Larval Chironomid Community Organization in Four Tropical Rivers: Human Impacts and Longitudinal Zonation

Chironomid (Diptera) larvae were collected between February 1997 and December 1998 from four Trinidadian rivers, two relatively impacted (the Arima and Guanapo Rivers), and two comparatively pristine (the Marianne and Paria Rivers), in order to assess community changes as a result of both land-use and the natural longitudinal gradient. Multivariate analyses indicated changes in chironomid community composition at the impacted sites, with the most dramatic changes occurring in the low-reaches of the Arima and Guanapo Rivers, which were subjected to nutrient enrichment and general urban runoff. For example, genus richness, measured using Marglef’s Index, was lower in the low-reaches of both the Arima and Guanapo Rivers (0.9 and 1.1, respectively), compared with the low-reaches of the Marianne and Paria Rivers (1.8 and 2.8, respectively). The average chironomid densities in the low-reaches of both the Arima and Guanapo Rivers (1346 and 1029 chironomids/1 m², respectively) were also much higher compared with those of the Marianne and Paria Rivers (130 and 123 chironomids/1 m², respectively), most likely being the result of nutrient enrichment. Furthermore, the low-reaches of both the Arima and Guanapo Rivers demonstrated a shift in community structure, favouring those taxa more tolerant of non-point source pollution, including Thienemanniella spp., Paratrichocladius sp., Corynoneura spp., Polypedilum spp., Chironomus spp., Saetheria sp., Rheotanytarsus sp., and Thienemannimyia sp. In terms of natural longitudinal changes, the chironomid community shifted from one numerically dominated by Orthocladiinae in the headwaters to one numerically dominated by Chironominae in the low-reaches. Overall, this study shows that land-use can have a greater influence on the structure and diversity of chironomid communities than the natural longitudinal gradient.     

An Evaluation of Plotless Sampling Using Vegetation Simulations and Field Data from a Mangrove Forest

In vegetation science and forest management, tree density is often used as a variable. To determine the value of this variable, reliable field methods are necessary. When vegetation is sparse or not easily accessible, the use of sample plots is not feasible in the field. Therefore, plotless methods, like the Point Centred Quarter Method, are often used as an alternative. In this study we investigate the accuracy of different plotless sampling methods. To this end, tree densities of a mangrove forest were determined and compared with estimates provided by several plotless methods. None of these methods proved accurate across all field sites with mean underestimations up to 97% and mean overestimations up to 53% in the field. Applying the methods to different vegetation patterns shows that when random spatial distributions were used the true density was included within the 95% confidence limits of all the plotless methods tested. It was also found that, besides aggregation and regularity, density trends often found in mangroves contribute to the unreliability. This outcome raises questions about the use of plotless sampling in forest monitoring and management, as well as for estimates of density-based carbon sequestration. We give recommendations to minimize errors in vegetation surveys and recommendations for further in-depth research.     

Predatory Mites in Tropical Australia: Local Species Richness complementarity1

Although little supporting data is available, mites (Acari) are often considered to be one of the ‘hyperdiverse’ taxa in tropical ecosystems. To test this assumption, we sampled single guilds of predatory mites (Hydracarina and Mesostigmata) in three different habitats (fresh water, rotting fungi, and forest foliage) across a range of sites in monsoonal, wet-tropical, and subtropical Australia. Most species (61%) were collected at a single site; as a result, all seven collector's curves rose steeply with little indication of reaching asymptotes. Regional faunas ranged from 87–94 percent distinct and of the 247 species identified, 114 (46%) were previously unknown in Australia and appear to be new. Even within taxonomically well-studied groups, such as the Hydracarina and Phytoseiidae, we found many new species (32% and 60%, respectively). Our results suggest that the diversity of tropical mites is very high and comparable to that of many insect taxa. We propose a simple model to explain our results, i.e. that in the tropics, high levels of complementarity between sites amplify local mite species richness. We tested this model by additional sampling, comparing within-site to between-site complementarity, and contrasting temperate with tropical foliar Mesostigmata. As predicted by the model, collecting at new sites continued to accumulate new species, complementarity was significantly greater between-sites than within-sites (72 vs. 25%), and temperate collections were more homogeneous and less diverse than tropical collections.     

Watershed Deforestation and Down-Estuary Transformations Alter Sources,Transport, and Export of Suspended Particles in Panamanian Mangrove Estuaries

We identified eight Panamanian watersheds in which conversion from wet tropical forest to pastures differed and assessed the effects of degree of deforestation, and down-estuary transformations, on the suspended particulate matter discharged from the watersheds, entering, traversing through mangrove estuaries, and emerging into coastal waters. Deforested watersheds discharged larger concentrations of suspended particulate matter, with lower % C and N, higher mineral content, and heavier isotopic signatures into fresh reaches of estuaries. Down-estuary, sediment entrainment increased non-organic content of particulates, and watershed-derived imprints of deforestation on composition of particulate matter were mostly erased by within-estuary transformations. Isotopic signatures of C, N, and S in particulate matter demonstrated strong land-sea couplings, and indicated that the direction of the coupling was asymmetrical, with terrestrial and estuarine sources delivering particulate materials to coastal waters and sediments. Mangrove estuaries therefore both act as powerful modulators of human activities on land, while also exporting particulate materials to sea.     

Comparison Between Tidally Driven Groundwater Flow and Flushing of Animal Burrows in Tropical Mangrove Swamps

Tidal groundwater in a mangrove swamp can return to the mangrove creek by one of two mechanisms: (a) it can either flow through the swamp soil due to the water table difference between the creek and the groundwater in the swamp; or (b) it can flow via tidal flushing of animal burrows. This paper compares the magnitude of these two mechanisms for different regions of a mangrove swamp. Direct groundwater flow rates resulting from water stored in the sediment as a consequence of infiltration, especially during and after tidal inundation, were calculated for every square meter in the surface of a mangrove forest from piezometer data. Flow rates of water due to burrow flushing were determined based on published surveys, by estimating the burrow volume and the percentage of the burrow water that is flushed at each tidal inundation. Although direct groundwater flux was found to decrease further away from the creek compared to close to the creek, it was also found to have a similar range as burrow flushing flow. Specifically, direct groundwater flow ranged from 0.004 to 0.04 m³/m²/day, whilst burrow flushing flux ranged from 0.01 to 0.04 m³/m²/day.Considering the errors involved in the experiments and calculations, these ranges can be considered as being the same and neither of the two processes can be considered as negligible compared to the other. As a consequence, surveys of groundwater processes in mangrove areas, and more generally in swamp and tidal areas where animal burrows are present, will need to consider both mechanisms. Investigations of the influence over flushing mechanisms of different residence times of the water in burrows and in the sediment body would also be recommended in order to establish salt and nutrient budget in mangrove swamps.     

Radiocarbon Dating and Wood Density Chronologies of Mangrove Trees in Arid Western Australia

Mangrove trees tend to be larger and mangrove communities more diverse in tropical latitudes, particularly where there is high rainfall. Variation in the structure, growth and productivity of mangrove forests over climatic gradients suggests they are sensitive to variations in climate, but evidence of changes in the structure and growth of mangrove trees in response to climatic variation is scarce. Bomb-pulse radiocarbon dating provides accurate dates of recent wood formation and tree age of tropical and subtropical tree species. Here, we used radiocarbon techniques combined with X-ray densitometry to develop a wood density chronology for the mangrove Avicennia marina in the Exmouth Gulf, Western Australia (WA). We tested whether wood density chronologies of A. marina were sensitive to variation in the Pacific Decadal Oscillation Index, which reflects temperature fluctuations in the Pacific Ocean and is linked to the instrumental rainfall record in north WA. We also determined growth rates in mangrove trees from the Exmouth Gulf, WA. We found that seaward fringing A. marina trees (∼10 cm diameter) were 48±1 to 89±23 years old (mean ± 1σ) and that their growth rates ranged from 4.08±2.36 to 5.30±3.33 mm/yr (mean ±1σ). The wood density of our studied mangrove trees decreased with increases in the Pacific Decadal Oscillation Index. Future predicted drying of the region will likely lead to further reductions in wood density and their associated growth rates in mangrove forests in the region.     

Productivity and Connectivity in Tropical Riverscapes of Northern Australia: Ecological Insights for Management

Flow regimes are fundamental to sustaining ecological characteristics of rivers worldwide, including their associated floodplains. Recent advances in understanding tropical river–floodplain ecosystems suggest that a small set of basic ecological concepts underpins their biophysical characteristics, especially the high levels of productivity, biodiversity and natural resilience. The concepts relate to (1) river-specific flow patterns, (2) processes ‘fuelled’ by a complex of locally generated carbon and nutrients seasonally mixed with carbon and nutrients from floodplains and catchments, (3) seasonal movements of biota facilitated by flood regimes, (4) food webs and overall productivity sustained by hydrological connectivity, (5) fires in the wet/dry tropical floodplains and riparian zones being major consumers of carbon and a key factor in the subsequent redistribution of nutrients, and (6) river–floodplains having inherent resilience to natural variability but only limited resilience to artificial modifications. Understanding these concepts is particularly timely in anticipating the effects of impending development that may affect tropical river–floodplains at the global scale. Australia, a region encompassing some of the last relatively undisturbed tropical riverine landscapes in the world, provides a valuable case study for understanding the productivity, diversity and resilience of tropical river–floodplain systems. However, significant knowledge gaps remain. Despite substantial recent advances in understanding, present knowledge of these highly complex tropical rivers is insufficient to predict many ecological responses to either human-generated or climate-related changes. The major research challenges identified herein (for example, those related to food web structure, nutrient transfers, productivity, connectivity and resilience), if accomplished in the next decade, will offer substantial insights toward assessing and managing ecological changes associated with human alterations to rivers and their catchments.     

Carbon and Nitrogen Sequestration of Melaleuca Floodplain Wetlands in Tropical Australia

Ecosystems - Wetlands of Melaleuca spp. in Australia form large forests that are highly threatened by deforestation and degradation. In America, Melaleuca has invaded large areas of native wetlands...     

Cane Toads on Cowpats: Commercial Livestock Production Facilitates Toad Invasion in Tropical Australia

Habitat disturbance and the spread of invasive organisms are major threats to biodiversity, but the interactions between these two factors remain poorly understood in many systems. Grazing activities may facilitate the spread of invasive cane toads (Rhinella marina) through tropical Australia by providing year-round access to otherwise-seasonal resources. We quantified the cane toad’s use of cowpats (feces piles) in the field, and conducted experimental trials to assess the potential role of cowpats as sources of prey, water, and warmth for toads. Our field surveys show that cane toads are found on or near cowpats more often than expected by chance. Field-enclosure experiments show that cowpats facilitate toad feeding by providing access to dung beetles. Cowpats also offer moist surfaces that can reduce dehydration rates of toads and are warmer than other nearby substrates. Livestock grazing is the primary form of land use over vast areas of Australia, and pastoral activities may have contributed substantially to the cane toad’s successful invasion of that continent.     

Vitellogenesis in Austracris guttulosa (Walker) (Orthoptera: Acrididae) in Tropical Australia

In tropical northern Australia, vitellogenesis in Austracris guttulosa only occurs if two requirements are met: daylength exceeds 13 h and vegetation is green. During the dry season (April-September), daylength is short and vitellogenesis is inhibited regardless of vegetation condition. Daylength exeeds 13 h by mid October, and vitellogenesis begins by late October if there is green vegetation produced by recent rain. But it is often dry during October and vitellogenesis is delayed until it rains either in November or even December.     

Macrofaunal Loss and Microhabitat Destruction: The Impact of Trampling in a Temperate Mangrove Forest, NSW Australia

Trampling paths are a feature of estuarine habitats in southeastern Australia. An experimental investigation quantified the impact of trampling over a 3 year period on the microhabitat features and macrofaunal assemblages in a temperate mangrove forest in New South Wales, Australia. The magnitude of the impact varied between 0 and 25 passes (representing a visit by 25 people), which reduced the biomass of the Bostrychia–Caloglossa algal association by 50%. The frequency of trampling varied between one and four, simulating a single and seasonal visits by a group of people to the mangrove forest. The main impact of trampling was an alteration to the microhabitat structures of the forest floor including a decrease in the number and vertical height of pneumatophores, an increase in the proportion of bent and broken pneumatophores in trampled lanes and a decrease in the biomass of the epiphytic Bostrychia–Caloglossa algal association. The macrofauna most impacted by trampling were the gastropods most commonly associated with the pneumatophores and algal assemblages and included Ophicardelus spp. and Assiminea buccinoides. Those gastropods least impacted by trampling were commonly associated with the surface of the sediment or were known to move over the surface and included Bembicium auratum and Cassidula zonata. There was little impact on the main burrowing crab, Heloecius cordiformis. There were some indirect impacts of trampling with the recruitment of Ophicardelus spp. Assiminea buccinoides and Salinator solida reduced in the subsequent year after trampling had ceased. People can create walking paths in mangrove forests that have long term effects on the habitat. An appropriate management emphasis may lie in maintaining the structural features of the habitat.     

Determination of Key Environmental Factors Responsible for Distribution Patterns of Fiddler Crabs in a Tropical Mangrove Ecosystem

In tropical regions, different species of fiddler crabs coexist on the mangrove floor, which sometimes makes it difficult to define species-specific habitat by visual inspection. The aim of this study is to find key environmental parameters which affect the distribution of fiddler crabs and to determine the habitats in which each species was most abundant. Crabs were collected from 19 sites within the mudflats of Sepang-Lukut mangrove forest. Temperature, porewater salinity, organic matter, water content, carbon and nitrogen content, porosity, chlorophyll content, pH, redox potential, sediment texture and heavy metals were determined in each 1 m² quadrate. Pearson correlation indicated that all sediment properties except pH and redox potential were correlated with sediment grain size. Canonical correspondence analysis (CCA) indicated that Uca paradussumieri was negatively correlated with salinity and redox potential. Sand dwelling species, Uca perplexa and Uca annulipes, were highly dependent on the abundance of 250 μm and 150 μm grain size particles in the sediment. Canonical Discriminative Analysis (CDA) indicated that variation in sediment grain size best explained where each crab species was most abundant. Moreover, U. paradussumieri commonly occupies muddy substrates of low shore, while U. forcipata lives under the shade of mangrove trees. U. annulipes and U. perplexa with the high number of spoon tipped setae on their second maxiliped are specialized to feed on the sandy sediments. U. rosea and U. triangularis are more common on muddy sediment with high sediment density. In conclusion, sediment grain size that influences most sediment properties acts as a main factor responsible for sediment heterogeneity. In this paper, the correlation between fiddler crab species and environmental parameters, as well as the interaction between sediment characteristics, was explained in order to define the important environmental factors in fiddler crab distributions.     

`Blue Lagoons and Coconut Palms‘: The Creation of a Tropical Idyll in Australia

The Great Barrier Reef is regarded as an ‘Australian icon’. It is an internationally recognised World Heritage site managed for its ‘natural’ values. However, it is a location where visitors rarely enjoy Australian landscapes. This paper contrasts the sensuous engagement of past visitors with contemporary tourist experiences. Analysis of historic and contemporary visual and written materials suggests that tourist landscapes of the Reef have been transformed significantly during the 20th century. In particular, experiences of Reef islands characterised by Australian bush have been displaced by those of a generic Pacific location. The coconut palm, as a symbol of earthly paradise, has played an important role in realising both an imagined landscape and the physical transformation of tourist locations. Whereas the tourism industry is often regarded as responsible for the promulgation of such generic images, this study suggests that they are the product of a shared imagination to which both the tourism industry and tourists subscribe.     

Boredom, Time, and Modernity: An Example from Aboriginal Australia

In this article, I explore an anthropologically underresearched topic, boredom, utilizing ethnographic data from the Australian Aboriginal settlement of Yuendumu and situating that research in a comparative perspective. I examine the concept's genesis and meaning at Yuendumu using the social-constructivist approach to boredom as proposed in literature studies, sociology, and philosophy. That approach provides an account of how the emergence of boredom in 18th-century Europe is linked to processes of modernity. That perspective, however, has led to claims that boredom is a Western phenomenon and that its existence elsewhere is because of "Westernization." In this article, I argue against that perspective by linking instances of boredom at Yuendumu to perceptions of personhood and to conceptualizations of being in time—particularly socioculturally specific ways of perceiving time and postcolonial temporalities as generating the emergence of boredom. This boredom is a historically and socioculturally specific phenomenon, arising out of distinct sociocultural engagements with locally particular processes of modernity.     

Variation in Life History of Land-Locked Lacustrine and Riverine Populations of Galaxias maculatus (Jenyns 1842) in Western Australia

Galaxias maculatus is one of the most naturally widely distributed species in the world yet there is no published literature on the biology of Western Australian populations. Galaxias maculatus in the rivers of the south coast of Western Australia inhabit an environment that is variable and at times unpredictable. By examining trends in gonadal development, lengths at first maturity, growth and appearance and persistence of larvae of G. maculatus in two saline rivers (Jerdacuttup River and Oldfield River) and one freshwater lake (Moates Lake), the life histories of populations in contrasting environments were determined. The stable conditions that prevail in the low salinity, cool waters of Moates Lake have fostered an extremely protracted breeding and larval recruitment period (year round) in comparison to limited recruitment in the riverine environments which experience less predictable flow events and water quality (salinity). An upstream migration for breeding in the Jerdacuttup and Oldfield rivers is similar to land-locked G. maculatus in south-eastern Australia and is a reversal of the downstream migration in Moates Lake or populations with marine larval dispersal, i.e. diadromous populations. Differences in population demographics existed between these habitats, with smaller maximum sizes and significantly smaller sizes at maturation occurring in the rivers compared to the lake. In Moates Lake approximately 93, 6 and 1% were 0+, 1+ and 2+, respectively; while cohort progression in the rivers suggests that most fish do not live beyond their first year. As an adaptable opportunist, G. maculatus can modify its life history strategy at a local level to persist in varied and sometimes unpredictable environments.     

Spatial and Temporal Examination of Bivalve Communities in Several Estuaries of Southern California and Northern Baja California,MX

A combination of historical bivalve surveys spanning 30–50 years and contemporary sampling were used to document the changes in bivalve community structure over time at four southern California and one northern Baja California estuaries. While there are limitations to the interpretation of historic data, we observed generally similar trends of reduced total bivalve species richness, losses of relatively large and/or deeper-dwelling natives, and gains of relatively small, surface dwelling introduced species across the southern California estuaries, despite fairly distinct bivalve communities. A nearly 50-year absence of bivalves from two wetlands surveyed in a Baja California estuary continued. A combination of site history and current characteristics (e.g., location, depth) likely contributes to maintenance of distinct communities, and both episodic and gradual environmental changes likely contribute to within-estuary temporal shifts (or absences). We highlight future research needed to determine mechanisms underlying patterns so that we can better predict responses of bivalve communities to future scenarios, including climate change and restoration.     

Optimal Fire Regimes for Soil Carbon Storage in Tropical Savannas of Northern Australia

Modification of fire regimes in tropical savannas can have significant impacts on the global carbon (C) cycle, and therefore, on the climate system. In Australian tropical savannas, there has been recent, large-scale implementation of fire management that aims to decrease Kyoto-compliant non-CO₂ greenhouse gas emissions by reducing late dry season intense fires through strategic early dry season burning. However, there is no accounting for changes to soil C stocks resulting from changes to savanna fire management, although impacts on these pools may be considerable. We present a hypothesis that soil C storage is greatest under low intensity fires with an intermediate fire return interval. Simulations using the CENTURY Soil Organic Matter Model confirmed this hypothesis with greatest soil C storage under a fire regime of one low intensity fire every 5 years. Key areas of uncertainty for CENTURY model simulations include fine root dynamics, charcoal production and nitrogen (N) cycling, and better understanding of these processes could improve model predictions. Soil C stocks measured in the field after 5 years of annual, 3 year and unburned fire treatments were not significantly different (range 41–58 t ha⁻¹), but further CENTURY modelling suggests that changes in fire management will take up to 100 years to have a detectable impact (+4 t ha⁻¹) on soil C stocks. However, implementation of fire management that reduces fire frequency and intensity within the large area of intact savanna landscapes in northern Australia could result in emissions savings of 0.17 t CO₂-e ha⁻¹ y⁻¹, four times greater than reductions of non-CO₂ emissions.