Soil organic carbon content at a range of north Australian tropical savannas with contrasting site histories

Soils play an important role in the global carbon cycle, and can be major source or sink of CO₂ depending upon land use, vegetation type and soil management practices. Natural and human impact on soil carbon concentration and storage is poorly understood in native north Australian savanna, yet this represents the largest carbon store in the ecosystem. To gain understanding of possible management impacts on this carbon pool, soil organic carbon (SOC) of the top 1m of red earth sands and sandy loams common in the region was sampled at 5 sites with different vegetation cover and site history (fire regime and tree removal). SOC was high when compared to other published values for savannas and was more comparable with dry-deciduous tropical forests. Sites sampled in this study represent high rainfall savannas of northern Australia (> 1700 mm annual rainfall) that feature frequent burning (2 in 3 years or more frequent) and a cycle of annual re-growth of tall C₄ grasses that dominate the savanna understorey. These factors may be responsible for the higher than expected SOC levels of the surface soils, despite high respiration rates. Medium term fire exclusion (15–20 years) at one of the sampled sites (Wildlife Park) dramatically reduced the grassy biomass of the understorey. This site had lower SOC levels when compared to the grass dominated and frequently burnt sites, which may be due to a reduction in detrital input to surface (0–30 cm) soil carbon pools. Exclusion of trees also had a significant impact on both the total amount and distribution of soil organic carbon, with tree removal reducing observed SOC at depth (100 cm). Soil carbon content was higher in the wet season than that in the dry season, but this difference was not statistically significant. Our results indicated that annual cycle of grass growth and wildfire resulted in small carbon accumulation in the upper region of the soil, and removal of woody plants resulted in significant carbon losses to recalcitrant, deep soil horizons greater than 80 cm depth.     

The influence of abrupt climate change on the ice‐age vegetation of the Central American lowlands

Aim To investigate the effects of abrupt climate change in the North Atlantic on the vegetation history of lowland Central America. We use palynological evidence from a Central American lake on the Yucatan Peninsula to evaluate the effects of rapid climate changes during the last ice age, between 65 and 8 ka. Location Lake Petén‐Itzá, lowlands of northern Guatemala. Methods Sediment core PI‐6 was sampled at c. 170‐year resolution for pollen and charcoal analysis in order to construct a temporal sequence of environmental change. Uni‐ and multivariate statistical analyses were performed on the pollen dataset to test whether there was an association between Heinrich events in the North Atlantic and vegetation changes in the Central American lowlands. Results Pollen analysis revealed that the composition of plant assemblages on the Yucatan Peninsula varied in phase with rapid changes in North Atlantic climate. Pine savannas were the main vegetation type between c. 60 and 47 ka. These savannas gave way to pine–oak (Pinus–Quercus) forests in the latter half of the last ice age. Marked episodes of replacement of the pine–oak assemblage by xeric‐tolerant taxa occurred during Heinrich events. The Last Glacial Maximum (LGM) was characterized by mesic conditions. Main conclusions The pollen record from Lake Petén‐Itzá showed that vegetation changes associated with Heinrich events were more significant than those associated with the LGM. Each Heinrich event produced a characteristic shift towards xeric taxa. Although colder than Heinrich events, the LGM on the Yucatan Peninsula was relatively moist, and the presumed savannization of the landscape during the maximum cooling of the last glacial was not supported by our data. Our findings suggest alternative scenarios for plant diversification and genetic interchange during glacial times, and also indicate that vegetation in tropical continental settings was not as stable as previously thought.     

Temperature can interact with landscape factors to affect songbird productivity

Increased temperatures and more extreme weather patterns associated with global climate change can interact with other factors that regulate animal populations, but many climate change studies do not incorporate other threats to wildlife in their analyses. We used 20 years of nest‐monitoring data from study sites across a gradient of habitat fragmentation in Missouri, USA, to investigate the relative influence of weather variables (temperature and precipitation) and landscape factors (forest cover and edge density) on the number of young produced per nest attempt (i.e., productivity) for three species of songbirds. We detected a strong forest cover × temperature interaction for the Acadian Flycatcher (Empidonax virescens) on productivity. Greater forest cover resulted in greater productivity because of reduced brood parasitism and increased nest survival, whereas greater temperatures reduced productivity in highly forested landscapes because of increased nest predation but had no effect in less forested landscapes. The Indigo Bunting (Passerina cyanea) exhibited a similar pattern, albeit with a marginal forest cover × temperature interaction. By contrast, productivity of the Northern Cardinal (Cardinalis cardinalis) was not influenced by landscape effects or temperature. Our results highlight a potential difficulty of managing wildlife in response to global change such as habitat fragmentation and climate warming, as the habitat associated with the greatest productivity for flycatchers was also that most negatively influenced by high temperatures. The influence of high temperatures on nest predation (and therefore, nest predators) underscores the need to acknowledge the potential complexity of species' responses to climate change by incorporating a more thorough consideration of community ecology in the development of models of climate impacts on wildlife.     

Tangible evidence of historic Australian indigenous savanna management

Knowledge of historic indigenous management practices in north Australian tropical savannas can benefit contemporary management by providing a long‐term ecological context. This study provides understanding of how indigenous peoples managed their resources during the period of colonization by Europeans. Traditional management practices and resource use were observed by the European explorers and missionaries, anthropologists and ethnographers who followed. The historic record shows that the savannas were managed intensively by indigenous peoples, even during the colonization era. Across the region they used fire throughout the dry seasons, which is recognized by ecologists today. Importantly, and not previously reported in the ecological literature, they constructed water wells that provided them with extended use of country into the dry seasons, built and managed fisheries to enhance and extend their food supplies, and created extensive walking paths. These findings are significant because previous ecological research has assumed implicitly that indigenous people in the region were dependent on natural waters and therefore subject to seasonal availability of water to enable them to penetrate and live in dry country, and has given scant acknowledgement of manipulation of resources. The anthropological studies were compromised by the devastating social disruptions caused by the colonizers (mostly cattle ranchers and miners) and subsequent missionaries and government administrators. Despite these disruptions, the evidence demonstrates continuity of knowledge and management practices in much of the region. This history provides contemporary ecologists and managers with evidence of consistent patterns of resource management from earlier times. The evidence also shows that indigenous people were less at the mercy of the environment than has been assumed previously. The combined evidence suggests that contemporary management should consider that traditional management practices over many thousands of years were active and ubiquitous, and continued into the present era and probably shaped the biota of the region.     

A new species of electric knifefish,genus Compsaraia (Gymnotiformes: Apteronotidae) from the Amazon River,with extreme sexual dimorphism in snout and jaw length

Abstract

A new species of the neotropical electric fish genus Compsaraia is described from the western Amazon of Peru and Brazil. Compsaraia samueli is distinguishable from all other apteronotids by sexual dimorphism in which mature males exhibit extreme elongation and slenderness of the snout and jaws. Compsaraia samueli is readily distinguishable from its only congener, C. compsa, by more caudal‐fin rays (17–18 vs. 13–16), a shorter caudal peduncle (mean length 9% vs. 34% body length to end of anal fin), a less tapering body shape in lateral profile (mean ratio of body depth at origins of anal fin and dorsal organ 93% vs. 75%), and a smaller maximum adult body size (230 vs. 305 mm). The genus Compsaraia is readily separated from other apteronotids by a pale antorbital stripe and a pale L‐shaped patch over the supra‐temporal canal. The phylogenetic position of C. samueliis estimated by inclusion in a previously published data matrixof osteological and other morphological characters. Comparisons of the cranial bones in apteronotids shows the derived morphology of C. samueli to be a composite of three developmentally and phylogenetically discrete characters: (1) positive allometric growth before sexual maturity in both sexes of the pre‐orbital region of the neurocranium, (2) positive allometric growth of the (oral) jaws, and (3) secondary sexual dimorphism of snout and jaw morphology. The genus Compsaraia represents one of at least three phylogenetically independent cases of snout elongation and one of at least four cases of jaw elongation within the Apteronotidae. Compsaraia samueli also represents one of at least four cases of secondary sexual dimorphism in snout and jaw length within the Apteronotidae. The phylogenetic distribution of snout and jaw characters within the Apteronotidae suggests the influence of both sexual and trophic functional influences on the evolution of head morphology.     

Poptella fuscata,a new Stethaprionini from the upper Amazon basin,Peru (Characiformes: Characidae)

A new species of Poptella is described from the Río Putumayo, Upper Río Amazon basin, Peru. The new species is distinguished from congeners by having a dense field of dark chromatophores homogeneously spread over the posterior half of the body, posterior humeral blotch extending to three to four horizontal scale rows below the lateral line, and a higher number of branched dorsal-fin rays. The new species can be readily distinguished from P. paraguayensis by having a comparatively shorter predorsal spine.     

When is a ‘forest’ a savanna,and why does it matter?

Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the high‐rainfall end of the tropical savanna biome, where heavily wooded mesic savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic savannas are predominantly mixed tree–C₄ grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C₄ grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life‐history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire‐resistant tree species in a C₄ grass‐dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them.     

A new endangered species of Megaleporinus (Characiformes: Anostomidae) from the Rio de Contas basin,eastern Brazil

A new species of Megaleporinus is described from the Rio de Contas, a coastal drainage of eastern Brazil, and its phylogenetic relationships are studied using molecular data. The new species is unique among Anostomidae by possessing two exclusive features: an irregular dark longitudinal stripe from supracleithrum to second midlateral blotch and anterior cranial fontanel partially closed. In addition, the new species is diagnosed by having three premaxillary teeth, three dentary teeth, 37 or 38 scales in lateral line, 16 scale rows around caudal peduncle, three dark midlateral blotches on body, and red fins in life. The new species is closely related to M. obtusidens from the São Francisco basin, corroborating previous studies that indicated that the latter represents a species complex as currently defined. The new species exhibits the first rib enlarged in mature males, a feature described for some congeners. The new species is herein considered to be Endangered under the IUCN criteria.     

Constrained preferences in nitrogen uptake across plant species and environments

Knowledge of determining factors for nitrogen uptake preferences and how they are modified in changing environments are critical to understand ecosystem nitrogen cycling and to predict plant responses to future environmental changes. Two ¹⁵N tracer experiments utilizing a unique differential labelled nitrogen source were employed in both African savannas and greenhouse settings. The results demonstrated that nitrogen uptake preferences were constrained by the climatic conditions. As mainly indicated by root δ¹⁵N signatures at 1:1 ammonium/nitrate ratio, in the drier environments, plants preferred nitrate and in the wetter environments they preferred ammonium. Nitrogen uptake preferences were different across different ecosystems (e.g. from drier to wetter environments) even for the same species. More significantly, our experiments showed that the plant progeny continued to exhibit the same nitrogen preference as the parent plants in the field, even when removed from their native environment and the nitrogen source was changed dramatically. The climatic constraint of nitrogen uptake preference is likely influenced by ammonium/nitrate ratios in the native habitats of the plants. The constancy in nitrogen preference has important implications in predicting the success of plant communities in their response to climate change, to seed bank use and to reforestation efforts.