The gradostat: A model of competition along a nutrient gradient

The general mathematical theory of the gradostat is presented for two competitors. The gradostat provides a mechanism for studying competition along a nutrient gradient. In the two vessel case, the results are complete and the conditions are testable. In then-vessel case, the relevant conditions are stated in terms of the stability modulii of certain matrices and are testable for any specific case.     

Frugivore diversity increases frugivory rates along a large elevational gradient

The effect of biodiversity on ecosystem functioning is increasingly well understood, but it has mainly been studied in small‐scale experiments of plant‐based ecosystem functions. In contrast, the relevance of biodiversity for animal‐mediated ecosystem functions like seed dispersal still poses an important gap in ecological knowledge. In particular, it is little understood how avian diversity affects frugivory rates, one of the most important parameters of seed dispersal rates, along large environmental gradients. Even less is known about the environmental context dependence of the frugivore–frugivory relationship. We used artificial fruits to analyze experimentally how the abundance and richness of three avian frugivore guilds (with incrementally more stringent classifications of frugivory) contributed to frugivory rates across 13 different habitat types along an elevational gradient from 870 to 4550 m a.s.l. at Mt Kilimanjaro, Tanzania. We further investigated how environmental context, in terms of local vegetation structure and natural fruit availability, modified the relationship between frugivores and frugivory rates. Our results demonstrate that the positive effect of avian diversity on frugivory rates holds along a large elevational gradient. We found marked differences in frugivory rates among the 13 habitat types, which were strongly related to the abundance and richness of obligate frugivorous birds. Vegetation structure had no significant effect on frugivory rates. An intermediate abundance of natural fruits enhanced frugivory rates, but this effect did not alter the positive frugivore–frugivory relationship. These results emphasize the fundamental importance of obligate frugivore diversity for frugivory rates and suggest that the positive effect of biodiversity on ecosystem functioning holds along large environmental gradients.     

Phytoplankton diversity loss along a gradient of future warming and brownification in freshwater mesocosms

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Importance and intensity of competition along a fertility gradient and across species

Questions: 1. Can the importance and the intensity of competition vary independently along a nutrient gradient? 2. Are these variations species dependent? Location: Sub‐alpine pastures of the northern French Alps. Methods: Competition intensity measures how much competition decreases the performances of an organism. Competition importance measures how much competition contributes to affect performance, among other processes (such as environmental stress or disturbance). Competition intensity and importance were measured on three co‐occurring species: Festuca rubra, a perennial grass, and two forbs of contrasting basal area, Chaerophyllum hirsutum and Alchemilla xantho‐chlora. A neighbour removal experiment was performed on Festuca rubra in three sub‐alpine grassland communities differing in fertility and on Chaerophyllum hirsutum and Alchemilla xanthochlora in the two more fertile of these communities. The importance of competition was quantified using an index proposed by Brooker et al. (2005). Results: Competition intensity and importance showed different patterns of variation along the fertility gradient for Festuca rubra: competition importance decreased with decreasing fertility whereas competition intensity did not change. The largest forb was the least affected by competition. Our results suggest that the importance of competition for all three species depended on their individual tolerance to low nutrient availability. Conclusions: 1. The distinction between the importance and the intensity of competition is helpful to explain conflicting results obtained on the variations of competition indices along productivity gradients. 2. The choice of a phytometer can affect the conclusions drawn from empirical studies.     

Metabolic rates of a widely distributed crayfish shift along a latitudinal gradient

Hydrobiologia - Species are often exposed to novel thermal regimes as a result of anthropogenic activities. Understanding the extent to which populations are locally adapted to the thermal regime...     

Herbivory and competition slow down invasion of a tall grass along a productivity gradient

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced aboveground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.Plant nomenclature follows Van der Meijden et al. (1990)     

Phytoplankton distribution and production along a wide environmental gradient in the South-West Atlantic off Uruguay

We investigated phytoplankton biomass, assemblage structure and production along an environmental gradient to evaluate if chlorophyll-a (as proxy for biomass) and primary production peaked under conditions hypothesised to favour phytoplankton growth. During Spring 2003, a wide area from shallow estuarine waters to the shelf slope off the Río de la Plata was sampled and routine measurements included CTD profiles, nutrients, chlorophyll-a, phytoplankton composition and abundance, seston and organic matter loads, downwelling light and, at selected stations, production versus irradiance experiments. Spatial differences in abiotic variables suggested distinct hydrographic zones that differed in phytoplankton biomass and productivity. Chlorophyll-a was highest under estuarine influence and peaked at low salinity when strong stratification developed in the outer estuary, and was minimum at the shelf break and slope. In that area, however, relatively high chlorophyll-a was associated to oceanographic fronts and to the occurrence of Sub Antarctic water within the photic depth range. Productivity was maximum in shallow waters, but biomass-specific productivity peaked at the outer shelf in oceanographic fronts or in upwelled Sub Antarctic waters. Over shelf and slope waters productivity and biomass were not tightly coupled, as indicated by situations of high biomass and low productivity (Station 9), low biomass and high productivity (Station 10), or both high biomass and productivity (Station 22). Ordination analysis of phytoplankton taxa suggested that assemblages changed gradually along the environmental gradient and correlated to abiotic variables defining geographic zones. Overall results were consistent with an interpretation that phytoplankton biomass and growth were modulated by light in estuarine and coastal waters, and by hydrographic processes on the continental shelf and slope. Handling editor: Luigi Naselli-Flores     

Phytoplankton, light and nutrients along a gradient of mixing depth: a field test of producer-resource theory

1. Variation in depth of the mixed surface layer of temperate lakes should affect phytoplankton dynamics because, with increasing mixing depth, average light intensity in and specific sedimentation losses out of the mixed layer both decrease. 2. Our aim was to test a recent dynamic model which relates phytoplankton biomass and the availability of production‐limiting resources (light and dissolved mineral nutrients) to mixing depth and nutrient supply from external sources. 3. During summer stratification we sampled the mixed layers of 30 dimictic, phosphorus‐limited, oligo‐ to mesotrophic, mostly non‐humic lakes north of the Alps. 4. The results agree well qualitatively with model expectations. Algal concentration in the mixed layer was negatively related to mixing depth or its surrogate log‐transformed lake area. Light intensity at the bottom of the mixed layer decreased whereas the concentration of available, inorganic phosphorus increased with increasing mixing depth. Across all depths, higher total phosphorus content was accompanied by higher phytoplankton biomass, lower light availability, and higher inorganic phosphorus concentration. 5. Our data match the predicted shift with increasing mixing depth from predominantly nutrient limitation towards increased light limitation of algal biomass.     

Release from competition and protection determine the outcome of plant interactions along a grazing gradient

Plant interactions are suggested to shift from competition to facilitation and collapse with increasing grazing pressure. The existence of this full range of plant interactions and the role of underlying mechanisms (i.e. release from competition and protecting effect) in response to herbivory remains poorly documented and mainly described in terrestrial systems. We use a large grazing disturbance gradient (five levels of grazing) to test its effect on the outcome of plant interactions and underlying mechanisms in freshwater ecosystems. In a mesocosm experiment, we manipulated the presence of neighbouring plants to test their negative (competition) or protective (facilitation) effects on target plants along the grazing pressure gradient. We predicted that plant interactions 1) shift from competition to indirect facilitation with increased grazing pressure, 2) indirect facilitation collapses at high levels of grazing, 3) release from competition mainly drives the outcome in lowly grazed conditions and, 4) decreased protection occurs in highly grazed conditions responsible for the collapse of facilitation. This study shows the occurrence of the full range of outcomes in plant interactions under a wide spectrum of grazing pressure and indicates how the complex combination of underlying mechanisms shapes variations in plant interactions. We show that both, the release from competition and the increased protection by neighbouring plants drove the shift from competition to indirect facilitation. Declined protection by neighbouring plants resulted in a collapse of indirect facilitation for survival under intense herbivory. Our study provides the first experimental evidence of indirect facilitation structuring freshwater ecosystems thereby validating important ecological concepts mainly developed for terrestrial ecosystems.     

Impact of competition on plant performances along a flooding gradient: a multi‐species experiment

Question: 1. How do the competitive response and the importance of competition vary between species and along a flooding gradient? 2. How does the role of competition in constraining species distribution limits along the gradient vary between lower and upper limits? Location: A 1‐ha meadow within the Alzette floodplain in Luxembourg. Methods: Competitive response and importance of competition were assessed on seven meadow species differing in their tolerance to flooding. Species were cultured in monocultures and in mixtures, in three water treatments reflecting the wet, the middle and the dry end of a natural flooding gradient. We developed two models based on a multiple regression in order to express each component of competition as a function of the neighbour biomass. Results: Five species showed variations in their competitive response across water treatments; however, these species achieved either their highest or their worst competitive response in their optimal water treatment (i.e. the treatment in which the species had the highest biomass in monoculture). Competition was more important for the flood‐tolerant species in the dry treatment than for the flood‐intolerant species in the wet treatment. Conclusions: 1. Variations in species competitive responses along flooding gradients may be the result of either an amplified effect between competition and hydrological stresses, or a hierarchical effect of stress over competition. 2. The role of competition is more important in constraining the upper distribution limits of the flood‐tolerant species than the lower limits of the flood‐intolerant species along flooding gradients.     

Evolution of phenotypic clusters through competition and local adaptation along an environmental gradient

We have analyzed the evolution of a quantitative trait in populations that are spatially extended along an environmental gradient, with gene flow between nearby locations. In the absence of competition, there is stabilizing selection toward a locally best-adapted trait that changes gradually along the gradient. According to traditional ideas, gradual spatial variation in environmental conditions is expected to lead to gradual variation in the evolved trait. A contrasting possibility is that the trait distribution instead breaks up into discrete clusters. Doebeli and Dieckmann (2003) argued that competition acting locally in trait space and geographical space can promote such clustering. We have investigated this possibility using deterministic population dynamics for asexual populations, analyzing our model numerically and through an analytical approximation. We examined how the evolution of clusters is affected by the shape of competition kernels, by the presence of Allee effects, and by the strength of gene flow along the gradient. For certain parameter ranges clustering was a robust outcome, and for other ranges there was no clustering. Our analysis shows that the shape of competition kernels is important for clustering: the sign structure of the Fourier transform of a competition kernel determines whether the kernel promotes clustering. Also, we found that Allee effects promote clustering, whereas gene flow can have a counteracting influence. In line with earlier findings, we could demonstrate that phenotypic clustering was favored by gradients of intermediate slope.     

Phytoplankton of cenotes and anchialine caves along a distance gradient from the northeastern coast of Quintana Roo,Yucatan Peninsula

This work details the taxonomic composition of suspended algae (phytoplankton and tycoplankton) communities in five cenotes (sinkholes) and two anchialine caves in northeastern Quintana Roo, Mexico. The sample set of cenotes are Casa, Nohoch Nah Chich, Maya Blue, Cristal, and Carwash, as well as the two associated caves leading from the cenotes of Maya Blue and Cristal. The site distribution represents a distance gradient with respect to the coastline with which we observe the effects of tidal movement and the mixing of waters (e.g. saline water and freshwater) on the composition of the suspended algae communities. Two sample sets were taken, one at the end of the dry season (March–April 1995) and the second at the end of the rainy season (September–October 1995) with the goal of comparing the contrasting climatic conditions of the region. A total of 79 species were identified, of which, diatoms were the most important with respect to species richness with a total of 75% of species. The floristic composition is very similar between the freshwater cenotes. The distance of a cenote site with respect to the coastline was a determining factor in the species composition. Casa Cenote is the most distinct of the sample set for the presence of marine species due to its proximity to the coastline. The tides are a large determining factor of the floristic composition of Casa Cenote with 24% all species identified in this study found exclusively in this system. The anchialine system species are transported from the cenotes and the adjacent cave systems. The largest percentage or species (95%) are freshwater, and only 5% of the total number of identified species are of marine origin. It is recognized that the most distant cenotes from the coast, Carwash and Cristal, as well as Maya Blue and Nohoch Nah Chich, are the most similar, despite being part of different cave systems. In these inland systems the marine species decreased drastically (2.4% in Nohoch Nah Chich and no marine species in the remaining cenotes). Marine species are found at the halocline of the caves.     

Phytoplankton competition in deep biomass maximum

Resource competition in heterogeneous environments is still an unresolved problem of theoretical ecology. In this article, I analyze competition between two phytoplankton species in a deep water column, where the distributions of main resources (light and a limiting nutrient) have opposing gradients and co-limitation by both resources causes a deep biomass maximum. Assuming that the species have a trade-off in resource requirements and the water column is weakly mixed, I apply the invasion threshold analysis (Ryabov and Blasius, Ecol Lett 14:220–228, 2011) to determine relations between environmental conditions and phytoplankton composition. Although species deplete resources in the interior of the water column, the resource levels at the bottom and surface remain high. As a result, the slope of resources gradients becomes a new crucial factor which, rather than the local resource values, determines the outcome of competition. The value of resource gradients nonlinearly depend on the density of consumers. This leads to complex relationships between environmental parameters and species composition. In particular, it is shown that an increase of both the incident light intensity or bottom nutrient concentrations favors the best light competitors, while an increase of the turbulent mixing or background turbidity favors the best nutrient competitors. These results might be important for prediction of species composition in deep ocean.     

Ecological fitness of a glyphosate-resistant Lolium rigidum population: Growth and seed production along a competition gradient

Repeated use of glyphosate has resulted in evolution of glyphosate-resistant Lolium rigidum populations in Australia. The relative growth, competitiveness and reproductive output of glyphosate resistant (R) and susceptible (S) L. rigidum phenotypes from a single population were compared in competition with wheat. Vegetative growth of R and S individuals was studied in response surface experiments in the glasshouse and seed production was measured using an additive neighbourhood design experiment conducted in pots outside during the normal growing season of L. rigidum in Australia. There were no significant differences in vegetative growth or competitiveness of the R and the S phenotypes. The mean weight of seeds produced by R plants was significantly greater than that produced by S individuals. In the absence of wheat and at low wheat densities, S plants produced more seeds than R plants. However, at higher crop densities, differences in seed production were not significant. This is the first study to compare components of fitness at key life history stages for glyphosate R and S phenotypes isolated from a single weed population. The results presented indicate important differences in resource allocation during the reproductive stage for R and S phenotypes. Subtle differences in life history strategies may be manipulated by agronomic management to exploit the potential ecological fitness costs of the R phenotype. Further studies are required to provide a greater understanding of the occurrence and extent of fitness costs associated with glyphosate resistance. This knowledge can then be incorporated into models that simulate resistance evolution to design management strategies to prevent and/or contain the spread of glyphosate resistance.     

Bacterial activity along a trophic gradient

Bacterial biomass, secondary production, and extracellular enzymatic activity [-glucosidase and leucine-aminopeptidase, measured as cleavage of artificial fluorogenic substrates 4-methyl umbelliferyl (MVF) -D-glucopyranoside and L-leucine 7-amido-4-methyl coumarin (MCA)] were measured along a trophic gradient in the Northern Adriatic Sea in four ecologically different situations. Bacterial parameters were compared with chlorophyll a and inorganic and organic nutrient concentrations. Bacterial secondary production and extracellular enzymatic activity markedly changed among different seasons and along the trophic gradient. Average bacterial secondary production increased from 0.61 to 2.09 µg Cl^–1 hour^–1 preceding a bloom, to 2.09 µg Cl^–1 hour^–1 during the bloom, decreasing again to 0.81 and 0.83 µg Cl^–1 hour^–1 in the post-bloom and summer periods, respectively (values from 0.5 m depth). Leucine-aminopeptidase activity showed more consistent trends than -glucosidase activity. Average values of leucine-aminopeptidase activity, measured by enzymatic release of MCA, increased from a pre-bloom value of 164.0 to 1,712.0 (nM MCA) hour^–1 released during a bloom, decreasing to 298.5 and 133.7 (nM MCA) hour^–1 released for the post-bloom and summer situation, respectively (values from 0.5 m depth). Average growth rates decreased during the bloom, whereas average extracellular enzymatic activity levels expressed on a cell basis increased by an average factor of 2. Along the trophic gradient, a consistent increase in bacterial secondary production could be observed in all but the summer situation (values from 0.5 m depth). Leucine-aminopeptidase activity also showed positive trends along the gradient, while -glucosidase activity did not exhibit such a clear trend. Bacterial biomass trends were less obvious considering both seasonal changes and the tropic gradient. Highly significant interrelations were detected between bacterial proteolytic activity, secondary production, chlorophyll a content, and nitrate concentrations, especially in the surface horizon. Send offprint requests to: G. J. Hemdl.     

Cryptic speciation along a bathymetric gradient

The deep ocean supports a highly diverse and mostly endemic fauna, yet little is known about how or where new species form in this remote ecosystem. How speciation occurs is especially intriguing in the deep sea because few obvious barriers exist that would disrupt gene flow. Geographic and bathymetric patterns of genetic variation can provide key insights into how and where new species form. We quantified the population genetic structure of a protobranch bivalve, Neilonella salicensis, along a depth gradient (2200–3800 m) in the western North Atlantic using both nuclear (28S and calmodulin intron) and mitochondrial (cytochrome c oxidase subunit I) loci. A sharp genetic break occurred for each locus between populations above 2800 m and below 3200 m, defining two distinct clades with no nuclear or mitochondrial haplotypes shared between depth regimes. Bayesian phylogenetic analyses provided strong support for two clades, separated by depth, within N. salicensis. Although no morphological divergence was apparent, we suggest that the depth‐related population genetic and phylogenetic divergence is indicative of a cryptic species. The frequent occurrence of various stages of divergence associated with species formation along bathymetric gradients suggests that depth, and the environmental gradients that attend changes in depth, probably play a fundamental role in the diversification of marine organisms, especially in deep water. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 897–913.     

Giant clams discriminate threats along a risk gradient and display varying habituation rates to different stimuli

It is often beneficial for animals to discriminate between different threats and to habituate to repeated exposures of benign stimuli. While much is known about risk perception in vertebrates and some invertebrates, risk perception in marine invertebrates is less extensively studied. One method to study risk perception is to habituate animals to a series of exposures to one stimulus, and then present a novel stimulus to test if it transfers habituation. Transfer of habituation is seen as a continued decrease in response while lack of transfer is seen either by having a similar or greater magnitude response. We asked whether giant clams (Tridacna maxima) discriminate between biologically relevant types of threats along a risk gradient. Giant clams retract their mantle and close their shell upon detecting a threat. While closed, they neither feed nor photosynthesize, and prior work has shown that the cost of being closed increases as the duration of their response increases. We recorded a clam's latency to emerge after simulated threats chosen to represent a risk gradient: exposure to a small shading event, a medium shading event, a large shading event (chosen to simulate fish swimming above them), tapping on their shell and touching their mantle (chosen to simulate different degrees of direct attack). Although these stimuli are initially perceived as threatening, we expected clams to habituate to them because they are ultimately non‐damaging and it would be costly for clams to remain closed for extended periods of time when there is no threat present. Clams had different initial latencies to emerge and different habituation rates to these treatments, and they did not transfer habituation to higher risk stimuli and to some lower risk stimuli. These results suggest that clams discriminated between these stimuli along a risk gradient and the lack of habituation transfer shows that the new stimulus was perceived as a potential threat. This study demonstrates that sessile bivalves can discern between levels of predatory threat. These photosynthetic clams may benefit from being able to categorize predator cues for efficient energy allocation.     

Goose grazing selectivity along a depletion gradient

The micro-habilat food selectivity along a gradient of increasing grazing intensity of wintering pink-footed geese Anser brachyrhynchus and barnacle geese Branta leucopsis foraging on winter wheat Triticum aestivum and smooth meadow grass Poa pratensis was investigated. Protein content of ungrazed primary and secondary leaves of winter wheat did not differ, whereas the protein content of tertiary leaves was significantly lower. Secondary leaves were significantly longer than both primary and tertiary leaves, whereas tertiary leaves were significantly longer than primary leaves. Protein content of primary and secondary leaves of Poa pratensis did not differ, whereas the protein content of tertiary leaves was significantly lower. Primary leaves were significantly shorter than both secondary and tertiary leaves, which did not differ significantly.
For all leaf categories, there was an increase in the removal with increasing goose grazing intensity. The geese showed a sequential selection of winter wheat leaves according to the protein content, initially preferring primary and secondary leaves, but increasingly selecting tertiary leaves of poorer quality as high-quality leaves became depleted. Foraging on Poa pratensis. geese selected secondary leaves according to the protein content, whereas tertiary leaves were preferred to primary leaves, although these had a protein content equal to secondary leaves. This, however, may be explained by the primary leaves being too short to maintain intake rates.
It is concluded that feeding preferences cannot be established on the basis of protein contents of food plants alone and that green biomass estimates not necessarily reflect resource availability.