Physiological responses by lagomorphs to resource limitations imposed by habitat fragmentation: implications for condition-sensitive predation

Abstract

Human land uses have resulted in landscape mosaics with habitat patches that vary in quality. Patch quality (including the abundance of food and the risk of predation) can affect the survival of animals that are sequestered in remnant patches of habitat. Recent investigations of the demography of New England cottontails (Sylvilagus transitionalis) have shown that cottontails on small (resource poor) patches were in poor physical condition (based on body mass) and often foraged at sites with limited cover. This resulted in a higher mortality rate than among rabbits occupying large (resource rich) patches. To gain additional insight into the consequences of habitat fragmentation, we tracked the physiological condition of rabbits occupying small and large patches during winter. The physiological condition of rabbits was determined using the urinary urea nitrogen:creatinine ratio, and the results were compared with similar indices obtained from captive rabbits. Consistent with our expectations, the nitrogen:creatinine ratios indicated that rabbits on small patches were nutrient limited for a longer period than rabbits on large patches. Transmitter-equipped rabbits on small patches had a lower survival rate and died earlier than rabbits on large patches. All mortalities were predator related. Using these data, we developed a simple model that supports the role of "condition-sensitive predation" as a major factor limiting populations of New England cottontails. Our results also demonstrate the utility of sampling physiological condition to provide an index of quality of lagomorph habitat in human-dominated landscapes.