Landscapes of facilitation Cornacchia, Loreta; Van De Koppel, Johan; Van Der Wal, Daphne ...
Ecology (Durham),
April 2018, Volume:
99, Issue:
4
Journal Article
Peer reviewed
Open access
Spatial heterogeneity plays a crucial role in the coexistence of species. Despite recognition of the importance of self-organization in creating environmental heterogeneity in otherwise uniform ...landscapes, the effects of such self-organized pattern formation in promoting coexistence through facilitation are still unknown. In this study, we investigated the effects of pattern formation on species interactions and community spatial structure in ecosystems with limited underlying environmental heterogeneity, using self-organized patchiness of the aquatic macrophyte Callitriche platycarpa in streams as a model system. Our theoretical model predicted that pattern formation in aquatic vegetation – due to feedback interactions between plant growth, water flow and sedimentation processes – could promote species coexistence, by creating heterogeneous flow conditions inside and around the plant patches. The spatial plant patterns predicted by our model agreed with field observations at the reach scale in naturally vegetated rivers, where we found a significant spatial aggregation of two macrophyte species around C. platycarpa. Field transplantation experiments showed that C. platycarpa had a positive effect on the growth of both beneficiary species, and the intensity of this facilitative effect was correlated with the heterogeneous hydrodynamic conditions created within and around C. platycarpa patches. Our results emphasize the importance of self-organized patchiness in promoting species coexistence by creating a landscape of facilitation, where new niches and facilitative effects arise in different locations. Understanding the interplay between competition and facilitation is therefore essential for successful management of biodiversity in many ecosystems.
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Microphytobenthos forms an important part of the diet of macrofauna (macrozoobenthos) in many intertidal ecosystems. It is unclear, however, whether the dependence of macrofauna on microphytobenthos ...varies spatially within and among tidal systems. We aim (1) to assess the spatial variability in the importance of microphytobenthos in the diet of macrofauna (i.e., between and within two tidal basins and as function of elevation), (2) to quantify grazing pressure of the macrofaunal community on different potential food sources (microphytobenthos, phytoplankton and terrestrial organic material) for several sites in two tidal basins and (3) to compare microphytobenthic production and summer/autumn grazing of the total macrofaunal community and grazing pressure per feeding type, with potential microphytobenthic production estimated from rates in early spring, when grazing was low. Using a natural stable isotope approach, we identified microphytobenthos as a more important food source for macrofauna than phytoplankton and terrestrial organic material. Microphytobenthos dependency differed between tidal basins for the genera Bathyporeia (sand digger shrimp), Macoma (Baltic tellin), and Peringia (mudsnail) and for sampled individuals of all genera combined, and did not vary as function of elevation. We showed that macrofaunal grazing on microphytobenthos is quantitatively important and, in some cases, approached microphytobenthic production rates in early spring. No positive relation between microphytobenthic production in early spring and macrofaunal grazing in summer/autumn was observed. This suggests that the studied consumer-resource interactions are coupled on a larger spatial scale (i.e., mesoscale, ≈10 to 100 km), rather than the fine (mm to m) scale.
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Salt marshes provide valuable ecosystem services including coastal protection by reducing wave loading on dikes and seawalls. If the topsoil is erosion resistant to fast‐flowing water, it may also ...reduce breach depth if a dike fails. In this experiment, we quantified the topsoil erosion resistance from marshes and bare tidal flats with different soil types to understand the extent to which they can help reduce breach depth. Intact soil samples were collected from 11 locations in the Netherlands at different tidal elevations and then exposed for 3 h to 2.3 m/s currents. To the samples that remained stable after flow exposure, an artificial crack was made to test their stability following soil disturbance. All samples from the tidal flats were completely eroded, regardless of sediment type. In contrast, all samples from well‐established marsh plateaus were stable as long as no disturbances were made, including those with sandy subsoils. After creating artificial cracks, samples with a thin cohesive top layer on top of sandy subsoil collapsed, while marshes with silty subsoils remained stable. Pioneer marshes on sandy substrate without a cohesive top layer were the only vegetated soils that completely eroded. The lower erosion of marshes with either sandy or silty soils compared to bare tidal flats was best explained by the presence of a top layer with belowground biomass, high organic content, high water content, and low bulk density. When analyzing the erodibility of marshes only, fine root density was the best predictor of erosion resistance. This study demonstrates the importance of preserving, restoring, or creating salt marshes, to obtain a topsoil that is erosion resistant under fast‐flowing water, which helps reduce breach dimensions if a dike fails. The probability of topsoil erosion in established marshes with sandy subsoil is higher than in silty marshes. A silty layer of cohesive sediment on top of the sand provides extra erosion resistance as long as it does not break. Pioneer marshes that have not developed a cohesive top layer are erosion sensitive, especially in sandy soils. For future marsh creations, using fine‐grained sediments or a mixture of sand with silt or clay is recommended.
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Complexity theory predicts that local feedback processes may strongly affect the organization of ecosystems on larger spatial scales. Whether complexity leads to increased resilience and stability or ...to increased vulnerability and criticality remains one of the dominant questions in ecology. We present a combined theoretical and empirical study of complex dynamics in mineralogenic salt marsh ecosystems that emerge from a positive feedback between clay accumulation and plant growth. Positive feedback induces self‐organizing within the ecosystem, which buffers for the strong physical gradient that characterizes the marine‐terrestrial boundary, and improves plant growth along the gradient. However, as a consequence of these self‐organizing properties, salt marshes approach a critical state as the edge of the salt marsh and the adjacent intertidal flat becomes increasingly steep and vulnerable to wave attack. Disturbance caused, for instance, by a storm may induce a cascade of vegetation collapse and severe erosion on the cliff edge, leading to salt marsh destruction. Our study shows that on short timescales, self‐organization improves the functioning of salt marsh ecosystems. On long timescales, however, self‐organization may lead to destruction of salt marsh vegetation.
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Many landscapes are characterized by a patchy, rather than homogeneous, distribution of vegetation. Often this patchiness is composed of single-species patches with contrasting traits, interacting ...with each other. To date, it is unknown whether patches of different species affect each other’s uptake of resources by altering hydrodynamic conditions, and how this depends on their spatial patch configuration. Patches of two contrasting aquatic macrophyte species (i.e., dense canopy-forming Callitriche and sparse canopy-forming Groenlandia) were grown together in a racetrack flume and placed in different patch configurations. We measured 15NH₄ ⁺ uptake rates and hydrodynamic properties along the centerline and the lateral edge of both patches. When the species with a taller, denser canopy (Callitriche) was located upstream of the shorter, sparser species (Groenlandia), it generated turbulence in its wake that enhanced nutrient uptake for the sparser Groenlandia. At the same time, Callitriche benefited from being located at a leading edge where it was exposed to higher mean velocity, as its canopy was too dense for turbulence to penetrate from upstream. Consistent with this, we found that ammonium uptake rates depended on turbulence level for the sparse Groenlandia and on mean flow velocity for the dense Callitriche, but Total Kinetic Energy was the best descriptor of uptake rates for both species. By influencing turbulence, macrophyte species interact with each other through facilitation of resource uptake. Hence, heterogeneity due to multispecific spatial patchiness has crucial implications for both species interactions and aquatic ecosystem functions, such as nitrogen retention.
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Mangrove trees influence their physical environment by exerting drag on tidal flows and waves while also stabilising the sediment bed of intertidal flats. These processes influence sediment ...accretion, the mangrove habitat and their resilience to sea level rise. However, little is known about the magnitude and spatial extent of the effects of mangrove forests on sediment transport and the morphology of the intertidal flat. We use manipulated simulations with an extended process‐based numerical model, to study the influence of mangrove forests on intertidal flat morphology on a yearly timescale. The model includes the influence of mangrove trees on tidal flows, waves and sediment dynamics. The model is calibrated and validated with a comprehensive set of measurement data including hydrodynamics, sediment transport and morphological processes from an expanding mangrove forest in the sediment‐rich Firth of Thames estuary in Aotearoa New Zealand.
Sediment accretion on the upper intertidal flat is predominantly influenced by the characteristic morphology of the established mangrove forest, with increased bed stability at higher mudflat elevations related to prolonged aerial exposure and drying of the bed. Our results show that, in comparison to the situation without mangroves, sediment accretion increases in the most seaward fringe area of the forest. The unvegetated intertidal flat fronting the mangrove forest captures less sediment compared to the situation without mangroves. The mangrove forest drag triggers the development of a steeper, convex‐up‐shaped, upper intertidal flat profile, especially during periods with higher water levels and waves. These effects are expected to influence the development and storm‐recovery of natural and restored mangrove forests and may contribute to the resilience and persistence of mangrove‐vegetated intertidal flats for coastal flood risk reduction.
Sediment accretion of mangrove‐vegetated intertidal mudflats is enhanced by the existing morphology, with increased bed stability in higher‐elevated areas due to prolonged aerial exposure. Mangrove drag and bed stabilisation effects further increase accretion rates in the mangrove forest fringe, but reduce accretion rates in the area fronting the forest. Mangroves thereby actively contribute to upper intertidal flat development and play a role in the resilience of upper intertidal flats and their persistence to reduce coastal flood risk.
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Combining natural saltmarsh habitats with conventional barriers can provide a sustainable and cost‐effective alternative for fully engineered flood protection, provided that a minimal salt marsh ...width can be guaranteed for a long period. Hence, it is essential to understand both the key factors and management options driving the lateral erodibility/stability of salt marshes.
We aimed to determine how salt marsh management (i.e. grazing by large vs. small grazers vs. artificial mowing), marsh elevation and marsh age affect soil stability (i.e. soil collapse) and intrinsic lateral erodibility of salt marshes (i.e. particle‐by‐particle detachment). Soil cores were collected in high and low marshes (above and below 0.5 m MHWL, respectively) of different ages. At these locations, we compared cores from grazed areas to cores inside grazer exclosures, with and without artificial mowing. All cores were exposed to waves in flumes to test their stability and lateral erodibility.
All soil cores were characterized by a stable fine‐grained layer deposited on top of readily erodible sand. The thickness of the fine‐grained layer was a key parameter in reducing salt marsh instability (cliff collapse). This layer thickness increased with marsh age and at lower elevations, but decreased with cattle grazing due to compaction.
The erosion resistance of the fine‐grained layer increased with (a) large grazers that compacted the soil by trampling, (b) mowing that excluded soil‐bioturbating species, and (c) grazing by small grazers that promoted vegetation types with higher root density.
Synthesis and applications. Overall, marshes with thinner cohesive and/or fine‐grained top layers are more sensitive to lateral erosion than marshes with deep cohesive soils, independently of the management. Grazing and artificial mowing can reduce the erodibility of fine‐grained soils, making salt marshes more resilient to lateral erosion. However, compaction by large grazers simultaneously leads to thinner fine‐grained layers and lower elevation, potentially leading to more inundation under sea‐level rise. Hence, to effectively manage salt marshes to enhance their contribution to coastal protection, we recommend (a) moderate/rotational livestock grazing, avoiding high intensity grazing in sediment‐poor systems sensitive to sea‐level rise and (b) investigating measures to preserve small grazers.
Overall, marshes with thinner cohesive and/or fine‐grained top layers are more sensitive to lateral erosion than marshes with deep cohesive soils, independently of the management. Grazing and artificial mowing can reduce the erodibility of fine‐grained soils, making salt marshes more resilient to lateral erosion. However, compaction by large grazers simultaneously leads to thinner fine‐grained layers and lower elevation, potentially leading to more inundation under sea‐level rise. Hence, to effectively manage salt marshes to enhance their contribution to coastal protection, we recommend (a) moderate/rotational livestock grazing, avoiding high intensity grazing in sediment‐poor systems sensitive to sea‐level rise and (b) investigating measures to preserve small grazers.
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Salt marshes fronting coastal structures, such as seawalls and dikes, may offer important ecosystem‐based coastal defence by reducing the wave loading and run‐up levels during storms. We question (i) ...how the long‐term salt marsh development in the Dutch Wadden Sea relates to the tidal‐flat foreshore bathymetry and (ii) how the wave run‐up onto dikes, which enhances the risk of dike failure, depends on foreshore bathymetry, the presence/absence of marshes, marsh vegetation properties, tidal range and wind exposure.
We analysed 15 years of vegetation and bathymetry maps along the entire Dutch Wadden Sea coast, in combination with detailed process‐based measurements at five locations during 3 years, to understand where salt marshes naturally form and what features determine their contribution to coastal protection.
The horizontal extent of marshes along the dikes remained relatively stable over the past decade. The presence of marshes was associated with higher elevations of adjacent tidal flats (above ~0.5 m NAP), while landward‐directed marsh retreat was associated with surface erosion of the fronting tidal flats.
Wave run‐up during storms was lower at sites with wider marshes and higher foreshore elevations. This was attributed to the marsh attenuation effect, which led to a reduction in wave heights at the dike toe. As the tidal range varies across the Dutch Wadden Sea, areas to the East with generally higher water levels experienced higher wave run‐up.
Synthesis and applications. We found that (i) marshes, where present, effectively protected the dikes from wave loading and (ii) the sites where marshes typically do not develop spontaneously were the most vulnerable to high wave run‐up. This catch‐22 problem implies that increasing reliance on nature‐based coastal defences along soft‐bottom coasts may require human interventions to stimulate marsh formation at the locations where it is most needed. Alternatively, ‘hard engineering’ solutions may remain necessary where implementing nature‐based solutions are either too costly, unachievable, or at the expense of other ecological values, such as causing the loss of mudflats that are important for migratory birds.
Samenvatting
Kwelders (ook wel schorren genaamd) die voor dijken liggen, kunnen de kosten van hoogwaterbescherming mogelijk verlagen doordat ze de hoogte van golfaanvallen en de golfoploop (te meten als de vloedlijn‐hoogte) tijdens stormen verminderen. Wij onderzoeken (1) hoe de lange‐termijn ontwikkeling van kwelders in de Nederlandse Waddenzee samenhangt met de hoogte van wadplaten en (2) hoe de golfoploop tegen de dijk wordt bepaald door de hoogte ligging van het voorland, de aan/afwezigheid van kwelders, vegetatie kenmerken, getij‐amplitude en blootstelling aan de wind.
Om te begrijpen waar kwelders kunnen voorkomen en welke eigenschappen belangrijk zijn voor hoogwaterbescherming, analyseerden we gedurende 15 jaar de verandering in vegetatie en wadplaat‐morfologie van de Nederlandse Waddenzee kust, en combineerden we dit met 3‐jaar meten op 5 locaties.
Het kwelder oppervlak van bleef relatief stabiel over het laatste decennium. Kwelders liggen vooral op plekken waar de naastgelegen wadplaten relatief hoog zijn (meer dan 0.5 m NAP), terwijl afkalving van kwelders vooral optrad waar de voorgelegen wadplaten door erosie verlagen.
Dijken ondervonden minder golfoploop tijdens stormen op plaatsen met bredere kwelders en hogere voorlanden. Doordat de getijdenamplitude (verschil tussen hoog en laag water) in de Waddenzee van west naar oost toeneemt, wordt ook de golfoploop op de dijk naar het oosten groter.
Synthese en toepassing. Uit het onderzoek blijkt dat (i) kwelders dijken effectief beschermen tegen golfaanvallen, maar (ii) dat juist op die plekken waar de golfoploop tegen de dijk het hoogst is, de kwelders niet spontaan ontwikkelen. Om kwelders dus daar te krijgen waar we ze het hardste nodig hebben om de golfoploop te verlagen, zal dat alleen lukken met menselijke ingrepen (zoals bijvb. kwelderwerken). Op plekken waar maatregelen om kweldervorming te stimuleren technisch onmogelijk zijn, te duur zijn of onwenselijk zijn omdat het ten kosten zou gaan van wadplaten die belangrijk zijn voor migrerende vogels, kan het beter zijn om voor ‘harde technische’ oplossingen te kiezen.
We found that (i) marshes, where present, effectively protected the dikes from wave loading and (ii) the sites where marshes typically do not develop spontaneously were the most vulnerable to high wave run‐up. This catch‐22 problem implies that increasing reliance on nature‐based coastal defences along soft‐bottom coasts may require human interventions to stimulate marsh formation at the locations where it is most needed. Alternatively, ‘hard engineering’ solutions may remain necessary where implementing nature‐based solutions are either too costly, unachievable, or at the expense of other ecological values, such as causing the loss of mudflats that are important for migratory birds.
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