2,914 research outputs found
The mid-domain effect: It’s not just about space
Ecologists and biogeographers have long sought to understand how and why diversity varies across space. Up until the late 20th century, the dominant role of environmental gradients and historical processes in driving geographical species richness patterns went largely undisputed. However, almost 20 years ago, Colwell & Hurtt (1994) proposed a radical reappraisal of ecological gradient theory that called into question decades of empirical and theoretical research. That controversial idea was later termed the ‘the mid-domain effect’: the simple proposition that in the absence of environmental gradients, the random placement of species ranges within a bounded domain will give rise to greatest range overlap, and thus richness, at the center of the domain (Colwell & Lees, 2000) (Fig. 1a). The implication of this line of reasoning is that the conventional null model of equal species richness regardless of latitude, elevation or depth should be replaced by one where richness peaks at some midpoint in geographical space.
Our intention here is to draw attention to a neglected, yet important manifestation of the mid-domain effect, namely the application of mid-domain models (also referred to as geometric constraint models) to non-spatial domains. If individual species have ranges that exist not just in geographical space but also in environmental factors, such as temperature, rainfall, pH, productivity or disturbance, shouldn’t we also expect mid-domain richness peaks along non-spatial gradients? A mid-domain model applied to non-spatial gradients predicts the maximum potential richness for every value of an environmental factor. As with spatial mid-domain models, realized richness would probably be less, but the limits to richness are still predicted to be hump-shaped. Indeed, hump-shaped relationships emerge with remarkably high frequency across various non-spatial gradients. For instance, two of ecology’s most fundamental, albeit controversial theories – the productivity–diversity relationship and the intermediate disturbance hypothesis – predict mid-domain peaks in species richness. However, the potential of non-spatial mid-domain models has gone largely ignored
Effects of oil-laden sediments on behavior and growth of juvenile flatfishes
Thesis (Ph.D.) University of Alaska Fairbanks, 1996Three species of juvenile Pacific flatfishes: yellowfin sole (Pleuronectes asper), rock sole (P. bilineatus), and Pacific halibut (Hippoglossus stenolepis) were exposed to sediments contaminated with Alaska North Slope crude oil to determine the behavior and growth of juveniles in polluted nursery grounds. Responses were correlated with known biomarkers of toxicant exposure. In the behavior experiments, fish exhibited a strong preference for fine grained sediments (500 microns) when presented with eight different sediment types ranging from mud to pebble. Juvenile yellowfin sole showed a preference for mud and mixed mud substrate, rock sole preferred sand substrates and halibut chose both mud and sand sediments. Flatfishes were able to detect and avoid heavily oiled (1400 g/g total petroleum hydrocarbons-TPH) sediments but did not avoid sediments at oil concentrations of 400 g/g TPH. Among yellowfin sole and rock sole, sediment preference altered behavioral response to oil whereas halibut did or did not avoid oil irrespective of sediment type. If flatfish do not avoid oil concentrations of 1600 g/g and higher on preferred sediment, growth reductions occur. Fish reared on oiled sediment grew slower than controls on non-oiled sediments. Growth reductions in all three species were significant following 30 days of exposure to 1600-1800 g/g TPH and became more pronounced over time. As the toxicant concentration or the length of exposure increased, growth per day decreased. By 90 days of exposure, fish exposed to 1600-1800 g/g TPH grew 38-57% slower than controls. Halibut had the greatest change in growth rate following oil exposure. Exposure of halibut to sand laden with 4700 g/g total hydrocarbons resulted in an 93% reduction in growth in 30 days. Condition factor was also most reduced in halibut. Changes in tissues and parasites indicated a reduction in fish health for all three species. There was an increase in fin erosion, liver lipidosis, gill hyperplasia and hypertrophy, and gill ciliate infestation combined with a decline in macrophage aggregates and gut parasites. Chronic marine oil pollution that results in hydrocarbon concentrations of 1600 g/g in nursery sediments has the potential to reduce growth and health of juvenile flatfishes. Recruitment of juveniles to the fishery would be reduced due to increased susceptibility to predation and slower growth to maturity
The evolution of the galactic morphological types in clusters
The morphological types of galaxies in nine clusters in the redshift range
0.1<z<0.25 are derived from very good seeing images taken at the NOT and the La
Silla Danish telescopes. With the purpose of investigating the evolution of the
fraction of different morphological types with redshift, we compare our results
with the morphological content of nine distant clusters studied by the MORPHS
group, five clusters observed with HST-WFPC2 at redshift z = 0.2-0.3, and
Dressler's (1980) large sample of nearby clusters. After having checked the
reliability of our morphological classification both in an absolute sense and
relative to the MORPHS scheme, we analyze the relative occurrence of
elliptical, S0 and spiral galaxies as a function of the cluster properties and
redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related
to the cluster morphology. In particular, in our cluster sample, clusters with
a high concentration of ellipticals display a low S0/E ratio and, vice-versa,
low concentration clusters have a high S0/E. At the same time, the trend of the
morphological fractions and ratios with redshift clearly points to a
morphological evolution: as the redshift decreases, the S0 population tends to
grow at the expense of the spiral population, whereas the frequency of Es
remains almost constant. We also analyze the morphology-density (MD) relation
in our clusters and find that -similarly to higher redshift clusters- a good MD
relation exists in the high-concentration clusters, while it is absent in the
less concentrated clusters. Finally, the comparison of the MD relation in our
clusters with that of the D97 sample suggests that the transformation of
spirals into S0 galaxies becomes more efficient with decreasing local density.Comment: 24 pages including 11 figures and 4 tables, accepted for publication
in Ap
Rotation curves and metallicity gradients from HII regions in spiral galaxies
In this paper we study long slit spectra in the region of H emission
line of a sample of 111 spiral galaxies with recognizable and well defined
spiral morphology and with a well determined environmental status, ranging from
isolation to non-disruptive interaction with satellites or companions. The form
and properties of the rotation curves are considered as a function of the
isolation degree, morphological type and luminosity. The line ratios are used
to estimate the metallicity of all the detected HII regions, thus producing a
composite metallicity profile for different types of spirals. We have found
that isolated galaxies tend to be of later types and lower luminosity than the
interacting galaxies. The outer parts of the rotation curves of isolated
galaxies tend to be flatter than in interacting galaxies, but they show similar
relations between global parameters. The scatter of the Tully-Fisher relation
defined by isolated galaxies is significantly lower than that of interacting
galaxies. The [NII]/H ratios, used as metallicity indicator, show a
clear trend between Z and morphological type, t, with earlier spirals showing
larger ratios; this trend is tighter when instead of t the gradient of the
inner rotation curve, G, is used; no trend is found with the interaction
status. The Z-gradient of the disks depends on the type, being almost flat for
early spirals, and increasing for later types. The [NII]/H ratios
measured for disk HII regions of interacting galaxies are higher than for
normal/isolated objects, even if all the galaxy families present similar
distributions of H Equivalent Width.Comment: accepted for publication in A&A (tables for HII region parameters
incomplete, contact [email protected] for the whole set of tables
Implications of new geochronological constraints on the Aberfeldy stratiform barite deposits, Scotland, for the depositional continuity and global correlation of the Neoproterozoic Dalradian Supergroup
The Dalradian Supergroup of largely clastic metasediments in Scotland and Ireland is a well-known record of Neoproterozoic to Cambrian sedimentation on the Laurentian margin. Its apparent long duration, from ∼ 800 to ∼ 500 Ma, has encouraged a search for tectonic breaks and unconformities within the sequence. One suggestion is that a break in sedimentation occurred within the upper part of the Argyll Group, specifically in the Easdale Subgroup. Here we report dates for demonstrably synsedimentary barite mineralization hosted by the Easdale Subgroup Ben Eagach Schist Formation. Bedded pyrite from the Foss and the Ben Eagach–Duntanlich barite deposits, stratigraphically < 100 m apart, yields Re-Os ages of 604.0 ± 7.2 Ma and 612.1 ± 18.6 Ma respectively with two-sigma overlap. These ages are similar to the previously constrained 601 ± 4 Ma Tayvallich Volcanic Formation lying 2–5 km stratigraphically above. The new ages suggest continuous sedimentation through this period and, in combination with existing time markers, increase the likelihood that the basal-Argyll, Port Askaig Formation diamictite represents the Marinoan glaciation rather than the Sturtian glacial episode which may not be represented in the Dalradian sequence. As such, we urge caution in using the Dalradian in global compilations of Neoproterozoic history until further geochronological constraints are obtained for the Cryogenian-Tonian parts of the stratigraphy, which is now a high priority
The WINGS Survey: a progress report
A two-band (B and V) wide-field imaging survey of a complete, all-sky X-ray
selected sample of 78 clusters in the redshift range z=0.04-0.07 is presented.
The aim of this survey is to provide the astronomical community with a complete
set of homogeneous, CCD-based surface photometry and morphological data of
nearby cluster galaxies located within 1.5 Mpc from the cluster center. The
data collection has been completed in seven observing runs at the INT and
ESO-2.2m telescopes. For each cluster, photometric data of about 2500 galaxies
(down to V~23) and detailed morphological information of about 600 galaxies
(down to V~21) are obtained by using specially designed automatic tools.
As a natural follow up of the photometric survey, we also illustrate a long
term spectroscopic program we are carrying out with the WHT-WYFFOS and AAT-2dF
multifiber spectrographs. Star formation rates and histories, as well as
metallicity estimates will be derived for about 350 galaxies per cluster from
the line indices and equivalent widths measurements, allowing us to explore the
link between the spectral properties and the morphological evolution in high-
to low-density environments, and across a wide range in cluster X-ray
luminosities and optical properties.Comment: 12 pages, 10 eps figures, Proceedings of the SAIt Conference 200
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