Ecology of Saguaro National Park
Saguaro (pronounced “suh-wah-roh”) National Park is located in southeastern Arizona near the city of Tucson within the Sonoran Desert, the only desert in the world that experiences two rainy seasons per year. Contrary to the popular notion that desert landscapes are barren and uninhabitable, Saguaro National Park is teeming with life.
The unique climate, combined with varied topography, leads to an incredible diversity of species throughout the Sonoran Desert and within Saguaro National Park. With over 550 animal species and over 2,000 plant species, the Sonoran Desert is considered the most biologically diverse desert in North America.
Saguaro National Park contains two separate districts: the Tucson Mountain District (TMD) and the Rincon Mountain District (RMD). The TMD is often referred to as Saguaro West, and the RMD as Saguaro East. Despite being part of the same national park and being relatively close in location, there are some important differences between the two. The groups of plant and animal organisms that live and interact with each other, known as biotic communities, of the TMD and RMD vary greatly and are typically defined by plant types and weather. Higher elevations, with more moisture and cooler average temperatures, can support a different range of organisms than drier, warmer areas at lower elevations.
The RMD is part of a Sky Island mountain range. With a maximum elevation of 8,664 feet at the peak of Mica Mountain, the RMD contains a wide variety of biotic communities, or biomes, including desert scrub, semi-desert grassland, oak woodland, Madrean pine-oak woodland, pine forest, and mixed conifer forest. The TMD, with a maximum elevation about half as high (4,678 feet) only contains the desert scrub and semi-desert grassland biotic communities.
Biotic Communities and Plant Species
Desert Scrub
Desert scrub is the driest and lowest-elevation (~1000-2500’) biotic community at Saguaro National Park, where the dominant forms of vegetation include shrubs and succulents. The most common shrubs include the mesquite tree (Prosopis genus), acacias (Acacia species), palo verdes (Cercidium species), and the creosote bush (Larrea tridentata). To adapt to the dry environment, creosote bushes have evolved a waxy coating on their leaves to reduce water loss through evaporation. When it rains, those waxy compounds are released into the air, creating a strong smell. The most common succulents found in the desert scrub biome include agave (Agave species), yucca (Yucca species), and cacti such as the barrel cactus (Ferrocactus and Echinocactus species), pincushion cacti (Mammalaria species), prickly pear (Opuntia species), and the cholla (Cylindropuntia species).
Perhaps the most defining plant of the region is the saguaro cactus (Carnegiea gigantea) for which the park gets its name. Saguaro cacti are only found in the Sonoran Desert, and they play a very important role in the ecosystem by providing shelter, food, and nesting sites for many animals.
All of these species play a key role in their ecosystem by fixing nitrogen from the environment into a usable form for animals using bacteria called Rhizobia in their root nodules.
Semi-Desert Grassland
The semi-desert grassland biotic community is found from approximately 2500-4500 feet in elevation. The vegetation in this biome is mainly comprised of short and mid-sized grasses. This biotic community is dominated by the velvet mesquite tree (Prosopis velutina).
Oak Woodland
The oak woodland biome, sometimes referred to as oak savannah, is composed of manzanita shrubs and trees (Arctostaphyos species) and oaks (Quercus species), with some annual and perennial grasses like in the semi-desert grassland.
Madrean Pine-Oak Woodland
With cooler temperatures at elevations around 5000-6000 feet, more oaks and gymnosperms (flowerless plants that don’t encase their seeds) are able to grow and flourish. Evergreen oaks such as the Emory oak (Quercus emoryi), Arizona white oak (Quercus arizonica), and Mexican blue oak (Quercus oblongifolia) are abundant. Common gymnosperms include the Mexican pinyon pine (Pinus cembroides) and alligator juniper (Juniperus deppeana). In this community, underbrush grasses are also common.
Pine Forest and Mixed Conifer Forest
At the highest elevation in Saguaro National Park, the pine and mixed conifer forest communities thrive in the cooler climate with more moisture. These biotic communities are dominantly comprised of gymnosperms such as, at increasing elevations, ponderosa pine (Pinus ponderosa) and other pine species (Pinus species), firs (Abies species), and spruces (Picea species).
Animal Species
Similar to the above plant species, many animals have adapted to survive the unique Sonoran Desert in Saguaro National Park, and a range of animal species can be found in each biotic community.
Amphibians and Reptiles
Saguaro National Park is home to several native reptiles and amphibians, referred to as herpetofauna, including: eight frog, one salamander, forty-eight lizard and snake, and three turtle species. Herpetofauna play an important role in the ecology of the American Southwest, acting as both predators and prey for many other animals. Reptiles and amphibians are ectotherms, meaning that their internal body temperature changes with the external temperature and can only be regulated through behavioral adaptations.
One way that reptiles regulate their body temperature is by being most active at cooler times of the day to prevent overheating. Many species of snakes, as well as the desert tortoises, are crepuscular, meaning that they are most active at dawn and dusk. Many snake species are also nocturnal, meaning that they’re most active at night. Some diurnal species, which are active during the daytime, regulate their body temperatures by alternating between basking in the sun and seeking shade. Some prominent reptile species of Saguaro National Park are Gila monsters (Heloderma suspectum), desert tortoises, coral snakes, and rattlesnakes (Crotalus species). Rattlesnakes, coral snakes, and Gila monsters are all highly venomous.
While reptiles have dry, scaly skin that helps them survive the arid desert habitat, the life cycle of amphibians is heavily dependent on water. Amphibians have three main stages throughout their life cycle: egg, larvae, and adult. Because amphibians don’t have amniotic eggs (protective coverings around their shells) like all other vertebrates, the eggs are susceptible to drying out, so they are heavily dependent on the presence of water. The larval stage is also dependent on water, with most tadpoles only being able to survive in water. Even after metamorphosis, when the species gains the ability to live on land, adult amphibians have incredibly porous skin that allows for gas exchange, making them prone to drying out in arid climates. Adults are also dependent on water to lay eggs and continue the life cycle.
To survive the dry desert climate, amphibians have adapted behaviors to help minimize water loss. Some species, including the tiger salamander (Ambystoma tigrinum), spend the majority of the year burrowed underground and only resurface during monsoon season to breed when water is abundant and temporary pools are created. Other species, such as the lowland leopard frog (Rana yavapaiensis) and the canyon treefrog (Hyla arenicolor) live in small bodies of water throughout the year but are threatened by various invasive species of fish, crayfish, and American bullfrogs (Lithobates catesbeianus). Because amphibians are so sensitive to changes in the environment, they are often used as indicators of ecosystem health.
Birds and Mammals
Saguaro National Park is home to many species of birds, including over 150 species at TMD and over 200 species at RMD. At lower elevations, roadrunners, Gila woodpeckers, and quails are common. Harris hawks (Parabuteo unicinctus) and red-tailed hawks (Buteo jamaicensis) can also be found nesting at or hunting from saguaro cacti. Within the desert scrub biome, black-throated sparrows (Setophaga nigrescens) and cactus wrens (Campylorhynchus brunneicapillus) are the most abundant. In oak woodland, the ash-throated flycatcher (Myiarchus cinerascens), Bewick’s wren (Thryomanes bewickii), and rufous-crowned sparrow (Aimophila ruficeps) are commonly found. At higher elevations in the pine-oak woodland and pine and mixed conifer forests, the spotted towhee (Pipilo maculatus), black-throated gray warbler (Setophaga nigrescens), yellow-eyed junco (Junco phaeonotus), and mountain chickadee (Poecile gambeli) are the most abundant species.
There are over 50 species of mammals found in Saguaro National Park. Fifteen species of bat are abundant throughout the park. Javelinas, also known as collared peccaries (Pecari tajacu), bobcats, deer, squirrels, and rabbits are also abundant. Larger carnivores such as black bears (Ursus americanus) and mountain lions (Puma concolor) have been spotted at higher elevations, with black bears only occurring in the RMD and mountain lions more abundant in the RMD than the TMD.
Wildfires
In recent years, wildfires have posed a large threat to the ecosystems of Saguaro National Park. Some upper-elevation biomes, including the pine-oak woodland and mixed conifer forest, are fire-adapted and actually depend on natural fires from every 5-15 years in order to clear out undergrowth vegetation and maintain biodiversity. Sometimes fires are even purposely started, after being very carefully planned and monitored, to facilitate the natural process or to reduce harmful vegetation.
However, some lower elevation communities, such as desert scrub and grassland, are maladapted to fires. Invasive species such as buffelgrass (Pennisetum ciliare), Lehmann lovegrass (Eragrostis lehmanniana), and red brome (Bromus rubens) have been linked with the recent increased frequency of fires in these communities. Aside from outcompeting local flora for natural resources, these invasive species are also highly flammable, causing and propagating wildfires rapidly. Many species such as succulents and the saguaro cactus cannot survive these high-intensity fires.
Fire activity also affects animals. A USGS study found that wildfires contribute to sediment that buries the already-vulnerable habitats of lowland leopard frogs. Saguaro National Park has implemented fire management practices to help mitigate the negative effects of wildfires. These practices include the close monitoring of natural fires at higher elevations and fire suppression when necessary to protect the communities.