Ecological dynamics of emerging bat virus spillover

Viruses that originate in bats may be the most notorious emerging zoonoses that spill over from wildlife into domestic animals and humans. Understanding how these infections filter through ecological systems to cause disease in humans is of profound importance to public health. Transmission of viruses from bats to humans requires a hierarchy of enabling conditions that connect the distribution of reservoir hosts, viral infection within these hosts, and exposure and susceptibility of recipient hosts. For many emerging bat viruses, spillover also requires viral shedding from bats, and survival of the virus in the environment. Focusing on Hendra virus, but also addressing Nipah virus, Ebola virus, Marburg virus and coronaviruses, we delineate this cross-species spillover dynamic from the within-host processes that drive virus excretion to land-use changes that increase interaction among species. We describe how land-use changes may affect co-occurrence and contact between bats and recipient hosts. Two hypotheses may explain temporal and spatial pulses of virus shedding in bat populations: episodic shedding from persistently infected bats or transient epidemics that occur as virus is transmitted among bat populations. Management of livestock also may affect the probability of exposure and disease. Interventions to decrease the probability of virus spillover can be implemented at multiple levels from targeting the reservoir host to managing recipient host exposure and susceptibility.

Ecological dynamics of emerging bat virus spillover

Viruses that originate in bats may be the most notorious emerging zoonoses that spill over from wildlife into domestic animals and humans. Understanding how these infections filter through ecological systems to cause disease in humans is of profound importance to public health. Transmission of viruses from bats to humans requires a hierarchy of enabling conditions that connect the distribution of reservoir hosts, viral infection within these hosts, and exposure and susceptibility of recipient hosts. For many emerging bat viruses, spillover also requires viral shedding from bats, and survival of the virus in the environment. Focusing on Hendra virus, but also addressing Nipah virus, Ebola virus, Marburg virus and coronaviruses, we delineate this cross-species spillover dynamic from the within-host processes that drive virus excretion to land-use changes that increase interaction among species. We describe how land-use changes may affect co-occurrence and contact between bats and recipient hosts. Two hypotheses may explain temporal and spatial pulses of virus shedding in bat populations: episodic shedding from persistently infected bats or transient epidemics that occur as virus is transmitted among bat populations. Management of livestock also may affect the probability of exposure and disease. Interventions to decrease the probability of virus spillover can be implemented at multiple levels from targeting the reservoir host to managing recipient host exposure and susceptibility.

Recruitment dynamics of invasive species in rainforest habitats following Cyclone Larry

In tropical forests, natural disturbance creates opportunities for species to claim previously utilized space and resources and is considered an important mechanism in the maintenance of species diversity. However, ecologists have long recognized that disturbance also promotes exotic plant invasions. Cyclones cause extensive defoliation, loss of major branches and multiple tree falls, resulting in a significantly more open canopy and increased light and heat levels in the understorey. The widespread and massive disturbance caused by cyclones provides ideal conditions for rapid recruitment and spread of invasive species. The ecological roles of invasive species in rainforest habitats following such a severe disturbance are poorly understood. Severe category 4 Cyclone Larry crossed the North Queensland coast in March 2006 causing massive disturbance to rainforest habitats from Tully to Cairns and west to the Atherton Tablelands. We established 10 plots in an area extensively damaged by this cyclone near El Arish in North Queensland. On each plot nine 2 × 2 m quadrats were established with three quadrats per plot in each of the following treatments: (i) complete debris removal down to the soil layer, (ii) removal of coarse woody debris only, and (iii) uncleared. We monitored recruitment, growth and mortality of all native and invasive species in the 90 quadrats every 3 months since the cyclone. Here we present the recruitment dynamics of invasive species across the study area in relation to the level of disturbance, the type of quadrat treatment, and the diversity and abundance of the native recruiting flora over the first 12 months post-cyclone. Our results suggest that invasive species will mostly comprise a transient component of the flora in the early stages of the successional response. However, some species may have longer-term effects on the successional trajectory of the rainforest and future forest composition and structure.

Population dynamics of the European rabbit (Oryctolagus cuniculus) in north eastern Australia: simulated responses to control

Wild European rabbits are a serious problem to agriculture in Australia, with an estimated annual cost of A$ 113 million. Biological control agents (myxomatosis and rabbit haemorrhagic disease virus) have caused large and sustained declines in rabbit populations throughout Australia. A simulation model incorporates these diseases as well as warren destruction as methods of controlling rabbit populations in Queensland, north eastern Australia. These diseases reduced populations by 90-99% and the combination of these and warren destruction led to 100% control in simulations at six sites across southern Queensland. Increasing monthly pasture growth by 15% had little effect on simulated populations whereas a 15% decrease reduced populations by 0-50%. An increase in temperature of 2.5 °C would lead to a 15-60% decrease in populations. These effects suggest that climate change will lead to a decrease in the population of rabbits in Queensland and a retraction in the northern limit of their distribution in Australia.

Stochastic demography and population dynamics in the red kangaroo Macropus rufus

1. Many organisms inhabit strongly fluctuating environments but their demography and population dynamics are often analysed using deterministic models and elasticity analysis, where elasticity is defined as the proportional change in population growth rate caused by a proportional change in a vital rate. Deterministic analyses may not necessarily be informative because large variation in a vital rate with a small deterministic elasticity may affect the population growth rate more than a small change in a less variable vital rate having high deterministic elasticity. 2. We analyse a stochastic environment model of the red kangaroo (Macropus rufus), a species inhabiting an environment characterized by unpredictable and highly variable rainfall, and calculate the elasticity of the stochastic growth rate with respect to the mean and variability in vital rates. 3. Juvenile survival is the most variable vital rate but a proportional change in the mean adult survival rate has a much stronger effect on the stochastic growth rate. 4. Even if changes in average rainfall have a larger impact on population growth rate, increased variability in rainfall may still be important also in long-lived species. The elasticity with respect to the standard deviation of rainfall is comparable to the mean elasticities of all vital rates but the survival in age class 3 because increased variation in rainfall affects both the mean and variability of vital rates. 5. Red kangaroos are harvested and, under the current rainfall pattern, an annual harvest fraction of c. 20% would yield a stochastic growth rate about unity. However, if average rainfall drops by more than c. 10%, any level of harvesting may be unsustainable, emphasizing the need for integrating climate change predictions in population management and increase our understanding of how environmental stochasticity translates into population growth rate.

The spatiotemporal dynamics of Tribolium castaneum (Herbst): adult flight and gene flow.

Tribolium castaneum (Herbst) has been used as a model organism to develop and test important ecological and evolutionary concepts and is also a major pest of grain and grain products globally. This beetle species is assumed to be a good colonizer of grain storages through anthropogenic movement of grain, and active dispersal by flight is considered unlikely. Studies using T. castaneum have therefore used confined or walking insects. We combine an ecological study of dispersal with an analysis of gene flow using microsatellites to investigate the spatiotemporal dynamics and adult flight of T. castaneum in an ecological landscape in eastern Australia. Flying beetles were caught in traps at grain storages and in fields at least 1 km from the nearest stored grain at regular intervals for an entire year. Significantly more beetles were trapped at storages than in fields, and almost no beetles were caught in native vegetation reserves many kilometres from the nearest stored grain. Genetic differentiation between beetles caught at storages and in fields was low, indicating that although T. castaneum is predominantly aggregated around grain storages, active dispersal takes place to the extent that significant gene flow occurs between them, mitigating founder effects and genetic drift. By combining ecological and molecular techniques, we reveal much higher levels of active dispersal through adult flight in T. castaneum than previously thought. We conclude that the implications of adult flight to previous and future studies on this model organism warrant consideration.

Soil seed bank dynamics in response to an extreme flood event in a riparian habitat

A significantly increased water regime can lead to inundation of rivers, creeks and surrounding floodplains- and thus impact on the temporal dynamics of both the extant vegetation and the dormant, but viable soil-seed bank of riparian corridors. The study documented changes in the soil seed-bank along riparian corridors before and after a major flood event in January 2011 in southeast Queensland, Australia. The study site was a major river (the Mooleyember creek) near Roma, Central Queensland impacted by the extreme flood event and where baseline ecological data on riparian seed-bank populations have previously been collected in 2007, 2008 and 2009. After the major flood event, we collected further soil samples from the same locations in spring/summer (November–December 2011) and in early autumn (March 2012). Thereafter, the soils were exposed to adequate warmth and moisture under glasshouse conditions, and emerged seedlings identified taxonomically. Flooding increased seed-bank abundance but decreased its species richness and diversity. However, flood impact was less than that of yearly effect but greater than that of seasonal variation. Seeds of trees and shrubs were few in the soil, and were negatively affected by the flood; those of herbaceous and graminoids were numerous and proliferate after the flood. Seed-banks of weedy and/or exotic species were no more affected by the flood than those of native and/or non-invasive species. Overall, the studied riparian zone showed evidence of a quick recovery of its seed-bank over time, and can be considered to be resilient to an extreme flood event.

Spawning stock dynamics of two penaeid prawns, Metapenaeus bennettae and Penaeus esculentus, in Moreton Bay, Queensland, Australia

Spawning stock dynamics of 2 commercially important penaeid prawns, Metapenaeus bennettae and Penaeus esculentus, from 9 stations in Moreton Bay (27°15'S, 153°15'E), southeast Queensland, Australia, were examined. An egg production index (EPI), based on the relative abundance, proportion that were mature or ripe, and size of adult females, was used as a measure of egg production in the 2 populations. Egg production by M. bennettae was 20 to 30 higher than that by P. esculentus, extended over 7 to 8 mo each year and peaked from February to March (late summer to early autumn). Monthly patterns in egg production by M. bennettae varied between years. In contrast, P. esculentus produced most of its eggs in a single, clearly defined peak in October (spring), although production continued to March (early autumn) each year. The seasonal onset and subsequent decline in maturation in P. esculentus were rapid. Egg production by M. bennettae was several times higher at the 5 northern stations than at the 4 southern stations and negatively correlated with salinity during the main spawning period. Egg production by P. esculentus was less varied among stations and positively correlated with depth. P. esculentus appeared more likely than M. bennettae to experience recruitment overfishing because (1) the peak spawning period for P. esculentus was dependent on relatively few adult females spawning over a short period, and (2) the selectivity of trawl nets used in the bay was much higher for P. esculentus spawners than for those of M. bennettae. Compared with more northern populations, P. esculentus in Moreton Bay matured at a larger size, had lower incidences of insemination and mature or ripe females, and had a shorter spawning period. These results suggest the likelihood of recruitment overfishing in P. esculentus increases with increasing latitude.

Dynamics of plant populations in Heteropogon contortus(black speargrass) pastures on a granite landscape in southern Queensland. 2. Seed production and soil seed banks of H. contortus

Seed production and soil seed hanks of H. contortus were studied in a subset of treatments within an extensive grazing study conducted in H. contortus pasture in southern Queensland between 1990 and 1996. Seed production of H. contortus in autumn ranged from 260 to 1800 seeds/m2 with much of this variation due to differences in rainfall between years. Seed production was generally higher in the silver-leaved ironbark than in the narrow-leaved ironbark land class and was also influenced by a consistent stocking rate x pasture type interaction. Inflorescence density was the main factor contributing to the variable seed production and was related to the rainfall received during February. The number of seeds per inflorescence was unaffected by seasonal rainfall, landscape position, stocking rate or legume oversowing. Seed viability was related to the rainfall received during March. Soil seed banks in spring varied from 130 to 520 seeds/m2 between 1990 and 1995 with generally more seed present in the silver-leaved ironbark than in the narrow-leaved ironbark land class. There were poor relationships between viable seed production and the size of the soil seed bank, and between the size of the soil seed bank and seedling recruitment. This study indicates that H. contortus has the potential to produce relatively large amounts of seed and showed that the seasonal pattern of rainfall plays a major role in achieving this potential

Dynamics of plant populations in Heteropogon contortus(black speargrass) pastures on a granite landscape in southern Queensland. 1. Dynamics of H. contortus populations

The dynamics of Heteropogon contortus (black speargrass) populations were measured in a subset of treatments contained within an extensive grazing study conducted between 1990 and 1996 in H. contortus pasture in southern Queensland. This subset included 2 landscape positions and 3 stocking rates in both native pasture and legume-oversown native pasture. Severe drought conditions throughout much of the study necessitated ongoing adjustments to the original stocking rates and, as a result, drought was the major influence on the dynamics of H. contortus populations. Plant density and basal area in the silver-leaved ironbark landscape were consistently higher than those in the narrow-leaved ironbark landscape. There was limited evidence of any impact by either light or moderate stocking rate but there was evidence of an impact at the heaviest stocking rate. There was minimal impact of legume oversowing. Relatively large fluctuations in plant density occurred during this study resulting from the death of existing plants, due mainly to drought, and seedling recruitment. Similarly, there were relatively large fluctuations in basal area caused mainly by changes in plant size. Rates for turnover of plant numbers were relatively high whereas plant turnover rates of basal areas were relatively low. Regular seedling recruitment appeared necessary to ensure the persistence of this species. Despite the high turnover, populations were maintained at reasonable levels indicating the overall resilience of H. contortus.

Dynamics of plant populations in Heteropogon contortus (black speargrass) pastures on a granite landscape in southern Queensland. 3. Dynamics of Aristida spp. populations

The dynamics of the unpalatable Aristida spp. (wiregrasses) were measured in a subset of treatments contained within an extensive grazing study conducted between 1990 and 1996 in H. contortus pasture in southern Queensland. This paper reports the results from these treatments which included 2 land classes (silver-leaved and narrowleaved ironbark), 3 stocking rates (0.3, 0.6 and 0.9 beasts/ha) in both native pasture and legumeoversown native pasture, all in the absence of fire. Changes in plant density and basal area of Aristida spp. reflected differences in both the survival and size of existing plants together with a large seedling recruitment in 1991. Two different taxa of Aristida spp. were distinguished; however, there were no clear differences in the response of these 2 taxa to the treatments. Grazing had the greatest impact on population dynamics through reducing basal area as stocking rate increased. Neither landscape position nor legume oversowing had a major impact on Aristida spp. The results suggest that populations of Aristida spp. will be highest under light grazing and that seedling recruitment may be episodic

Dynamics of plant populations in Heteropogon contortus (black speargrass) pastures on a granite landscape in southern Queensland. 4. The effects of burning on H.contortus and Aristida spp. populations

This paper reports an experiment undertaken to examine the impact of burning in spring together with reduced grazing pressure on the dynamics of H. contortus and Aristida spp. In H. contortus pasture in south-eastern Queensland. The overall results indicate that spring burning in combination with reduced grazing pressure had no marked effect on the density of either grass species. This was attributed to 2 factors. Firstly, extreme drought conditions restricted any increase in H. contortus seedling establishment despite the presence of an adequate soil seed bank prior to summer; and secondly, some differences occurred in the response to fire of the diverse taxonomic groupings in the species of Aristida spp. present at the study site. This study concluded that it is necessary to identify appropriate taxonomic units within the Aristida genus and that, where appropriate, burning in spring to manage pasture composition should be conducted under favorable rainfall conditions using seasonal forecasting indicators such as the Southern Oscillation Index

Henipaviruses: Emerging Paramyxoviruses Associated with Fruit Bats

Two related, novel, zoonotic paramyxoviruses have been described recently. Hendra virus was first reported in horses and thence humans in Australia in 1994; Nipah virus was first reported in pigs and thence humans in Malaysia in 1998. Human cases of Nipah virus infection, apparently unassociated with infection in livestock, have been reported in Bangladesh since 2001. Species of fruit bats (genus Pteropus ) have been identified as natural hosts of both agents. Anthropogenic changes (habitat loss, hunting) that have impacted the population dynamics of Pteropus species across much of their range are hypothesised to have facilitated emergence. Current strategies for the management of henipaviruses are directed at minimising contact with the natural hosts, monitoring identified intermediate hosts, improving biosecurity on farms, and better disease recognition and diagnosis. Investigation of the emergence and ecology of henipaviruses warrants a broad, cross-disciplinary ecosystem health approach that recognises the critical linkages between human activity, ecological change, and livestock and human health.

Perennial grassland dynamics on fertile plains: Is coexistence mediated by disturbance?

The response of grasslands to disturbance varies with the nature of the disturbance and the productivity of the landscape. In highly productive grasslands, competitive exclusion often results in decreased species richness and grazing may allow more species to coexist. Once widespread, grasslands dominated by Dichanthium sericeum (Queensland bluegrass) and Astrebla spp. (Mitchell grass) occur on fertile plains but have been reduced in extent by cultivation. We tested the effects of exclusion of livestock grazing on these grasslands by comparing the floristic composition of sites in a nature reserve with an adjacent stock reserve. In addition, sites that had been cultivated within the nature reserve were compared with those where grazing but no cultivation had occurred. To partition the effects of temporal variation from spatial variation we sampled sites in three different years (1998, 2002 and 2004). Some 194 taxa were recorded at the nature reserve and surrounding stock routes. Sampling time, the occurrence of past cultivation and livestock grazing all influenced species composition. Species richness varied greatly between sampling periods relating to highly variable rainfall and water availability on heavy clay soils. Native species richness was significantly lower at previously cultivated sites (13-22 years after cultivation), but was not significantly influenced by grazing exclusion. After 8 years it appears that reintroducing disturbance in the form of livestock grazing is not necessary to maintain plant species richness in the reserve. The highly variable climate (e.g. droughts) probably plays an important role in the coexistence of species by negating competitive exclusion and allowing interstitial species to persist.

Habitat dynamics in the bed sediments of an intermittent upland stream

The effects of the hydrological regime on temporal changes to physical characteristics of substratum habitat, sediment texture of surface sediments (<10 cm), were investigated in a sub-tropical headwater stream over four years. Surface discharge was measured together with vertical hydraulic gradient and groundwater depth in order to explore features of sediment habitat that extend beyond the streambed surface. Whilst the typical discharge pattern was one of intermittent base flows and infrequent flow events associated with monsoonal rain patterns, the study period also encompassed a drought and a one-in-a-hundred-year flood. Rainfall and discharge did not necessarily reflect the actual conditions in the stream. Although surface waters were persistent long after discharge ceased, the streambed was completely dry on several occasions. Shallow groundwater was present at variable depths throughout the study period, being absent only at the height of the drought. The streambed sediments were mainly gravels, sand and clay. Finer sediment fractions showed a marked change in grain size over time, although bedload movement was limited to a single high discharge event. In response to a low discharge regimen (drought), sediments characteristically showed non-normal distributions and were dominated by finer materials. A high-energy discharge event produced a coarsening of sands and a diminished clay fraction in the streambed. Particulate organic matter from sediments showed trends of build-up and decline with the high and low discharge regimes, respectively. Within the surface sediment intersticies three potential categories of invertebrate habitat were recognised, each with dynamic spatial and temporal boundaries.