Seabirds as vectors for nutrients and pollutants to islands

Seabird colonies are integral locations for the input of marine-derived nutrients. Seabirds transport these nutrients from their marine feeding grounds, and deposit them through guano and other allochthonous inputs, such as feathers and eggs, when they return to land. Of these, guano is the most influential to terrestrial environments as it is an excellent natural fertiliser and contributes to the surrounding ecosystem in a myriad of ways. The effects of seabirds are often more pronounced on islands compared with mainland environments as these areas often have limited nutrient inputs. As such, many islands are dependent on seabirds and their guano as it is a continual source of nutrient replenishment.

However, seabirds can also transport inorganic and organic pollutants and marine plastics to their colonies. Unlike nutrients, these contaminants do not have positive impacts for terrestrial environments and instead can cause hotspots of pollution. This can result in negative consequences such as poor soil quality which can lead to flow on effects to the entire ecosystem.

In addition to this, seabirds face increasing pressures from a variety of sources, including invasive alien species, bycatch in fisheries, and climate change, and this is having a detrimental effect on their populations worldwide. With seabird populations in decline, so too are the nutrients transported from sea to land. The potential adverse consequences of nutrient decline are many and potentially could lead to drastic changes to ecosystems, with island environments most as risk. Thus, the highly beneficial, nutrient rich guano of seabirds worldwide is becoming less abundant in many areas, and at the same time, more contaminated.

Through this thesis I investigated the nutrients and contaminants carried from marine to terrestrial environments by seabirds. First, I conducted a systematic literature review and meta-analysis on the topic of seabirds as transporters of nutrients and contaminants to their terrestrial breeding, roosting, and nesting grounds, with a particular focus on guano (Chapter 2). This provided insights into commonly studied species, types and concentrations of nutrients and metals transported, and the overall effects and influence of seabirds on their terrestrial environments. I found that seabird guano is a highly beneficial fertiliser, boosting the nutrient content of colony soils compared to control soils (i.e., areas without guano deposition). I identified common methodological approaches, such as using stable isotope analysis as a tracer of marine nitrogen from seabirds, as well as gaps in our knowledge regarding what species are lacking research, which provided me with an important foundation of understanding for my subsequent thesis chapters.

In the following three chapters I used the Flesh-footed Shearwater (Ardenna carneipes) population on Lord Howe Island to explore the quantities of nutrients, metal contaminants, and plastics that are transported from the marine environment by the birds. This is the first time that these data have been reported and provide an important assessment for this vulnerable species, as well as demonstrating the cumulative pressures that this species face. Flesh-footed Shearwaters ingest high quantities of marine plastics and have experienced significant population decline over the last few decades. There is concern among the Lord Howe Island community and scientists alike that the kentia palm (Howea forsteriana) forests that appear to rely on the nutrients transported by the shearwaters may not thrive if the population continues to decline.

I first explored the nutrients found in shearwater guano and examined soil, kentia palm leaves, and a soil invertebrate (leopard slug Limax maximus) in colonies and compared these data to areas on Lord Howe Island without shearwaters (Chapter 3). Guano was rich in nitrogen, phosphorous, and potassium, and soils from colonies were enriched in the latter two, however soil nitrogen concentrations were not significantly different across sites. This indicates that there are other factors that influence nitrogen concentrations on Lord Howe Island, such as volatilisation. Furthermore, nitrogen stable isotopes in soils were also not significantly different which suggests the presence of historic seabird colonies at the control site.

Secondly, I analysed the guano of shearwaters for a suite of metals (Chapter 4). Like the previous chapter, I determined whether these contaminants were accumulating in soil, vegetation, and soil invertebrates within the colonies. Guano contained arsenic, mercury, lead, cadmium, copper, and chromium, and contributed to significantly higher concentrations of all metals (except lead) in colony soils compared to control soils. The concentrations of these metals in kentia palm leaves and leopard slugs were more variable and there were few significant differences observed among sites. Metal concentrations in shearwater guano were higher than in the guano of other Procellariiformes compared in my systematic review. This potentially may be caused by the significant quantities of plastics that Flesh-footed Shearwaters ingest acting as another source of metals, in addition to dietary sources.

In my last data chapter (Chapter 5), I quantified the annual deposition rate of plastics to the shearwater colonies on Lord Howe Island. I estimated that the shearwaters are transporting 688,480 (95% CI: 582,409 – 800,877) plastic pieces to the island every year, with the plastics weighing a total of 165,204 g (95% CI: 141,060 – 192,763 g). This demonstrates that seabirds are transporters of marine-derived plastics and can reintroduce items littered, dumped, and lost from urban and marine areas back into terrestrial environments.

My research demonstrates that Flesh-footed Shearwaters are important transporters of nutrients to their colonies on Lord Howe Island. Equally important, but often less described, is the birds’ ability to transport metal contaminants and marine plastics to their breeding grounds as well. As pollution of the marine environment is predicted to increase in coming years, it is likely that the shearwaters will continue to deposit contaminants, and at potentially higher rates.

While my thesis focused on one seabird species from one area, there are patterns that ring true for many other seabirds around the world. It is highly like that seabirds are depositing contaminants and plastics on remote islands, many of those that are otherwise pristine and free from direct human interaction or destruction. Seabirds are inadvertently damaging the very environments that they rely upon for their survival, just as these environments rely upon the seabirds and their nutrients. Therefore, seabirds are intrinsically linked with land, yet as their populations continue to decline, far-reaching changes could occur that may potentially have a raft of negative consequences. Thus, it is important to continue to manage seabird populations for not only their continued survival, but the survival of island ecosystems worldwide.