Density-dependent facilitation cascades determine epifaunal community structure in temperate Australian mangroves

Ecological restoration at pilot-scale employing site-specific rationales for small-patch degraded mangroves in Indian Sundarbans

To date, degraded mangrove ecosystem restoration accomplished worldwide primarily aligns towards rehabilitation with monotypic plantations, while ecological restoration principles are rarely followed in these interventions. However, researchers admit that most of these initiatives' success rate is not appreciable often. An integrative framework of ecological restoration for degraded mangroves where site-specific observations could be scientifically rationalized, with co-located reference pristine mangroves as the target ecosystem to achieve is currently distinctively lacking. Through this experimental scale study, we studied the suitability of site-specific strategies to ecologically restore degraded mangrove patches vis-à-vis the conventional mono-species plantations in a highly vulnerable mangrove ecosystem in Indian Sundarbans. This comprehensive restoration framework was trialed in small discrete degraded mangrove patches spanning ~ 65 ha. Site-specific key restoration components applied are statistically validated through RDA analyses and Bayesian t-tests. 25 quantifiable metrics evaluate the restoration success of a ~ 3 ha degraded mangrove patch with Ridgeline distribution, Kolmogorov-Smirnov (K-S) tests, and Mahalanobis Distance (D2) measure to prove the site's near-equivalence to pristine reference in multiple ecosystem attributes. This restoration intervention irrevocably establishes the greater potential of this framework in the recovery of ecosystem functions and self-sustenance compared to that of predominant monoculture practices for vulnerable mangroves.

Density-dependent species interactions modulate alpine treeline shifts

Species interactions such as facilitation and competition play a crucial role in driving species range shifts. However, density dependence as a key feature of these processes has received little attention in both empirical and modelling studies. Herein, we used a novel, individual-based treeline model informed by rich in situ observations to quantify the contribution of density-dependent species interactions to alpine treeline dynamics, an iconic biome boundary recognized as an indicator of global warming. We found that competition and facilitation dominate in dense versus sparse vegetation scenarios respectively. The optimal balance between these two effects was identified at an intermediate vegetation thickness where the treeline elevation was the highest. Furthermore, treeline shift rates decreased sharply with vegetation thickness and the associated transition from positive to negative species interactions. We thus postulate that vegetation density must be considered when modelling species range dynamics to avoid inadequate predictions of its responses to climate warming.

Facilitation of macrofaunal assemblages in marinas by the habitat-forming invader Amathia verticillata (Bryozoa: Gymnolaemata) across a spatiotemporal scale

Widespread habitat-forming invaders inhabiting marinas, such as the spaghetti bryozoan Amathia verticillata, allow exploring facilitation processes across spatiotemporal contexts. Here we investigate the role of this bryozoan as habitat for native and exotic macrofaunal assemblages across different ecoregions of Western Mediterranean and East Atlantic coasts, and a monthly variation over a year. While only 7 (all peracarid crustaceans) of the 54 associated species were NIS, they dominated macrofaunal assemblages in terms of abundance, raising the potential for invasional meltdown. NIS richness and community structure differed among marinas but not among ecoregions, highlighting the importance of marina singularities in modulating facilitation at spatial scale. Despite facilitation did not depend on bryozoan abundance fluctuations, it was affected by its deciduous pattern, peaking in summer and disappearing in late winter. Monitoring A. verticillata in marinas, especially in summer periods, may improve the detection and management of multiple associated NIS.

Facilitation of animals is stronger during summer marine heatwaves and around morphologically complex foundation species

Foundation species create biogenic habitats, modify environmental conditions, augment biodiversity, and control animal community structures. In recent decades, marine heatwaves (MHWs) have affected the ecology of foundation species worldwide, and perhaps also their associated animal communities. However, no realistic field experiment has tested how MHWs affect animals that live in and around these foundation species. We therefore tested, in a four-factorial field experiment, if colonisation by small mobile marine animals (epifauna) onto plates with attached single versus co-occurring foundation species of different morphological complexities, were affected by 3-5°C heating (that mirrored a recent extreme MHW in the study area) and if the heating effect on the epifauna varied within and between seasons. For this experiment mimics of turf seaweed represented the single foundation species and holdfasts of seven common canopy-forming seaweed represented the co-occurring foundation species with different morphological complexities. We found that the taxonomic richness and total abundance of epifauna, dominated by copepods, generally were higher on heated plates with complex seaweed holdfasts in warmer summer trials. Furthermore, several interactions between test-factors were significant, e.g., epifaunal abundances, were, across taxonomic groups, generally higher in warmer than colder summer trials. These results suggest that, in temperate ecosystems, small, mobile, short-lived, and fast-growing marine epifauna can be facilitated by warmer oceans and morphologically complex foundation species, implying that future MHWs may increase secondary production and trophic transfers between primary producers and fish. Future studies should test whether these results can be scaled to other ecological species-interactions, across latitudes and biogeographical regions, and if similar results are found after longer MHWs or within live foundation species under real MHW conditions.

Interspecific sociality alters the colonization and extinction rates of birds on subtropical reservoir islands

Island biogeography theory has proved a robust approach to predicting island biodiversity on the assumption of species equivalency. However, species differ in their grouping behaviour and are entangled by complex interactions in island communities, such as competition and mutualism. We here investigated whether intra- and/or interspecific sociality may influence biogeographic patterns, by affecting movement between islands or persistence on them. We classified bird species in a subtropical reservoir island system into subcommunities based on their propensity to join monospecific and mixed-species flocks. We found that subcommunities which had high propensity to flock interspecifically had higher colonization rates and lower extinction rates over a 10-year period. Intraspecific sociality increased colonization in the same analysis. A phylogenetically corrected analysis confirmed the importance of interspecific sociality, but not intraspecific sociality. Group-living could enable higher risk crossings, with greater vigilance also linked to higher foraging efficiency, enabling colonization or long-term persistence on islands. Further, if group members are other species, competition can be minimized. Future studies should investigate different kinds of island systems, considering positive species interactions driven by social behaviour as potential drivers of community assembly on islands. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

A persistent green macroalgal mat shifts ecological functioning and composition of associated species on an Eastern Tropical Pacific coral reef

Global evidence of phase shifts to alternate community types is of particular concern because these new communities can provide fundamentally different and often novel ecosystem functions and services compared to the original community. Shifts of a diverse range of marine communities to dominance by green macroalgal mats have occurred worldwide, making it critical to understand their emerging functions and roles. We observed a green algal mat on two reefs in the Eastern Tropical Pacific, with one persisting for >10 years on a reef with stable herbivore populations and no known sources of anthropogenic nutrients. These mats supported a more speciose macroalgal community with fewer taxa present in the adjacent coral community and facilitated growth of an associated understory macroalgal species by reducing herbivory pressure and possibly enhancing nutrient supplies within the mat community state. These results demonstrate a weakening in the processes controlling reef community structure as a result of the shift in composition associated with the macroalgal mat, creating a positive feedback supporting mat persistence. These novel ecosystem functions generated by this alternate community state illustrate the importance of further research on community shifts, which will become increasingly common in the Anthropocene.

A realized facilitation cascade mediated by biological soil crusts in a sagebrush steppe community

Biological soil crusts can have strong effects on vascular plant communities which have been inferred from short-term germination and early establishment responses. However, biocrusts are often assumed to function as an "organizing principle" in communities because their effects can "cascade" to interactions among crust-associated plant species. We conducted surveys and experiments to explore these cascades and found that biocrusts were positively associated with large patches (> 10 m diameter) of a dominant shrub Artemisia tridentata. At the smaller scale of individual shrubs and the open matrices between shrubs, biocrusts were negatively associated with Artemisia. Juveniles of Artemisia were found only in biocrusts in intershrub spaces and never under shrubs or in soil without biocrusts. In two-year field experiments, biocrusts increased the growth of Festuca and the photosynthetic rates of Artemisia. Festuca planted under Artemisia were also at least twice as large as those planted in open sites without crusts or where Artemisia were removed. Thus, biocrusts can facilitate vascular plants over long time periods and can contribute to a "realized" cascade with nested negative and positive interactions for a range of species, but unusual among documented cascades in that it includes only autotrophs.

Remnant oyster reefs as fish habitat within the estuarine seascape

Interest in oyster reef conservation and restoration is growing globally, but particularly in Australia, it is unclear the extent to which oyster reefs complement (versus replicate) habitat provisioning by other structured habitats in the seascape. Remote underwater video surveys of two east Australian estuaries revealed that at high tide, oyster reefs not only supported distinct fish communities to bare sediments but also to adjacent seagrass beds and mangrove forests. Fish observations in oyster reefs were close to double that of mangroves and seagrass, with species richness, abundance, feeding and wandering behaviours similar. Several species of blenny and goby were unique to oyster reefs and oyster-containing mangroves, whilst recreationally fished species such as bream and mullet were more abundant on oyster reefs than in other habitats. Resolving the association between oyster reefs and fish species within the broader seascape will assist in developing restoration and management strategies that maximise fisheries benefit.

Exploring coexistence mechanisms in a three-species assemblage

Interactions among species are essential in shaping ecological communities, although it is not always clear under what conditions they can persist when the number of species involved is higher than two. Here we describe a three-species assemblage involving the seagrass Cymodocea nodosa, the pen shell Pinna nobilis and the herbivore sea urchin Paracentrotus lividus, and we explore the mechanisms allowing its persistence through field observations and manipulative experiments. The abundance of pen shells was higher in seagrass beds than in bare sand, suggesting a recruitment facilitation. The presence of sea urchins, almost exclusively attached or around pen shells, indicated habitat facilitation for sea urchins, which overgrazed the meadow around the pen shells forming seagrass-free halos. Our results suggest that this system persists thanks to: (i) the behavioral reluctance of sea urchins to move far from pen shells, making their impact on seagrass strictly local, (ii) the sparse distribution of pen shells and (iii) the plant's resistance mechanisms to herbivory. Unpacking these mechanisms allows a better understanding of how ecological communities are assembled.

Heterogeneity within and among co-occurring foundation species increases biodiversity

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.

Species interactions that can enhance habitat heterogeneity such as facilitation cascades of foundation species have been overlooked in biodiversity models. This study conducted 22 geographically distributed experiments in different ecosystems and biogeographical regions to assess the extent to which biodiversity is explained by three axes of habitat heterogeneity in facilitation cascades.

Differential Response of Macrobenthic Abundance and Community Composition to Mangrove Vegetation

The mass planting of mangroves has been proposed as a mitigation strategy to compensate for mangrove loss. However, the effects of mangrove vegetation on the abundance and community composition of macrobenthos remain controversial. The macrobenthic communities in four intact mangrove forests with different conditions and the adjacent nonvegetated mudflats of two mangrove species with distinct stand structures on the western coast of Taiwan were examined. Some macrobenthic taxa occurred only in the mangroves, suggesting macrobenthic critical habitats. Seasonal shift in community composition was more pronounced in the mudflats than in the mangroves, possibly due to the rich food supply, low temperature, and shelter function provided by mangrove forests. However, crab density was always lower in the mangroves than in the mudflats. There was a negative relationship between the stem density of Kandelia obovata (S., L.) and infaunal density. The pneumatophore density of Avicennia marina (Forsk.) correlated negatively with epifaunal density. Our results show that the response of macrobenthic abundance and community composition to mangrove vegetation was inconsistent. We reason that mangroves are critical habitats for the macrobenthos in the mudflats. However, if mangrove tree density is high, we predict that the macrobenthic density will decrease. This suggests that at some intermediate level of mangrove tree density, where there are enough mangrove trees to harbor a macrobenthic community but not enough trees to significantly reduce this density, mangroves management can be optimally achieved to promote the presence of a diverse and dense macrobenthic community.

Density-dependence tips the change of plant-plant interactions under environmental stress

Facilitation studies typically compare plants under differential stress levels with and without neighbors, while the density of neighbors has rarely been addressed. However, recent empirical studies indicate that facilitation may be density-dependent too and peak at intermediate neighbor densities. Here, we propose a conceptual model to incorporate density-dependence into theory about changes of plant-plant interactions under stress. To test our predictions, we combine an individual-based model incorporating both facilitative response and effect, with an experiment using salt stress and Arabidopsis thaliana. Theoretical and experimental results are strikingly consistent: (1) the intensity of facilitation peaks at intermediate density, and this peak shifts to higher densities with increasing stress; (2) this shift further modifies the balance between facilitation and competition such that the stress-gradient hypothesis applies only at high densities. Our model suggests that density-dependence must be considered for predicting plant-plant interactions under environmental change.

Size and density mediate transitions between competition and facilitation

Species simultaneously compete with and facilitate one another. Size can mediate transitions along this competition-facilitation continuum, but the consequences for demography are unclear. We orthogonally manipulated the size of a focal species, and the size and density of a heterospecific neighbour, in the field using a model marine system. We then parameterised a size-structured population model with our experimental data. We found that heterospecific size and density interactively altered the population dynamics of the focal species. Size determined whether heterospecifics facilitated (when small) or competed with (when large) the focal species, while density strengthened these interactions. Such size-mediated interactions also altered the pace of the focal's life history. We provide the first demonstration that size and density mediate competition and facilitation from a population dynamical perspective. We suspect such effects are ubiquitous, but currently underappreciated. We reiterate classic cautions against inferences about competitive hierarchies made in the absence of size-specific data.

The mechanisms by which oysters facilitate invertebrates vary across environmental gradients

The effective use of ecosystem engineers to conserve biodiversity requires an understanding of the types of resources an engineer modifies, and how these modifications vary with biotic and abiotic context. In the intertidal zone, oysters engineer ecological communities by reducing temperature and desiccation stress, enhancing the availability of hard substrate for attachment, and by ameliorating biological interactions such as competition and predation. Using a field experiment manipulating shading, predator access and availability of shell substrate at four sites distributed over 900 km of east Australian coastline, we investigated how the relative importance of these mechanisms of facilitation vary spatially. At all sites, and irrespective of environmental conditions, the provision of hard substrate by oysters enhanced the abundance and richness of invertebrates, in particular epibionts (barnacles and oyster spat) and grazing gastropods. Mobile arthropods utilised the habitat provided by disarticulated dead oysters more than live oyster habitat, whereas the abundance of polychaetes and bivalves were much greater in live oysters, suggesting the oyster filter-feeding activity is important for these groups. In warmer estuaries, shading by oysters had a larger effect on biodiversity, whereas in cooler estuaries, the provision of a predation refuge by oysters played a more important role. Such knowledge of how ecosystem engineering effects vary across environmental gradients can help inform management strategies targeting ecosystem resilience via the amelioration of specific environmental stressors, or conservation of specific community assemblages.

A Facilitation Cascade Enhances Local Biodiversity in Seagrass Beds

Invertebrate diversity can be a key driver of ecosystem functioning, yet understanding what factors influence local biodiversity remains uncertain. In many marine and terrestrial systems, facilitation cascades where primary foundation and/or autogenic ecosystem engineering species promote the settlement and survival of a secondary foundation/engineering species have been shown to enhance local biodiversity and ecosystem functioning. We experimentally tested if a facilitation cascade occurs among eelgrass (Zostera marina), pen clams (Atrina rigida), and community diversity in temperate seagrass beds in North Carolina, U.S.A., and if this sequence of direct positive interactions created feedbacks that affected various metrics of seagrass ecosystem function and structure. Using a combination of surveys and transplant experiments, we found that pen clam density and survivorship was significantly greater in seagrass beds, indicating that eelgrass facilitates pen clams. Pen clams in turn enhanced local diversity and increased both the abundance and species richness of organisms (specifically, macroalgae and fouling invertebrate fauna)—the effect of which scaled with increasing clam density. However, we failed to detect an impact of pen clams on other seagrass functions and hypothesize that functioning may more likely be enhanced in scenarios where secondary foundation species specifically increase the diversity of key functional groups such as epiphyte grazers and/or when bivalves are infaunal rather than epifaunal. Our findings add to the growing amount of literature that demonstrates that secondary foundation species are important drivers of local biodiversity in marine ecosystems. Further experimentation is needed that directly examines (i) the role of functional versus overall diversity on seagrass functions and (ii) the relative importance of life-history strategy in determining when and where engineering bivalves increase biodiversity and/or functioning of seagrass beds.

Algal Epibionts as Co-Engineers in Mussel Beds: Effects on Abiotic Conditions and Mobile Interstitial Invertebrates

Mussels and macroalgae have long been recognized as physical ecosystem engineers that modulate abiotic conditions and resources and affect the composition of rocky shore assemblages. Their spatial distributions in the intertidal zone frequently overlap, as many algal species thrive as epibionts on mussel beds. Nonetheless, their potential for combined engineering effects has not been addressed to date. Here we illustrate that Porphyra sp.—a desiccation-resistant macroalga that develops mostly epiphytically onto mussel beds—affects temperature, desiccation levels, and mobile interstitial invertebrates in mussel beds. Specifically, we observed that Porphyra cover (a) reduced temperature at the surface of the mussel bed but not at their base, (b) reduced desiccation both at the surface and base of the mussel bed and, (c) increased the densities of an abundant interstitial species—the amphipod Hyale grandicornis—in several study sites/dates. Additionally, we found that the positive responses of these grazing amphipods to Porphyra were driven by physical habitat modification (engineering) rather than food availability. This suggests that co-engineering by Porphyra and mussels generates abiotic states and focal species responses that would not be predictable from their individual effects. We expect that increased appreciation of co-engineering aids our understanding of complex ecological dynamics.

Large-scale distribution patterns of mangrove nematodes: A global meta-analysis

Mangroves harbor diverse invertebrate communities, suggesting that macroecological distribution patterns of habitat-forming foundation species drive the associated faunal distribution. Whether these are driven by mangrove biogeography is still ambiguous. For small-bodied taxa, local factors and landscape metrics might be as important as macroecology. We performed a meta-analysis to address the following questions: (1) can richness of mangrove trees explain macroecological patterns of nematode richness? and (2) do local landscape attributes have equal or higher importance than biogeography in structuring nematode richness? Mangrove areas of Caribbean-Southwest Atlantic, Western Indian, Central Indo-Pacific, and Southwest Pacific biogeographic regions. We used random-effects meta-analyses based on natural logarithm of the response ratio (lnRR) to assess the importance of macroecology (i.e., biogeographic regions, latitude, longitude), local factors (i.e., aboveground mangrove biomass and tree richness), and landscape metrics (forest area and shape) in structuring nematode richness from 34 mangroves sites around the world. Latitude, mangrove forest area, and forest shape index explained 19% of the heterogeneity across studies. Richness was higher at low latitudes, closer to the equator. At local scales, richness increased slightly with landscape complexity and decreased with forest shape index. Our results contrast with biogeographic diversity patterns of mangrove-associated taxa. Global-scale nematode diversity may have evolved independently of mangrove tree richness, and diversity of small-bodied metazoans is probably more closely driven by latitude and associated climates, rather than local, landscape, or global biogeographic patterns.

Restoring rocky intertidal communities: Lessons from a benthic macroalgal ecosystem engineer

As coastal population growth increases globally, effective waste management practices are required to protect biodiversity. Water authorities are under increasing pressure to reduce the impact of sewage effluent discharged into the coastal environment and restore disturbed ecosystems. We review the role of benthic macroalgae as ecosystem engineers and focus particularly on the temperate Australasian fucoid Hormosira banksii as a case study for rocky intertidal restoration efforts. Research focussing on the roles of ecosystem engineers is lagging behind restoration research of ecosystem engineers. As such, management decisions are being made without a sound understanding of the ecology of ecosystem engineers. For successful restoration of rocky intertidal shores it is important that we assess the thresholds of engineering traits (discussed herein) and the environmental conditions under which they are important.

Cockles, barnacles and ascidians compose a subtidal facilitation cascade with multiple hierarchical levels of foundation species

Facilitation cascades occur when multiple foundation species in a community are involved in a hierarchy of positive interactions, and consist of a primary facilitator which positively affects secondary facilitators, each supporting a suit of dependent species. There is no theoretical limit to the number of levels in a facilitation cascade, yet the existence of more than two has rarely been examined. We manipulated biogenic substrate produced by a primary facilitator (cockle shells) and a secondary facilitator (barnacles and their empty tests) in a space-limited subtidal community to test the hypothesis that solitary ascidians would be the third-level facilitator. In the field, most ascidians were found on barnacles, and most barnacles occupied cockle shells. To produce this pattern, barnacles could nurse ascidians (a longer 'facilitation chain') or outcompete them from cockle shells (a shorter chain). Experimental results clearly supported the nursing hypothesis providing evidence for a facilitation cascade with three hierarchical levels of foundation species. Our findings confirm that like predation and competition, positive interspecific interactions nest into multi-tier hierarchies with numerous levels. While the number of foundation species should increase community stability and resilience as it increases diversity and reduces environmental stress, facilitation chain length may have the opposite effect.

Species coexistence and the superior ability of an invasive species to exploit a facilitation cascade habitat

Facilitation cascades generated by co-occurring foundation species can enhance the abundance and diversity of associated organisms. However, it remains poorly understood how differences among native and invasive species in their ability to exploit these positive interactions contribute to emergent patterns of community structure and biotic acceptance. On intertidal shorelines in New England, we examined the patterns of coexistence between the native mud crabs and the invasive Asian shore crab in and out of a facilitation cascade habitat generated by mid intertidal cordgrass and ribbed mussels. These crab species co-occurred in low intertidal cobbles adjacent to the cordgrass–mussel beds, despite experimental findings that the dominant mud crabs can kill and displace Asian shore crabs and thereby limit their successful recruitment to their shared habitat. A difference between the native and invasive species in their utilization of the facilitation cascade likely contributes to this pattern. Only the Asian shore crabs inhabit the cordgrass–mussel beds, despite experimental evidence that both species can similarly benefit from stress amelioration in the beds. Moreover, only Asian shore crabs settle in the beds, which function as a nursery habitat free of lethal mud crabs, and where their recruitment rates are particularly high (nearly an order of magnitude higher than outside beds). Persistence of invasive adult Asian shore crabs among the dominant native mud crabs in the low cobble zone is likely enhanced by a spillover effect of the facilitation cascade in which recruitment-limited Asian shore crabs settle in the mid intertidal cordgrass–mussel beds and subsidize their vulnerable populations in the adjacent low cobble zone. This would explain why the abundances of Asian shore crabs in cobbles are doubled when adjacent to facilitation cascade habitats. The propensity for this exotic species to utilize habitats created by facilitation cascades, despite the lack of a shared evolutionary history, contributes to species coexistence and the acceptance of invasives into a diverse community.

A sixth-level habitat cascade increases biodiversity in an intertidal estuary

Many studies have documented habitat cascades where two co-occurring habitat-forming species control biodiversity. However, more than two habitat-formers could theoretically co-occur. We here documented a sixth-level habitat cascade from the Avon-Heathcote Estuary, New Zealand, by correlating counts of attached inhabitants to the size and accumulated biomass of their biogenic hosts. These data revealed predictable sequences of habitat-formation (=attachment space). First, the bivalve Austrovenus provided habitat for green seaweeds (Ulva) that provided habitat for trochid snails in a typical estuarine habitat cascade. However, the trochids also provided habitat for the nonnative bryozoan Conopeum that provided habitat for the red seaweed Gigartina that provided habitat for more trochids, thereby resetting the sequence of the habitat cascade, theoretically in perpetuity. Austrovenus is here the basal habitat-former that controls this "long" cascade. The strength of facilitation increased with seaweed frond size, accumulated seaweed biomass, accumulated shell biomass but less with shell size. We also found that Ulva attached to all habitat-formers, trochids attached to Ulva and Gigartina, and Conopeum and Gigartina predominately attached to trochids. These "affinities" for different habitat-forming species probably reflect species-specific traits of juveniles and adults. Finally, manipulative experiments confirmed that the amount of seaweed and trochids was important and consistent regulators of the habitat cascade in different estuarine environments. We also interpreted this cascade as a habitat-formation network that describes the likelihood of an inhabitant being found attached to a specific habitat-former. We conclude that the strength of the cascade increased with the amount of higher-order habitat-formers, with differences in form and function between higher and lower-order habitat-formers, and with the affinity of inhabitants for higher-order habitat-formers. We suggest that long habitat cascades are common where species traits allow for physical attachment to other species, such as in marine benthic systems and old forest.

Foundation species' overlap enhances biodiversity and multifunctionality from the patch to landscape scale in southeastern United States salt marshes

Although there is mounting evidence that biodiversity is an important and widespread driver of ecosystem multifunctionality, much of this research has focused on small-scale biodiversity manipulations. Hence, which mechanisms maintain patches of enhanced biodiversity in natural systems and if these patches elevate ecosystem multifunctionality at both local and landscape scales remain outstanding questions. In a 17 month experiment conducted within southeastern United States salt marshes, we found that patches of enhanced biodiversity and multifunctionality arise only where habitat-forming foundation species overlap--i.e. where aggregations of ribbed mussels (Geukensia demissa) form around cordgrass (Spartina alterniflora) stems. By empirically scaling up our experimental results to the marsh platform at 12 sites, we further show that mussels--despite covering only approximately 1% of the marsh surface--strongly enhance five distinct ecosystem functions, including decomposition, primary production and water infiltration rate, at the landscape scale. Thus, mussels create conditions that support the co-occurrence of high densities of functionally distinct organisms within cordgrass and, in doing so, elevate salt marsh multifunctionality from the patch to landscape scale. Collectively, these findings suggest that patterns in foundation species' overlap drive variation in biodiversity and ecosystem functioning within and across natural ecosystems.We therefore argue that foundation species should be integrated in our conceptual understanding of forces that moderate biodiversity--ecosystem functioning relationships, approaches for conserving species diversity and strategies to improve the multifunctionality of degraded ecosystems.

Relationships between an invasive crab, habitat availability and intertidal community structure at biogeographic scales

At local scales, habitat availability influences interactions between native and invasive species. Habitat availability may also predict patterns in native communities and invasive species at biogeographic scales when both native and invasive species have specific habitat requirements. The New Zealand porcelain crab, Petrolisthes elongatus, has invaded intertidal rocky shores around Tasmania, Australia, where it is found in high densities (>1800 m(2)) under rocks. A hierarchical sampling approach was used to investigate 1) the relationship between habitat availability (rock cover) and the biomass and abundance of P. elongatus, and 2) the relationship between P. elongatus biomass and native communities at local and regional scales. Invertebrate communities and habitat availability were sampled at multiple sites in the north and south regions of Tasmania. P. elongatus biomass and abundance were positively correlated with rock cover and patterns were consistent at the biogeographic scale (between regions). P. elongatus biomass was positively correlated with native species richness, biomass and abundance highlighting their co-dependence on rock cover. However, multivariate analyses indicated a different native community structure with increasing P. elongatus biomass. Flat, strongly adhering gastropods (chitons and limpets) were positively correlated with P. elongatus biomass, whereas mobile gastropods and crabs were negatively correlated with P. elongatus biomass. Despite local scale variation, there were clear consistent relationships between habitat-availability and the biomass of P. elongatus, and between native communities and the biomass of P. elongatus suggesting that the relationships between native and invasive species may be predictable at large spatial scales. Moreover, the strong relationships between P. elongatus biomass and changes in native community structure suggest a greater understanding of its impact is needed so that appropriate management plans can be developed.

Morphological traits and density of foundation species modulate a facilitation cascade in Australian mangroves

Facilitation cascades are critical to the maintenance of biodiversity in a variety of habitats. Through a series of two experiments, we examined how the morphological traits and density of interacting foundation species influence the establishment and persistence of a facilitation cascade in temperate Australian mangrove forests. In this system, mangrove pneumatophores trap the free-living alga, Hormosira banksii, which, in turn, supports dense and diverse assemblages of epifaunal mollusks. The first experiment, which manipulated pneumatophore height and density, revealed that these two traits each had additive negative effects on the establishment, but additive positive effects on the persistence of the cascade. High densities of tall pneumatophores initially served as a physical barrier to algal colonization of pneumatophore plots, but over the longer-term enhanced the retention of algae. The increased algal biomass, in turn, facilitating epifaunal colonization. The second experiment demonstrated that the retention of algae by pneumatophores was influenced more by algal thallus length than vesicle diameter, and this effect occurred independent of pneumatophore height. Our study has extended facilitation theory by showing that the morphological traits and density of basal and intermediary facilitators influence both the establishment and persistence of facilitation cascades. Hence, attempts to use foundation species as a tool for restoration will require an understanding not only of the interactions among these, but also of the key traits that modify interrelationships.