Despite its 1989 designation as a threatened species under the federal Endangered Species Act, the desert tortoise, Gopherus agassizii, has declined in numbers by ninety percent since the 1980s. Although federal protection made it illegal to harm desert tortoises or remove them from the wild of the southwestern North American deserts, this measure has been insufficient to reverse the species’ decline, and further intervention is required.
Recovery has been slow, partly due to the desert tortoise’s low reproductive potential. Females breed only after reaching full size at fifteen to twenty years of age, and even then may only lay eggs when adequate forage is available. Although the number of eggs in each clutch varies, and each female might lay a few clutches in one season, the average mature female produces only a few eggs annually. From these precious eggs, hatchlings emerge wearing soft shells that will harden slowly into protective armor over the next five years. The vulnerable young are entirely neglected by adult tortoises, and only five percent ultimately reach adulthood.
Predators are blamed for most tortoise deaths; ravens, specifically, are estimated to cause more than half of the juvenile tortoise deaths in the Mojave Desert. Tortoise eggs and juveniles, with their delicate shells, can fall prey to many birds, mammals, and other reptiles. For protection from predators, as well as from desert temperature extremes, tortoises of all ages burrow into the earth. However, if rabbits and rodents are scarce, larger mammalian predators may dig tortoises out of their burrows, devouring even mature tortoises despite their hardened shells.
Even with current protections from human interference, the desert tortoise faces a tough recovery, so additional measures must be taken. First, the limited habitat of desert tortoises, with soil suitable for their burrows, must be protected from development. Next, urban expansion often has the unintended effect of increasing raven populations, so aggressive measures to control the birds are necessary to increase desert tortoise hatchling survival rates. Finally, released captive tortoises typically perish, and can pass upper respiratory tract disease into the wild population with devastating consequences.
Though desert tortoises are well adapted to arid habitats, and adults can survive a year without access to water, they rely heavily on moisture in the vegetation consumed in spring, when they surface from their hibernal dormancy. The loss of native plants to grazing livestock and invasive plant species, then, may lessen the tortoise’s resistance to pathogens, though the tortoises do also dig precipitation basins in the soil and linger near one when rain is impending.
However, if rabbits and rodents are scarce, larger predators may exhume tortoises from their burrows, devouring even mature tortoises despite their hardened shells. Further, tortoises are susceptible to a wide range of pathogens. The population decline is partly due to upper respiratory tract disease (URTD), characterized by nasal and ocular discharge and palpebral edema. In 2006, more than 80 percent of captive desert tortoises had anti-mycoplasma antibodies, seropositive indication of the disease. Released captive tortoises can rapidly spread URTD into the wild population with devastating consequences. Though desert tortoises are well adapted to arid habitats, and adults can survive a year without access to water, they rely heavily on moisture in the vegetation consumed in spring, when they surface from their hibernal dormancy.
1. It can be inferred from the passage that the desert tortoise mortality rate would be most likely to decrease if which of the following were true?(A) Desert tortoise burrows were cooler.
(B) Male and female tortoises mated more frequently.
(C) Adult tortoises provided better care for their young.
(D) Forage plants were abundant in the habitat of the desert tortoise.
(E) Rabbits were abundant in the habitat of the desert tortoise.
2. The passage mentions each of the following as reasons that the desert tortoise is a threatened species EXCEPT(A) expansion of urban areas near the desert tortoise habitat
(B) the low reproductive rate of desert tortoises
(C) desert temperature extremes
(D) predation by ravens
(E) the release of captive tortoises by pet owners
3. The primary intent of the passage is to do which of the following?(A) Describe the lifecycle of a species
(B) Advocate future actions
(C) Discuss a problem
(D) Evaluate past actions
(E) Criticize the government
4. Previous efforts to protect the desert tortoise are regarded by the author with(A) weary skepticism
(B) complete satisfaction
(C) implied opposition
(D) qualified approval
(E) overt disdain
5. The passage implies that precipitation in the desert tortoise’s habitatA. falls approximately once a year
B. increases the tortoise’s resistance to pathogens
C. falls mainly in the spring
D. is consumed primarily by grazing livestock
E. is collected by the tortoises intentionally
6. The passage implies that a mature female desert tortoiseA. has little contact with its offspring after birth
B. might produce only one surviving offspring each decade
C. has a softer shell than a mature male desert tortoise
D. produces fewer offspring than a mature male desert tortoise
E. reaches reproductive maturity at a later age than a male desert tortoise
7. The author uses the adjective “precious” in the sixth sentence of the first paragraph most likely to emphasize that:
(A) the number of eggs produced by female desert tortoises is limited
(B) the shells of desert tortoise eggs are delicate and easily broken
(C) desert tortoise eggs have high economic value
(D) the eggs are fiercely protected by the female desert tortoise
(D) predators, such as mammals and other reptiles, forage for desert tortoise eggs
8. The primary purpose of the passage is toA. describe the lifecycle of a species
B. discuss the threats faced by a certain species
C. advocate research on a disease wide-spread among captive animals
D. evaluate potential solutions to a wildlife problem
E. criticize federal legislation protecting a certain species
9. The author mentions “anti-mycoplasma antibodies” (Highlighted) in the second paragraph in order toA. claim that captive tortoises should not be released into the wild
B. refute claims that captive tortoises are more healthy than those in the wild
C. suggest an explanation for certain disease symptoms
D. emphasize how widespread a disease is in a population
E. discuss the effects of diet on the desert tortoise’s susceptibility to disease