Facilitation Cascades Research Articles

Incorporating generalist seagrasses enhances habitat restoration in a changing environment

Abstract Coastal habitat‐forming species provide protection and essential habitat for fisheries but their ability to maintain these services are under threat from novel stressors including rising temperatures. Coastal habitat restoration is a powerful tool to help mitigate the loss of habitat‐forming species, however, many efforts focus on reintroducing a single, imperilled species instead of incorporating alternatives that are more conducive to current and future conditions. Seagrass restoration has seen mixed success in halting local meadow declines but could begin to specifically utilize generalist seagrasses with climate change‐tolerant and opportunistic life history traits including high reproduction rates and rapid growth. Here, we built on decades of successful eelgrass (Zostera marina) restoration in the Chesapeake Bay by experimentally testing seed‐based restoration potential of widgeongrass (Ruppia maritima)—a globally distributed seagrass that can withstand wide ranges of salinities and temperatures. Using field experiments, we evaluated which seeding methods yielded highest widgeongrass survival and growth, tested if seeding widgeongrass adjacent to eelgrass can increase restoration success, and quantified how either seagrass species changes restored bed structure, invertebrate communities, and nitrogen cycling. We found that widgeongrass can be restored via direct seeding in the fall, and that seeding both species maximized total viable restored area. Our pilot restoration area increased by 98% because we seeded widgeongrass in shallow, high temperature waters that are currently unsuitable for eelgrass survival and thus, would remain unseeded via only eelgrass restoration efforts. Restored widgeongrass had higher faunal diversity and double animal abundance per plant biomass than restored eelgrass, whereas restored eelgrass produced three times greater plant biomass per unit area and higher nitrogen recycling in the sediment. Synthesis and applications. Overall, we provide evidence that supplementing opportunistic, generalist species into habitat restoration is a proactive approach to combat climate change impacts. Specifically, these species can increase trait diversity which, for our study, increased total habitat area restored—a key factor to promote seagrass beds' facilitation cascades, stability, and grass persistence through changing environments. Now, we call for tests to determine if the benefits of restoration with generalist species alone or in conjunction with historically dominant taxa are broadly transferrable to restoration in other marine and terrestrial habitats.

Mutual facilitation between foundation species Mytilus edulis and Lanice conchilega promotes habitat heterogeneity on tidal flats

Foundation species that modify their habitat can facilitate other species, including other foundation species. Most studies focus solely on a single foundation species, overlooking such facilitation cascades. In this study, we investigated the interactions between the two coastal foundation species Mytilus edulis (blue mussel) and Lanice conchilega (sand mason worm). We investigated whether these species engage in facilitative interactions or if their association simply reflects a shared ecological niche on the soft-sediment intertidal flats of the Dutch Wadden Sea. To do so, we performed species distribution modeling, manipulative field experiments, and field surveys. We found a positive association between both foundation species, with a 2.45 times higher occurrence of both species compared to a random distribution. In addition, these species partially occupied the same ecological niche. We demonstrated that L. conchilega provided settlement substrate for M. edulis spat, increasing densities by 400 times compared to bare plots. Furthermore, M. edulis reefs facilitated L. conchilega occurrence in the wake of the reef. Biogenic reef development revealed that this interspecific facilitation resulted in spatial habitat heterogeneity. Therefore, we conclude that interspecific facilitation can significantly enhance the occurrence of these two important intertidal foundation species. Acknowledgment of such complex facilitation interactions has an untapped potential for improving the success of restoration and conservation programs.

Facilitation cascades across space: Monitoring estuarine foundation species from satellites to the microscope

AbstractEstuaries are productive ecosystems that are vulnerable to human impacts and environmental disturbances. Estuaries are often inhabited by foundation species that facilitate other organisms by creating and modifying habitats. However, spatiotemporal variability and underpinning drivers of facilitation from estuarine foundation species are poorly studied, especially in the context of monitoring and conservation. Here, we combined the analyses of satellite, drone, and camera images, close‐up field sampling of quadrats and individuals, and laboratory microscopy, to quantify co‐occurrences between the cockle Austrovenus stutchburyi and green macroalga Ulva spp., and their associated invertebrate communities, across spatial resolutions and environmental gradients in a New Zealand estuary. We found higher abundance of Ulva in winter and higher abundance of Austrovenus at a site near the ocean. Furthermore, Ulva was, across space and time, often attached to Austrovenus, either live or dead shells, suggesting a hierarchy between a primary (cockle) and secondary (macroalgae) foundation species. Finally, Ulva provided a better habitat for mobile invertebrates than Austrovenus, and thereby increased estuarine diversity through a density‐dependent facilitation cascade, which also was consistent in space and time. Overall, our study showed that facilitation cascades can be indirectly inferred and monitored through remote sensing of foundation species using supplementary imaging methods, and that integration of data across scales allows researchers to better understand the drivers and consequences of facilitation from foundation species. These insights may aid in the development of efficient monitoring strategies and ultimately improve the management of estuarine ecosystems.

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.

Temporal consistency of a facilitation cascade on shallow rocky reefs

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.

Climate‐driven substitution of foundation species causes breakdown of a facilitation cascade with potential implications for higher trophic levels

Abstract Climate change can alter ecological communities both directly, by driving shifts in species distributions and abundances, and indirectly by influencing the strength and direction of species interactions. Within benthic marine ecosystems, foundation species such as canopy‐forming macro‐algae often underpin important cascades of facilitative interactions. We examined the wider impacts of climate‐driven shifts in the relative abundances of foundation species within a temperate reef system, with particular focus on a habitat cascade whereby kelp facilitate epiphytic algae that, in turn, facilitate mobile invertebrates. Specifically, we tested whether the warm‐water kelp Laminaria ochroleuca, which has proliferated in response to recent warming trends, facilitated a secondary habitat‐former (epiphytic algae on stipes) and associated mobile invertebrates, to the same degree as the cold‐water kelp Laminaria hyperborea. The facilitative interaction between kelp and stipe‐associated epiphytic algae was dramatically weaker for the warm‐water foundation species, leading to breakdown of a habitat cascade and impoverished associated faunal assemblages. On average, the warm‐water kelp supported >250 times less epiphytic algae (by biomass) and >50 times fewer mobile invertebrates (by abundance) than the cold‐water kelp. Moreover, by comparing regions of pre‐ and post‐range expansion by L. ochroleuca, we found that warming‐impacted kelp forests supported around half the biomass of epiphytic algae and one‐fifth of the abundance of mobile invertebrates, per unit area, compared with unimpacted forests. We suggest that disruption to this facilitation cascade has the potential to impact upon higher trophic levels, specifically kelp forest fishes, through lower prey availability. Synthesis. Climate‐driven shifts in species' distributions and the relative abundances of foundation organisms will restructure communities and alter ecological interactions, with consequences for ecosystem functioning. We show that climate‐driven substitutions of seemingly similar foundation species can alter local biodiversity and trophic processes in temperate marine ecosystems.

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.

Epibiont community composition of red mangroves Rhizophora mangle are contingent on root characteristics

Foundation species traits that structure communities are rarely experimentally examined; thus, a predictive understanding of their functions lags behind patterns of observed species associations. Red mangrove Rhizophora mangle roots form complex living habitats that support diverse epibiont communities, making them a model system for testing links between variation in foundation species traits and associated biodiversity. Here, we compared epibiont community composition between living and non-living mangrove roots, as well as root mimics, to test how foundation species traits affect community structure. We also quantified the community structure of associated mobile invertebrates to examine their relationship with secondary foundation species (e.g. sponges, bivalves) that grow on the roots. After 14 mo of colonization and succession, substrate composition (i.e. mangrove, wood, PVC) had significant effects on community composition, richness, and abundance of sessile epibionts and mobile invertebrates. Non-living mangrove roots were 5 times more likely to deteriorate, and consequently had the lowest epibiont richness and abundance. We found strong positive relationships between mobile invertebrate richness and the abundance, measured as biomass, and richness of sponges and bivalves, suggesting that variation among roots in secondary foundation species play an important role in mediating mobile invertebrate community composition. This study highlights the functional role of habitat structure and how rapidly that function can be lost without biogenic maintenance. Our results indicate the importance of facilitation cascades in fostering diverse mobile invertebrate communities and highlight both advantages and limitations in using artificial structures in restoration programs.

Effects of a trophic cascade on a multi-level facilitation cascade.

The role of cascades in natural communities has been extensively studied, but interactions between trophic and facilitation cascades are unexplored. In the White Sea (65°N) shallow subtidal, bivalve primary facilitators provide hard substrate for secondary facilitator barnacles, that, in turn, provide substrate for conspecifics, ascidians, red algae and multiple associated organisms, composing a multi-level facilitation cascade. Previous research revealed that predation by the whelk Boreotrophon clathratus accounts for ~7% of adult barnacle mortality. Low whelk abundance limits their effect, with barnacles living on conspecifics several times more vulnerable to predation than those living on primary substrate. Trophic cascades can selectively shield foundation species from consumers, and hence may affect the structure and length of facilitation cascades. We tested the hypothesis that low abundance of the whelks results from mesopredator predation on their juveniles. Depending on the magnitude of the effect, this would mean that a trophic cascade controls the abundance of barnacles on all substrates or only barnacles living on conspecifics. We also suggested that barnacles on primary substrates and conspecifics facilitate different dependent assemblages. We manipulated the presence of crab and shrimp mesopredators in field caging experiments to assess their effect on whelk recruitment. In a field survey, we compared the assemblages of sessile macrobenthic organisms associated with barnacles living on different substrates. Caging experiments evidenced that crab and shrimp mesopredators reduce whelk recruitment by 4.6 times. Field data showed that barnacles on primary substrate and on conspecifics promote different dependent assemblages including secondary facilitator ascidians. Although mesopredators do not shield barnacles from elimination, their absence would restrict them from living on conspecifics. Barnacles on conspecifics are functionally different from barnacles on primary substrate, and can be considered a separate level of the facilitation cascade. Trophic cascades thus can generate community-wide effects on facilitation cascades by affecting their structure and possibly length.

The bright side of parasitic plants: what are they good for?

Parasitic plants are mostly viewed as pests. This is caused by several species causing serious damage to agriculture and forestry. There is however much more to parasitic plants than presumed weeds. Many parasitic plans exert even positive effects on natural ecosystems and human society, which we review in this paper. Plant parasitism generally reduces the growth and fitness of the hosts. The network created by a parasitic plant attached to multiple host plant individuals may however trigger transferring systemic signals among these. Parasitic plants have repeatedly been documented to play the role of keystone species in the ecosystems. Harmful effects on community dominants, including invasive species, may facilitate species coexistence and thus increase biodiversity. Many parasitic plants enhance nutrient cycling and provide resources to other organisms like herbivores or pollinators, which contributes to facilitation cascades in the ecosystems. There is also a long tradition of human use of parasitic plants for medicinal and cultural purposes worldwide. Few species provide edible fruits. Several parasitic plants are even cultivated by agriculture/forestry for efficient harvesting of their products. Horticultural use of some parasitic plant species has also been considered. While providing multiple benefits, parasitic plants should always be used with care. In particular, parasitic plant species should not be cultivated outside their native geographical range to avoid the risk of their uncontrolled spread and the resulting damage to ecosystems.

Positive cascading effects of epiphytes enhance the persistence of a habitat‐forming macroalga and the biodiversity of the associated invertebrate community under increasing stress

AbstractEnhanced persistence of stress sensitive species in harsh environments due to amelioration of physical stress by habitat‐forming species has been widely documented. By contrast, less attention has been given to positive species interactions that may enhance stress tolerance in habitat‐forming species, with positive cascading effects on biodiversity and ecosystem functioning.We used a combination of field surveys and experiments to assess the effects of the epiphyteJania rubens, on its subtidal macroalgal hostHalopithys incurva, and their cascading effects on the associated invertebrate community, along a depth gradient associated with increasing stress (i.e. light stress at shallower depths).Correlative studies showed that the biomass and photosynthetic efficiency ofH. incurvaincreased with depth, along with a decrease of epiphyte biomass. The experimental manipulation of epiphytes (removal in shallow water and addition in deep water) revealed that their effects onH. incurvaphysiology varied from positive in shallow habitats to negative in deep habitats. These results suggest that reduction of light/UV stress byJ. rubensallowsH. incurvato persist in shallow‐water habitats that would be otherwise unsuitable. In addition, colonization by large clumps ofJ. rubensincreased the abundance and diversity of associated invertebrates in shallow habitat, suggesting that positive feedbacks betweenH. incurva, acting as the primary habitat‐former, andJ. rubens, acting as a secondary habitat‐former, generate a facilitation cascade.Synthesis. Our study suggests that the persistence of some habitat‐forming species and their ability to support biodiversity may rely upon physical stress reduction by other species (e.g. epiphytes in our case) that are generally neglected when planning conservation strategies. A deeper understanding of context dependency in positive species interactions is essential for predicting patterns of species distribution under increasingly stressful climate scenarios.

Facilitation cascades create a predation refuge for biodiversity in a novel connected habitat

AbstractCommunities structured via facilitation cascades whereby one habitat‐forming species promotes a secondary habitat‐forming species with synergistic effects on biodiversity are increasingly documented. Habitat‐formers can also extend the realized niche of other habitat‐forming species by facilitating their recruitment into novel environments across heterogeneous landscapes. However, further understanding of whether secondary habitat‐formers in these novel environments differ in their structure or support different communities from those settled in their natal habitat is required. Here, we investigated whether abiotic and biotic conditions in a novel connected habitat influence biodiversity within a secondary habitat‐former facilitated outside its natal habitat. We contrasted the morphology of the habitat‐forming alga, Sirophysalis trinodis, and its associated fish and epifaunal community occurring on rocky reef (natal habitat) and on nearby a biogenic hard substrate, live clam shells (novel habitat) in soft sediments. The algae on clams had a different morphology to those on the rocky shore and supported increased abundance of some epifaunal species. A reciprocal transplant experiment of S. trinodis individuals among habitats found that differences in the abundance of epifauna were explained by the increased abundance and consumption rates of predatory fish on the reef habitat compared to soft‐sediment habitats, and not by differences in algal morphology between habitats. We demonstrate that facilitation promotes a secondary habitat‐former into a novel habitat, which then enhances the abundance of associated epifauna by providing a predation refuge. This study contributes to a small but growing body of research demonstrating landscape‐scale effects of facilitation cascades, and that basal habitat‐formers can extend the realized niche of secondary habitat‐formers and their associated communities.

Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem.

Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.

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.

When one foundation species supports another: Tubeworms facilitate an extensive kelp bed in a soft‐sediment habitat

AbstractThe role of foundation species—dominant organisms that provide habitat for other organisms—in enhancing the diversity and abundance of associated species is now well known. Less well studied are interactions between multiple, co‐occurring foundation species, which collectively provide habitat for other organisms. Here, I document a unique interaction between tubeworms (Eudistylia vancouveri) and two kelp species (Alaria marginata and Saccharina latissima). Kelps—large brown macroalgae in the order Laminariales—require hard substrates for attachment and are therefore typically restricted to rocky reefs. However, while exploring a remote coastline in southeastern Alaska, USA, I observed an extensive kelp bed, composed of both Alaria and Saccharina, growing in a location where the substrate was a mixture of mud and sand. The leathery tubes of Eudistylia formed dense hummocks that protruded from the sediment and provided a hard substratum for attachment by the kelps. Given the role that kelps, including Alaria and Saccharina, play as important facilitators of fish and invertebrates, this interaction likely represents an example of a facilitation cascade, which occurs when one species facilitates another species that subsequently enhances the diversity or abundance of associated organisms. Furthermore, the high kelp biomass likely contributes detrital inputs to the local ecosystem, potentially enhancing productivity. More generally, this observation highlights the need to understand the habitat requirements of foundation species, including the potential for other species to facilitate them.

Positive and negative interactions control a facilitation cascade

AbstractFacilitation cascades, which enhance the diversity of ecological communities in many ecosystems, have been viewed as the net outcome of positive species’ interactions. The strength and direction of these interactions, and thus the realized biodiversity, however, are likely to vary with the density and traits of the habitat‐formers and via negative interactions among interacting species. To test this, we manipulated the density and status (alive vs. dead) of a secondary habitat‐former, the razor clam Pinna sp., and measured responses by the primary habitat‐former, the seagrass Zostera muelleri, associated epifauna and infauna, and fish foraging behavior. At the plot level, for both live and dead clams, the total abundance of epifauna increased with clam density. However, for individual clams, the density of epifauna/cm2 decreased with increasing clam density. Video image analysis showed higher fish predation of epifauna on dead compared to live clams at high but not low densities and path analysis indicated that these strong negative trophic interactions increased with dead clam density via both direct and indirect pathways. By contrast, an increasing density of live but not dead clams was negatively correlated with seagrass faunal densities. However, seagrass growth and standing biomass were unaffected by clam density or status. Our study illustrates that the realized facilitation cascade is a function of nested negative and positive interactions which change as a function of the density of clams and whether they were dead or alive, and therefore do not represent a collection of hierarchical positive interactions.

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.

Evidence of Facilitation Cascade Processes as Drivers of Successional Patterns of Ecosystem Engineers at the Upper Altitudinal Limit of the Dry Puna.

Facilitation processes constitute basic elements of vegetation dynamics in harsh systems. Recent studies in tropical alpine environments demonstrated how pioneer plant species defined as “ecosystem engineers” are capable of enhancing landscape-level richness by adding new species to the community through the modification of microhabitats, and also provided hints about the alternation of different ecosystem engineers over time. Nevertheless, most of the existing works analysed different ecosystem engineers separately, without considering the interaction of different ecosystem engineers. Focusing on the altitudinal limit of Peruvian Dry Puna vegetation, we hypothesized that positive interactions structure plant communities by facilitation cascades involving different ecosystem engineers, determining the evolution of the microhabitat patches in terms of abiotic resources and beneficiary species hosted. To analyze successional mechanisms, we used a “space-for-time” substitution to account for changes over time, and analyzed data on soil texture, composition, and temperature, facilitated species and their interaction with nurse species, and surface area of engineered patches by means of chemical analyses, indicator species analysis, and rarefaction curves. A successional process, resulting from the dynamic interaction of different ecosystem engineers, which determined a progressive amelioration of soil conditions (e.g. nitrogen and organic matter content, and temperature), was the main driver of species assemblage at the community scale, enhancing species richness. Cushion plants act as pioneers, by starting the successional processes that continue with shrubs and tussocks. Tussock grasses have sometimes been found to be capable of creating microhabitat patches independently. The dynamics of species assemblage seem to follow the nested assemblage mechanism, in which the first foundation species to colonize a habitat provides a novel substrate for colonization by other foundation species through a facilitation cascade process.

C3-C4 intermediacy in grasses: organelle enrichment and distribution, glycine decarboxylase expression, and the rise of C2 photosynthesis.

Photorespiratory glycine shuttling and decarboxylation in bundle sheath (BS) cells exhibited by C2 species is proposed to be the evolutionary bridge to C4 photosynthesis in eudicots. To evaluate this in grasses, we compare anatomy, cellular localization of glycine decarboxylase (GDC), and photosynthetic physiology of a suspected C2 grass, Homolepis aturensis, with these traits in known C2 grasses, Neurachne minor and Steinchisma hians, and C3 S laxum that is sister to S hians We also use publicly available genome and RNA-sequencing data to examine the evolution of GDC subunits and enhance our understanding of the evolution of BS-specific GDC expression in C2 and C4 grasses. Our results confirm the identity of H aturensis as a C2 species; GDC is confined predominantly to the organelle-enriched BS cells in H aturensis and S hians and to mestome sheath cells of N minor Phylogenetic analyses and data obtained from immunodetection of the P-subunit of GDC are consistent with the hypothesis that the BS dominant levels of GDC in C2 and C4 species are due to changes in expression of a single GLDP gene in M and BS cells. All BS mitochondria and peroxisomes and most chloroplasts in H aturensis and S hians are situated centripetally in a pattern identical to C2 eudicots. In S laxum, which has C3-like gas exchange patterns, mitochondria and peroxisomes are positioned centripetally as they are in S hians This subcellular phenotype, also present in eudicots, is posited to initiate a facilitation cascade leading to C2 and C4 photosynthesis.