Premise of the Study Phenology is the study of biological life cycle events, such as flowering and migration. Climate patterns can alter these life history events, having ecosystem‐wide ramifications. For example, warmer springs are associated with earlier leaf‐out for many species, impacting species interactions and growing‐season carbon dynamics. While phenological research has been conducted extensively in temperate regions, relatively little is known about the phenological responses in arid and semi‐arid regions. Methods In this study we looked at the flowering phenology of a keystone species in the Sonoran Desert, the saguaro cactus (Carnegiea gigantea). The timing and abundance of flowering was observed on 151 individuals for 10 years at a site near Tucson, Arizona, USA. Using six phenological traits, we explored the relationship between saguaro size and flowering and the climatic drivers of flowering. Key Results Our analyses demonstrated how the calculation of phenological traits at the individual versus the population level can yield differing responses to climate variability, suggesting that not all studies examining the same trait (e.g., first day of bloom) are directly comparable. We found that larger cacti began flowering earlier, flowered for longer, and produced more flowers. Warmer temperatures were correlated with advanced onset and higher bloom yields, while increased precipitation appeared to delay onset and reduce bloom yields. Conclusions Given that climate models predict that the Southwestern USA will become increasingly warmer with more variable precipitation, saguaros may begin flowering earlier in the season and flower more intensely, which could impact pollen availability and population dynamics.
The pallid bat (Antrozous pallidus) is a species of western North America, inhabiting ecoregions ranging from desert to oak and pine forest. They are primarily insectivorous predators on large arthropods that occasionally take small vertebrate prey, and are at least seasonally omnivorous in certain parts of their geographic range where they take nectar from cactus flowers and eat cactus fruit pulp and seeds. Until recently, mesquite bugs were primarily tropical-subtropical inhabitants of Mexico and Central America but have since occupied the southwestern United States where mesquite trees occur. Using a noninvasive method, we investigated the bats’ diet at the Cienega Creek Natural Preserve, Arizona, by collecting food parts discarded beneath three night roosts in soil-piping cavities in a mesquite bosque. We also made phenological and behavioral observations of mesquite bugs, Thasus neocalifornicus, and their interactions with the mesquite trees. We determined that the bats discarded inedible parts of 36 species in 8 orders of mainly large-bodied and nocturnal insects below the night-roosts. In addition, one partial bat wing represents probable predation upon a phyllostomid bat, Choeronycteris mexicana. About 17 of the insect taxa are newly reported as prey for pallid bats, as is the bat C. mexicana. The majority of culled insect parts (88%) were from adult mesquite bugs. Mesquite bug nymphs did not appear in the culled insect parts. After breeding in late summer, when nighttime low temperatures dropped below 21 °C, the adult bugs became immobile on the periphery of trees where they probably make easy prey for opportunistic foliage-gleaning pallid bats. Proximity of night-roosts to mesquite bug habitat probably also enhances the bats’ exploitation of these insects in this location.
The pallid bat (Antrozous pallidus) is a species of arid and semiarid western North America, inhabiting ecoregions ranging from desert to oak and pine forest. Considered primarily insectivorous predators on large arthropods but taking occasional small vertebrate prey, pallid bats were recently shown to be at least seasonally omnivorous; they demonstrate unusual dietary flexibility and opportunism in certain parts of their geographic range and at different times of year. In a few areas they take nectar from cactus flowers and eat cactus fruit pulp and seeds. Until recently mesquite bugs were primarily tropical-subtropical inhabitants of Mexico and Central America but have since occupied the southwestern United States where mesquite trees occur. Pallid bats regularly use night roosts as temporary shelters in which to process and consume large arthropods caught near their foraging areas. Using a noninvasive method, we investigated the bats’ diet by collecting food parts discarded by the bats beneath three night roosts in soil-piping cavities at the Cienega Creek Natural Preserve, Arizona. We also made phenological and behavioral observations of the mesquite bugs, Thasus neocalifornicus, and their interactions with the mesquite trees. The bats discarded inedible parts of at least 36 species in 8 orders of mainly large-bodied and nocturnal insects below the night-roosts. In addition, one partial bat wing represents predation upon a phyllostomid bat, Choeronycteris mexicana. About 17 of the insect taxa are newly reported as prey for pallid bats, as is the bat C. mexicana. The large majority of culled insect parts (88.8%) were from adult mesquite bugs. As nymphs, mesquite bugs are aposematically colored and secrete noxious pheromones; nymphs did not appear in the bat-culled insect parts. Adult mesquite bugs are darkly colored and secrete different noxious pheromones than the nymphs. During daytime hours in the summer adult bugs are abundant, flying around the canopy and alighting on the edges of the trees. In late summer and early fall they breed and lay eggs that overwinter on the mesquite branches to hatch in January. Soon after breeding, the adult bugs die. When summer heat diminishes and nighttime low temperatures drop below 21°C, the adult bugs become immobile on the periphery of the trees where they probably make easy prey for foliage-gleaning pallid bats. The historically subtropical-tropical mesquite bugs may have moved into the southwestern United States with the spread of cattle and mesquites. In this area of Arizona, pallid bats provide an important natural control on the local mesquite bug population. The high diversity of other insect remains and the remains of another species of bat provide additional supportive evidence of a diet for pallid bats that reflects their plasticity across a variety of habitats. This behavioral plasticity probably enhances the bats’ survival across their range in the face of climate change.
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