Plant sex influences on riparian communities and ecosystems

Over the past several decades, we have increased our understanding of the influences of plant genetics on associated communities and ecosystem functions. These influences have been shown at both broad spatial scales and across many plant families, creating an active subdiscipline of ecology research focused on genes-to-ecosystems connections. One complex aspect of plant genetics is the distinction between males and females in dioecious plants. The genetic determinants of plant sex are poorly understood for most plants, but the influences of plant sex on morphological, physiological, and chemical plant traits are well-studied. We argue that these plant traits, controlled by plant sex, may have wide-reaching influences on both terrestrial and aquatic communities and ecosystem processes, particularly for riparian plants. Here we systematically review the influences of plant sex on plant traits, influences of plant traits on terrestrial community members, and how interactions between plant traits and terrestrial community members can influence terrestrial ecosystem functions in riparian forests. We then extend these influences into adjacent aquatic ecosystem functions and aquatic communities to explore how plant sex might influence linked terrestrial-aquatic systems as well as the physical structure of riparian systems. This review highlights data gaps in empirical studies exploring the direct influences of plant sex on communities and ecosystems but draws inference from community and ecosystem genetics. Overall, this review highlights how variation by plant sex has implications for climate change adaptations in riparian habitats, the evolution and range shifts of riparian species and the methods used for conserving and restoring riparian systems.

Ranking buffel: Comparative risk and mitigation costs of key environmental and socio-cultural threats in central Australia

Changed fire regimes and the introduction of rabbits, cats, foxes, and large exotic herbivores have driven widespread ecological catastrophe in Australian arid and semi-arid zones, which encompass over two-thirds of the continent. These threats have caused the highest global mammal extinction rates in the last 200 years, as well as significantly undermining social, economic, and cultural practices of Aboriginal peoples of this region. However, a new and potentially more serious threat is emerging. Buffel grass (Cenchrus ciliaris L.) is a globally significant invader now widespread across central Australia, but the threat this ecological transformer species poses to biodiversity, ecosystem function, and culture has received relatively little attention. Our analyses suggest threats from buffel grass in arid and semi-arid areas of Australia are at least equivalent in magnitude to those posed by invasive animals and possibly higher, because unlike these more recognized threats, buffel has yet to occupy its potential distribution. Buffel infestation also increases the intensity and frequency of wildfires that affect biodiversity, cultural pursuits, and productivity. We compare the logistical and financial challenges of creating and maintaining areas free of buffel for the protection of biodiversity and cultural values, with the creation and maintenance of refuges from introduced mammals or from large-scale fire in natural habitats. The scale and expense of projected buffel management costs highlight the urgent policy, research, and financing initiatives essential to safeguard threatened species, ecosystems, and cultural values of Aboriginal people in central Australia.

Returning a lost process by reintroducing a locally extinct digging marsupial

The eastern bettong (Bettongia gaimardi), a medium-sized digging marsupial, was reintroduced to a predator-free reserve after 100 years of absence from the Australian mainland. The bettong may have the potential to restore temperate woodlands degraded by a history of livestock grazing, by creating numerous small disturbances by digging. We investigated the digging capacity of the bettong and compared this to extant fauna, to answer the first key question of whether this species could be considered an ecosystem engineer, and ultimately if it has the capacity to restore lost ecological processes. We found that eastern bettongs were frequent diggers and, at a density of 0.3–0.4 animals ha⁻¹, accounted for over half the total foraging pits observed (55%), with echidnas (Tachyglossus aculeatus), birds and feral rabbits (Oryctolagus cuniculus) accounting for the rest. We estimated that the population of bettongs present dug 985 kg of soil per ha per year in our study area. Bettongs dug more where available phosphorus was higher, where there was greater basal area of Acacia spp. and where kangaroo grazing was less. There was no effect on digging of eucalypt stem density or volume of logs on the ground. While bettong digging activity was more frequent under trees, digging also occurred in open grassland, and bettongs were the only species observed to dig in scalds (areas where topsoil has eroded to the B Horizon). These results highlight the potential for bettongs to enhance soil processes in a way not demonstrated by the existing fauna (native birds and echidna), and introduced rabbit.

Risks to carbon dynamics in semi-arid woodlands of eastern Australia under current and future climates

Extreme disturbance events, such as wildfire and drought, have large impacts on carbon storage and sequestration of forests and woodlands globally. Here, we present a modelling approach that assesses the relative impact of disturbances on carbon storage and sequestration, and how this will alter under climate change. Our case study is semi-arid Australia where large areas of land are managed to offset over 122 million tonnes of anthropogenic carbon emissions over a 100-year period. These carbon offsets include mature vegetation that has been protected from clearing and regenerating vegetation on degraded agricultural land. We use a Bayesian Network model to combine multiple probabilistic models of the risk posed by fire, drought, grazing and recruitment failure to carbon dynamics. The model is parameterised from a review of relevant literature and additional quantitative analyses presented here. We found that the risk of vegetation becoming a net source of carbon due to a mortality event, or failing to realise maximum sequestration potential, through recruitment failure in regenerating vegetation, was primarily a function of rainfall in this semi-arid environment. However, the relative size of an emissions event varied across vegetation communities depending on plant attributes, specifically resprouting capacity. Modelled climate change effects were variable, depending on the climate change projection used. Under 'best-case' or 'most-likely' climate scenarios for 2050, similar or increased projections of mean annual precipitation, associated with a build-up of fuel, were expected to drive an increase in fire activity (a 40-160% increase), but a decrease in drought (a 20-35% decrease). Under a 'worst-case' climate scenario, fire activity was expected to decline (a 37% decrease), but drought conditions remain similar (a 5% decrease). These projected changes to the frequency of drought and fire increase the risk that vegetation used for carbon offsetting will fail to provide anticipated amounts of carbon abatement over their lifetime.

Rabbit biocontrol and landscape-scale recovery of threatened desert mammals

Funding for species conservation is insufficient to meet the current challenges facing global biodiversity, yet many programs use expensive single-species recovery actions and neglect broader management that addresses threatening processes. Arid Australia has the world's worst modern mammalian extinction record, largely attributable to competition from introduced herbivores, particularly European rabbits (Oryctolagus cuniculus) and predation by feral cats (Felis catus) and foxes (Vulpes vulpes). The biological control agent rabbit hemorrhagic disease virus (RHDV) was introduced to Australia in 1995 and resulted in dramatic, widespread rabbit suppression. We compared the area of occupancy and extent of occurrence of 4 extant species of small mammals before and after RHDV outbreak, relative to rainfall, sampling effort, and rabbit and predator populations. Despite low rainfall during the first 14 years after RHDV, 2 native rodents listed by the International Union for Conservation of Nature (IUCN), the dusky hopping-mouse (Notomys fuscus) and plains mouse (Pseudomys australis), increased their extent of occurrence by 241-365%. A threatened marsupial micropredator, the crest-tailed mulgara (Dasycercus cristicauda), underwent a 70-fold increase in extent of occurrence and a 20-fold increase in area of occupancy. Both bottom-up and top-down trophic effects were attributed to RHDV, namely decreased competition for food resources and declines in rabbit-dependent predators. Based on these sustained increases, these 3 previously threatened species now qualify for threat-category downgrading on the IUCN Red List. These recoveries are on a scale rarely documented in mammals and give impetus to programs aimed at targeted use of RHDV in Australia, rather than simply employing top-down threat-based management of arid ecosystems. Conservation programs that take big-picture approaches to addressing threatening processes over large spatial scales should be prioritized to maximize return from scarce conservation funding. Further, these should be coupled with long-term ecological monitoring, a critical tool in detecting and understanding complex ecosystem change.

Barking up the wrong tree? Are livestock or rabbits the greater threat to rangeland biodiversity in southern Australia?

Increasing provision of permanent water points has put most Australian pastoral rangelands within grazing distance of sheep, cattle, kangaroos and large feral herbivores, and there is concern that grazing-sensitive native plants will be lost as a result. Proposals have been developed to conserve plant biodiversity by permanently excluding livestock from any areas that are remote from permanent water, or to buy back pastoral properties and remove existing water points to create large reserves. There is, however, little evidence that water-remote areas provide refuge for grazing-sensitive plants, nor consistent evidence of plant biodiversity loss along gradients of increasing livestock grazing pressure in proximity to water. One of the reasons why that evidence might be lacking is that most livestock studies have not considered the grazing impact of sympatric European rabbits, the most widespread and abundant wild herbivore in southern Australia. Numerous studies have shown that rabbit grazing has a major impact on rangeland vegetation and can prevent regeneration at densities which may be too low to be thought important. Plant biodiversity gradients are readily discernible along gradients of rabbit density in livestock-free reserves. Rabbits are therefore likely to be a significant confounding factor when assessing livestock impacts, or possibly the primary cause of observed patterns of plant diversity. If so, attempts to preserve plant biodiversity by removing livestock are destined to fail in rabbit-grazed areas. Public funds for biodiversity conservation in the pastoral zone might be better spent on co-investment with pastoralists for rabbit control on conservatively stocked properties, rather than on restricting pastoral use of land in livestock-free, rabbit-infested reserves.

Rangeland Condition Monitoring: A New Approach Using Cross-Fence Comparisons of Remotely Sensed Vegetation

A need exists in arid rangelands for effective monitoring of the impacts of grazing management on vegetation cover. Monitoring methods which utilize remotely-sensed imagery may have comprehensive spatial and temporal sampling, but do not necessarily control for spatial variation of natural variables, such as landsystem, vegetation type, soil type and rainfall. We use the inverse of the red band from Landsat TM satellite imagery to determine levels of vegetation cover in a 22,672km² area of arid rangeland in central South Australia. We interpret this wealth of data using a cross-fence comparison methodology, allowing us to rank paddocks (fields) in the study region according to effectiveness of grazing management. The cross-fence comparison methodology generates and solves simultaneous equations of the relationship between each paddock and all other paddocks, derived from pairs of cross-fence sample points. We compare this ranking from two image dates separated by six years, during which management changes are known to have taken place. Changes in paddock rank resulting from the cross-fence comparison method show strong correspondence to those predicted by grazing management in this region, with a significant difference between the two common management types; a change from full stocking rate to light 20% stocking regime (Major Stocking Reduction) and maintenance of full 100% stocking regime (Full Stocking Maintained) (P = 0.00000132). While no paddocks had a known increase in stocking rate during the study period, many had a reduction or complete removal in stock numbers, and many also experienced removals of pest species, such as rabbits, and other ecosystem restoration activities. These paddocks generally showed an improvement in rank compared to paddocks where the stocking regime remained relatively unchanged. For the first time, this method allows us to rank non-adjacent paddocks in a rangeland region relative to each other, while controlling for natural spatio-temporal variables such as rainfall, soil type, and vegetation community distributions, due to the nature of the cross-fence experimental design, and the spatially comprehensive data available in satellite imagery. This method provides a potential tool to aid land managers in decision making processes, particularly with regard to stocking rates.