The stress-gradient hypothesis predicts that interactions among plants are context dependent, shifting from facilitation to competition as environmental stress decreases. Although restricted to ...facilitation/competition, the mechanistic model behind the hypothesis is easily modified to include other negative interactions that are as important as competition in structuring natural communities, e.g., herbivory. To evaluate this hypothesis we experimentally tested if the balance between the facilitative and trophic effect of an intertidal, burrowing, herbivorous crab in marsh plants is context dependent and shifts from positive to negative as stress decreases. By sampling salt marshes differing in sediment size characteristics, we show that sites with larger sediment particle size had less stressful oxygen levels than sites with fine sediment particles, and that the level of stress was reduced by the presence of crab burrows. We then conducted a factorial experiment manipulating sediment size and crab presence. Results show that, by decreasing soil anoxic stress, crabs increase plant growth in stressful zones, but their ecological importance as herbivores increases in more benign zones. Our findings suggest that the balance between positive and negative interactions along stress gradients is more important than previously perceived and also applies to facilitation and herbivory between animals and plants.
While the effects of top-down and bottom-up forces on aboveground plant growth have been extensively examined, less is known about the relative impacts of these factors on other aspects of plant life ...history. In a fully-factorial, field experiment in a salt marsh in Virginia, USA, we manipulated grazing intensity (top-down) and nutrient availability (bottom-up) and measured the response in a suite of traits for smooth cordgrass (Spartina alterniflora). The data presented within this manuscript are unpublished, original data that were collected from the same experiment presented in Silliman and Zieman 2001. Three categories of traits and characteristics were measured: belowground characteristics, litter production, and reproduction, encompassing nine total responses. Of the nine response variables measured, eight were affected by treatments. Six response variables showed main effects of grazing and/ or fertilization, while three showed interactive effects. In general, fertilization led to increased cordgrass belowground biomass and reproduction, the former of which conflicts with predictions based on resource competition theory. Higher grazing intensity had negative impacts on both belowground biomass and reproduction. This result contrasts with past studies in this system that concluded grazer impacts are likely relegated to aboveground plant growth. In addition, grazers and fertilization interacted to alter litter production so that litter production disproportionately increased with fertilization when grazers were present. Our results revealed both predicted and unexpected effects of grazing and nutrient availability on understudied traits in a foundational plant and that these results were not fully predictable from understanding the impacts on aboveground biomass alone. Since these diverse traits link to diverse ecosystem functions, such as carbon burial, nutrient cycling, and ecosystem expansion, developing future studies to explore multiple trait responses and synthesizing the ecological knowledge on top-down and bottom-up forces with trait-based methodologies may provide a promising path forward in predicting variability in ecosystem function.
Just as some species are used as model systems in organismal biology (e.g., physiology, genetics), many ecosystems are commonly used as model systems in ecology. Salt marshes, for instance, are great ...models to perform manipulative field experiments, and thus, were historically used to understand the drivers of community and ecosystem function. Decades of experimental work, indeed, made a strong contribution to community ecology as a discipline, but most of the emerged hypotheses and models were grounded in a few sites. When studies from new sites came onboard, looking to enlarge generalities, their results challenged the prevailing ideas. Here, we review more than 25 years of intense experimentation in South West Atlantic salt marshes, which helped not only to increase the knowledge about salt marsh functioning, but also to expand this knowledge beyond salt marshes helping to refine community and ecosystem function theory. We show that results coming from SW Atlantic marshes significantly contribute to understand 1) the separate and interactive effect of biotic and abiotic stress for species distribution and even for ecosystem stability, 2) the integrated role of species that can function as ecosystem engineers and as consumers, 3) the balance between stochastic and deterministic forces as drivers of community structure and 4) the regulation of cross-ecosystem fluxes. Nevertheless, we believe SW Atlantic salt marshes still have a lot more to offer, not only as conceptual models that help satisfy our intellectual curiosity, but also as key ecosystems that provide valuable benefits to our societies.
Según la teoría del nicho ecológico, la composición de especies en un lugar dado debería estar determinada por la conjunción de los factores bióticos y abióticos que allí actúan. En cambio, la teoría ...neutral propone que las especies son equivalentes y que la composición de ensambles locales está dada, entonces, por procesos de colonización y extinción independientes de las especies. Hoy en día se sabe que es difícil encontrar en la naturaleza ensambles puramente neutrales o puramente basados en el nicho ecológico, sino que lo que priman son ensambles intermedios. Los ensambles con una impronta fuerte del nicho ecológico están muy influidos por las interacciones interespecíficas o por los forzantes ambientales, mientras que los ensambles neutrales están más influidos por la deriva ecológica o por las dinámicas de extinción y colonización en base a las abundancias de las especies. El concepto de beta diversidad se tornó especialmente popular durante la última década y ha sido utilizado con diferentes finalidades en una gran variedad de ambientes. En particular, ganó mucha relevancia como herramienta para estimar la importancia relativa del nicho ecológico en los ensambles de especies. Esto se puede evaluar de forma sencilla con el uso de modelos neutrales ad hoc. En un ensamble puramente neutral, la variabilidad en la composición del ensamble entre muestras no debería ser diferente a la esperada como producto de una selección de especies al azar del total regional de especies (diversidad gama) para cada muestra (diversidad alfa). Cuanto más se aleje la variabilidad a la esperable por azar, mayor será la importancia relativa del nicho ecológico. Recientemente, hubo un gran avance en entender qué tipos de factores promueven uno u otro ensamble. Sin embargo, al ser un campo relativamente nuevo queda aún mucho por saber (e.g., interacción de factores, relación tiempoespacio, diferencias entre niveles tróficos). En este trabajo revisamos los alcances y la metodología de una temática que está viviendo un desarrollo marcado dentro de la ecología de comunidades, y discutimos algunos de los aspectos relevantes y relacionados que, por el momento, no fueron estudiados.
Increasing evidence has shown that nutrients and consumers interact to control primary productivity in natural systems, but how abiotic stress affects this interaction is unclear. Moreover, while ...herbivores can strongly impact zonation patterns in a variety of systems, there are few examples of this in salt marshes. We evaluated the effect of nutrients and herbivores on the productivity and distribution of the cordgrass Spartina densiflora along an intertidal stress gradient, in a Southwestern Atlantic salt marsh. We characterized abiotic stresses (salinity, ammonium concentration, and anoxia) and manipulated nutrients and the presence of the herbivorous crab Neohelice (Chasmagnathus) granulata, at different tidal heights with a factorial experiment. Abiotic stress increased at both ends of the tidal gradient. Salinity and anoxia were highest at the upper and lower edge of the intertidal, respectively. Nutrients and herbivory interacted to control cordgrass biomass, but their relative importance varied with environmental context. Herbivory increased at lower tidal heights to the point that cordgrass transplants onto bare mud substrate were entirely consumed unless crabs were excluded, while nutrients were most important where abiotic stress was reduced. Our results show how the impact of herbivores and nutrients on plant productivity can be dependent on environmental conditions and that the lower intertidal limits of marsh plants can be controlled by herbivory.
Salt marshes are among the most productive systems of the world, with plant primary production limited by soil oxygen deficiency and nutrient availability. Nevertheless, root adaptations to anoxia ...and nutrient acquisition are different and often incompatible. The SW Atlantic salt marshes are characterized by high densities of the deep (up to 1 m) burrowing crab
Neohelice granulata (Dana, 1851) that may change soil physical and chemical characteristics by burrow construction. In this work, we experimentally evaluated the hypothesis that crab burrowing can enhance soil oxygenation, causing changes in
Spartina densiflora Brongniart below ground tissues from structures adapted to anoxia to systems adapted for nutrient acquisition. This response, in turn, would enhance plant productivity. Results from field observations show that oxygen availability is higher in zones with high burrow densities. As burrow densities increased, the plant root distribution changed from shallow (associated to low oxygen availability) to deeper and vertically homogeneous, with a positive correlation between burrow density and plant aboveground biomass. Experimental exclusion of crabs shows that they induce changes of root strategies from anoxia toleration to nutrient efficient acquisition, with increasing plant productivity. The invasive success that this plant shows in other parts of the world is likely to be due to their ability to tolerate harsh environmental conditions. Our results suggest that the morphological plasticity of
S. densiflora is also important in their native zone given the characteristics of their specific habitat.
Size advantage in male-male competition over mates, combined with male preference over large females, is a common feature that can drive to size assortative mating and, eventually, sexual selection. ...In crabs, appendage autotomy can affect assortative mating and opportunity for sexual selection by affecting size advantage in mating contests. In this work, we evaluate the effect of size and appendage autotomy in generating assortative mating in the mud crab Cyrtograpsus angulatus. Field observations of guarding pairs in two different populations show a positive correlation between carapace width of males and females in both the populations. In one of the populations, incidence of appendage autotomy was low and the variability in the size of reproductive males was lower than the variability in the size of randomly collected males (i.e. only larger males were successful in getting a female), whereas there was no differences in the other population (i.e. most male sizes were successful) where the incidence of appendage autotomy was very high, indicating that the importance of size is higher when the incidence of autotomy is low. In this context, experiments (in both populations) show that, in contests for a female, larger males outcompete smaller ones only when they had intact appendages. When males had missing chelipeds, winning or loosing against smaller males was random. This may lead to a decrease in the importance of male size in populations with high incidence of cheliped autotomy, affecting assortative mating and opportunity for selection and, thus, affecting selective pressures.
Climate change is generating extreme climate events, affecting ecosystem integrity and function directly through increases in abiotic stress and disturbance and indirectly through changes in the ...strength of biotic interactions. As consumers play an essential role in ecosystem functioning and have been shown to be highly sensitive to climate conditions, improved understanding of their role under changing environmental conditions is necessary to accurately anticipate climate change impacts on ecosystem integrity.
We evaluated if prolonged periods of extreme rain, a climatic event increasing in severity in many places around the world, and coincident increases in coastal flooding duration intensify consumer control of foundational salt marsh grass structure and quantify the consequences of flooding–consumer interactions on salt marsh range extent. To achieve this, we analysed: historic trends in crab grazing; crab numbers and activity in and out of rainy years on the low marsh edge; vegetation retreat from the low marsh edge at a plot scale in a manipulative exclosure experiment; vegetation retreat at a landscape‐scale from drone image analyses; and the vertical erosion in the lowest edge of an Argentinean salt marsh.
During flooded periods, crabs congregated in the low marsh, resulting in localized overgrazing of salt marsh grass and the rapid horizontal retreat of the marsh edge (98.5 cm on average). Salt marsh edge retreat resulted in a loss of ~4.5% of the total marsh area at the landscape scale. Inside crab exclusion plots, although grass cover declined slightly during the study period, the marsh edge did not retreat.
Synthesis. This study provides experimental evidence that an extreme climate event can destabilize a local consumer–prey interaction, indirectly triggering the range contraction of a critical coastal habitat. This work contributes to a growing body of research demonstrating that consumers can be unleashed, rather than suppressed, by extreme climatic events. Moreover, in cases where consumer fronts form during such events, the result can be not only local (along habitat edges) but also landscape‐scale extinction of foundation species and the habitats they biogenically create. Together, this supports the general idea that models of future climate scenarios integrate the indirect effects on ecosystem‐regulating food web interactions.
Resumen
El cambio climático está generando fenómenos climáticos extremos que afectan a la integridad y el funcionamiento de los ecosistemas directamente a través del aumento del estrés abiótico y los disturbios, e indirectamente a través de los cambios en la fuerza de las interacciones bióticas. Dado que los consumidores desempeñan un papel esencial en el funcionamiento de los ecosistemas, y se ha demostrado que son muy sensibles a las condiciones climáticas, es necesario mejorar la comprensión de su papel en condiciones ambientales cambiantes para anticipar con precisión los impactos del cambio climático en la integridad de los ecosistemas.
Aquí evaluamos si inundaciones más prolongadas provocadas por precipitaciones extremas, un fenómeno que se está dando en muchos lugares del mundo, intensifican el control de los consumidores sobre las plantas fundacionales de marismas afectando al funcionamiento de estos sistemas. Para ello, analizamos: (i) las tendencias históricas de precipitaciones y herbivoría de cangrejos; (ii) el número y la actividad de los cangrejos antes y durante años lluviosos en el borde de la marisma baja; (iii) el retroceso de la vegetación en el límite inferior de la marisma baja a escala de parcela en un experimento de exclusión de cangrejos; (iv) el retroceso de la vegetación a escala de paisaje a partir del análisis de imágenes tomadas con drones; y (v) la erosión vertical en el límite inferior de una marisma argentina.
Durante los periodos de inundación asociados a precipitaciones extremas, los cangrejos se congregaron en la marisma baja, lo que provocó un sobreconsumo de la vegetación y una rápida retracción de su límite inferior de distribución (98,5 cm en promedio). Este retroceso resultó en una pérdida de ~4,5% de la superficie total de la marisma a escala del paisaje. Dentro de las parcelas de exclusión de cangrejos, aunque la cobertura de plantas disminuyó ligeramente durante el periodo de estudio, el límite de distribución no se vio afectado.
Síntesis. Este estudio proporciona pruebas experimentales de que un evento climático extremo puede desestabilizar una interacción consumidor‐presa, desencadenando indirectamente la retracción del rango de un hábitat costero crítico. Este trabajo contribuye a un creciente cuerpo de investigación que demuestra que los consumidores pueden ser liberados, en lugar de suprimidos, por eventos climáticos extremos. Además, en los casos en los que se forman frentes de consumo durante esos fenómenos, el resultado puede ser no sólo locales (a escala de parche), sino también a escala de paisaje. En conjunto, esto apoya la idea más general de que los modelos de escenarios climáticos futuros necesitan incorporar los efectos directos e indirectos de las interacciones tróficas que regulan los ecosistemas.
Extremely atypical rainfall promotes longer flooding periods in salt marshes. This concentrates crab feeding activity in the low salt marsh edge, and propagates upwards, leading to die‐off vegetation zones and the retreat of the lower edge of salt marsh vegetation. Vegetation loss potentially accelerates shoreline vertical erosion, ultimately affecting the delivery of ecosystem services (i.e. shoreline protection and C accumulation).
Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a ...competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non-nitrogen-fixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.
Human alterations to nutrient cycles and herbivore communities are affecting global biodiversity dramatically. Ecological theory predicts these changes should be strongly counteractive: nutrient ...addition drives plant species loss through intensified competition for light, whereas herbivores prevent competitive exclusion by increasing ground-level light, particularly in productive systems. Here we use experimental data spanning a globally relevant range of conditions to test the hypothesis that herbaceous plant species losses caused by eutrophication may be offset by increased light availability due to herbivory. This experiment, replicated in 40 grasslands on 6 continents, demonstrates that nutrients and herbivores can serve as counteracting forces to control local plant diversity through light limitation, independent of site productivity, soil nitrogen, herbivore type and climate. Nutrient addition consistently reduced local diversity through light limitation, and herbivory rescued diversity at sites where it alleviated light limitation. Thus, species loss from anthropogenic eutrophication can be ameliorated in grasslands where herbivory increases ground-level light.
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