It’s been suggested that the birds were killed to protect lambs. Tragically, not only was this illegal cull unnecessary – evidence suggests that eagles do not often kill livestock – but it could also have ecological consequences right across Australia.
There are two main categories of wedge-tailed eagles, based on their age class: sedentary breeding adults, which stay in a home range with nest sites; and highly nomadic juvenile birds that can cover huge distances. There are usually fewer adult birds in one place, because they are territorial.
The very high number of birds affected make it likely that they were largely juveniles. There is currently no accurate data on how many wedge-tailed eagles are in Australia, but this single culling event could have serious effects on future generations’ breeding capacity.
Sites of persecution can have impacts to eagle populations if they become “ecological sinks”. These are places that draw birds in from a wide area, perhaps because of an unnaturally abundant food source, and then result in birds dying. If these ongoing “mortality black holes” cause hundreds of birds to die in relatively short periods of time, this can start impacting the population.
Do eagles kill lambs?
The wedge-tailed eagle is a powerful predator that kills a variety of mammals. Anecdotal observations by landowners describe birds attacking live lambs and even half-grown sheep. There are also cases in the literature of them working in tandem to hunt larger prey such as kangaroos – behaviour that has been widely documented for large eagle species.
However, evidence gathered during extensive research in Australia has shown that in most cases, eagles seen feeding on lamb or sheep carcasses are “cleaning up” after other predators like foxes and crows, which were actually the direct cause of death.
There are no documented cases of wedge-tailed eagles causing significant economic impacts to the sheep industry. But even if they did, there are other options besides culling. Carcasses placed near livestock would provide easier alternative food sources, for example. Shepherds can effectively guard flocks and protect lambs. Finally, given that wedge-tailed eagles are protected, it may be appropriate for the government to pay compensation for livestock losses.
It must also be emphasised that eagles prey on a range of other species that are considered to be agricultural pests, such as overabundant native kangaroos, cockatoos, and feral species like rabbits and foxes.
Some eagles live, and some die. Such is life on this amazing, arid continent. Death itself is a normal ecological phenomenon, but unnatural deaths on such a large scale can have disastrous consequences for long-lived raptors like the wedge-tailed eagle. We must as a community respect the critical role that predators play in the landscape.
Coral species have a varied sex life. The majority of species are simultaneously male and female (hermaphrodites) and typically pack both eggs and sperm (gametes) into tight, buoyant bundles that are released after dark with remarkable synchronisation. The bundles float to the surface and open, allowing the eggs meet compatible sperm.
Less commonly, some coral species have separate sexes, and a few species even release asexually produced clones of themselves. For all species with sexual reproduction fertilised eggs develop into mobile larvae that settle on the sea floor and become polyps: the beginning of a new coral colony on the reef.
Mass spawnings are spectacular events, in which dozens of coral species release their gametes at specific times. Sometimes more than 100 species spawn on a single night, or over a few successive nights.
This iconic celebration of sex on the reef was first described in the central Great Barrier Reef in 1984 by a group of early-career scientists. The discovery earned them a prestigious Australian Museum Eureka Award for Environmental Research in 1992.
The precise timing of this seasonal phenomenon is linked to seawater temperature, lunar phases, and other factors such as the daily cycle of light and dark. Mass coral spawning is the dominant reproductive mode for corals on the Great Barrier Reef, and has also been recorded on reefs around the world.
The release of egg and sperm bundles is the culmination of many months of development. In years when the full moon falls early in October and November, many colonies are not quite ready and delay spawning for another lunar cycle. That’s why this year will see some action in November and another mass spawning event after the December full moon.
An important date in the scientific calendar
Spawning can be replicated in aquarium settings, which provide unique opportunities to researchers. All three of us work in the Australian Institute of Marine Science’s (AIMS) unique Sea Simulator, where large numbers of coral larvae are produced for scientific experiments.
Scientists from the Institute and around the world work through the spawning nights to collect gamete bundles, separate sperm and fertilise the eggs, then rear millimeter-long larvae and juveniles. Many experiments continue for days, weeks and even years to address critical knowledge gaps in how corals respond to and recover from stress.
New tools for coral reef management
The extensive coral death in the northern Great Barrier Reef following back-to-back bleaching events in 2016 and 2017 highlights the impacts of rapidly changing ocean conditions. AIMS scientists focus on developing ways to help coral adapt and restore damaged reefs.
Corals reefs are at a crossroads, but there is still hope. Experiments during this year’s spawning season will test whether surviving corals from recent bleaching events are naturally adapted to warmer reef temperatures, and if they produce more heat-tolerant young.
This knowledge underpins the development of active reef management tools such as assisted gene flow.
Assisted gene flow involves moving heat-tolerant corals (or their young) to reefs that are warming. This technique proposes to improve the overall heat tolerance of local coral populations, to help the buffer the reef against future bleaching events caused by warmer than normal water temperatures.
More local threats to corals include poor water quality and pollution from coastal development. The early stages of a coral’s life are very sensitive to exposure to pesticides, oil spills and sediments from dredging.
Carefully controlled experiments with aquarium-reared coral larvae provide insights into the role of these local pressures on the rate of recovery and replenishment following large-scale disturbances.
The present reality for coral reefs is one of increasing strain from climate change, cyclones, crown-of-thorns starfish predation, and declining water quality. The ability of coral reef ecosystems to recover from these challenges relies on the success of mass coral spawning both on the reef and advances in the laboratory to generate new options to enhance reef resilience.
Exploring reef restoration and adaptation needs to go hand-in-hand with ongoing (and increasing) efforts in conventional management, such as climate change mitigation, regional management of water quality and control of crown-of-thorns starfish.
We now know that greenhouse gases are rising faster than at any time since the demise of dinosaurs, and possibly even earlier. According to research published in Nature Geoscience this week, carbon dioxide (CO₂) is being added to the atmosphere at least ten times faster than during a major warming event about 50 million years ago.
In the past, rapid increases in greenhouse gases have been associated with mass extinctions. It is therefore important to understand how unusual the current rate of atmospheric CO₂ increase is with respect to past climate variability.
Into the ice ages
There is no doubt that atmospheric CO₂ concentrations and global temperatures have changed in the past.
During this time, the Earth oscillated between cold ice ages and warm “interglacials”. To move from an ice age to an interglacial, you need to increase CO₂ by roughly 100 ppm. This increase repeatedly melted several kilometre-thick ice sheets that covered the locations of modern cities like Toronto, Boston, Chicago or Montreal.
The most prominent of these events was the Palaeocene Eocene Thermal Maximum (PETM). This event resulted in one of the largest known extinctions of life forms in the deep ocean. Atmospheric temperatures increased by 5-8C within a few thousand years.
The new research, led by Professor Richard Zeebe of the University of Hawaii, analysed ocean sediments to quantify the lag between warming and changes in the carbon cycle during the PETM.
Although climate archives become less certain the further we look back, the authors found that the carbon release must have been below 1.1 billion tonnes of carbon per year. That is about one-tenth of the rate of today’s carbon emissions from human activities such as burning fossil fuels.
What happens when the brakes are off?
Although the PETM resulted in one of the largest known deep sea extinctions, it is a small event when compared to the five major extinctions in the past.
The Permian-Triassic Boundary extinction, nicknamed “The Great Dying”, wiped out 90% of marine species and 70% of land vertebrate families 250 million years ago. Like its four brothers, this extinction event happened a very long time ago. Climate archives going that far back lack the resolution needed to reliably reconstruct rates of change.
There is, however, evidence for extensive volcanic activity during the Great Dying, which would have led to a release of CO₂ as well as the potential release of methane along continental margins. Ocean acidification caused by high atmospheric CO₂ concentrations and acid rain have been put forward as potential killer mechanisms.
These past warming events occurred without human influence. They point to the existence of positive feedbacks within the climate system that have the power to escalate warming dramatically. The thresholds to trigger these feedbacks are hard to predict and their impacts are hard to quantify.
Some examples of feedbacks include the melting of permafrost, the release of methane hydrates from ocean sediments, changes in the ocean carbon cycle, and changes in peatlands and wetlands. All of these processes have the potential to quickly add more greenhouse gases to the atmosphere.
Given that these feedbacks were strong enough in the past to wipe out a considerable proportion of life forms on Earth, there is no reason to believe that they won’t be strong enough in the near future, if triggered by sufficiently rapid warming.
Today’s rate of change in atmospheric CO₂ is unprecedented in climate archives. It outpaces the carbon release during the most extreme abrupt warming events in the past 66 million years by at least an order of magnitude.
We are therefore unable to rely upon past records to predict if and how our ecosystems will be able to adapt. We know, however, that mass extinctions have occurred in the past and that these extinctions, at least in the case of the PETM, were triggered by much smaller rates of change.
Katrin and Kaitlin will be on hand for an Author Q&A between 2 pm and 3 pm AEDT on Thursday March 24. Post your questions in the comment section below.