35 research outputs found
Turbulence and galactic structure
Interstellar turbulence is driven over a wide range of scales by processes
including spiral arm instabilities and supernovae, and it affects the rate and
morphology of star formation, energy dissipation, and angular momentum transfer
in galaxy disks. Star formation is initiated on large scales by gravitational
instabilities which control the overall rate through the long dynamical time
corresponding to the average ISM density. Stars form at much higher densities
than average, however, and at much faster rates locally, so the slow average
rate arises because the fraction of the gas mass that forms stars at any one
time is low, ~10^{-4}. This low fraction is determined by turbulence
compression, and is apparently independent of specific cloud formation
processes which all operate at lower densities. Turbulence compression also
accounts for the formation of most stars in clusters, along with the cluster
mass spectrum, and it gives a hierarchical distribution to the positions of
these clusters and to star-forming regions in general. Turbulent motions appear
to be very fast in irregular galaxies at high redshift, possibly having speeds
equal to several tenths of the rotation speed in view of the morphology of
chain galaxies and their face-on counterparts. The origin of this turbulence is
not evident, but some of it could come from accretion onto the disk. Such high
turbulence could help drive an early epoch of gas inflow through viscous
torques in galaxies where spiral arms and bars are weak. Such evolution may
lead to bulge or bar formation, or to bar re-formation if a previous bar
dissolved. We show evidence that the bar fraction is about constant with
redshift out to z~1, and model the formation and destruction rates of bars
required to achieve this constancy.Comment: in: Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note, Eds., K. Freeman, D. Block, I. Puerari, R. Groess,
Dordrecht: Kluwer, in press (presented at a conference in South Africa, June
7-12, 2004). 19 pgs, 5 figure
Exotic fish in exotic plantations: a multi-scale approach to understand amphibian occurrence in the mediterranean region
Globally, amphibian populations are threatened by a diverse range of factors including habitat
destruction and alteration. Forestry practices have been linked with low diversity and
abundance of amphibians. The effect of exotic Eucalyptus spp. plantations on amphibian
communities has been studied in a number of biodiversity hotspots, but little is known of its
impact in the Mediterranean region. Here, we identify the environmental factors influencing
the presence of six species of amphibians (the Caudata Pleurodeles waltl, Salamandra salamandra,
Lissotriton boscai, Triturus marmoratus and the anurans Pelobates cultripes and
Hyla arborea/meridionalis) occupying 88 ponds. The study was conducted in a Mediterranean
landscape dominated by eucalypt plantations alternated with traditional use (agricultural,
montados and native forest) at three different scales: local (pond), intermediate (400
metres radius buffer) and broad (1000 metres radius buffer). Using the Akaike Information
Criterion for small samples (AICc), we selected the top-ranked models for estimating the
probability of occurrence of each species at each spatial scale separately and across all
three spatial scales, using a combination of covariates from the different magnitudes. Models
with a combination of covariates at the different spatial scales had a stronger support
than those at individual scales. The presence of predatory fish in a pond had a strong effect
on Caudata presence. Permanent ponds were selected by Hyla arborea/meridionalis over
temporary ponds. Species occurrence was not increased by a higher density of streams,
but the density of ponds impacted negatively on Lissotriton boscai. The proximity of ponds
occupied by their conspecifics had a positive effect on the occurrence of Lissotriton boscai
and Pleurodeles waltl. Eucalypt plantations had a negative effect on the occurrence of the
newt Lissotriton boscai and anurans Hyla arborea/meridionalis, but had a positive effect on
the presence of Salamandra salamandra, while no effect on any of the other species was detected.
In conclusion, eucalypts had limited effects on the amphibian community at the intermediate
and broad scales, but predatory fish had a major impact when considering all the
scales combined. The over-riding importance of introduced fish as a negative impact suggests that forest managers should prevent new fish introductions and eradicate fish
from already-occupied ponds whenever possible