps. Specifically, my research questions were: (1) in which habitats has facilitation most frequently been observed?, (2) how have publications on facilitation changed over time?, (3) in which geographic regions has facilitation most frequently been observed?, and (4) are there any trends in the types of studied facilitative interactions (e.g. direct vs. indirect, inter- vs. intraspecific, mention of the Stress-Gradient Hypothesis, etc.)? Answers to these questions will help inform restoration design and identify where research needs to be done in order to further establish facilitative theory and most effectively incorporate facilitation into the practice of intertidal restoration.

METHODS

I conducted a comprehensive search of all published scientific literature involving facilitation in salt marshes, mangroves, oyster reefs, and rocky shores following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. The search was conducted in Web of Science in June 2020 with the following search string: TS = ((“intertidal reef*” OR “rocky intertidal” OR “rocky shore*” OR “rocky reef*” OR “saltmarsh*” OR “salt

marsh*” OR “tidal marsh*” OR “oyster*” OR “mangrove*”) AND (“facilitat*”)), Timespan = All Years, Search Language = English, Databases = WOS, BCI, CCC, DRCI DIIDW, KJD, MEDLINE, RSCI, SCIELO, ZOOREC. This search produced 2,397 papers, 2,280 of which were unique (Figure 1). In order to be included in the database, papers must: be in the English language; have an abstract; be peer-reviewed (no government reports, book chapters, theses, etc.); be only field-based, primary research (no syntheses, meta-analyses, modeling studies, lab experiments, mesocosm experiments, etc.); relate to one of the intertidal habitats specified in the research question (i.e. salt marshes, mangroves, oyster reefs, or rocky shores); include the word ‘facilitation’; and involve one-on-one, inter- or intraspecific facilitation between species.

Title and abstract screening eliminated 2,095 papers, leaving 185 papers to be screened at the full-text level (Figure 1). Based on time constraints, I decided to focus exclusively on papers from salt marshes, mangroves, and oyster reefs, and all rocky intertidal papers were excluded from the full-text screening. After the full-text screening, 78 papers were included in the final database for the qualitative synthesis (Figure 1). From each of these articles, data was extracted on: (1) bibliographic information (authors, journal name, publication year, etc.); (2) geographic information (continent, country(ies), state(s), and latitude, as applicable); (3) the habitat where the facilitative interaction occurred; (4) the species involved in the facilitative relationship; and (5) the outcomes of each facilitative interaction.

Before data extraction could begin, a few decisions had to be made. First, if multiple facilitative interactions were discovered, the above data was extracted for each interaction. Within the 78 studies included in the database, 212 different facilitative interactions were recorded. Second, a few definitions had to be set. The latitude of each study site and interaction was recorded. Interactions were classified into 5° latitudinal blocks and also classified as occurring in tropical latitudes at or below the Tropic of Cancer/Capricorn (0°-23.5°), mid- latitudes (23.6°-66.5°), or polar latitudes at or above the Arctic/Antarctic Circles (66.6°-90°) (Val et al., 2005; Leddy, 2020). Whether or not a primary foundation species was involved in each interaction was also recorded. For the purposes of this review, the Angelini et al. (2011) definition of a primary foundation species as one that creates “complex habitats in which associated organisms find refuge from biological and physical stress” and are “fundamental to the structure and resilience of terrestrial and marine ecosystems” was used. Whether or not a

disturbance was involved in each interaction was also recorded, and the Platt & Connell (2003) definition was used. They defined a disturbance as a discrete event that damages or kills residents on a site (Platt & Connell, 2003).

RESULTS

In total, the database includes 78 studies, 59 of which were in salt marshes, 19 in mangroves, and only 2 in oyster reefs (Table 1). Two papers were in a salt marsh-mangrove ecotone and are thus included in the paper count for both salt marshes and mangroves. The earliest paper to study facilitation was in salt marshes and published in 1984. The first paper on facilitation in mangroves was published in 1990 and the first paper in oyster reefs was not published until 2016. In general, studies on facilitation in intertidal habitats have increased drastically since the mid-1990s (Figure 2). The 78 studies in the database were published in 40 different journals, with 22 of the studies published in Ecology followed by 5 published in the Journal of Vegetation Science. Of the 78 studies, 51 were experimental, 5 were observational, and 22 had a component of both.

The 78 studies produced 212 unique, facilitative interactions, as many of the studies involved multiple interactions across various geographic landscapes and ecological parameters. These interactions took place at 154 independent study sites located all over the world (Figure 3). Facilitative interactions were recorded on every continent except Antarctica. Of the 212 interactions, 97 were recorded in North America, 31 in Asia, 29 in South America, 27 in Australia/Oceania, 26 in Europe, and 2 in Africa. The majority of the interactions were in the United States (n = 91), followed by Australia (n = 27) and Argentina (n = 22). In total, interactions were recorded in 18 different countries and across a wide range of latitudes. The majority of observed facilitative interactions occurred in mid-latitudes (n = 190) followed by 20 in tropical latitudes and only two in polar latitudes, which largely mirrored the distribution of study sites (Figure 4).

Of the 212 interactions, 132 were recorded in salt marshes, 77 were in mangroves, and 3 were in oyster reefs. Salt marshes were most frequently studied in North America, mangroves were most frequently studied in Asia, and oyster reefs were only studied in Europe. Across all

ecosystems, plant-animal interactions were found most frequently (n = 96), followed by plant- plant interactions (n = 75), and animal-animal interactions (n = 31) (Figure 5). Algae was only involved in 10 interactions. With respect to trophic level, autotroph-autotroph interactions were most common (n = 80), followed by autotroph-herbivore interactions (n = 60) (Figure 6). Of the 212 interactions, 167 involved direct facilitation leaving only 45 involving indirect facilitation. Twenty-four of the 45 indirect interactions were in mangroves, accounting for 31% of mangrove interactions (Figure 7). In addition, only 17 of the interactions were intraspecific and accounted for 8% (n = 11) of salt marsh interactions and 8% (n = 6) of mangrove interactions (Figure 8). A majority of the facilitative interactions involved the primary foundation species (n = 174), leaving only 38 interactions that did not (Figure 9). Only 13 of the 212 interactions were trophic (consumptive) and all were in salt marshes (Figure 10). Twenty-four interactions involved a disturbance, with the majority occurring in salt marshes (n = 19). The Stress-Gradient Hypothesis was addressed with 22 of the interactions, accounting for 11% (n = 14) of salt marsh interactions and 10% (n = 8) of mangrove interactions.

Of the 195 interspecific interactions, only one species benefited in 179 interactions and both species benefited in 16 interactions. These 16 mutualistic interactions only occurred in salt marshes (Figure 11). Of the 212 interactions, 106 involved two different trophic levels. The higher trophic level benefited in 72 interactions, the lower trophic level benefited in 18 interactions, and both trophic levels benefited in 16 interactions (Figure 12).

DISCUSSION

This review summarizes general trends and identifies research gaps in the study of facilitative interactions in intertidal, marine habitats. Ecological facilitation is a relatively new concept on the ecological time scale, thus, even though a lot of research has been published, there is still much that needs to be done in order to determine the full scope and potential of facilitation as a type of positive species interaction that can be used in restoration of intertidal habitats (Bronstein, 2009; Silliman et al., 2015).

The majority of the 78 studies in the database focused on salt marshes, which is likely because salt marshes are readily accessible to most scientists. In the United States, for example,

9 there are roughly 1,700,000 hectares of salt marsh (equivalent to over 688,000 football fields) spread all around the country (Zedler et al., 2008). This makes them convenient for coastal ecologists looking to study intertidal interactions. They are also distributed widely around the world, as they can be found on most coastlines at temperate and upper latitudes (Zedler et al., 2008). This is in contrast to, say, mangroves that are largely located in the equatorial region. The largest percentage of mangroves is found between 5°N and 5°S, which makes mangrove research much harder to perform since it requires significant travel for scientists that might be most likely to publish results in English-language journals (Giri et al., 2011). However, it should also be considered that salt marshes are where all the research on intertidal facilitation started. Dr. Mark Bertness was one of the pioneers in the study of intertidal facilitation, and he almost exclusively worked in salt marshes. Thus, it is possible that there are more salt marsh studies because they had a head-start on the other habitats, and over time these other habitats will catch up.

Oysters are one of the only species that can live aboveground without being consumed because of their associational defenses (Bible et al., 2017). Thus, it was surprising to find that only two of the 78 studies took place in oyster reefs. There has historically been an extreme bias towards studying negative interactions in oyster reefs. However, Reeves et al. (2020) recently performed a review of positive interactions in oyster reefs in a restoration context. Of the 96 studies in the database, 35 included confirmed mutualistic (and thus facilitative, by definition) interactions (Reeves et al., 2020). This led me to believe that the present study would have included more studies in oyster reefs. The fact that my database only included two studies in oyster reefs was likely a product of the search string and inclusion criteria. Reeves et al. (2020) included laboratory and modeled studies and considered subtidal reefs as well. They also looked at all kinds of positive interactions and did not make use of the word ‘facilitation’ mandatory for inclusion (Reeves et al., 2020). The inclusion criteria for the present study required papers to be field-based, be intertidal, and use the word ‘facilitation’ to describe the positive interaction, which most likely explains why only two studies qualified for inclusion.

Studies that have observed ecological facilitation in intertidal habitats have increased exponentially since the mid-1990s. As previously mentioned, mutualisms are a type of facilitation (based on the definition used for this study). However, research on ecological

mutualisms, specifically, has a much deeper history than research on ecological facilitation. Research on mutualistic theory can be traced as far back as Darwin and Aristotle, and field experiments started to take off in the mid-1980s (Bronstein, 2009; Bruno et al., 2003). Thus, since mutualism is a type of facilitation, one could consider facilitative theory to trace back this far as well. However, if facilitation is considered as a sole concept (where it is possible for only one species to benefit as long as the other is unharmed), this timeline is not the same. While examples of mutualisms go as far back as Ancient Greece, it is believed the earliest documented case of facilitation was in 1914 when G.A. Pearson demonstrated that conifers regenerated more successfully after fires when grown within quaking aspen clones (Bronstein, 2009; Pearson, 1914). This is relatively recent on the ecological timescale and explains why the first study on intertidal facilitation was not published until 1984. Once researchers discovered the potential for systematically harnessing facilitative interactions for restoration purposes, publications started increasing exponentially. However, theories involving ecological facilitation are still lacking, and many primary research questions still need to be asked and answered in order to have a more complete and widely accepted facilitation theory.

Almost half of the observed interactions were in the United States and mostly in east coast salt marshes. One possible explanation involves the location of the scientists currently interested in the topic. The most frequently published scientists studying intertidal facilitation are Dr. Mark Bertness at Brown University in Rhode Island, United States and Dr. Brian Silliman at Duke University in North Carolina, United States. The top published scientists are both on the east coast of the United States where there is significant salt marsh habitat, which means a significant number of the 78 studies or 212 interactions were there as well. The language bias is also a potential factor. Papers needed to be in the English language in order to be included in the database. It is possible that this relatively increased the number of papers in English speaking countries, including the United States. Other than this distinction, geographic location of the studies seemed to largely mirror the geographic ranges of the habitats.

The majority of facilitative interactions involved plants, with plant-animal and plant-plant interactions being most common. Trophic level interactions agreed with this result, as the majority were autotroph-autotroph and autotroph-herbivore. After discovering that 76 of the 78 papers in the database were in salt marshes and mangroves, the fact that many interactions

involved plants/autotrophs was an expected result. However, it was interesting to see how few interactions involved upper trophic level predators. Many studies have shown the potential of predator facilitation in plant communities (Kotler et al., 1992; Gehr et al., 2018). Because the database documented so few predator facilitative interactions, it is unknown if they are uncommon in intertidal habitats or if they are just understudied. This needs to be further researched.

The majority of the 212 interactions involved direct facilitation. This was expected, as initial studies looking at facilitation were almost exclusively looking at direct interactions (Bertness & Hacker, 1994; Bertness & Shumway, 1993; Callaway & King, 1996; Carlsson & Callaghan, 1991). Indirect facilitation was really not studied until 1999 when Levine observed indirect facilitation in a riparian community (Brooker et al., 2008; Levine, 1999). One very interesting result was the high occurrence of a primary foundation species in the interactions. It has long been recognized that primary foundation species have facilitated other species in their communities through habitat creation, but it was interesting to see how strongly the data reinforced how much of a positive impact primary foundation species can have on the habitats they reside in (Bertness & Callaway, 1994; Dayton, 1972; Ellison et al., 2005; Stachowicz, 2001). However, it should be considered that this result could have been amplified by the wording of the search string, which included the habitat forming species. For example, it is possible that putting ‘mangrove’ in the search string relatively increased the number of studies that include mangroves, the primary foundation species, in the facilitative interaction. The majority of the interactions were non-trophic, which coincides with the fact that the majority were also direct interactions that did not involve upper trophic levels. Trophic interactions are usually indirect (consumptive) and frequently involve an upper trophic level (Shurin et al., 2002). Not very many of the interactions involved a disturbance, and those that did were mostly in salt marshes. It was expected that more of the interactions would involve a disturbance based on the many studies that have shown facilitation following a disturbance that would not necessarily occur if the disturbance did not occur, especially in marine environments (Roff et al., 2015; Wright & Gribben, 2017). The Stress-Gradient Hypothesis was addressed in rather few interactions. It was expected that it would be addressed more frequently because the hypothesis was introduced in 1994, around the same time that studies on facilitation started taking off, and facilitation is an essential part of the hypothesis (Bertness & Callaway, 1994).

It was also interesting to see which species benefited from the interactions. The majority of the interactions resulted in only one of the two species benefiting. The few studies where both species benefited were all in salt marshes. This result was expected based on the parameters defined in the search string. ‘Mutualism’ (where both species benefit) was not included. Thus, all studies where both species benefited were not likely included in the resulting database unless they specifically address mutualism as being a form of facilitation. Finally, it was determined that the higher trophic level benefited in the majority of interactions. This confirmed my hypothesis because, in general, organisms that are higher in the food chain rely more heavily on lower trophic levels to survive (e.g. as a source of food) (Fretwell, 1987).

Between the three habitats, mangroves and oyster reefs most need to be studied further. Both habitats have shown the potential for facilitative interactions, and if more of these positive interactions could be identified, they could be used in beneficial ways, just as Silliman et al. (2015) discovered that clumped planting of marsh plants led to facilitative interactions that increased the restoration success of degraded marshes. In addition, facilitation in these habitats should be studied across a wider geographic range in order to determine if the discovered facilitative interactions are applicable globally and if there are new interactions that occur elsewhere that have not yet been discovered. More effort should also be put into looking for indirect and trophic facilitation within these habitats. It is easier to observe and hypothesize the existence of direct interactions between species, so more effort should be put into searching for indirect interactions that might be otherwise overlooked. This could also reveal more predator facilitation relationships. Facilitation that results from a disturbance should also be further studied. Based on the work that has been done, there is immense potential here that will become even more applicable as climate change progresses, inevitably causing more disturbances globally. Finally, the work being done should more frequently be looked at in terms of the Stress-Gradient Hypothesis. I think many of the published studies could have been applied to the Hypothesis and were not. The Hypothesis provides immense potential for understanding the ecological interactions across landscapes, and it should be applied to as many situations as possible.

The 78 studies in this database revealed multiple trends with intertidal facilitation that were previously unknown. This information can go directly towards use in restoration design.

This study also revealed where more work needs to be done to develop facilitation theory more thoroughly, and because facilitative research is still relatively new, there is quite a bit. However, the positive implications of systematically harnessing nature’s facilitative interactions for restoration are immense and will make it well worth the effort.

ACKNOWLEDGEMENTS

I would like to thank Dr. Carter Smith, Dr. Brian Silliman, and Morgan Rudd for their help with this project. Dr. Smith and Dr. Silliman were invaluable in helping me conceptualize the idea, teaching me how to perform a systematic review, and reviewing drafts of my paper, and Morgan Rudd provided valuable advice about the Web of Science search string. The entire Silliman Lab has been amazing in their support and encouragement throughout this process.

I would also like to thank my family and friends for their unwavering support during the execution of this Master’s Project and graduate school in general, especially during these unprecedented times. I truly could not have done it without them.

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Literature Included in Database

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APPENDIX

Table 1: Included studies

Reference Journal Country Habitat Hughes, 2012 Ecology United States Salt marsh Bertness & Coverdale, 2013 Ecology United States Salt marsh Daleo & Iribarne, 2009 Ecology Argentina Salt marsh Guo et al., 2013 Global Change Biology United States Salt marsh/Mangrove Bertness & Shumway, 1993 The American Naturalist United States Salt marsh Rand, 2004 Ecology United States Salt marsh Marine Ecology Progress Vozzo & Bishop, 2019 Australia Mangrove Series Bertness & Yeh, 1994 Ecology United States Salt marsh Alberti et al., 2008 Ecology Argentina Salt marsh Buschbaum et al., 2016 Journal of Sea Research Netherlands/Germany Oyster reef Bishop et al., 2012 Ecology Australia Mangrove Wetlands Ecology and Pranchai et al., 2018 Brazil Mangrove Management Mendez et al., 2015 Aquatic Ecology Argentina Salt marsh Smith, 2015 Journal of Crustacean United States Salt marsh Daleo et al., 2007 Ecology Letters Argentina Salt marsh Ma et al., 2014 Conservation Biology China Salt marsh Wetlands Ecology and Salmo and Duke, 2010 Philippines Mangrove Management Hacker and Bertness, 1999 Ecology United States Salt marsh Duggan-Edwards et al., 2020 Journal of Applied Ecology United Kingdom Salt marsh Egerova et al., 2003 Wetlands United States Salt marsh Bruno et al., 2017 PeerJ United States Salt marsh Callaway, 1994 Ecology United States Salt marsh Altieri et al., 2013 Ecology United States Salt marsh Smith et al., 2009 Marine Biology United States Mangrove Bertness, 1985 Ecology United States Salt marsh Gittman & Keller, 2013 Ecology United States Salt marsh Journal of Experimental Nomann & Pennings, 1998 United States Salt marsh Marine Biology and Ecology Proceedings of the Royal Angelini et al., 2015 Society B: Biological United States Salt marsh Sciences Proceedings of the National Silliman & Newell, 2003 United States Salt marsh Academy of Sciences Journal of Shellfish Hutchens & Walters, 2006 United States Salt marsh Research Crotty & Angelini, 2020 Current Biology United States Salt marsh Reynolds et al., 2017 Ecosphere United States Salt marsh

Van der Graaf et al., 2004 Polar Biology Russian Federation Salt marsh Pennings & Callaway, 1996 Ecology United States Salt marsh Marine Ecology Progress Cutajar et al., 2012 Australia Salt marsh Series Hughes et al., 2014 Oikos United States Salt marsh Bertness et al., 2015 Oecologia United States Salt marsh Noto & Shurin, 2017 Ecology and Evolution United States Salt marsh Sueiro et al., 2013 Acta Oecologica Argentina Salt marsh Zhang et al., 2012 Ecology China Mangrove Crain, 2008 Journal of Ecology United States Salt marsh Bertness, 1991 Ecology United States Salt marsh Yuan et al., 2013 PLoS One China Salt marsh Philosophical Transactions Huxham et al., 2010 of the Royal Society B: Sri Lanka/Kenya Mangrove Biological Sciences Journal of Vegetation Proenca et al., 2019 France Salt marsh Science Zhang & Wang, 2016 Plant Ecology China Salt marsh Journal of Vegetation Howison et al., 2015 Netherlands Salt marsh Science McKee et al., 2007 Ecological Applications Belize Mangrove Qiu et al., 2019 Marine Pollution Bulletin China Salt marsh Hacker & Bertness, 1995 Ecology United States Salt marsh Bishop et al., 2013 Ecology Australia Mangrove He & Cui, 2015 Scientific Reports China Salt marsh Derksen-Hooijberg et al., 2018 Journal of Applied Ecology United States Salt marsh Journal of Vegetation Rubio-Casal et al., 2001 Spain Salt marsh Science Teutli-Hernandez et al., 2019 Ecological Engineering Mexico Mangrove Can der Wal et al., 2000 Ecology Netherlands Salt marsh Marine Ecology Progress Aquino-Thomas & Proffitt, 2014 United States Mangrove Series Grewell, 2008 Ecology United States Salt marsh Bertness & Hacker, 1994 The American Naturalist United States Salt marsh Bortolus et al., 2002 Ecology Argentina Salt marsh Ren et al., 2008 Ecological Research China Mangrove Christianen et al., 2018 Marine Biology Research Netherlands Oyster reef Bertness, 1984 Ecology United States Salt marsh Johnson et al., 2018 Ecosphere United States Salt marsh Kim et al., 2009 Journal of Coastal Research Denmark Salt marsh Deis et al., 2017 PeerJ United States Salt marsh Proffitt et al., 2005 Journal of Ecology United States Salt marsh Dijkstra et al., 2012 Oikos United States Salt marsh Journal of Vegetation Espinar et al., 2002 Spain Salt marsh Science

Stahl et al., 2006 Functional Ecology Netherlands Salt marsh Pakistan Journal of Marine Nazim et al., 2009 Pakistan Mangrove Sciences Journal of Experimental Garside & Bishop, 2014 Australia Mangrove Marine Biology and Ecology Journal of Experimental Ellison & Farnsworth, 1990 Belize Mangrove Marine Biology and Ecology Vogt et al., 2014 Basic and Applied Ecology Brazil Mangrove Canepuccia et al., 2008 Estuaries and Coasts Argentina Salt marsh Journal of Vegetation Tessier et al., 2002 France Salt marsh Science Peterson & Bell, 2012 Ecology United States Salt marsh/Mangrove Donnelly & Walters, 2014 Estuaries and Coasts United States Mangrove

Figure 1: PRISMA flow diagram for article selection Figure 2: Line graph of cumulative publications for each habitat separately and together over time Figure 3: Pinpoint map of all study sites. Note that many articles used multiple study sites, and therefore 154 unique study sites were identified from the 78 articles in the database. Figure 4: Bar chart of the number of study sites and the number of interactions in each five- degree latitude block. Note that of the 78 articles in the database, 154 unique study sites were identified. In addition, of the 212 interactions identified, 14 interactions occurred in multiple five-degree latitude blocks. This made the total number of interactions 226 for this analysis. Figure 5: Bar chart showing the number of interactions for each combination between plants, animals, and algae. Figure 6: Heat chart of the number of each interaction combination between trophic levels, including: autotrophs, detritivores, herbivores, omnivores, and carnivores. Figure 7: Stacked bar chart showing the percent of direct and indirect interactions for each habitat type. Figure 8: Stacked bar chart showing the percent of interspecific and intraspecific interactions for each habitat type. Figure 9: Stacked bar chart showing the percent of interactions that involve a primary foundation species for each habitat type. Figure 10: Stacked bar chart showing the percent of trophic and non-trophic interactions for each habitat type. Figure 11: Stacked bar chart showing the percent of interactions where both species benefited (mutualism) and where only one species benefited. Intraspecific interactions were not included in this analysis. Figure 12: Bar chart showing the number of interactions where the higher trophic level benefited, the lower trophic level benefited, and both trophic levels benefited. Note that only studies that involved interactions between different trophic levels were included in this analysis, totaling 106 interactions.

Figure 1

Records identified through Additional records identified database searching through other sources

(n = 2,397) (n = 0)

Identification

Records after duplicates removed

(n = 2,280)

Screening Records screened Records excluded

(n = 2,280) (n = 2,095)

Full-text articles Full-test articles assessed for eligibility excluded, with reasons (n = 185) (n = 107) Eligibility

Studies included in qualitative synthesis (n = 78)

Included Studies included in

quantitative synthesis (meta-analysis) (n = N/A)

Figure 2

50 Salt Marsh Mangrove 40 Oyster Reef

30 TOTAL Total Number Publications of Number Total 20

Figure 3

Figure 4

Count 0 5 10 15 20 25 30 35 40

85-90 N 80-85 N 75-80 N 70-75 N 65-70 N 60-65 N 55-60 N 50-55 N 45-50 N 40-45 N 35-40 N 30-35 N 25-30 N 20-25 N 15-20 N 10-15 N 5-10 N 0-5 N 0-5 S 5-10 S

Degrees Latitude 10-15 S 15-20 S 20-25 S 25-30 S 30-35 S 35-40 S 40-45 S 45-50 S 50-55 S 55-60 S 60-65 S 65-70 S 70-75 S 75-80 S 80-85 S 85-90 S

Number of Study Sites Number of Interactions

Figure 5

120

100

40 Number of Facilitative Interactions 20

0 Plant-Animal Plant-Plant Animal-Animal Plant-Algae Animal-Algae Algae-Algae Interaction Type

Figure 6 Autotroph Detritivore Herbivore Omnivore Carnivore Autotroph 80 21 60 10 10 Detritivore 0 2 1 0 Herbivore 26 1 1 Omnivore 0 0 Carnivore 0

Figure 7

100

50 Indirect 40 Direct

30 Percent of Total Interactions (%) 20