Effect of precipitation variability on net primary production and soil respiration in a Chihuahuan Desert grassland

Precipitation regimes are predicted to become more variable with more extreme rainfall events punctuated by longer intervening dry periods. Water‐limited ecosystems are likely to be highly responsive to altered precipitation regimes. The bucket model predicts that increased precipitation variability will reduce soil moisture stress and increase primary productivity and soil respiration in aridland ecosystems. To test this hypothesis, we experimentally altered the size and frequency of precipitation events during the summer monsoon (July through September) in 2007 and 2008 in a northern Chihuahuan Desert grassland in central New Mexico, USA. Treatments included (1) ambient rain, (2) ambient rain plus one 20 mm rain event each month, and (3) ambient rain plus four 5 mm rain events each month. Throughout two monsoon seasons, we measured soil temperature, soil moisture content (θ), soil respiration (R_s), along with leaf‐level photosynthesis (A_net), predawn leaf water potential (Ψ_pd), and seasonal aboveground net primary productivity (ANPP) of the dominant C₄ grass, Bouteloua eriopoda. Treatment plots receiving a single large rainfall event each month maintained significantly higher seasonal soil θ which corresponded with a significant increase in R_s and ANPP of B. eriopoda when compared with plots receiving multiple small events. Because the strength of these patterns differed between years, we propose a modification of the bucket model in which both the mean and variance of soil water change as a consequence of interannual variability from 1 year to the next. Our results demonstrate that aridland ecosystems are highly sensitive to increased precipitation variability, and that more extreme precipitation events will likely have a positive impact on some aridland ecosystem processes important for the carbon cycle.     

Predicting small‐mammal responses to climatic warming: autecology,geographic range,and the Holocene fossil record

Forecasting how species will respond to climatic change requires knowledge of past community dynamics. Here we use time‐series data from the small‐mammal fossil records of two caves in the Great Basin of the American West to evaluate how contrasting and variable local paleoclimates have shaped small‐mammal abundance dynamics over the last ～7500 years of climatic change. We then predict how species and communities will respond to future scenarios of increased warming and aridity coupled with continued spread of an invasive annual grass (Bromus tectorum). We find that most community‐level responses to climatic change occur in the mammalian abundance structure at both sites; the dominance of the community by individuals from species with a southern geographic affinity increases with climatic warming. This suggests that responses occurred in situ rather than by the immigration of new taxa over this time interval. Despite predictability at the community‐scale, species‐level relationships between abundance and climate are variable and are not necessarily explained by a species' geographic affinity. Species present at both sites, however, exhibit remarkably similar responses to climate at each site, indicating that species autecology (specifically dietary functional group) is important in determining response to climatic warming. Regression‐tree analyses show remarkable concordance between the two cave faunas and highlight the importance of a granivorous dietary strategy in this desert ecosystem. Under projections of increased temperature and decreased precipitation over the next 50 years, our results indicate that granivores should thrive as communities become more dominated by individuals with a southern geographic affinity. Granivores, however, are negatively impacted by the invasion of cheatgrass. The last century of anthropogenic impacts has thus placed granivores at a greater risk of extinction than predicted under climate‐only scenarios.     

Effects of fishing and acidification‐related benthic mortality on the southeast Australian marine ecosystem

Oceanic uptake of anthropogenic carbon dioxide (CO₂) is altering the carbonate chemistry of seawater, with potentially negative consequences for many calcifying marine organisms. At the same time, increasing fisheries exploitation is impacting on marine ecosystems. Here, using increased benthic‐invertebrate mortality as a proxy for effects of ocean acidification, the potential impact of the two stressors of fishing and acidification on the southeast Australian marine ecosystem to year 2050 was explored. The individual and interaction effects of the two stressors on biomass and diversity were examined for the entire ecosystem and for regional assemblages. For 61 functional groups or species, the cumulative effects of moderate ocean acidification and fishing were additive (30%), synergistic (33%), and antagonistic (37%). Strong ocean acidification resulted in additive (22%), synergistic (40%), and antagonistic (38%) effects. The greatest impact was on the demersal food web, with fishing impacting predation and acidification affecting benthic production. Areas that have been subject to intensive fishing were the most susceptible to acidification effect, although fishing also mitigated some of the decline in biodiversity observed with moderate acidification. The model suggested that ocean acidification and long‐term fisheries exploitation could act synergistically with the increasing sensitivity to change from long‐term (decades) fisheries exploitation potentially causing unexpected restructuring of the pelagic and demersal food webs. Major regime shifts occur around year 2040. Greater focus is needed on how differential fisheries exploitation of marine resources may exacerbate or accelerate effects of environmental changes such as ocean acidification.     

How useful are the recommended counts and indices in the systematics of the Octopodidae (Mollusca: Cephalopoda)

A morphological dataset based on 14 standard counts and indices was constructed for 68 specimens comprising 12 species of octopuses. This was used to construct distance matrices based on morphological characters. These matrices were compared with genetic distance matrices compiled during molecular phylogenetic analyses of the same 12 species using four mitochondrial and two nuclear genes. Mantel tests showed that there was significant congruence between the phenetic and genetic matrices, suggesting that the genetic signal is reflected in the morphological data set. Matrices of geographical distance were constructed for the 12 species based on the latitude, longitude, and depth of capture of 1726 individuals. These matrices never showed significant congruence with genetic data or with morphological data. Multivariate analysis of the morphological dataset suggests that these counts and indices, traditionally used for discriminating between species in cephalopods, do not show great discrimination at species level, but provide excellent discrimination at the generic level, and, as such, might be useful for resolving the generic placement of some problematic taxa. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 205–218.