Seabird declines threatening coastal nutrient supply

The loss of bird poo from global coastlines will make waves in habitats around the world. 

Seabirds are on the front line of the Anthropocene as they face a myriad of threats, ranging from pollution to habitat loss and food shortages. As bird populations decline, the flow of nutrients such as nitrogen and phosphorous from the sea to land will be severely impacted. 

With these elements vital to the growth and metabolism of life, this could have unexpected effects beyond the birds themselves. 

Dr Alex Bond is the Principal Curator and Curator in Charge of Birds at the Museum and co-authored this paper alongside Megan Grant and Jennifer Lavers from the University of Tasmania. He says, 'We have lost more than half of all seabirds in the last 50 years in terms of numbers, which equates to a significant nutrient impact.  

'They have an incredible influence on terrestrial ecosystems that we don't really think about, and it's only when you see really obvious cases, such as plastics on Lord Howe Island, where it hits home.  

'As a result of this, we could potentially start to see some knock-on effects on invertebrate and plant communities on some of these islands.' 

The paper was published in the Journal of Animal Ecology.

Why is bird poo important? 

Though it may not be the topic of polite conversation, bird poo is vitally important to coastal and island habitats. In areas where nutrients may otherwise be in short supply, the excrement can provide a significant amount. 

The impact of these nutrients can transform habitats as the nutrients are moved from sea to land. For instance, the volcanic island of Surtsey emerged off the Icelandic coast in 1963. The barren rock was colonised within a few years by seabirds, which have been credited with enabling more than 60 plant species to arrive on the island by carrying seeds and transforming its soil. 

These impacts are known to date back thousands of years. Research in the Falkland Islands showed that the arrival of seabirds such as penguins to the archipelago was linked with a dramatic change in soil composition 5,000 years ago, closely followed by a pollen spike as plants moved in. 

Many isolated islands around the world play host to similar bird populations, with their droppings building up into a solid mass, known as guano. This has been found to help boost surrounding marine life, as nutrients leaching from guano into the surrounding seas support coral reefs and associated communities. 

Its fertilisation properties have also had impacts on human culture. Some suggest that some of the world's first conservation measures were introduced by the Incans, who used limited amounts of the guano found on breeding islands off the South American coast as fertiliser. 

Anyone disturbing or killing guano birds was liable to be sentenced to death under the laws in place at the time. 

Subsequently, colonial powers discovered its extraordinary value, stripping islands around the world of guano to feed growing populations. Millions of seabirds were killed by the guano industry as their breeding grounds were torn apart. 

Almost 150 years after the end of the guano era, seabirds are being put in peril once again – as is the poo they produce. 

What do we understand about nutrient movement by seabirds? 

Seabirds are faced with a multitude of threats, including overfishing, hunting and climate change. It is estimated that almost half of all seabird species are in decline, with a third of species considered to be threatened with extinction.  

To understand how their loss could impact global ecosystems, the scientists compared research on how birds moved nutrients and pollutants between the sea and land, both through their poo but also through their eggs and feathers.  

Collectively, the research showed the importance of seabirds to these environments. Islands with seabird populations have higher nitrogen and phosphorous levels than those without, contributing to the growth of plant and invertebrate populations. 

While generally found to be beneficial, seabird guano can also have negative impacts on ecosystems. In addition to causing eutrophication at particularly high densities, the presence of heavy metals and other toxins in the bodies of these birds was regularly detected. 

Microplastics were also identified in more recent studies, with previous research indicating that these small polymer pieces are able to alter the blood chemistry of seabirds. 

'Guano is one of the main ways that birds get rid of toxic materials from their body, from plastics and contaminants to heavy metals,' Alex explains. 'Plastics are easy to study if you're walking around a seabird colony, as you can see plastics on a colony floor.  

'However, there have only been a few studies on that, as the focus has much more been on heavy metal and chemical contaminants.' 

While research into this area is becoming more common, the researchers noted that there were significant gaps in our knowledge of these nutrient flows.  

Certain areas of the world, such as Africa and southeast Asia, have had few studies investigating how they are affected by bird poo, meaning potential impacts could go unnoticed. Certain bird species were also understudied, especially those which are vulnerable to extinction.  

The scientists recommend that researchers begin to fill in these knowledge gaps, in order to mitigate the impacts of the Anthropocene on these seabirds as much as possible.