Resource Subsidies Alter Spatial Ecology of Gila Monsters

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Resource subsidies can have profound effects on animal behavior, reproduction, and growth. Resource subsidies often result from human-altered environments that augment natural resources, creating small-scale enriched landscapes (“terrestrial islands”) juxtaposed with less productive natural landscapes. Effects of artificially enriched landscapes can be more pronounced in arid-land systems where resource availability is already limited. We analyzed patterns of space use and movement of Gila Monsters (Heloderma suspectum) based on radio telemetry data from a subsidized resource (water and prey) environment (Stone Canyon Golf Club) and compared them to individuals from a natural environment (Owl Head Buttes). After adjusting estimates of area use for sex, number of relocations, and year, males in the subsidized resource environment had a mean home range area (minimum convex polygon) of 13.6 ha (range: 4.4 – 24.0 ha), while females had mean home ranges of 8.3 ha (2.0 – 35.8 ha). In the natural environment, males had an average home range area of 43.2 ha (21.5 – 82.7 ha) while females had an area of 23.6 ha (7.9 – 47.6). Gila Monsters from the two environments exhibited seasonal differences in movement patterns, primarily between the dry and monsoon seasons, with home ranges in the subsidized environment being smaller (13.0 ha for males, 10.5 ha for females) and slightly more stable than home ranges in the natural environment (23.7 ha for males, 23.6 ha for females). To explore spatial arrangements within and between sexes, we examined overlap of home range areas. There were differences in home range overlap within and between sexes; in the subsidized population, there was very little male-male overlap (only two occurrences), but a high degree of female-female overlap (7 occurrences). In the natural population, there was very little female-female overlap (one occurrence) but a high degree of male-male overlap (7 occurrences), likely because home ranges were approximately twice as large in the natural population. Male home ranges extensively overlapped female home ranges at both sites. Gila Monster home ranges may be smaller and packed more densely together in subsidized resource environments than those of natural environments due to increases in available resources.