What do matrix population models reveal about the sustainability of non-timber forest product harvest?
Abstract
Understanding how management activities impact plant population dynamics is necessary to conserve at-risk species, control invasive species and sustainably harvest non-timber forest products (NTFP). For NTFP, knowledge about how the sustainability of harvest varies by plant life-form and part harvested is limited and needed to inform management of the thousands of species providing income to millions of people world-wide. Matrix population models are commonly used to generate management recommendations. We reviewed studies of 46 NTFP species that used matrix models and synthesize the current knowledge on harvest effects. For 19 species with harvested and control populations, we assessed the impacts of harvest on projected population growth rates (λ) using meta-analysis and analysed trends in population responses to harvest across species, life-forms and plant part harvested using elasticity and life table response experiment (LTRE) analyses, and the combination of both, to assess vulnerability to harvest. NTFP harvest significantly reduced λ across species. On the scale of individual studies, however, λ provided little information about harvest sustainability unless replication was sufficiently high. Most studies had low levels of replication over space or time and did not include contrasting levels of harvest. Whole-plant harvest of herbs and bark harvest from trees were not sustainable largely because of decreases in survival. Palm leaf or fruit harvest and rattan stem harvest were potentially sustainable. Combined elasticity-LTRE analysis was especially valuable in assessing the sustainability of harvest when differences in λ between harvested and control populations were small, for studies with limited replicates, and where harvest effects varied regionally. Synthesis and applications. The use of matrix models to assess the impacts of NTFP harvest is still rare in regions where trade of wild plants is heaviest and for several commonly harvested life-forms. Given the high variance in estimates for most NTFP species, λ does not provide a precise assessment of harvest impacts. We recommend that managers consider the combined elasticity-LTRE analysis in addition to λ in making management decisions for NTFP. NTFP research that accounts for environmental drivers of population dynamics in addition to harvest should be prioritized.