Climate change‐induced temperature stress is affecting the performance and survival of plants across terrestrial and aquatic ecosystems. For terrestrial plants, below‐ground microbes can enhance plant performance in response to environmental stress and recent evidence suggests a similar role for marine plants. Despite this, the potential for below‐ground microbes to enhance marine plant resilience against climate change‐induced marine heatwaves (MHWs), an ocean temperature stress that is increasing in frequency and intensity globally, remains unclear. We experimentally manipulated microbial communities in Zostera muelleri rhizosphere and bulk sediments through root sterilisation and sediment autoclaving to determine their influence on seagrass growth and survival under two marine heatwave scenarios: recent MHW profile and an end‐of‐century scenario. Seagrasses with an experimentally disrupted rhizosphere microbiome showed reduced growth under all temperature and sediment treatments. In contrast, an intact bulk sediment microbiome hindered plant growth under the future marine heatwave scenario and disruption of these communities had a positive effect on plant performance. Future marine heatwave treatments had a lower relative abundance of potentially beneficial microbes in bulk sediments (i.e. Akkermansiaceae) and were enriched with potential plant pathogens (i.e. Xanthomonadaceae). In addition, the rhizosphere of plants in intact bulk sediments showed a lower relative abundance of potential plant‐growth‐promoting bacteria. Synthesis. This study provides experimental evidence that marine heatwaves can negatively affect seagrass performance via changes in bulk sediment microbiota and that the benefits provided by rhizosphere microbiota to plants may not be enough to overcome such effects. Our experiment highlights for the first time how below‐ground microbes influence seagrass responses to heat stress. Furthermore, our findings emphasise the need to consider below‐ground microbial interactions in future seagrass research and suggest that shifts in microbial communities may play a role in seagrass resilience to climate change. These insights may be critical for restoration efforts, as integrating below‐ground microbial communities into seagrass management strategies may enhance the success of restoration initiatives under changing environmental conditions.

Local spatial attributes of habitat patches (e.g., area, rugosity) are well-known to affect biodiversity, but such effects may interact with regional environmental processes resulting in variations in those relationships at large spatial scales. Hence, it is unclear whether associations between spatial attributes and biodiversity can be generalized to inform biodiversity conservation and restoration. This study investigated: (i) the relationships between epifauna assemblages and spatial attributes of Sydney rock oyster (Saccostrea glomerata) reefs, including within-patches (e.g. rugosity, distance to patch edge and elevation), whole-patch (size and shape) and amongst-patch attributes (i.e., landscape connectivity) in three estuaries spanning ∼250 km coastline of eastern Australia; and (ii) the variation in epifauna assemblages among the three estuaries to understand the variation in those relationships at regional scales. At each estuary, we collected random oyster cores from different oyster patches and counted and identified all epifauna >1 mm. Multivariate assemblage composition of epifauna showed large variations among estuaries, with little associations to spatial attributes. At local scales, attributes such as elevation, patch area and patch isolation were strong predictors of richness and total abundance in some estuaries. However, none of these relationships were consistent in strength and/or direction across estuaries. Thus, results suggest that the influence of local spatial attributes of oyster reefs on epifaunal assemblages is small and likely related to variations in species pools and environmental setting among estuaries. A greater understanding of the drivers of assemblages, especially at larger spatial scales, may lead to improved outcomes of management and restoration strategies for biodiversity associated with foundation species.

Biological invasions pose a significant threat to local ecosystems and native species worldwide. For macrophytes, the interaction of factors such as native species biomass loss and propagule pressure of invasive species have important consequences for invasion success. How these factors are mediated by below-ground processes is, however, poorly understood. Through a field survey and manipulative experiment, we simultaneously investigated the influences of native seagrass (Zostera muelleri) biomass, invasive propagule pressure and below-ground bacterial communities (evaluated through 16S amplicon sequencing) in mediating the establishment of the invasive green alga, Caulerpa taxifolia. Results from the field survey showed that rhizosphere bacterial communities and predicted metabolism were clearly distinct between Z. muelleri and C. taxifolia, but bulk sediment microbes and sediment characteristics (i.e. proportion of fine sediments and organic carbon) did not differ. Increasing seagrass or C. taxifolia biomass only affected rhizosphere bacterial richness, and this was apparent on specific bacterial groups involved in sulphur and nitrogen metabolism. Comparatively, the experimental manipulation showed that seagrass biomass significantly decreased C. taxifolia biomass, but this effect depended on invasive propagule levels. More specifically, C. taxifolia biomass was lower in treatments with higher Z. muelleri biomass although the magnitude of this effect was stronger in plots with high propagule pressure. Overall, increasing C. taxifolia propagule pressure may intensify negative interspecific competition among invading individuals, while higher seagrass biomass also reduces overall invasion success. These effects could potentially be mediated by root-associated microbial communities and directly influence invasion success.

Current rates of ocean warming are predicted to exacerbate ongoing declines in seagrass populations. Above-ground responses of seagrass to increasing temperatures have been studied from a direct physiological perspective while indirect effects, including changes to microbially-mediated below-ground processes, remain poorly understood. To test potential effects of increased temperature on seagrass growth and associated microbial communities, we sampled seagrass beds experiencing ambient and elevated water temperatures at Lake Macquarie, Australia. Sites with warmer water were associated with a plume from a power station discharge channel with temperatures analogous to conditions predicted by 2100 under current rates of ocean warming (+3°C). The microbial community composition in both sediments and leaf tissues varied significantly between warm and ambient water temperatures with higher relative abundances of putative sulphate-reducing bacteria such as Desulfocapsaceae, Desulfobulbaceae and Desulfosarcinaceae in sedimentary communities in warm water. Above-ground biomass and seagrass growth rates were greater at warm sites while below-ground biomass and detrital decomposition rates showed no difference suggesting potential buffering of temperature effects below-ground. These findings suggest a 3°C rise in temperate regions is unlikely to induce mortality in seagrass however, it may shift microbial communities towards more homogenous structure and composition.

Understanding how habitat attributes (e.g., patch area and sizes, connectivity) control recruitment and how this is modified by processes operating at larger spatial scales is fundamental to understanding population sustainability and developing successful long-term restoration strategies for marine foundation species-including for globally threatened reef-forming oysters. In two experiments, we assessed the recruitment and energy reserves of oyster recruits onto remnant reefs of the oyster Saccostrea glomerata in estuaries spanning 550 km of coastline in southeastern Australia. In the first experiment, we determined whether recruitment of oysters to settlement plates in three estuaries was correlated with reef attributes within patches (distances to patch edges and surface elevation), whole-patch attributes (shape and size of patches), and landscape attributes (connectivity). We also determined whether environmental factors (e.g., sedimentation and water temperature) explained the differences among recruitment plates. We also tested whether differences in energy reserves of recruits could explain the differences between two of the estuaries (one high- and one low-sedimentation estuary). In the second experiment, across six estuaries (three with nominally high and three with nominally low sedimentation rates), we tested the hypothesis that, at the estuary scale, recruitment and survival were negatively correlated to sedimentation. Overall, total oyster recruitment varied mostly at the scale of estuaries rather than with reef attributes and was negatively correlated with sedimentation. Percentage recruit survival was, however, similar among estuaries, although energy reserves and condition of recruits were lower at a high- compared to a low-sediment estuary. Within each estuary, total oyster recruitment increased with patch area and decreased with increasing tidal height. Our results showed that differences among estuaries have the largest influence on oyster recruitment and recruit health and this may be explained by environmental processes operating at the same scale. While survival was high across all estuaries, growth and reproduction of oysters on remnant reefs may be affected by sublethal effects on the health of recruits in high-sediment estuaries. Thus, restoration programs should consider lethal and sublethal effects of whole-estuary environmental processes when selecting sites and include environmental mitigation actions to maximize recruitment success.

Over 85 % of oyster reefs have been lost globally due to disease, overharvesting, global warming, and pollution. Consideration of the ecosystem services provided by healthy oyster reefs (e.g., coastal protection, water purification and carbon burial) has driven recent research and restoration efforts worldwide. However, hydrodynamic studies, specifically looking at the effects of different levels of wave exposure on the ecomorphodynamics of oyster reefs, are scarce. In this study, we consider oyster reefs in microtidal estuaries under different levels of relative wave exposure to determine how hydrodynamics may shape reef morphology and how reef morphology affects wave dissipation. We quantify oyster reef morphology through spatial analysis, using morphometrics and spatial density and relate these to the ability of oyster reefs to dissipate wave energy. Field campaigns were undertaken at three microtidal sites in southeast Australia with different hydrodynamic exposure and morphology: Gamay (Botany Bay), Port Hacking and Crookhaven River. We found that reef morphology and orientation is related to estuarine hydrodynamic conditions and thus we propose an ecomorphodynamic model with a continuum of morphologies from sparse reefs aligned perpendicular to the tidal currents and incoming waves (patch reefs), through broken up barriers semi -aligned or obliquely to the tidal flows (string reefs), to the total barrier that exists under the lowest hydrodynamic conditions (fringing reefs). The highest dissipative ability of locally generated wind waves occurred at Crookhaven (patch reef, 165 kW/m 2 ), and lowest at Gamay (string reef, 11.66 kW/m 2 ). Our results suggest that reef morphology, and orientation to currents and waves, influence wave dissipation, and that hydrodynamic conditions in turn influence reef morphology. These findings are important to inform future reef restoration under increasingly severe climate change conditions to optimise ecosystem services on restored oyster reefs.

Foundation species are being restored into inherently variable landscapes with multiple, interspersed habitats. However, understanding of the influence of different neighbouring habitats on community assembly and the survival of restored species is limited, despite their significant potential to affect restoration outcomes. We tested how habitat‐setting (being next to seagrass, seagrass and mangroves, or unvegetated sediments) and predation (by meso‐ and/or large predators) influenced macroinvertebrate community assembly and the survival of juvenile Sydney rock oysters ( Saccostrea glomerata ) on experimental oyster reef units in the Port Hacking estuary, New South Wales, Australia. Each habitat‐setting produced a distinct macroinvertebrate community on experimental reefs, whereas predation had limited effects on community structure. Juvenile oysters were instead highly predated everywhere, and oyster predation was dominated by the large, transient fish Acanthropagrus australis . Our findings allow practitioners to predict and tailor the communities which establish on restored oyster reefs by strategically placing them next to different habitats. If sites have a high predation risk but require seeding for reefs to establish, then caging or complex substrates must be used to increase seeded oyster survival.

Bioturbation in coastal sediments plays a crucial role in biogeochemical cycling. However, a key knowledge gap is the extent to which bioturbation influences bacterial community diversity and ecosystem processes, such as nitrogen cycling. This study paired bacterial diversity, bioturbation activity and in situ flux measurements of oxygen and nitrogen from bioturbated sediments at six estuaries along the East coast of Australia. Bacterial community diversity, composition and predicted functional profiles were similar across burrow and surface sediments but were significantly influenced by bioturbator activity (measured as number of burrows) at sites with higher fine grain content. Sediment oxygen demand increased with bioturbator activity but changes in nitrogen cycling (as measured by fluxes and predicted bacterial functional gene analysis) were more spatially variable and were unrelated to bioturbator activity and bacterial community shifts. This study highlights how bioturbator activity influences bacterial community structure and functioning and what implications this has for biogeochemical cycles in estuarine sediments.

Oyster reefs play a crucial role in the removal of nitrogen (N) from aquatic systems by facilitating nutrient regeneration and denitrification, both in their tissues and shells and surrounding sediments. However, we still have a limited understanding about the contribution of each component of the reefs (e.g. oysters vs sediments) to N processes, and whether rates are dependent on site-specific characteristics. To address these knowledge gaps, we conducted an experiment across six oyster reefs along 1080 km of the Eastern Australian coast with different sediment characteristics. By using in-situ clear and dark incubation chambers, we assessed how benthic metabolism, nutrient and dinitrogen gas (N2) fluxes varied among the following treatments: 'oysters', 'sediments', and 'sediments + oysters' that were used to represent components of the whole reef habitat (i.e. reef matrix vs surrounding sediments vs the interaction among them, respectively), and sites. We found that during dark conditions and at siltier sites, N2 effluxes from oysters can be up to 23 times higher than sediments, while N2 effluxes from chambers with both sediments and oysters were similar to sediment treatments, and lower than oyster treatments. These results can be explained by sediment processes including nutrient assimilation by benthic microalgae and/or lower nutrient diffusion into interstitial space. Additionally, oyster treatments showed an uptake of nitrate (NO3-) that was likely converted into N2, whereas sediment treatments showed an overall release of NO3-. In dark conditions, ammonium (NH4+) fluxes remained consistent across treatments and sites, indicating that any exports from oyster excretion (in those treatments including oysters) were either counterbalanced by or comparable to exports from sediments. This study provides evidence that the crucial contribution of oyster reefs to N removal is dependent on interactions between reef components and environmental factors.

Eutrophication is a worldwide issue that can disrupt ecosystem processes in sediments. Studies have shown that macrofauna influences sediment processes by engineering environments that constrain microbial communities. Here, we explored the effect of different sizes of the Sydney cockle (Anadara trapezia), on bacterial and archaeal communities in natural and experimentally enriched sediments. A mesocosm experiment was conducted with two enrichment conditions (natural or enriched) and 5 cockle treatments (small, medium, large, mixed sizes and a control). This study was unable to detect A. trapezia effects on microbial communities irrespective of body size. However, a substantial decrease of bacterial richness, diversity, and structural and functional shifts, were seen with organic enrichment of sediments. Archaea were similarly changed although the magnitude of effect was less than for bacteria. Overall, we found evidence to suggest that A. trapezia had limited capacity to affect sediment microbial communities and mitigate the effects of organic enrichment.

Ecosystem engineers have profound effects on ecosystem structure and functioning. While habitat-forming ecosystem engineers like trees and corals are acknowledged for their habitat-modifying function, habitat-modifying mobile organisms are not typically recognised for their habitat-forming function. Limpets are appreciated as important modifiers of benthic assemblages through their grazing activity, but little is known about the potential role of their shells in providing habitat for either their competitors or the species they consume. We show that in the Northeast Atlantic, limpets are simultaneous consumers and facilitators of algae on rocky shores, with species-specific outcomes. When limpets are abundant, only the most grazing-resistant algae persist on the rocks; however, when one particular species (Patella ulyssiponensis) is present, rich algal assemblages develop on their shells. This facilitatory role is less pronounced in other species (Patella vulgata, Patella depressa). Manipulative experiments suggest that aggressive behaviour in P. ulyssiponensis moderates mutual grazing on shells, providing an important associational refuge for algae from consumption. Patella vulgata shells provide an associational refuge for small individuals of other limpets from competition on primary rock substrata. Limpets not only modify the surrounding environment but also constitute whole microcosms supporting diverse and dense epibiotic communities. Not only is this the case in natural habitats, but even more so in artificial environments where smooth engineered habitats like sea walls favour limpet dominance. This can lead to the emergence of 'limpet barrens' (akin to subtidal urchin barrens), where high limpet densities coupled with maximal grazing efficiency prevent recruitment of other taxa to the surrounding substratum. These results are not just relevant for theoretical ecology but also for practical wildlife management in a changing world.

Anthropogenic environmental stressors have significantly reduced biodiversity and the capacity of remnant natural habitats to deliver ecosystem functions and services in urban areas. To mitigate these impacts and recover biodiversity and function, ecological restoration strategies are needed. While habitat restoration is proliferating in rural and peri-urban areas, strategies purposely designed to succeed under the environmental, social and political pressures of urban areas are lacking. Here, we propose that ecosystem health in marine urban areas can be improved by restoring biodiversity to the most dominant habitat, unvegetated sediments. We reintroduced a native ecosystem engineer, the sediment bioturbating worm Diopatra aciculata, and assessed their effects on microbial biodiversity and function. Results showed that worms can affect the diversity of microbes, but effects varied between locations. Worms caused shifts in microbial community composition and function at all locations. Specifically, the abundance of microbes capable of chlorophyll production (i.e. benthic microalgae) increased and the abundance of microbes capable of methane production decreased. Moreover, worms increased the abundances of microbes capable of denitrification in the site with lowest sediment oxygenation. Worms also affected microbes capable of degrading the polycyclic aromatic hydrocarbon toluene, although the direction of that effect was site-specific. This study provides evidence that a simple intervention such as the reintroduction of a single species can enhance sediment functions important for the amelioration of contamination and eutrophication, although further studies are needed to understand the variation in outcomes between sites. Nevertheless, restoration strategies targeting unvegetated sediments provide an opportunity to combat anthropogenic stressors in urban ecosystems and may be used for precondition before more traditional forms of habitat restoration such as seagrass, mangrove and shellfish restoration.

Below‐ground microbiota play an important role in mediating environmental conditions with important consequences for plant performance. Micro‐organisms involved in plant–soil interactions may be associated with roots or bulk soil; however, the relative influence of these below‐ground microbial assemblages on plant performance is poorly known, particularly for marine plants. We separately manipulated the root and sediment microbial assemblages of the seagrass Zostera muelleri in a fully factorial experiment to determine how these assemblages determined plant response (e.g. growth) to nutrient enrichment, a major stressor in marine systems. Under ambient nutrient conditions, seagrass growth was maintained regardless of root microbial assemblage disruption. Under high nutrient stress, however, seagrasses with disrupted root microbiota had reduced growth, whereas growth was maintained in seagrasses with an intact root microbiota. Disruption of bulk‐sediment microbiota did not affect seagrass growth. Nutrient elevation was correlated with enhanced abundances of several putatively beneficial microbial taxa (e.g. sulphide‐oxidising Beggiatoaceae and denitrifying Geofilum rubicundum ) associated with roots. Synthesis . Our results suggest that under ambient nutrient conditions micro‐organisms play a reduced role in influencing plant performance, but under more stressful conditions positive plant–root micro‐organism interactions strengthened. These results are among the first to experimentally determine that interactions between marine plants and the root‐associated microbiota are key drivers of seagrass performance under human‐induced environmental changes. This suggests that as in terrestrial systems, marine plant resilience depends on the stress‐mitigating functions of their root‐associated microbiota and disturbance to those plant–microbiota interactions can be deleterious for plant performance. Improving our understanding of these plant–micro‐organism interactions may be critical for understanding the functioning and resilience of threatened marine plants and developing more effective restoration strategies for them.

Context Gamay is a coastal waterway of immense social, cultural and ecological value. Since European settlement, it has become a hub for industrialisation and human modification. There is growing desire for ecosystem-level management of urban waterways, but such efforts are often challenged by a lack of integrated knowledge.Aim and methods We systematically reviewed published literature and traditional ecological knowledge (TEK), and consulted scientists to produce a review of Gamay that synthesises published knowledge of Gamay's aquatic ecosystem to identify knowledge gaps and future research opportunities.Key results We found 577 published resources on Gamay, of which over 70% focused on ecology. Intertidal rocky shores were the most studied habitat, focusing on invertebrate communities. Few studies considered multiple habitats or taxa. Studies investigating cumulative human impacts, long-term trends and habitat connectivity are lacking, and the broader ecological role of artificial substrate as habitat in Gamay is poorly understood. TEK of Gamay remains a significant knowledge gap. Habitat restoration has shown promising results and could provide opportunities to improve affected habitats in the future.Conclusion and implications This review highlights the extensive amount of knowledge that exists for Gamay, but also identifies key gaps that need to be filled for effective management.

Facilitation cascades are increasingly recognized as key drivers of biodiversity in a variety of habitats, yet their temporal variability remains poorly investigated. On shallow subtidal rocky reefs, positive interactions between the canopy-forming species, Halopithys incurva, and its epiphyte, Jania rubens, enhance the abundance and diversity of the associated mobile invertebrate assemblage. By means of a field experiment manipulating the presence of J. rubens on H. incurva plants, we investigated whether the effects of this facilitation cascade varied over time scales of months. Despite seasonal fluctuations in its biomass, J. rubens enhanced total invertebrate abundance and species richness by 64% and 45%, respectively, throughout the experiment. Our results suggest that the presence of J. rubens, likely by providing novel microhabitats and increasing resource availability, sustains invertebrate biodiversity of shallow macroalgal forests consistently between warm and cold periods. Understanding the temporal dynamics of facilitation cascades and their mechanisms can help inform management strategies targeting biodiversity conservation and restoration under current and future climates.

Microbes are sensitive indicators of estuarine processes because they respond rapidly to dynamic disturbance events. As most of the world's population lives in urban areas and climate change-related disturbance events are becoming more frequent, estuaries bounded by cities are experiencing increasing stressors, at the same time that their ecosystem services are required more than ever. Here, using a multidisciplinary approach, we determined the response of planktonic microbial assemblages in response to seasonality and a rainfall disturbance in an urban estuary bounded by Australia's largest city, Sydney. We used molecular barcoding (16S, 18S V4 rRNA) and microscopy-based identification to compare microbial assemblages at locations with differing characteristics and urbanisation histories. Across 142 samples, we identified 8,496 unique free-living bacterial zOTUs, 8,175 unique particle associated bacterial zOTUs, and 1,920 unique microbial eukaryotic zOTUs. Using microscopy, we identified only the top <10% abundant, larger eukaryotic taxa (>10 µm), however quantification was possible. The site with the greater history of anthropogenic impact showed a more even community of associated bacteria and eukaryotes, and a significant increase in dissolved inorganic nitrogen following rainfall, when compared to the more buffered site. This coincided with a reduced proportional abundance of Actinomarina and Synechococcus spp., a change in SAR 11 clades, and an increase in the eukaryotic microbial groups Dinophyceae, Mediophyceae and Bathyoccocaceae, including a temporary dominance of the harmful algal bloom dinoflagellate Prorocentrum cordatum (syn. P. minimum). Finally, a validated hydrodynamic model of the estuary supported these results, showing that the more highly urbanised and upstream location consistently experienced a higher magnitude of salinity reduction in response to rainfall events during the study period. The best abiotic variables to explain community dissimilarities between locations were TDP, PN, modelled temperature and salinity (r = 0.73) for the free living bacteria, TP for the associated bacteria (r = 0.43), and modelled temperature (r = 0.28) for the microbial eukaryotic communities. Overall, these results show that a minor disturbance such as a brief rainfall event can significantly shift the microbial assemblage of an anthropogenically impacted area within an urban estuary to a greater degree than a seasonal change, but may result in a lesser response to the same disturbance at a buffered, more oceanic influenced location. Fine scale research into the factors driving the response of microbial communities in urban estuaries to climate related disturbances will be necessary to understand and implement changes to maintain future estuarine ecosystem services.

To investigate nitrogen (N) cycling in oyster reef habitats along the East coast of Australia, we assessed N-cycling gene abundances in oyster shell biofilms and surrounding sediments, and explored their correlation with environmental factors and respective N rates. We found higher abundances of the denitrification gene nosZII in oyster shell biofilms, while there were not significant differences in the denitrification genes nirS and nirK between oyster biofilms and sediments. Additionally, oyster shell biofilms had a lower (nirS + nirK)/nosZII ratio, indicating a greater capacity for N removal and limited nitrous oxide release compared to sediments. Abundance of nirS, nirK, and dissimilatory nitrate reduction to ammonium (nrfA) genes in sediments decreased with increasing content of organic material, suggesting the influence of large-scale environmental conditions. N-cycling gene abundances did not relate to N rates, emphasising the importance of investigating microbial genes to enhance our understanding of the N cycle in oyster reef habitats.

Abstract Habitat‐forming organisms provide three‐dimensional structure that supports abundant and diverse communities. Variation in the morphological traits of habitat formers will therefore likely influence how they facilitate associated communities, either via food and habitat provisioning, or by altering predator–prey interactions. These mechanisms, however, are typically studied in isolation, and thus, we know little of how they interact to affect associated communities. In response to this, we used naturally occurring morphological variability in the alga Sargassum vestitum to create habitat units of distinct morphotypes to test whether variation in the morphological traits (frond size and thallus size) of S. vestitum or the interaction between these traits affects their value as habitat for associated communities in the presence and absence of predation. We found morphological traits did not interact, instead having independent effects on epifauna that were negligible in the absence of predation. However, when predators were present, habitat units with large fronds were found to host significantly lower epifaunal abundances than other morphotypes, suggesting that large frond alga provided low‐value refuge from predators. The presence of predators also influenced the size structure of epifaunal communities from habitat units of differing frond size, suggesting that the refuge value of S. vestitum was also related to epifauna body size. This suggests that habitat formers may chiefly structure associated communities by mediating size‐selective predation, and not through habitat provisioning. Furthermore, these results also highlight that habitat traits cannot be considered in isolation, for their interaction with biotic processes can have significant implications for associated communities.

Habitat heterogeneity is considered a primary causal driver underpinning patterns of diversity, yet the universal role of heterogeneity in structuring biodiversity is unclear due to a lack of coordinated experiments testing its effects across geographic scales and habitat types. Furthermore, key species interactions that can enhance heterogeneity, such as facilitation cascades of foundation species, have been largely overlooked in general biodiversity models. Here, we performed 22 geographically distributed experiments in different ecosystems and biogeographical regions to assess the extent to which variation in biodiversity is explained by three axes of habitat heterogeneity: the amount of habitat, its morphological complexity, and capacity to provide ecological resources (e.g. food) within and between co-occurring foundation species. We show that positive and additive effects across the three axes of heterogeneity are common, providing a compelling mechanistic insight into the universal importance of habitat heterogeneity in promoting biodiversity via cascades of facilitative interactions. Because many aspects of habitat heterogeneity can be controlled through restoration and management interventions, our findings are directly relevant to biodiversity conservation.