Date of Award

Spring 2020

Document Type

Open Access Thesis

Degree Name

Master of Science (MS)



First Advisor

Christine Maher


When two species compete for food, one species typically outcompetes the other. Subordinate species can alleviate costs of competition by reducing diet overlap, promoting coexistence. Non-native coyotes (Canis latrans) and historically native gray foxes (Urocyon cineroargenteus) have expanded their range in Maine and may compete with native red foxes (Vulpes vulpes). To investigate competition, I analyzed stable isotopes and stomach contents to determine if coyotes, red foxes, and gray foxes differed in use of anthropogenic foods and trophic position. I predicted that coyotes utilize anthropogenic foods the least (lowest δ13C), and red foxes utilize anthropogenic foods the most (highest δ13C), and that coyotes occupy the highest trophic position (highest δ15N), with gray foxes at the lowest trophic position (lowest δ15N). Furthermore, I predicted that gray fox diets consist of primarily plant matter and overlap to a greater degree with red foxes than with coyotes, and conversely that coyote diets consist primarily of animal prey and overlap most with red foxes. I compared δ13C and δ15N from muscle and hair samples to assess relative resource use among species, and I compared frequency (%) of prey items taken from stomach contents to assess diets. Red foxes consumed anthropogenic foods the most in fall and early winter, gray foxes consumed anthropogenic foods the most in summer, and coyotes consumed anthropogenic foods the least in all seasons. Coyotes held the highest relative trophic position in fall and early winter, red foxes held the highest relative trophic position in summer, and gray foxes held the lowest relative trphic position in all seasons. Based on stomach contents in fall and early winter, gray foxes had the broadest diet and consumed the most plants, and coyotes had the narrowest diet. My results suggest that all species compete in fall and early winter. Red foxes were the only species to show isotopic niche overlap with both potential competitors in all seasons, though competition is likely highest in fall and early winter as food availability declines. Red foxes are likely most susceptible to competitive exclusion among canids in Maine.



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