The role of habitat‐providing species in facilitating associated species abundance and diversity is recognized as a key structuring force in many ecosystems. Reciprocal facilitation by associates, often involving multiple species, can be important for the maintenance of the host species. As with other multi‐species interactions (e.g. multiple predator effects), non‐additive relationships may be common among these associates, yet relatively few studies have examined potential interactions among multiple facilitator species. We combined field surveys and a mesocosm experiment to examine the independent and interactive effects of two co‐occurring facilitator species, ribbed mussels Geukensia demissa and fiddler crabs Uca pugilator, on their host salt marsh plant species, cordgrass Spartina alterniflora. We also experimentally examined how these relationships varied across different host plant genotypes. Overall, facilitator effects increased with increasing facilitator density. There was a significant interaction between mussel and fiddler crab presence, indicating that the effects of each species on cordgrass were dependent on the presence of the other facilitator species. In addition, there were strong interactions among mussels, fiddler crabs, and plant genotype, with greater variation in the performance of individual genotypes when fiddler crabs were absent. Our work reinforces the importance of considering multiple responses when assessing the functional redundancy of co‐occurring facilitators, as species are seldom completely redundant across the range of services they provide. It also highlights that the strength and direction of species interactions can vary due to genetic variation within the interacting species.

Oceans are warming at rates faster than marine species have experienced over the previous 400,000 years. Our quantitative review across a wide range of marine taxa revealed that increases in temperature that occur above mean annual temperature (MAT) generally lower population survival and reproduction. Furthermore, these effects are likely to be increasingly detrimental, particularly for survival, as the magnitude of warming increases. Although we found evidence for considerable variation within and between populations in response to increasing temperatures, whether this existing variation will allow for evolutionary rescue is uncertain. Importantly, a simple metric (ie MAT) facilitates evaluation of the effects of increasing temperatures on the survivorship and reproduction of most populations, even those for which limited data on thermal sensitivity are available. This approach improves our understanding of population‐level responses to climate change, which is critical for adaptation planning that aims to sustain and restore vulnerable populations.

Climate change has far-reaching effects on human and ecological systems, requiring collaboration across sectors and disciplines to determine effective responses. To inform regional responses to climate change, decision-makers need credible and relevant information representing a wide swath of knowledge and perspectives. The southeastern U. S. State of Georgia is a valuable focal area for study because it contains multiple ecological zones that vary greatly in land use and economic activities, and it is vulnerable to diverse climate change impacts. We identified 40 important research questions that, if answered, could lay the groundwork for effective, science-based climate action in Georgia. Top research priorities were identified through a broad solicitation of candidate research questions (180 were received). A group of experts across sectors and disciplines gathered for a workshop to categorize, prioritize, and filter the candidate questions, identify missing topics, and rewrite questions. Participants then collectively chose the 40 most important questions. This cross-sectoral effort ensured the inclusion of a diversity of topics and questions (e.g., coastal hazards, agricultural production, ecosystem functioning, urban infrastructure, and human health) likely to be important to Georgia policy-makers, practitioners, and scientists. Several cross-cutting themes emerged, including the need for long-term data collection and consideration of at-risk Georgia citizens and communities. Workshop participants defined effective responses as those that take economic cost, environmental impacts, and social justice into consideration. Our research highlights the importance of collaborators across disciplines and sectors, and discussing challenges and opportunities that will require transdisciplinary solutions.

Plants and their interactions with root-associated fungi have wide-reaching ecological effects, from influencing individual traits to modifying interactions with other species to affecting biodiversity and biogeochemical cycles (Johnson et al., 2012). While mycorrhizal fungi have long been of interest, there is growing evidence that other root-associated fungi markedly influence host plant structural and physiological traits, particularly when mycorrhizal fungi are absent or in low abundance (Porras-Alfaro and Bayman, 2011; Clemmensen et al., 2013; Kivlin et al., 2013; Berthelot et al., 2019). Plant trait differences can result from functions provided by fungal endophytes, including the production of secondary metabolites with antibacterial properties and hormones that influence plant growth and biomass allocation (Andrade-Linares and Franken, 2013; Berthelot et al., 2019). Dark septate endophytes (DSE) are a polyphyletic fungal endophyte group with high colonization rates in the roots of a broad range of host plants in nearly all biomes (Mandyam and Jumpponen, 2008, 2014; Newsham, 2011) and with melanized hyphae that may allow them to withstand extreme conditions (Jumpponen and Trappe, 1998; Kivlin et al., 2013). Unlike mycorrhizal fungi, DSE do not have specialized structures for resource exchange with host plants, but their presence is associated with increased plant shoot nitrogen and phosphorus concentrations (Newsham, 2011). Recent genomic evidence suggests that DSE have a spectrum of enzymes capable of degrading organic matter (Knapp et al., 2018), consistent with an ability to increase nutrient availability to plants, particularly in nutrient-poor soils (Newsham, 2011; Knapp et al., 2018). Dark septate endophytes can also protect against plant pathogens (Andrade-Linares and

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