Ecological drivers of rare desert milkweed persistence amid utility-scale solar energy
Desert ecosystems in southwestern North America face multiple direct and indirect effects of disturbance by utility-scale renewable energy development. For plants, these may include population reduction, fragmentation, and altered herbivore and pollinator densities. At five sites, including two directly impacted by utility-scale energy development, we assessed survival, adult and seed dormancy, growth, and fecundity of a rare iteroparous desert perennial (Ascelpias nyctaginifolia) over seven seasons, spring and fall of each year spring 2011 - fall 2014. We also deployed herbivore exclusions at two unimpacted sites to assess the demographic effects of herbivores > 1 cm2. From field observations on adult plants and recurits and both field and laboratory experiments on seed viability and germination, we calculated site- and season-specific vital rates. We built population projection models and used life table response experiments to assess differences between populations, seasons, and treatments.
We found significant reduction of plant performance and its correlates, including seed production, adult dormancy, and herbivory rates within the solar field and in mitigation transplants. In undisturbed sites, exclusion of herbivores >1 cm significantly improved vegetative growth and fruit production, but reduced seeds/fruit, suggesting that pollination by large pollinators is increases fecundity. Seeds/fruit were also significantly lower in the solar field than in undisturbed populations. Site- and season-specific population growth rates (λ) were highly responsive to local weather conditions. Lambda generally fluctuated near and below the replacement rate, except for at two sites that responded with significant increases in population growth after a deluge in fall of 2013.
Performance of plants within the solar field was comparable to undisturbed sites with respect to plant growth and fruit production. However, elevated herbivory, increased adult dormancy, and reduced seed production may eventually threaten persistence of plants within the solar field. Survival, growth, and recruitment within this lower-impact design solar facility provide clear benefit over standard designs that eliminate habitat entirely. Additional research on the A. nyctaginifolia trophic system, including effects on its invertebrate herbivores and their avian predators, is necessary to determine the long-term effects of the solar installation and guide adaptive management.