Resilience of palm populations to disturbance is determined by interactive effects of fire, herbivory and harvest

Little is known about the interactive effects of multiple forms of disturbance - natural or anthropogenic - on plant population dynamics. This limits our ability to understand the drivers of these dynamics and effectively manage plant populations in the face of changing disturbance regimes. Fire, grazing and harvest of wild plants are three widespread, commonly co-occurring land management activities in the tropics that contribute to altered disturbance regimes. In this study, we investigate the effects of and interactions among these activities on mountain date palm (Phoenix loureiroi) populations in the Western Ghats, India with data from 14 palm populations, including over 2300 palm ramets monitored from 2009 to 2011. We construct integral projection models (IPMs) of palm population dynamics based on the response of palm individuals to fire and varying intensities of grazing and leaf harvest and carry out Markov Chain Monte Carlo simulations to understand the implications of different scenarios of disturbance for population persistence. We find that palm individuals and populations were resilient to low levels of fire, grazing and harvest. There are also important interactions among these forms of disturbance: intensities of harvest and grazing are highest in populations one to two years after fire, and the intensity of harvest is lower in heavily grazed populations. These interactions mean that the combined effects of multiple forms of disturbance differ from what would be assumed from simply adding their separate effects. While simulations show that sustained high intensities of harvest and grazing would have severely negative consequences for palm populations, interactions between these two forms of disturbance prevent palm populations from actually experiencing such extreme conditions. On the other hand, because intensities of grazing and harvest both increase moderately following fire, their combined negative effects on population growth rates are exacerbated by fire. Thus, multiple, interacting disturbances determine the resilience of these palm populations and the sustainability of their management.Synthesis. This study illustrates the importance of explicitly incorporating realistic interactions among multiple forms of disturbance when evaluating plant populations' resilience to changing disturbance regimes and the sustainability of plant harvest.