Optimal Harvesting Strategies for Timber and Non-Timber Forest Products in Tropical Ecosystems
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Author's personal copy Theor Ecol DOI 10.1007/s12080-015-0286-4 ORIGINAL PAPER Optimal harvesting strategies for timber and non-timber forest products in tropical ecosystems Orou G. Gaoue1,2 · Jiang Jiang3 · Wandi Ding4 · Folashade B. Agusto5 · Suzanne Lenhart3 Received: 20 August 2015 / Accepted: 12 November 2015 © Springer Science+Business Media Dordrecht 2016 Abstract Harvesting wild plants for non-timber forest growth species have lower optimal harvesting rates, objec- products (NTFPs) can be ecologically sustainable–without tive functional values and profits than fast growth species. long-term consequences to the dynamics of targeted and However, contrary to expectation, the effect of species lifes- associated species–but it may not be economically satisfy- pan on optimal harvesting rates was weak suggesting that ing because it fails to provide enough revenues for local life history is a better indicator of species resilience to har- people over time. In several cases, the same species can vest than lifespan. Overall, lethal or nonlethal harvest rates be harvested for NTFP and also logged for timber. Three must be <40 % to ensure optimality. This optimal rate is decades of studies on the sustainability of NTFP harvest for lower than commonly reported sustainable harvest rates for local people’s livelihood have failed to successfully inte- non-timber forest products. grate these socio-economic and ecological factors. We apply optimal control theory to investigate optimal strategies for Keywords Differential equations model · Harvest model · the combinations of non-lethal (e.g., NTFP) and lethal (e.g., Life history · Lifespan · Non-timber forest products · timber) harvest that minimize the cost of harvesting while Optimal control theory · Sustainable timber harvest maximizing the benefits (revenue) that accrue to harvesters and the conservation value of harvested ecosystems. Opti- mal harvesting strategies include starting with non-lethal Introduction NTFP harvest and postponing lethal timber harvesting to begin after a few years. We clearly demonstrate that slow Identifying sustainable harvest limits for renewable resources and how these limits are constrained by socio- economic and environmental factors represent some of the most debated issues in conservation biology. Non-timber Orou G. Gaoue forest products (NTFPs) such as fruits, leaves, and resins [email protected] are increasingly harvested from wild populations as source 1 of food and medicine to local people worldwide and are Department of Botany, University of Hawaii at Manoa, part of a growing interest from pharmaceutical firms (Bawa Honolulu, HI 96822, USA et al. 2004). Harvesting NTFP provides a range of benefits 2 Universite de Parakou, 123, Parakou, Benin for local people and can contribute to poverty alleviation (Shackleton and Shackleton 2004; Shackleton et al. 2011). 3 National Institute for Mathematical and Biological Synthesis, In developing countries, more than 80 % of the population University of Tennessee, Knoxville, TN 37996, USA relies on medicinal plants for primary healthcare (Hamilton 4 Department of Mathematical Sciences and Computational 2004), and 72,000 species of medicinal plants are used Science Program, Middle Tennessee State University, regularly by local people with 3000 species as part of the Murfreesboro, TN 37132, USA international trade (Schippmann et al. 2003, 2006). During 5 Department of Ecology & Evolutionary Biology, the dry season when agricultural products are scarce and University of Kansas, Lawrence, KS 66045, USA herbaceous pasture burned, several fruits are harvested for Author's personal copy Theor Ecol direct consumption, and tree foliage is harvested to feed excelsa) are also harvested for medicinal and food purposes cattle (Emanuel et al. 2005; Avocevou Ayisso et al. 2009; (Ndoye and Tieguhong 2004). Similarly, in Benin, most of Gaoue and Ticktin 2010). The NTFP market is estimated at the fodder tree species (e.g., Khaya senegalensis, Afzelia more than US$90 billion (Pimentel et al. 1997). From an africana, Pterocarpus erinaceus) whose foliage are repeat- economic perspective, NTFP harvesting provides revenue edly harvested by local people to feed cattle during the dry to families who collect these products directly from the for- season are also the main timber tree species (Gaoue and est, and even higher net margins for bundlers and industries Ticktin 2007, 2009). In Brazil, Carapa guianensis is valu- at the top of the value chain. However, harvesting comes able for its NTFP (for its seeds which produce prized oils) with a huge ecological cost in some cases (Schmidt et al. and also for its timber (Klimas et al. 2012a). 2011; Ticktin 2015). Studies on the sustainability of NTFP or timber harvest Several plant species are harvested too frequently and have developed independently, and even within the litera- at very high intensity such that they are threatened with ture on NTFP harvest, there are two parallel sets of studies. extinction (Hall and Bawa 1993; Peres et al. 2003; While NTFP harvest studies are rarely concerned with opti- Silvertown 2004; Potts and Vincent 2008; van Andel and mal harvesting strategies, early studies on timber harvest Havinga 2008). For example, Prunus africana is a tree focused on determining the optimal harvesting rotation or whose bark is harvested in Central and Southern Africa age (Hardie et al. 1984; Plantinga 1998; Chang 1998), and to treat prostatic hypertrophy. As a consequence of over- on the effects of price fluctuation on optimal harvesting harvesting, multiple populations of this tropical tree are strategies (Newman et al. 1985; Brazee and Mendelsohn extinct (Cunningham and Mbenkum 1993). Brazil nuts have 1988). On the other hand, studies on the sustainability of been harvested for decades (Silvertown 2004)andinsome NTFP harvest have historically been divided into two cat- parts of South America, populations of Bertholletia excelsa egories. A first set of studies focus on the socio-economic are declining as a result of chronic fruit harvest (Peres et al. sustainability of harvesting. There is an extensive body of 2003). Recent reviews of the ecological impact of NTFP work on the role that NTFP harvesting plays in alleviat- harvest suggest that most species are facing decline as a ing (or not) poverty (Cosyns et al. 2011; Shackleton and consequence of harvesting (Schmidt et al. 2011; Ticktin Pandey 2014; Shackleton and Shackleton 2004), on the 2004). In most cases, harvesting NTFP (non-lethal) does value of NTFP and on the average revenues that accrue to not lead to the death of harvested individuals. For several local collectors of NTFP (Klimas et al. 2012b; Godoy et al. NTFP plants, local people remove fruits, bark, and leaves 2000; Gopalakrishnan et al. 2005; Vodouheetal.ˆ 2009). A from standing plants. For example, harvesting Brazil nuts second set of studies focus on ecological sustainability of (Zuidema and Boot 2002) or fruits from Sclerocarya birrea harvest and use matrix projection models (Caswell 2001) (Emanueletal.2005) does not involve cutting down har- to estimate the long-term population growth rate, λ,(the vested trees. However, for some NTFP harvesting such as dominant eigenvalue) and used simulations to identify sus- Amla fruits from Phyllantus emblica in India (Ticktin et al. tainable NTFP harvesting thresholds (Bernal 1998; Ticktin 2012), or Gaharu, a fragrant resinous wood, from Aquilaria et al. 2002, 2012; Zuidema et al. 2007; Gaoue et al. 2011; spp. in Indonesia (Soehartono and Newton 2001), individu- Mondragon and Ticktin 2011; Klimas et al. 2012a). Unfor- als plants could be cut down prior to harvesting these NTFP. tunately, these two lines of research on the sustainability For P.africana, some harvesters girdle trees to maximize the of NTFP exploitation by local people have also evolved amount of bark they harvest from each trees and this leads independently and understanding the socio-economic and to the death of individual trees (Stewart 2009). Nearly 28 % ecological sustainability of such activities remains elu- of the 188 medicinal plants species surveyed in Suriname sive (for a broader discussion see Armsworth et al. 2010). were harvested lethally (van Andel and Havinga 2008). Hernandez Barrios et al. (2015), in an attempt to combine Although most studies on the sustainability of forest both ecological and economic data to define sustainable leaf resources harvest considered either NTFP or timber har- harvest, focused on maximizing the economic profit under vesting, in reality, several species in tropical ecosystems the constraint that survival and growth of individuals are not are often harvested both for timber and non-timber forest significantly reduced. Macpherson et al. (2012) also utilized products (van Andel and Havinga 2008; Guariguata et al. a similar approach, where economic data are used to update 2009, 2010; Klimas et al. 2012b; Rist et al. 2012; Grogan the outcome from an ecological model. Although these et al. 2014). Guariguata et al. (2010) in an extensive review approaches are valid, they fail to directly integrate costs provides evidence for several cases of combined timber and benefits with the conservation value of the remaining and non-timber forest products harvest on the same species stands. or the same forest. For example, more than half of the Here, we use for the first time, optimal control theory main timber species harvested in Cameroon (e.g., Triplochi- to investigate sustainable harvest strategies for non-timber