When hundreds of animals die from bird flu, what happens next?

Hundreds of birds have died from avian flu in Australia. What happens next? In a matter of weeks, Australia has moved from its first mainland H5N1 bird flu detections to hundreds of confirmed cases and several wildlife mass mortality events from the disease. A series of mass deaths off South Australia, including more than 1,000 terns, marked an important shift in Australia's encounter with H5N1 bird flu. Federal authorities warn they won't be the last, although it is far too early to know what exactly comes next. But what we do know is mass mortality events are an increasing challenge for ecologists and wildlife managers. Warning: This story contains images that some readers may find distressing. Mass mortality events are sudden episodes in which unusually large numbers of animals die over a relatively short period, often near each other. Disease is one cause. But they can also be triggered by heatwaves, drought, fire, floods, harmful algal blooms, starvation and other environmental extremes. Besides bird flu mortalities, there has recently been a mysterious mass death of pelicans at the Menindee Lakes, NSW, and the disappearance of cuttlefish from the Upper Spencer Gulf, SA. Mass mortality events don't simply remove animals from ecosystems. They create masses of dead and decaying flesh (carrion), redistribute nutrients, attract scavengers and change interactions between species. Yet despite their scale, we know surprisingly little about these events and their longer-term ecological consequences. Mass mortality events are an increasing ecological challenge Mass mortality events are inherently difficult to study. We rarely know when or where one will occur, making it difficult to collect the "before" data ecologists normally rely on. Marine heatwaves, fires, droughts and other extreme events are already capable of killing wildlife on enormous scales. Mass death events are expected to become more frequent as weather and climate extremes intensify. H5N1 bird flu adds another potential driver of mass mortality to Australian ecosystems. By the time an event is detected, it may already be well underway, while events in remote areas may go unnoticed altogether. Much of the attention understandably focuses on the animals that have died: how many, which species and why. But the ecological effects of a mass mortality event don't end with those deaths. One carcass is very different from 1,000 A single carcass is a relatively small and temporary food resource. But hundreds or thousands of carcasses arriving over a short period in close proximity can represent a substantial and concentrated pulse of food into an ecosystem. The ecological impact of this sudden abundance of carrion will be different from the normal amount of dead animal flesh in an area. When an animal dies in nature, there is usually something willing to eat it. Flies and other invertebrates can find carcasses remarkably quickly. Birds and mammals often follow. Some are familiar scavengers: ravens and other corvids, raptors, dingoes, and foxes. But scavenging is far more widespread than we often assume. Many animals that we don't usually think of as scavengers will eat decaying flesh when the opportunity arises. In our own work in Australia, for example, we've recorded brushtail possums frequently scavenging animal carcasses. Other research has documented opportunistic scavenging across a huge diversity of vertebrates. Most of the time this is an important ecological process. Scavengers consume dead animals, helping ecosystems cope with sudden increases in carcasses while returning nutrients and energy to the food web (a complex system of food chains). But large amounts of carrion could shift where scavengers spend their time and how long they remain in an area. Some species may dominate access to a large number of carcasses, changing opportunities for other scavengers. It is also possible that scavenger communities are overwhelmed by too many carcasses and can't consume it all. The effects don't necessarily stop at scavengers. As carcasses decompose, large quantities of nutrients are released into the soil. This can change soil chemistry and, in turn, vegetation. Depending on the size of the mortality event, those effects can persist long after the dead animals themselves have gone. So mass mortality events don't simply remove animals from an ecosystem; they can temporarily restructure how energy and nutrients move through it. But when the animals die from an infectious disease, another layer is added; each carcass may also be a potential source of exposure. Why scavenging matters for bird flu During a disease outbreak scavenging becomes more complicated. With H5N1 bird flu, an infected animal can remain a source of infectious material after it has died. Experimental work has shown the virus can persist in carcasses with environmental conditions, such as temperature, influencing for how long. In one study, red foxes fed infected bird carcasses became infected and shed virus for several days. This is particularly relevant because scavengers are mobile. A fox feeding on a seabird carcass isn't restricted to the beach where it found it. Raptors can cover large areas. Gulls routinely move between beaches, wetlands, rubbish tips, urban areas and agricultural landscapes. Animals can also physically remove parts of carcasses and consume them somewhere else. This means a carcass can connect species and habitats that would otherwise have relatively little direct contact. However it is important to note that finding a virus in a scavenger does not automatically mean that animal is important in spreading it onwards. Exposure, infection and onward transmission are separate things, and we still have major gaps in our understanding of each. But this is why scavenging deserves more attention in Australia's H5N1 response. For many scavenger species, we have little idea how susceptible they are to H5N1 bird flu, how often they will encounter infected carrion, or what happens when they do. And we shouldn't assume that scavenging automatically makes a disease outbreak worse. In some circumstances, scavengers may actually help. Efficient scavenger communities can rapidly remove carrion, potentially reducing how long carcasses and their associated risks persist in the landscape. What can we do about mass deaths? Australia knew H5 bird flu was coming for years. While considerable work has gone into surveillance and response planning, there have also been warnings that major gaps remain in our ability to respond to its impacts on wildlife, particularly during mass mortality events. Surveillance of living animals remains critical, as does protecting poultry, identifying outbreaks and preparing responses for threatened species. There won't be one answer. Removing every dead animal from the Australian environment would be impossible and probably undesirable. Carrion is a natural part of ecosystems. But during very large mortality events, carcass dispersal or removal may sometimes be warranted when carrion exceeds what scavenger communities can efficiently process. This may be particularly important at breeding colonies, where infected carcasses can accumulate in a relatively small area, or where threatened species are at risk. We need evidence to know when intervention helps and when it doesn't. Monitoring how long carcasses persist, what consumes them and where that material goes should therefore sit alongside disease surveillance from the beginning. Dr Patrick Finnerty is a conservation ecologist at the University of Sydney and former participant in the ABC Top 5 science media residency.