Quantifying off-highway vehicle impacts on density and survival of a threatened dune-endemic plant

Off-highway vehicle (OHV) recreation impacts species and ecosystems around the world, and is particularly prevalent in the United States. Our study examines the impact of OHVs on one of several species restricted within the United States to the Algodones Dunes, California. This dune system attracts many OHV enthusiasts annually, and interest in determining the impact of OHVs on Astragalus magdalenae var. peirsonii (Peirson’s milk-vetch) has motivated multiple investigations. Several of these investigations have arrived at seemingly contradictory conclusions. Our study objectives were to quantify the degree by which A. m. var. peirsonii densities differed between high- and low-OHV use areas and determine whether OHV impact was a plausible factor for affecting plant density. Density estimate comparisons indicated 4–5 times fewer plants occupied a study area open to OHVs relative to two nearby study areas legally closed to OHVs. A manipulative mark-resight experiment compared the survival probabilities of control plants to those purposefully struck by OHVs and found that, for plants with canopy diameters of <50 cm, OHV impact reduced survival by 33%. Density estimates from one of the study areas recently closed to OHVs were surprisingly large, which may suggest that periodic closure of occupied habitat during favorably wet years would assist in ensuring A. m. var. peirsonii productivity and thereby persistence. An improved understanding of this phenomenon may indicate a dunes-wide management strategy that would allow OHV activity to coexist with A. m. var. peirsonii.