PS 32-39
Habitat fragmentation induced by urban sprawl compromises the interactions between desert plants and fruit-consuming mammals
The growth of metropolitan areas can engender the annexation and fragmentation of wilderness areas in the adjacent hinterlands. This fragmentation can alter the plant and animal assemblages that live in the resulting fragments and, as a result, alter the interspecific interactions experienced by the residents of the fragments. Here, we explore the consequences of habitat fragmentation for dispersal of cactus fruit in the Sonoran Desert landscape within and adjacent to Tucson, Arizona. We use the barrel cactus, Ferocactus wislizeni, as a model system to diagnose fruit utilization by an assemblage of mammals that can occupy desert fragments. In Jan 2014, we inspected 896 plants in 21 fragments to test the hypothesis that inter-site variation in the interactions between the Ferocactus fruit crops and animal communities can be predicted based on inter-site differences in area that constrain the mammal species likely to be residents of particular sites. Further, we tested the hypothesis that inter-site differences in the phenology of fruit removal would compromise the ability of seedling to exploit precipitation delivered in the winter and spring.
Results/Conclusions
Fruit removal rates were 50% greater in sites larger than the threshold size identified as sufficient for supporting resident frugivores (>1.8ha), relative to smaller sites. Tall plants were significantly more susceptible to frugivore failure (i.e., failure of fruits to be removed), and were also significantly more likely to have fruits collected by rodents rather than hares, canids, peccaries or mule deer. The threshold height where this change in the nature of the plant-animal interactions became apparent increases predictably with fragment area. As a result of slower fruit removal rates, the seed crops in small fragments forfeit access to an increasing proportion of the rains between October and March. At a minimum, the mean forfeiture for seed crops in sites <1 and <10 ha is 0.29 ± 0.13 and 0.09 ± 0.07 of the available rains (mean ± SD), respectively. In contrast, seeds at our largest site (350 ha) likely forfeit access to less than 3% of the precipitation. We expect these inter-site differences to compromise the integrity of plant populations in small and intermediate-sized fragments.