Print PagePlant populations experience multiple forms of environmental variation and disturbance concurrently, many of which likely interact to influence population dynamics. Despite this, studies of plant demography rarely test for the effects of or interactions among multiple environmental factors on population dynamics. Determining the nature of such interactions is key to understanding the relative importance the many environmental factors plants experience to patterns and processes at multiple ecological scales. Fire, livestock grazing, and wild plant harvest commonly co-occur within the tropical human-modified ecosystems where the majority of the world’s remaining terrestrial biodiversity resides. These factors, individually and in combination, have the potential to significantly impact plant population dynamics. We test the effects of, and interactions among, fire, grazing, and leaf harvest on wild populations of mountain date palm (Phoenix loureiri Kunth) in the Western Ghats region of India. We use two approaches, with data from 2009-2011: 1) An observational study of 14 palm populations across areas with and without commercial palm harvest and with varying times since fire, and 2) A manipulative factorial-design experiment that tests for the effects of and interactions among fire, leaf harvest and grazing on nearly 400 palm individuals.
Results/Conclusions
We find that palm population dynamics are affected by multiple interactions among harvest, grazing, and fire. Projected population growth rates (λ) based on matrix models show that fire and harvest together reduce population growth more than would be expected from the impact of either alone. Results from a life table response experiment reveal that this interactive effect is due largely to reduced adult survival in harvested populations exposed to recent fire, even though harvest and fire do not directly kill palm individuals. Harvest also reduces projected population growth rates through reductions in fertility and survival of vegetative sprouts. Fire reduces survival of adults and juveniles. Results from our observational study and manipulative experiment show that the negative impact of recent fire on palm population dynamics is due in part to increased browsing on palm plants – by both livestock and wild animals – following fire. Because leaf harvest intensity and grazing both increase following fire, fire return interval is vital to the management of harvested palm populations. The interactions we observe among multiple environmental factors are likely common, especially in human-managed ecosystems. Accounting for such interactions can improve the conservation of harvested or threatened plant species and control of invasive species.
