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Butterfly colors and global change: anthropogenic influence on a sexually selected wing trait in the cabbage white butterfly
AdvisorForister, Matthew L.
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Habitat changes (e.g. deforestation, urbanization, etc.), the introduction of exotic species, human harvesting (e.g. fisheries), climate change, and pollution are creating novel environments at an unprecedented rate, affecting ecological and evolutionary processes in a countless number of ways. One massive shift that has occurred in the United States over the last few centuries has been land-use change primarily into agricultural and urban areas. These shifts in land types have affected the nitrogen cycle through fertilizer and atmospheric nitrogen deposition. There is evidence that these nutritional shifts are changing nitrogen availability which is having ecological effects as well as an evolutionary effect on animal communication. However, condition-dependent signals, or honest signals that relate information about mate quality, have been less studied in terms of responses to increased nitrogen pools. Using a combination of field-based observations and lab-based experiments, I have investigated how land use change is affecting the sexually selected phenotypic characteristics of the cabbage white butterfly, Pieris rapae. We know this butterfly is nitrogen limited, and the white color of the cabbage white butterfly comes from nitrogen-rich pterin pigments. Coloration was quantified using reflectance measurements as a proxy for pigment concentration. Using these methods, I was able to perform the investigations described in this dissertation. First, I start by introducing land-type changes in the Anthropocene, honest signals, my focal species (the cabbage white butterfly), and brief methods used for wing coloration metrics. In chapter one I looked at color differences in male and female cabbage white butterflies across multiple land types across the United States. Chapter two investigated the effects of varying nitrogen availability (through manipulated artificial diets) on the sequestration and allocation of nitrogen to cabbage white head, thorax, abdomen, and wing tissue in addition to coloration. Finally, in chapter three, I looked at how four populations, with differing levels of surrounding agricultural land, varied in wing coloration, investment in reproductive tissue, and mating behavior. Taken together, these three chapters provide evidence that wing coloration is affected by land use and increased nitrogen availability that has a direct relationship with reproductive tissues, however not in the way that had been predicted at the start of my investigations. I conclude with a summary of my findings, the importance of research into anthropogenically induced effects on honest signals, and future directions.