Disentangling the effects of multiple anthropogenic drivers on the decline of two tropical dry forest trees


Correspondence author. E-mail: ticktin@hawaii.edu


1. Tropical plant populations are often subject to multiple types of anthropogenic disturbance. Effective management requires disentangling the effects of these disturbances and prioritizing interventions for the driver(s) most responsible for population decline. However, the effects of multiple drivers on plant population dynamics are rarely examined.

2. We assessed the independent and combined effects of common anthropogenic disturbances on the transient and long-term population dynamics of two economically important, declining tree species in an Indian dry forest. Specifically, we drew on 10 years of demographic monitoring to assess the effects of non-timber forest product (NTFP) harvest and two invasive species (an understorey shrub Lantana camara and a mistletoe Taxillus tomentosus) on amla (Phyllanthus emblica and Phyllanthus indofischeri) populations.

3. Although fruit harvest has been blamed for declining amla populations, the current policy and management strategies implemented to restrict it have little effect on long-term stochastic growth rates (λs) of amla both with and without invasive species. In contrast, mistletoes significantly decreased λs of both species.

4. Lantana had both direct and indirect effects on P. emblica, causing a regeneration bottleneck. Lantana had a direct negative effect on seeding and sapling growth, whereas populations without lantana experienced higher levels of grazing by wild animals. Over 10 years, P. emblica populations dropped to 16% of their original size in areas with invasive species.

5.Synthesis and applications. Our results illustrate that mistletoe and lantana, not fruit harvest, are the main drivers of amla decline, and these species are likely to be driving the decline of other Indian dry forest tree species. Management directed only at limiting fruit harvest will be ineffective. Instead, control of both invasive species combined with temporary protection from grazing is urgently needed. The ban on fruit harvest in Indian protected areas is not an effective conservation policy for these species. Harvest is not necessarily the main cause of decline for NTFP species. Management plans for NTFP and other at-risk species must consider the relative effects of different drivers of decline, including direct and indirect effects of invasive species.