The construction of roads within desert tortoise habitat presents several challenges for the long-term persistence of tortoise populations. Habitat fragmentation divides tortoise home ranges and may prevent tortoises from accessing food and appropriate shelter. Isolation inhibits gene flow and leads to inbreeding depression. Direct mortality resulting from traffic collisions removes otherwise healthy individuals from
the population. Roadways also allow people to more easily come into contact with desert tortoises. The stress of human-tortoise interactions may affect the animal’s ability to survive, while collection of desert tortoises removes the animal from the population altogether and can be viewed as an additive source of mortality.
Fragmentation and direct mortality have been attributed to population-level declines in desert tortoises; however, the prevalence of illegal collection has not been addressed. In this study, a live tortoise decoy was used to estimate the probability of specific motorist behaviors when encountering a desert tortoise along three road types: 2-lane paved, maintained gravel and non-maintained gravel.
Live Decoy Setup
Each monitoring session consisted of a four person crew: 1 central observer, 2 lookouts (1 of them an AGFD officer) and 1 tortoise. Lookouts were posted approximately ¼ mile in either direction from the central observer and notified him or her of approaching traffic via radio. The central observer then placed the decoy on the side of the road opposite oncoming traffic and recorded human-tortoise interactions from a concealed location. Interactions were placed into one of six categories:
- no response
- slowed vehicle
- stopped and viewed
- moved decoy off roadway
- collected decoy
In the event that the decoy was collected, Law Enforcement personnel immediately stopped the vehicle, interviewed the occupants and retrieved the decoy.
Our approach to data analysis reflected the fact that illegal tortoise collection can be decomposed into two probabilities: (1) the probability of detection and (2) the probability of collection. We used multinomial logistic regression to estimate the probabilities of motorist response to the presence of a tortoise across the three road types. Our analysis consisted of two steps. First, we estimated the probability of detection for each road category. Second, we estimated the predicted probabilities of motorist responses given that the tortoise was detected. Overall probabilities of four motorist behaviors along each of the three road types were then calculated as the product of the predicted probability of detection and the predicted probability for each of the behaviors.
Results and Discussion
We conducted a total of 50 monitoring sessions at 50 sites and obtained a total of 561 observations of human-tortoise interactions. No collisions with the live tortoise decoy occurred during the study. Motorists traveling along maintained gravel roads had the highest probability of detecting the decoy. Heavy vegetation and rough terrain likely contributed to low tortoise detection on non-maintained roads, while high speeds may be the cause on paved roads. Once the decoy was noticed, the probability of specific motorist behaviors varied greatly among road types. People traveling along 2-lane paved roads were most likely to slow the vehicle without stopping. On maintained gravel roads, motorists had the highest probability of stopping to view the tortoise without touching it. Motorists on non-maintained gravel roads were most likely to move the tortoise off of the roadway. Between 1 and 3% of the motorists that detected the tortoise illegally collected it and the likelihood of collection was highest on maintained gravel roads.
Motorists that collected the decoy expressed ignorance regarding state laws restricting collection. Increased educational efforts, such as posting signs on gravel roadways within tortoise habitat may help to reduce collection. Additionally, exclusion fencing to keep tortoises from accessing the road surface will reduce the number of direct mortalities from collisions while also decreasing detection rates of passing motorists.
Desert tortoises are long-lived and have low reproductive rates. The loss of a single healthy, adult female may affect local population persistence. Based on reproductive and survival estimates for desert tortoises in the Mojave Desert, a single female may contribute up to 6.85 tortoises that will survive to reach sexual maturity over her lifetime. Significant increases in naturally low juvenile survival rates would have to be achieved to compensate for the effective increase in adult mortality due to illegal collection.
This study was funded by the Bureau of Land Management and the Arizona Game and Fish Department
For more information contact
David Grandmaison, Research Biologist
Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086