Introduced and invasive species are a hot topic in ecology. Even when brought in for good reason, introduced species can have unforeseen negative impacts on the environment and the species around them. Take Cane Toads, for example. They were introduced to Australia to control a particular bug, but ended up eating everything they could fit in their mouths, especially native, endangered species. Or look at the mongoose, brought to Hawaii to control rat populations. While it does enjoy the invasive rodents, it also feeds on the eggs of native birds, decimating their populations. Now, there's another species to add to the list of dangerous invasives: The Japanese White-Eye.
Japanese white-eyes (Zosterops japonicus) are really, really cute little birds (see me with one on the R). At just under 5 inches with bright green feathers and a yellow throat, the white-eye is a colorful and attractive bird. It's name comes from the distinctive white ring around its eyes. You'll find them all over the Hawaiian Islands where they were introduced intentionally in the 1929 to control bug populations. Populations have grown exponentially since, devouring bugs and nectar wherever the white-eyes can survive. Since they're cute, few people have thought twice about the invasion of this pretty little bird. The federal fish and wildlife commission here doesn't think they're a problem, and has even revoked the permits of scientists studying the bird's effects. But, a paper soon to be published in Current Biology begs to differ.
You see, there were already nectar-feeders in Hawaii before the white-eye was introduced. These native honeycreepers, like the Akepa and the Amakihi (on L), are a little larger and breed slower than the white-eye. But what's worse for them is that the white-eye didn't come alone. It brought with it parasites like avian malaria that the native species had never encountered before. The sickness and competition for food have taken their toll. Since the arrival of the white-eye, native Hawaiian bird populations have plummeted.
That's what Dr. Lenny Freed and Dr. Rebecca Cann have shown using 20 years of mark and recapture data in the federal preserve Hakalau on the big island of Hawaii. The birds were caught using mist nets and all kinds of data was taken for each bird - weight, morphological measurements, age, sex, parasitic infections, and more.
The data revealed that over the years, where the white-eyes flourished, the overall survival of juvenile native birds has dropped. Specifically, the native species are being out competed for food resources, leading to signals of malnourishment. The bill length and overall size of the native species has decreased dramatically.
"Just as there are permanent effects of stunted growth in human children, there are permanent effects in adult birds," explains Dr. Rebecca Cann from the University of Hawaii at Manoa. "Birds cannot use their shorter bills to feed efficiently for themselves or when feeding their young. Stunted birds have higher death rates than normal size birds. The Japanese white-eye is causing this problem for native Hawaiian birds by depleting the food available for growth, survival, and breeding."
Where the white-eyes have yet to invade or are in low numbers, the native honeycreepers are healthier, with larger bills and higher survival rates.
In the end, the data suggested that white-eyes are most likely responsible for the decline of 7 of 8 native forest birds in a major portion of the Hakalau refuge. While there are other threats like malaria and parasitoids, the fact that juvenile birds fared well wherever white-eyes are not is pretty damning. Another paper, out of the same lab in Hawaii, has shown that decreased food availability is altering sex ratios of the native Akepa (PDF), leading to too few females - another way in which the competition for food with the white-eye is dooming native Hawaiian bird species.
The white-eye is yet another example of how invasive species can severely damage the communities they take over. And despite how many biocontrol methods have failed horribly, nations and scientists continue to consider them viable options to control pest species. It's a tangled web we weave when we add or subtract species from an ecosystem - we shouldn't do it so lightly.
Leonard A. Freed, & Rebecca L. Cann (2009). Negative Effects of an Introduced Bird Species on Growth and Survival in a Native Bird Community Current Biology
Leonard A. Freed, Rebecca L. Cann, & Karl Diller (2009). Sexual dimorphism and the evolution of seasonal variation in sex allocation in the Hawaii akepa Evolutionary Ecology Research, 11, 731-757