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Jacobsen D.,Copenhagen University | Dangles O.,Pontifical Catholic University of Ecuador | Dangles O.,French National Center for Scientific Research | Dangles O.,University Paris - Sud
Global Ecology and Biogeography | Year: 2012

Aim To test for a possible effect of environmental harshness on large-scale latitudinal and elevational patterns in taxon richness of macrofauna in arctic and alpine glacier-fed streams. Location Svalbard (79°N), Iceland (65°N), Norway (62°N), Switzerland and Italy (46°N), France (43°N), New Zealand (43°S) and Ecuador (0°), covering an elevational gradient from sea level to 4800m a.s.l. Methods We gathered data from 63 sites along 13 streams and created an index of glacial influence (the glacial index, GI) as an integrative proxy for environmental harshness. The explicative power of the GI, environmental variables, latitude and elevation on taxon richness was tested in generalized linear models. Taxon richness along geographical gradients was analysed at standardized levels of GI in contour plots. Beta diversity and assemblage similarity was calculated at different GI intervals and compared with a null-model. Results Overall, taxon richness decreased exponentially with increased GI (r 2= 0.64), and of all included factors, GI had the highest explicative power. At low values of GI we found that local taxon richness varied along the coupled gradients of latitude and elevation in a hump-shaped manner. However, this pattern disappeared at high values of GI, i.e. when environmental harshness increased. Beta diversity increased, while similarity among assemblages decreased towards high GI values. Main conclusions In our study system, the number of taxa able to cope with the harshest conditions was largely independent of the regional taxon pool, and environmental harshness constituted a 'fixed' constraint for local richness, irrespective of latitude and elevation. Contrary to expectations, we found that beta diversity was highest and similarity lowest among the harshest sites, suggesting that taxon richness was not solely driven by niche selection based on environmental tolerances, but also stochastic ecological drift, leading to dispersal-limited communities. © 2011 Blackwell Publishing Ltd.

Hietz P.,University of Natural Resources and Life Sciences, Vienna | Valencia R.,Pontifical Catholic University of Ecuador | Joseph Wright S.,Smithsonian Tropical Research Institute
Functional Ecology | Year: 2013

Wood density (WD) affects plant biomechanics, drought and decay resistance. As a consequence, WD is an important functional trait related to plant demography and ecosystem processes, which is also used to estimate tree biomass. Radial variation in WD (changes from the centre of the stem to the cambium) affects the strength of the entire stem, but also reflects any changes in wood functional properties that might occur during a tree's lifetime. To understand how WD and radial WD gradients, which were defined as the slope of the relationship between WD and distance to the centre, are related to demographic traits of species, we investigated WD in 335 tree species from a Panamanian moist forest and 501 species from an Ecuadorian rain forest and radial density gradients in 118 and 186 species, respectively, and compared WD with tree growth, mortality and size. WD was negatively related to tree growth and mortality. WD tended to increase towards the outside in trees with low initial density and to decrease towards the outside in trees with high initial density. Radial WD gradients were largely unrelated to tree size and demographic traits, but some families had higher or lower WD gradients at a given inner WD. Inner WD was by far the best predictor of radial WD gradients (r2 = 0·39 for Panama and 0·45 for Ecuador) and this relationship was indistinguishable between the two rain forests. This suggests a broadly uniform function of WD variation, likely responding to mechanical requirements during ontogeny. We discuss the factors potentially driving radial increases or decreases in WD and suggest ways to elucidate the relative importance of tree mechanics, hydraulic safety or decay resistance. We also discuss that not accounting for radial WD gradients may result in substantial errors in WD of the whole stem and consequently biomass estimates, and recommend sampling density gradients when obtaining density data from tree cores. © 2013 The Authors. Functional Ecology © 2013 British Ecological Society.

Menendez-Guerrero P.A.,Pontifical Catholic University of Ecuador | Graham C.H.,State University of New York at Stony Brook
Ecography | Year: 2013

Amphibians are declining at alarming rates worldwide; however, the causes of these declines remain somewhat elusive. Here we evaluated three major threats implicated in declines of populations and disappearance of Ecuadorian amphibians: chytridiomicosis, climate change, and habitat loss. We assessed spatial patterns of these key threats to Ecuadorian amphibians using a multi-species database of endemic frogs along with information on the pathogen's distribution and environmental requirements, species sensitivity to climate change (indirectly based on species geographical distribution and ecological properties) and habitat loss. Our results show that amphibians display a non-random pattern of extinction risk, both geographically and taxonomically. Further, climate change, chytridiomicosis, and their synergetic effects, are likely currently exerting the greatest impact on amphibians in Ecuador, while habitat loss does not seem to be causing precipitous declines. The most threatened species under the IUCN extinction risk categories are exactly those that appear to be the most affected by these threats. By examining multiple potential causes of amphibian threat level in a spatially explicit framework our study provides new insights about what combination of factors are most important in amphibian declines in a tropical diversity hotspot. Further, our approach and conclusions are useful for studying declines in other regions of the world. © 2013 The Authors.

Elinson R.P.,Duquesne University | del Pino E.M.,Pontifical Catholic University of Ecuador
Wiley Interdisciplinary Reviews: Developmental Biology | Year: 2012

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes. © 2011 Wiley Periodicals, Inc.

Metz M.R.,University of California at Berkeley | Sousa W.P.,University of California at Berkeley | Valencia R.,Pontifical Catholic University of Ecuador
Ecology | Year: 2010

Negative density-dependent mortality can promote species coexistence through a spacing mechanism that prevents species from becoming too locally abundant. Negative density-dependent seedling mortality can be caused by interactions among seedlings or between seedlings and neighboring adults if the density of neighbors affects the strength of competition or facilitates the attack of natural enemies. We investigated the effects of seedling and adult neighborhoods on the survival of newly recruited seedlings for multiple cohorts of known age from 163 species in Yasuní National Park, Ecuador, an ever-wet, hyper-diverse lowland Amazonian rain forest. At local scales, we found a strong negative impact on firstyear survival of conspecific seedling densities and adult abundance in multiple neighborhood sizes and a beneficial effect of a local tree neighborhood that is distantly related to the focal seedling. Once seedlings have survived their first year, they also benefit from a more phylogenetically dispersed seedling neighborhood. Across species, we did not find evidence that rare species have an advantage relative to more common species, or a community compensatory trend. These results suggest that the local biotic neighborhood is a strong influence on early seedling survival for species that range widely in their abundance and life history. These patterns in seedling survival demonstrate the role of density-dependent seedling dynamics in promoting and maintaining diversity in understory seedling assemblages. The assemblage-wide impacts of species abundance distributions may multiply with repeated cycles of recruitment and density-dependent seedling mortality and impact forest diversity or the abundance of individual species over longer time scales. © 2010 by the Ecological Society of America.

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