Coastal vegetated ecosystems are acknowledged for their capacity to sequester organic carbon (OC), known as blue C. Yet, blue C global accounting is incomplete, with major gaps in southern hemisphere ...data. It also shows a large variability suggesting that the interaction between environmental and biological drivers is important at the local scale. In southwest Atlantic salt marshes, to account for the space occupied by crab burrows, it is key to avoid overestimates. Here we found that southern southwest Atlantic salt marshes store on average 42.43 (SE = 27.56) Mg OC·ha
(40.74 (SE = 2.7) in belowground) and bury in average 47.62 g OC·m
·yr
(ranging from 7.38 to 204.21). Accretion rates, granulometry, plant species and burrowing crabs were identified as the main factors in determining belowground OC stocks. These data lead to an updated global estimation for stocks in salt marshes of 185.89 Mg OC·ha
(n = 743; SE = 4.92) and a C burial rate of 199.61 g OC·m
·yr
(n = 193; SE = 16.04), which are lower than previous estimates.
Both bottom-up (e.g., nutrients) and top-down (e.g., herbivory) forces structure plant communities, but it remains unclear how they affect the relative importance of stochastic and deterministic ...processes in plant community assembly. Moreover, different-sized herbivores have been shown to have contrasting effects on community structure and function, but their effects on the processes governing community assembly (i.e., how they generate the impacts on structure) remain largely unknown. We evaluated the influence of bottom-up and top-down forces on the relative importance of deterministic and stochastic processes during plant community assembly. We used the data of a 7-yr factorial experiment manipulating nutrient availability (ambient and increased) and the presence of vertebrate herbivores (>1 kg) of different body size in a floodplain grassland in The Netherlands. We used a null model that describes a community composition expected by chance (i.e., stochastic assembly) and compared the plant community composition in the different treatments with this null model (the larger the difference, the more deterministically assembled). Our results showed that herbivore exclusion promoted a more stochastic plant community assembly, whereas increased nutrients played a relatively minor role in determining the relative importance of stochasticity in community assembly. Large herbivores facilitated intermediate-sized mammal herbivores, resulting in synergistic effects of enhanced grazing pressure and a more deterministic and convergent plant community assembly. We conclude that herbivores can act as strong deterministic forces during community assembly in natural systems. Our results also reveal that although large- and intermediate-sized mammal herbivores often have contrasting effects on many community and ecosystem properties, they can also synergistically homogenize plant communities.
Abstract
Eutrophication usually impacts grassland biodiversity, community composition, and biomass production, but its impact on the stability of these community aspects is unclear. One challenge is ...that stability has many facets that can be tightly correlated (low dimensionality) or highly disparate (high dimensionality). Using standardized experiments in 55 grassland sites from a globally distributed experiment (NutNet), we quantify the effects of nutrient addition on five facets of stability (temporal invariability, resistance during dry and wet growing seasons, recovery after dry and wet growing seasons), measured on three community aspects (aboveground biomass, community composition, and species richness). Nutrient addition reduces the temporal invariability and resistance of species richness and community composition during dry and wet growing seasons, but does not affect those of biomass. Different stability measures are largely uncorrelated under both ambient and eutrophic conditions, indicating consistently high dimensionality. Harnessing the dimensionality of ecological stability provides insights for predicting grassland responses to global environmental change.
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.
Interactions among plants have been hypothesized to be context dependent, shifting between facilitative and competitive in response to variation in physical and biological stresses. This hypothesis ...has been supported by studies of the importance of positive and negative interactions along abiotic stress gradients (e.g., salinity, desiccation), but few studies have tested how variation in biotic stresses can mediate the nature and strength of plant interactions. We examined the hypothesis that herbivory regulates the strength of competitive and facilitative interactions during succession in Argentinean marshes dominated by Spartina densiflora and Sarcocornia perennis. Spartina densiflora is preferred by the dominant herbivore in the system, the crab Chasmagnathus granulatus. We experimentally manipulated crab herbivory, plant structure, and shade, and we found that, when herbivory was low in the spring and summer, competitive interactions between plants were dominant, but in the fall, when herbivory was highest, facilitative interactions dominated, and Spartina densiflora survival was completely dependent upon association with Sarcocornia perennis. Moreover, experimental removal of Sarcocornia perennis across recently disturbed tidal flats revealed that, while Sarcocornia perennis positively affected small Spartina densiflora patches by decreasing herbivory, as patch size increases and they can withstand the impact of herbivory, competitive interactions predominated and Spartina densiflora ultimately outcompeted Sarcocornia perennis. These results show that herbivory can mediate the balance between facilitative and competitive processes in vascular plant communities and that the strength of consumer regulation of interactions can vary seasonally and with patch size.
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.
Biotic and abiotic factors interact with dominant plants—the locally most frequent or with the largest coverage—and nondominant plants differently, partially because dominant plants modify the ...environment where nondominant plants grow. For instance, if dominant plants compete strongly, they will deplete most resources, forcing nondominant plants into a narrower niche space. Conversely, if dominant plants are constrained by the environment, they might not exhaust available resources but instead may ameliorate environmental stressors that usually limit nondominants. Hence, the nature of interactions among nondominant species could be modified by dominant species. Furthermore, these differences could translate into a disparity in the phylogenetic relatedness among dominants compared to the relatedness among nondominants. By estimating phylogenetic dispersion in 78 grasslands across five continents, we found that dominant species were clustered (e.g., co‐dominant grasses), suggesting dominant species are likely organized by environmental filtering, and that nondominant species were either randomly assembled or overdispersed. Traits showed similar trends for those sites (<50%) with sufficient trait data. Furthermore, several lineages scattered in the phylogeny had more nondominant species than expected at random, suggesting that traits common in nondominants are phylogenetically conserved and have evolved multiple times. We also explored environmental drivers of the dominant/nondominant disparity. We found different assembly patterns for dominants and nondominants, consistent with asymmetries in assembly mechanisms. Among the different postulated mechanisms, our results suggest two complementary hypotheses seldom explored: (1) Nondominant species include lineages adapted to thrive in the environment generated by dominant species. (2) Even when dominant species reduce resources to nondominant ones, dominant species could have a stronger positive effect on some nondominants by ameliorating environmental stressors affecting them, than by depleting resources and increasing the environmental stress to those nondominants. These results show that the dominant/nondominant asymmetry has ecological and evolutionary consequences fundamental to understand plant communities.
We found a prevalent disparity between the dominant and nondominant species (measured as the standardized effect size of the mean nearest taxonomic distance), with the former more clustered than the latter, suggesting a disparity in the mechanisms organizing both groups. We also found several clades more likely to have nondominant species than dominant species, measured as the probability of finding a species of a given clade among the third less abundant species in the sites where that clade occurred. Unexpectedly, many nondominant clades have a large number of species, mainly were comprised of nonwoody species, and often appeared in the phylogeny. Together, these findings suggest dominance and nondominance are conserved and that their differences have ecological consequences.
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.
The objective of this article has been to use Fuzzy Set Theory (FST) to assess the construction risks of three mega transport projects (MTPs) in Latin America and illustrate the relative importance ...of ambiguous risk factors in this region. The selected multiple-case study comprised the following three cases: Ferroanel from Sao Paulo in Brazil; Central Railway of Uruguay; and Line 1 of the Bogota Metro in Colombia. By applying FST to this set of MTPs, which includes projects developed in different institutional contexts, this paper shows that ill-defined construction risks are especially important in megaprojects in Latin America. Consequently, it also reveals that risk assessment should not be circumscribed to the sole use of numeric, probability-based analyses, which are incapable of considering ambiguous risks. On the contrary, this paper demonstrates the potential of using other methodologies, such as FST, which also incorporate perceptions of the institutional contexts in order to improve risk management in megaprojects of the region.
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