Darwin’s finches in the Galapagos are the poster children of adaptive radiation. These birds have formed many species that differ in their beaks. The size and shape of beaks make them uniquely suited to take advantage of different food resources. Hard big beaks are great for eating large hard seeds, small beaks for small seeds. But despite these differing preferences in seeds, they all agree on one thing: chips and cookies are pretty delicious, and that could mean trouble for the future of Galapagos finches.
Urban growth brings more roads, less trees, more humans and more snacks
Luis de Leon has been studying how human impacts in the Galapagos affect the ecology and morphology of finches. Specifically they have been studying finches in Santa Cruz. This island has changed dramatically in the last 15 years. Urban growth in Santa Cruz has led to more roads and sidewalks (55 hectares growth) and a less vegetation (109 hectare lost). In addition, human population has doubled. These changes can lead to fewer natural food sources and greater encounters with human food. Because of that animals can now find our food in trash, directly from people, and sometimes by taking the food right off of our plate!
You are what you eat. Is eating human food bad?
Many animals have been shown to eat human food when this is available. Classic foraging theory would predict that when human food is plentiful and the cost of obtaining it is low, human food sources should be used more frequently by animals. Thus a theoretical human food snacker reduces the cost of traditional foraging by obtaining its food from stable, abundant sources. This means that human food can be very rewarding due to its abundance and how easy it is to get it. Research in gulls showed that male gulls that eat human food are heavier and have greater body conditions. However, consumption of human food has been postulated to increase the likelihood of disease by concentrating animal in smaller areas. A study with coyotes found that urban coyotes were protein deficient and in more disease ridden. In previous posts on this blog, we talked about the possibility of consuming anthropogenic foods leading to evolutionary change as well as how to measure blood glucose to quantify the physiological response of human food consumption.
How does snacking on human food impact Darwin’s finches?
Luis de León and his team found that within the small island of Santa Cruz (96.53 miles squared) a single species of finch (Geospiza fortis) showed remarkable differences in their beak size and shape. Finches living in forested areas had beaks that were bigger, deeper and with stronger biting force compared to finches living at Academy Bay, a moderately disturbed site near the town of Puerto Ayora (which has a population of around 12,000 people).
These findings were surprising because hard and soft seeds were available at both sites. Finches were expected to have similar beaks because the natural food sources were similar. Could access to human food expand the finches’ diet? To answer that question Luis and his team conducted a follow up study where they tested the finches’ preference for human or natural food. This time, however, they looked at all of the finch species present (G. fortis, G. fuliginosa, G. scandens, and G. magnirostris).
They tested the food preferences at sites that differed in urban development and number of humans visitors. At the urban site, they observed 79% of finches consuming human food in the wild! In cafeteria food experiments, finches from areas that were more developed (like Puerto Ayora) and had more tourists with food (like beaches) predominately chose human food. Finches ate more chips and cookies than they did rice. Furthermore, at the beach finches came more eagerly to feed when they heard the noise of chips bags opening, which implies that finches from the beach have learned to associate chip noise with food. Other populations, including the highly urban population in Puerto Ayora did not react as strongly to the chip noise. In short, they found that finches at the developed sites not only eat human food but prefer it, and at the beach site they rely on audio cues to help them grab a quick bite! Significantly, this was associated with an increase in niche breadth and niche overlap of G. fortis and G. fuligonosa at the two urban sites, with 100% overlap in Puerto Ayora.
This shift in diet could have major repercussions for Darwin’s finches. When the niches of different species of finches converge, the mechanisms maintaining their distinctness as species are eroded. This is particularly true considering selection on beak morphology in Galapagos finches happens during periods of resource scarcity, but human foods are present and abundant year round! This relaxation of selection and increased niche overlap may lead to morphological convergence in beak shape across species and increased hybridization between species. Luis explores these possibilities and hypothesizes that consumption of anthropogenic food could have longterm evolutionary impacts on the Galapagos finches.
What animals have you seen consuming urban foods?
Interest in the topic of human food consumption by wildlife has been growing recently. Indirect domestication of wildlife could result in part from greater tolerance to humans as animals settle in cities to forage from our food. I’ve seen people in Puerto Rico leaving mounds of rice for birds, which often gets eaten by the the pearly eyed thrasher. What human food items have you seen eaten by animals?
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