Predicting the impacts of palm heart and fruit harvesting using Integral Projection Models

Abstract

Due to the increasing human impact on natural resources, we assessed the harvesting of non-timber forest products (NTFP) to verify demographic changes in populations of native palm trees. Euterpe edulis is native to the Atlantic Forest in Brazil, characterized by high deforestation and fragmentation. This palm is also targeted for palm heart and fruit harvesting. The threats posed by such factors motivated this study, as they might lead to a decrease in natural populations. The viability of sustainably harvesting the species in small fragmented areas is unknown. We performed simulations for palm heart and fruit harvesting in three small populations (entitled: SH, AJ, and ES) sampled in a 1-year interval (2010–2011) to verify whether these practices were sustainable. Different harvesting scenarios were simulated: (1) no harvesting; (2) harvesting of palm heart of reproductive individuals; (3) harvesting of palm heart of large individuals (diameter at ground level > 65 mm, including reproductive or not). and (4) fruit harvesting. Each scenario was simulated at different harvesting intensities (percentage of individuals or fruits harvested). Integral projection models were used to calculate two demographic parameters, namely, population growth rate (λ) and elasticity. In the no harvesting scenario, the populations had λ > 1 (SH = 1.0655, AJ = 1.0184, and ES = 1.0862). Palm heart harvesting proved to be sustainable in both scenarios, but at a higher intensity in scenario (2) (SH = 83%, AJ = 14%, and ES = 35%) than in scenario (3) (SH = 17%, AJ = 4%, and ES = 16%). Fruit harvesting was sustainable at any intensity for all three populations. As the survival of large individuals has a high impact on λ, palm heart harvesting was in most cases sustainable only at low intensities. In contrast, as fecundity and seedling survival have a low impact on λ, fruit harvesting still proved sustainable at high intensities. Although the populations are tolerant to harvesting to some degree, it must be conducted carefully. As populations are fragile due to the current condition of reduced population size, the removal of palms at any rate can affect population persistence and generate possible cascade effects on the forest.

Population responses to environmental stochasticity are primarily driven by survival-reproduction trade-offs and mediated by aridity

“…Additionally, we suggest that, because the effects of reproduction on population responses to environmental stochasticity are highly predictable by evolutionary history, there is an opportunity to improve the parameterisation and accuracy of demographic models in population viability analyses 53 . This opportunity includes models examining early impacts of exploitation, such seed harvesting 54 . When survival (most important and less predictable vital rate) cannot be fitted in the model parameterization, reducing uncertainty in other vital rates such reproduction is imperative 53 .…”