National Autonomous University of Mexico

About: National Autonomous University of Mexico is a education organization based out in Mexico City, Distrito Federal, Mexico. It is known for research contribution in the topics: Population & Galaxy. The organization has 72868 authors who have published 127797 publications receiving 2285543 citations. The organization is also known as: UNAM & Universidad Nacional Autónoma de México.

Vertebrates on the brink as indicators of biological annihilation and the sixth mass extinction.

Abstract: The ongoing sixth mass species extinction is the result of the destruction of component populations leading to eventual extirpation of entire species. Populations and species extinctions have severe implications for society through the degradation of ecosystem services. Here we assess the extinction crisis from a different perspective. We examine 29,400 species of terrestrial vertebrates, and determine which are on the brink of extinction because they have fewer than 1,000 individuals. There are 515 species on the brink (1.7% of the evaluated vertebrates). Around 94% of the populations of 77 mammal and bird species on the brink have been lost in the last century. Assuming all species on the brink have similar trends, more than 237,000 populations of those species have vanished since 1900. We conclude the human-caused sixth mass extinction is likely accelerating for several reasons. First, many of the species that have been driven to the brink will likely become extinct soon. Second, the distribution of those species highly coincides with hundreds of other endangered species, surviving in regions with high human impacts, suggesting ongoing regional biodiversity collapses. Third, close ecological interactions of species on the brink tend to move other species toward annihilation when they disappear-extinction breeds extinctions. Finally, human pressures on the biosphere are growing rapidly, and a recent example is the current coronavirus disease 2019 (Covid-19) pandemic, linked to wildlife trade. Our results reemphasize the extreme urgency of taking much-expanded worldwide actions to save wild species and humanity's crucial life-support systems from this existential threat.

Economic value of the pest control service provided by Brazilian free-tailed bats in south-central Texas

Abstract: Brazilian free-tailed bats (Tadarida brasiliensis) form enormous summer breeding colonies, mostly in caves and under bridges, in south-central Texas and northern Mexico. Their prey includes several species of adult insects whose larvae are known to be important agricultural pests, including the corn earworm or cotton bollworm (Helicoverpa zea). We estimate the bats' value as pest control for cotton production in an eight-county region in south-central Texas. Our calculations show an annual value of $741 000 per year, with a range of $121 000–$1 725 000, compared to a $4.6–$6.4 million per year annual cotton harvest.

Sinking satellites and the heating of galaxy discs

Abstract: We have carried out a set of self-consistent N-body simulations to study the interaction between disc galaxies and merging satellites with the aim of determining the disc kinematical changes induced by such events. We explore a region of the parameter space embracing satellites with different masses and internal structure and orbits of various eccentricities. We find that the analytic estimates of Toth & Ostriker are high; overestimating the disc heating and thickening resulting from the accretion process by a factor of about 2--3. We find that the heating and thickening of the disc differ for satellites on prograde and retrograde orbits. The former tend to heat the stellar disc while the latter primarily produce a coherent tilt. For instance, a satellite of a Milky Way type galaxy with an initial mass of 20 per cent of that of the disc and on a retrograde orbit increases the velocity ellipsoid at the solar neighbourhood by (ΔσR, Δσπ, Δσz)⊙≈ (11,9,6) km s-1 and produces a maximum increment of the vertical scalelength and the stability parameter Q, inside the solar radius, of 300 pc and 0.8, respectively, increases of about 43 and 53 per cent. The same satellite, but on a prograde orbit, leads to changes of (ΔσR, Δσπ , Δσz)⊙≈ (22,15,12) km s-1, Δ zo,⊙≈ 550 pc and Δ Q⊙≈ 1.2. Thus, disc galaxies may accrete quite massive satellites without destroying the disc, particularly, if the orbits are retrograde. We also find that a massive bulge may play a role in reducing these effects. We have quantified the importance of the responsiveness of the halo by replacing it by a rigid potential in several simulations. In these cases, the increase of the vertical scalelength is larger by a factor of 1.5--2, indicating that a self-consistent treatment is essential for obtaining realistic results. A frequent by-product of the accretion process is the formation of weak stellar warps and asymmetric discs. Finally, we have checked how well Chandrasekhar's dynamical friction formula reproduces the sinking rates in several of our experiments. We find that it works well provided a suitable value is chosen for the Coulomb logarithm and the satellite mass is taken to be the mass still bound to the satellite at each moment.

Multiple ecological pathways to extinction in mammals

Abstract: As human population and resource demands continue to grow, biodiversity conservation has never been more critical. About one-quarter of all mammals are in danger of extinction, and more than half of all mammal populations are in decline. A major priority for conservation science is to understand the ecological traits that predict extinction risk and the interactions among those predictors that make certain species more vulnerable than others. Here, using a new database of nearly 4,500 mammal species, we use decision-tree models to quantify the multiple interacting factors associated with extinction risk. We show that the correlates of extinction risk vary widely across mammals and that there are unique pathways to extinction for species with different lifestyles and combinations of traits. We find that risk is relative and that all kinds of mammals, across all body sizes, can be at risk depending on their specific ecologies. Our results increase the understanding of extinction processes, generate simple rules of thumb that identify species at greatest risk, and highlight the potential of decision-tree analyses to inform conservation efforts.

Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions

Abstract: We study the formation of giant dense cloud complexes and of stars within them by means of SPH numerical simulations of the mildly supersonic collision of gas streams (``inflows'') in the warm neutral medium (WNM). The resulting compressions cause cooling and turbulence generation in the gas, forming a cloud that then becomes self-gravitating and undergoes global collapse. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulations show that: a) The clouds are not in a state of equilibrium. Instead, they undergo secular evolution. Initially, their mass and gravitational energy |Eg| increase steadily, while the turbulent energy Ek reaches a plateau. b) When |Eg| becomes comparable to Ek, global collapse begins, causing a simultaneous increase in both that maintains a near-equipartition condition |Eg| ~ 2 Ek. c) Longer inflow durations delay the onset of global and local collapse, by maintaining a higher turbulent velocity dispersion in the cloud over longer times. d) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. e) The column densities of the local star-forming clumps are very similar to reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. The MC formation mechanism discussed here naturally explains the apparent ``virialized'' state of MCs and the ubiquitous presence of HI halos around them. Within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (>~ 15 Myr) accumulation periods do occur during which the clouds build up their gravitational energy, and which are expected to be spent in the atomic phase.