1,805 research outputs found
Effect of multiple disturbances on food web vulnerability to biodiversity loss in detritus-based systems
Global biodiversity is affected by human pressure and climate change, and the present rate of biodiversity loss is probably higher than ever before. Community composition is also changing, and interspecific interactions are under severe pressure. The extinction of one species within a food web can result in further secondary extinctions, due to bottom-up effects that can be even more intense and less predictable than the direct effects of disturbance, undermining our capacity for ecosystem management and conservation. Here we investigated a metric for assessing the structural stability of food webs in the face of species loss, referred to as "Resistance'', based on two fundamental web properties: (1) the proportion of key species in the web, a "key'' species being one whose deletion leads to at least one secondary extinction, and (2) the mean number of secondary extinctions observed per key species deletion. We compared web Resistance with web Robustness (Dunne et al. 2002) based on 12 detritus-based riverine food webs under four species extinction scenarios on various temporal and spatial scales. We investigated the effect of multiple disturbances (extreme flood and river basin urbanization) on community vulnerability to biodiversity loss, assessing the behavior of Robustness and Resistance under the applied species extinction scenarios and testing their dependence on web topology. We estimated the contribution of the rarest and the most dominant species, and that of the most and least connected species, to web Resistance.
Urbanization negatively affected community vulnerability to biodiversity loss. Only food web Resistance showed a significant flood effect and interaction between flood and urbanization. The most connected species contributed the most to food web resistance, whereas the rarest and the most abundant species had a similar, intermediate structural importance. Both food web Resistance and the role of selected key species varied across web description scales. Food web Resistance values were coherent across species extinction scenarios, demonstrating the suitability of the proposed approach for quantifying community vulnerability to species loss and the importance of considering food webs in monitoring and impact assessment programs. The approach is thus seen to be a promising research pathway supporting ecosystem management
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Modelling the disk atmosphere of the low mass X-ray binary EXO 0748-676
Low mass X-ray binaries exhibit ionized emission from an extended disk
atmosphere that surrounds the accretion disk. However, its nature and geometry
is still unclear. In this work we present a spectral analysis of the extended
atmosphere of EXO 0748-676 using high-resolution spectra from archival
XMM-Newton observations. We model the RGS spectrum that is obtained during the
eclipses. This enables us to model the emission lines that come only from the
extended atmosphere of the source, and study its physical structure and
properties. The RGS spectrum reveals a series of emission lines consistent with
transitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line
fitting and photoionization modelling. Our results suggest that there are two
photoionization gas components, out of pressure equilibrium with respect to
each other. One with ionization parameter of 2.5 and a large opening angle, and
one with 1.3. The second component is possibly covering a smaller fraction of
the source. From the density diagnostics of the O vii triplet using
photoionization modelling, we detect a rather high density plasma of > 10^13
cm^-3 for the lower ionization component. This latter component also displays
an inflow velocity. We propose a scenario where the high ionization component
constitutes an extended upper atmosphere of the accretion disk. The lower
ionization component may instead be a clumpy gas created from the impact of the
accretion stream with the disk.Comment: A&A accepted, 10 pages, 9 figure
The cooling, mass and radius of the neutron star in EXO 0748-676 in quiescence with XMM-Newton
We analyse four XMM-Newton observations of the neutron-star low-mass X-ray
binary EXO 0748676 in quiescence. We fit the spectra with an absorbed
neutron-star atmosphere model, without the need for a high-energy (power-law)
component; with a 95 per cent confidence the power-law contributes less than 1
per cent to the total flux of the source in keV. The fits show
significant residuals at around 0.5 keV which can be explained by either a hot
gas component around the neutron star or a moderately broad emission line from
a residual accretion disc. The temperature of the neutron-star has decreased
significantly compared to the previous observation, from 124 eV to 105 eV, with
the cooling curve being consistent with either an exponential decay plus a
constant or a (broken) power law. The best-fitting neutron-star mass and radius
can be better constrained if we extend the fits down to the lowest possible
energy available. For an assumed distance of 7.1 kpc, the best-fitting
neutron-star mass and radius are and
km if we fit the spectrum over the keV range, but
and km if we restrict the
fits to the keV range. We finally discuss the effect of the assumed
distance to the source upon the best-fitting neutron-star mass and radius. As
systematic uncertainties in the deduced mass and radius depending on the
distance are much larger than the statistical errors, it would be disingenuous
to take these results at face value.Comment: 12 pages, 6 figure
Effect of habitat degradation on competition, carrying capacity, and species assemblage stability
In human-impacted rivers, nutrient pollution
has the potential to disrupt biodiversity organisation
and ecosystem functioning, prompting calls for
effective monitoring and management. Pollutants,
together with natural variations, can modify the
isotopic signature of aquatic organisms. Accordingly,
we explored the potential of isotopic variations as an
indicator of drainage basin influences on river food
webs. We assessed stable N and C isotopes within six
food webs along a river affected by multiple pollution
sources. CORINE land cover maps and Digital
Elevation Models (DEMs) were also applied to
understand the impact on surface waters of anthropogenic
pressures affecting the catchment. N isotopic
signatures of taxa fell in association with ammonium
inputs from agriculture, indicating that nitrogen pollution
was related to synthetic fertilizers. Isotopic
variations were consistent across trophic levels, highlighting
site-specific communities and identifying taxa
exposed to pollutants. This allowed us to locate point
sources of disturbance, suggesting that food web
structure plays a key role in pollutant compartmentalisation
along the river. Thematic maps and DEMs
helped understand how the anthropogenic impact on
river biota is mediated by hydro-geomorphology.
Thus, the integration of site-scale analyses of
stable isotopes and land use represents a promising
research pathway for explorative nutrient pollution
monitoring in human-impacted rivers
Time- and depth-wise trophic niche shifts in Antarctic benthos
Climate change is expected to affect resource-consumer interactions underlying stability in polar food webs. Polar benthic organisms have adapted to the marked seasonality characterising their habitats by concentrating foraging and reproductive activity in summer months, when inputs from sympagic and pelagic producers increase. While this enables the persistence of biodiverse food webs, the mechanisms underlying changes in resource use and nutrient transfer are poorly understood. Thus, our understanding of how temporal and spatial variations in the supply of resources may affect food web structure and functioning is limited. By means of C and N isotopic analyses of two key Antarctic benthic consumers (Adamussium colbecki, Bivalvia, and Sterechinus neumayeri, Echinoidea) and Bayesian mixing models, we describe changes in trophic niche and nutrient transfer across trophic levels associated with the long- and short-term diet and body size of specimens sampled in midsummer in both shallow and deep waters. Samplings occurred soon after the sea-ice broke up at Tethys Bay, an area characterised by extreme seasonality in sea-ice coverage and productivity in the Ross Sea. In the long term, the trophic niche was broader and variation between specimens was greater, with intermediate-size specimens generally consuming a higher number of resources than small and large specimens. The coupling of energy channels in the food web was consequently more direct than in the short term. Sediment and benthic algae were more frequently consumed in the long term, before the sea-ice broke up, while consumers specialised on sympagic algae and plankton in the short term. Regardless of the time scale, sympagic algae were more frequently consumed in shallow waters, while plankton was more frequently consumed in deep waters. Our results suggest a strong temporal relationship between resource availability and the trophic niche of benthic consumers in Antarctica. Potential climate-driven changes in the timing and quality of nutrient inputs may have profound implications for the structure of polar food webs and the persistence of their constituent species, which have adapted their trophic niches to a highly predictable schedule of resource inputs
Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems
In the study of food webs, the existence and explanation of recurring patterns, such as the scale
invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing
issues. Our study focused on litter-associated food webs and explored the influence of detritivore and
predator niche width (as d13C range) on web topological structure. To compare patterns within and
between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed
42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and
cornfield), using an experimental approach with litterbags. The results suggest that although web
differences exist between ecosystems, patterns are more similar within than between aquatic and
terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of
predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in
the community. The tendency was more marked in terrestrial ecosystems and can be explained by a
lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance
with these results, the number of links increased with the number of species but with a significantly
sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found
to be directly correlated to niche width, increased with the total number of species in terrestrial webs,
whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the
total number of species. In both types of ecosystem, web robustness to rare species removal increased
with connectance and the niche width of predators. In conclusion, although limited to litter-associated
macroinvertebrate assemblages, this study highlights structural differences and similarities between
aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in
determining the structure of detritus-based food webs and posing foraging optimisation constraints as a
general mechanistic explanation of food web complexity differences within and between ecosystem
types
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