Linear and landscape disturbances alter Mojave desert tortoise movement behavior

Hromada, Steven J.; Esque, Todd C.; Vandergast, Amy G.; Drake, K. Kristina; Chen, Felicia; Gottsacker, Ben; Swart, Jordan; Nussear, Kenneth E.

doi:10.3389/fevo.2023.971337

Cited by 2 publications

(4 citation statements)

References 73 publications

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“…Further, when in this state, they moved even greater distances than when in the traveling state and far from the highway. This finding was opposite that of other recent research, which found that Mojave desert tortoises moved shorter distances when near a highway 34 . We hypothesize that the difference may be due to either (a) analysis design, as Hromada et al 34 pooled males and females for their distance to highway covariate and did not interact sex*transition probability*distance to highway, or (b) forage differences, as their highway site was on the western side of the tortoise’s range which is characterized by lower elevation and drier precipitation regimes than our study in the Eastern Mojave 38 , 40 , 51 .…”

Section: Discussioncontrasting

confidence: 99%

“…However, we did document a tortoise crossing the highway twice via a culvert. Such short-term excursions across linear barriers have been previously documented in Mojave desert tortoises, and indicate that the road effect on tortoise movement did not render the highway as a completely impermeable barrier 34 – 36 .…”

Section: Discussionsupporting

confidence: 55%

“…Previous research identifying “road effect” zones for the Mojave desert tortoise predominantly concluded that direct mortality of local populations was the most likely cause of lower tortoise abundance near highways 20 , 28 . Roadside exclusionary fencing may alleviate this mechanism, but recent tracking of free-moving Mojave desert tortoises has continued to find avoidance of roads or altered movement near roads 32 , 34 , 35 , 57 . The mechanisms behind this indirect effect may be noise or vibrations permeating from the highway outwards.…”

Section: Discussionmentioning

confidence: 99%

“…In addition to being an impermeable barrier, there is conflicting evidence on how roads indirectly affect movement of Mojave desert tortoises. In some cases, tortoises have longer steplengths when near low-traffic roads and shorter steps near highways 32 , 34 , but sometimes tortoises avoid roads altogether 35 . These long-term indirect impacts of exclusionary fencing on near-highway tortoise movement behaviors are not yet well-understood.…”

Section: Introductionmentioning

confidence: 99%

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Indirect impacts of a highway on movement behavioral states of a threatened tortoise and implications for landscape connectivity

Harju,

Cambrin,

Berg

2024

Sci Rep

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Roads have often been identified as barriers to the movement of free-ranging animals. However, whether restoration of landscape connectivity across roadways can mitigate barriers to movement is insufficiently understood in light of indirect effects of roads on wildlife movement. We GPS-tagged free-ranging Mojave desert tortoises (Gopherus agassizii) to quantify movement behavioral states using hidden Markov models in relation to a major highway and to document use of existing, permeable culverts. We then used the observed movement behaviors to parameterize simulations of tortoise movement to evaluate alternative culvert designs and placements for enhancing connectivity across the roadway. Tortoises were most active during mid-day, in warm temperatures, and when close to the highway. The highway affected transition probabilities between movement states, as females were more likely than males to switch to an energy-demanding traveling movement state, remain in that state, and move farther than usual within that state. In contrast, males were more likely than females to continue in the low-energy resting state when close to the highway, but if traveling, to travel farther than usual. We observed two highway crossings by a tagged tortoise, which was a higher rate of crossing than in simulated tortoises. Simulated crossing rates increased with culvert size and culvert density, and size and density appeared more important for crossing than if culverts were placed singly or in pairs. Existing culvert densities across the region appeared potentially sufficient for long-term genetic connectivity, but only if retrofitted to allow for tortoise access and passing. We concluded that existing highway traffic may indirectly depress tortoise populations adjacent to the highway, particularly via negative impacts to female movements, and that existing culverts in washes should be retrofitted to allow for periodic tortoise crossings to improve structural connectivity for occasional passage.

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