Spatial heterogeneity and irreversible vegetation change in semi-arid grazing systems

Recent theoretical studies have shown that spatial redistribution of surface water may explain the occurrence of patterns of alternating vegetated and degraded patches in semiarid grasslands. These results implied, however, that spatial redistribution processes cannot explain the collapse of production on coarser scales observed in these systems. We present a spatially explicit vegetation model to investigate possible mechanisms explaining irreversible vegetation collapse on coarse spatial scales. The model results indicate that the dynamics of vegetation on coarse scales are determined by the interaction of two spatial feedback processes. Loss of plant cover in a certain area results in increased availability of water in remaining vegetated patches through run-on of surface water, promoting within-patch plant production. Hence, spatial redistribution of surface water creates negative feedback between reduced plant cover and increased plant growth in remaining vegetation. Reduced plant cover, however, results in focusing of herbivore grazing in the remaining vegetation. Hence, redistribution of herbivores creates positive feedback between reduced plant cover and increased losses due to grazing in remaining vegetated patches, leading to collapse of the entire vegetation. This may explain irreversible vegetation shifts in semiarid grasslands on coarse spatial scales

Signatures of chaos in animal search patterns

One key objective of the emerging discipline of movement ecology is to link animal movement patternsto underlying biological processes, including those operating at the neurobiological level. Nonetheless,little is known about the physiological basis of animal movement patterns, and the underlying searchbehaviour. Here we demonstrate the hallmarks of chaotic dynamics in the movement patterns ofmud snails (Hydrobia ulvae) moving in controlled experimental conditions, observed in the temporaldynamics of turning behaviour. Chaotic temporal dynamics are known to occur in pacemaker neuronsin molluscs, but there have been no studies reporting on whether chaotic properties are manifest in themovement patterns of molluscs. Our results suggest that complex search patterns, like the Lévy walksmade by mud snails, can have their mechanistic origins in chaotic neuronal processes. This possibilitycalls for new research on the coupling between neurobiology and motor properties

Patterning in mussel beds explained by the interplay of multi-level selection and spatial self-organization

Cooperation, ubiquitous in nature, is difficult to explain from an evolutionary perspective. Many modeling studies strive to resolve this challenge, but their simplifying assumptions on population and interaction structure are rarely met in ecological settings. Here we use a modeling approach that includes more ecological detail to investigate evolution of cooperation in spatially self-organized mussel beds. Mussels cooperate with each other through aggregative movement and attachment using byssal threads. These cooperative behaviors shape the spatial structure of the mussel bed, which can range from scattered distributions to labyrinth-like patterns and dense mussel clumps. The spatial pattern in turn impacts an individual’s fitness at two levels: (i) proper attachment to neighboring individuals decreases predation risk, and (ii) attachment to a sufficiently large group prevents dislodgement by wave stress. Without this second level of selection, our simulations do typically not result in evolutionary attractors that lead to the labyrinth-like spatial patterns that are characteristic for natural mussel beds. Yet, when group-level selection is included, labyrinth-like patterns emerge under a wide range of conditions. Our model demonstrates that multiple selection factors working at different spatial scales – predation of individuals and dislodgement of entire mussel clumps – combinedly determine evolution of cooperative traits in mussels and thereby result in emergence of the labyrinth-like spatial patterns that we observe in natural mussel beds

Algal-Induced Biogeomorphic Feedbacks Lay the Groundwork for Coastal Wetland Development

Ecosystem establishment under adverse geophysical conditions is often studied within the “windows of opportunity” framework, identifying disturbance-free periods (e.g., calm wave climate) where species can overcome establishment thresholds. However, the role of biogeophysical interactions in this framework is less well understood. The establishment of saltmarsh vegetation on tidal flats, for example, is limited by abiotic factors such as hydrodynamics, sediment stability and drainage. On tidal flats, raised sediment ridges colonized by algal mats (Vaucheria sp.) appear to accomodate high densities of plant seedlings. Such ridges were previously found to have higher sediment strength than substratum without algae. Here, we investigate whether these measurements can be explained by geophysical factors only, or that biological (Vaucheria-induced) processes influence tidal marsh establishment by forming stabilized bedforms. We performed two experiments under controlled mesocosm conditions, to test the hypotheses that (a) Vaucheria grows better on elevated topographic relief, that (b) the binding force of their algal filaments increases sediment strength, and that (c) Vaucheria consequently creates elevated topographic relief that further facilitates algal growth. Our experimental results confirm the existence of this algal-induced biogeomorphic feedback cycle. These findings imply that benthic algae like Vaucheria may contribute significantly to tidal marsh formation by creating elevated and stabilized substratum. This suggests biogeophysical feedbacks can “widen” the windows of opportunity for further ecosystem establishment. Our results could be useful for the design of managed realignment projects aimed at restoring the unique ecosystem services of coastal wetlands, such as habitat biodiversity, carbon sequestration potential and nature-based flood defense

Mussel seed is highly plastic to settling conditions:The influence of waves versus tidal emergence

Phenotypic plasticity is important for organisms to adjust to a new environment.Therefore, the transplantation success of an organism to a new environment can be increased with knowledge of its capacity for phenotypic plasticity in different life stages, and the phenotypic adjustments it needs to make in specific environmental situations. Both the capacity for phenotypic plasticity and the necessary phenotypic adjustments for transplantation were tested in a mesocosm experiment using blue mussels Mytilus edulis as a model organism. This study tested (1) to what extent mussel seed coming from collectors in the water column are still capable of adjusting their phenotype, and (2) whether exposure to air or wave action is more important as a driver of phenotypic adjustments for mussels living in intertidal conditions. We found that musselseed had a high capacity for phenotypic plasticity, and were capable of adjusting their morphology to accommodate different intertidal hydrodynamic conditions. Exposure to air influenced the shell shape, condition, byssal attachment strength and aggregation behaviour, but exposure to waves played the most important role in determining the phenotype of mussels. Wave-exposedmussels grew bigger, rounder, had thicker shells and a stronger byssal attachment strength than mussels exposed to either calm tidal or calm submerged environments. This knowledge is important for selecting a suitable source population and transplantation location

Spatial interactions and resilience in arid ecosystems

We present a mathematical analysis of the consequences of spatial interactions between vegetation patches by means of water flow for the functioning of arid systems. Our model results suggest that spatial exchange of water improved the resilience to disturbances and increased the resistance to human-induced loss of cover. Furthermore, spatial exchange of water allowed vegetation to persist at lower rainfall levels compared to systems that lack spatial interactions. Our analysis stresses the general importance of spatial interactions for the functioning of ecological systems

Ecosystem engineering by large grazers enhances carbon stocks in a tidal salt marsh

Grazers can have a large impact on ecosystem processes and are known to change vegetation composition. However, knowledge of how the long-term presence of grazers affects soil carbon sequestration is limited. In this study, we estimated total accumulated organic carbon in soils of a back-barrier salt marsh and determined how this is affected by long-term grazing by both small and large grazers in relation to age of the ecosystem. In young marshes, where small grazers predominate, hare and geese have a limited effect on total accumulated organic carbon. In older, mature marshes, where large grazers predominate, cattle substantially enhanced carbon content in the marsh soil. We ascribe this to a shift in biomass distribution in the local vegetation towards the roots in combination with trampling effects on the soil chemistry. These large grazers thus act as ecosystem engineers: their known effect on soil compaction (based on a previous study) enhances anoxic conditions in the marsh soil, thereby reducing the oxygen available for organic carbon decomposition by the local microbial community. This study showed that the indirect effects of grazing can significantly enhance soil carbon storage through changing soil abiotic conditions. This process should be taken into account when estimating the role of ecosystems in reducing carbon dioxide concentration in the atmosphere. Ultimately, we propose a testable conceptual framework that includes 3 pathways by which grazers can alter carbon storage: (1) through above-ground biomass removal, (2) through alteration of biomass distribution towards the roots and/or (3) by changing soil abiotic conditions that affect decomposition.</p

Are all patterns created equal?:Cooperation is more likely in spatially simple habitats

Cooperative behaviours, such as aggregation with neighbouring conspecifics, canenhance resilience in habitats where risks (i.e. predation, physical disturbances) are high, exerting positive feedback loops to maintain a healthy population. At the same time, cooperation behaviours can involve some extra energy expenditures and in‐ creasing resource competition. For sessile reefs, like mussels, simulation models predict increased cooperation under increasing levels of environmental stress. Predation risk is viewed as a behaviour‐modifying stressor, but its role on cooperation mechanisms, such as likelihood of reciprocity, has not yet been empirically tested. This study harnesses this framework to understand how cooperation changes under different perceived levels of predation risk, using mussel beds as model of a complex“self‐organised” system. Hence, we assessed the context dependency of cooperation response in different “landscapes of fear,” created by changes in predator cues, sub‐ stratum availability and body size. Our experiments demonstrated that i) cooperation in a mussel bed system increases when predator cues are present, but that this relationship was found to be both, ii) strongly context‐dependent, particularly upon substratum availability and iii) size‐dependent. That is, while cooperation is in general greater for larger individuals, the response to risk results in greater cooperation when alternative attachment substratum is absent, meaning that simpler landscapes may be perceived as riskier. The context dependency of structural complexity is also an essential finding to consider in a changing world where habitats are losing complexity and cooperative strategies should be maximised

Bioengineering promotes habitat heterogeneity and biodiversity on mussel reefs

Loss of biodiversity is among the most pressing global problems. Yet, despite its pertinent nature, the biological processes involved in the maintenance of biodiversity are poorly understood. Habitat heterogeneity is widely regarded as a key factor underpinning the biodiversity of land- and sea-scapes. However, it remains unclear how species coexist in many of those ecosystems that lack conspicuous heterogeneity. We demonstrate how spatially self-organized mussel reefs create microhabitats/heterogeneity that facilitate diverse invertebrate communities. By comparing seawater filled pools with open inlets in a mussel reef, we found that natural reef pools, emerging due to the habitat engineering of the mussels, strongly increased variation in organic enrichment and promoted beta-diversity compared to the surrounding tidal flat. These findings significantly extend the scale of influence typically described for self-organized habitats and highlight the importance of bioengineering and its positive effects on habitat heterogeneity and community diversity