Good news! Rodent Removal Results in Rebound of Seabird Populations
By Andrew Bousfield
For most people, rodent infestations are considered a nightmare that they hope to avoid. Seabirds share this nightmare, but unfortunately lack our rodent removing abilities. Luckily conservationists can help these island nesting birds through strategic eradications of these unwanted pests!
What’s Wrong with Rodents?
While to most it’s obvious why rats and mice can be problematic in our world, it can be easy to overlook how these tiny pests heavily impact animals that are residents of small islands. Seabirds for example rely on islands to breed and rear their offspring. These birds benefit the island in a variety of ways, such as seed dispersal and plant pollination. This balanced relationship is often present in environments like islands because if an organism takes too much from their surroundings, they risk throwing off that necessary balance.
Introduced species however typically aren’t well suited for this tight balance, as the environments they evolved in may have different climates, food resources, predators, and more. Rodents for example are opportunistic feeders, meaning they’ll eat whatever food source is available to them in a given setting. This diet hurts island Seabirds both directly through predation and indirectly through competing for limited island resources. On Surprise Island off of New Caledonia, the black rat and house mouse were the introduced pests that became a problem for our feathered friends.
A Simple Solution?
The strategy is simple, remove the rodents and count the birds! In their study following the effects of rodent eradication on seabird populations, Philippe-Lesaffre and colleagues focused on the brown booby, red-footed booby, masked booby, black noddy, brown noddy, great frigatebird, and lesser frigatebird. To monitor the short term impacts of their study, they began a yearly bird count in late 2002 using a strategy called transect sampling. With this method, they essentially created ten parallel lines that they would walk along and collect their data. Information such as number of seabirds counted, age of the birds, and the type of vegetation present were recorded. Finally in 2005 the rodent removal campaign was completed, and rodent eradication was confirmed in 2006! The yearly island surveys would continue until 2009.
A Flap in the Right Direction
In the four years post removal, it was clear that some of the birds were responding better then others. While the effect of removal was less certain for the three booby species, the increase in breeding pairs for the noddies and frigatebirds was undeniable. To understand the long term effectiveness of this conservation strategy, additional surveys both prior and following the rodent removal were analyzed. Once again, the noddies and frigatebirds showed the largest growth in numbers.
But where did this short term growth come from? While long term it makes sense for the breeding pairs to naturally increase from previously successful nests, it’s unlikely all of the growth seen in the four years post eradication came from previous residents alone. Philippe-Lesaffre points out that much of that initial growth could be due to settlement from birds from nearby islands. This would especially make sense for the frigatebirds, who are strong nest defenders. It’s likely these settling frigatebirds, noticing the removal of a potential stressful neighbor, opted to lay their eggs on Surprise Island. While the initial growth of the frigatebirds was likely due to an initial settlement, the long term growth seen in the noddies is suggested to be the result of the rodent removal. These smaller birds reach maturity faster than frigatebirds, so the quick growth is more likely due to the loss of a nest predator.
Overall, the growth in population of the frigatebirds and noddies has increased the diversity of seabirds present on Surprise Island. The removal of introduced species removed a potential conflict for nesting birds, allowing new birds to settle the island and resident birds to live with one less stress inducing neighbor. While settlement could have negative implications for the islands that lost their resident birds, the net growth in seabird mating pairs should be considered a win for conservation. Removing introduced species while challenging, can benefit resident species in many ways and restore the balance needed for ecosystems to thrive!
References
Philippe‐Lesaffre, M., Thibault, M., Caut, S., Bourgeois, K., Berr, T., Ravache, A., Vidal, É., Courchamp, F., & Bonnaud, E. (2023). Recovery of insular seabird populations years after rodent eradication. Conservation Biology, 37(3). https://doi.org/10.1111/cobi.14042
