The tangled causes of population decline in two harvested plant species: A comment on Ticktin et al. (2012)

Abstract

Ticktin et al. (2012) attempted to disentangle multiple stressors impacting harvested populations of amla (Phyllanthus emblica and P. indofischeri) [Ticktin et al. (2012) Disentangling the effects of multiple anthropogenic drivers on the decline of two tropical dry forest trees. Journal of Applied Ecology, 49, 774-784.]. We propose that an unbalanced design and substitution of missing data for key parameters render their matrix models flawed. They conclude that the main threats to recruitment were lantana and mistletoe invasion and recommend revoking the ban on amla harvest. Our re-analyses of their data set, without substituting unavailable data, showed that cessation of harvests significantly increased stochastic growth rates of amla, despite high lantana and mistletoe cover. Management recommendations by Ticktin et al. (2012) were based on the assumptions of invasiveness of a native mistletoe and grazing impacts of wild ungulates. However, interactions with amla are complex, and the recommendations made by them could have deleterious repercussions on native biota. Synthesis and applications. Developing an objective understanding of harvest consequences by incorporating earlier findings and considering uncertainties in results is critical for maintaining livelihoods and ecological processes linked to amla populations. © 2013 British Ecological Society.