Assessing Sustainability of Nontimber Forest Product Extractions: How Fire Affects Sustainability

Sustainable use of nontimber forest products (NTFPs) can be affected by levels of extractions as well as by other anthropogenic influences such as fire and grazing. We examined the effects of fire on the demography of Phyllanthus emblica, an important NTFP in the forests of Biligiri Rangan Hills, India. We then assessed demographic responses to the combined effects of fire and current fruit harvesting patterns. Fruits of Phyllanthus are commercially harvested by an indigenous forest dwelling people. Using matrix population models, we compared demographic indices across a chronosequence of time since last fire. Population growth rates (λ) ranged from 0.7692 to 1.1443 across the five times since last fire. λ was the lowest at times since last fire of 2 and 3 yr. Frequent fires increased time to maturity by altering growth and survival rates, thereby causing a demographic shift from growth to regressions or negative growth. Elasticity analysis revealed that stasis of adults makes the biggest contribution to λ. Simulations of periodic and stochastic fire regimes suggest that higher λ and population persistence can be achieved at fire-return intervals of ≥7 and ≥9 yr, respectively. These fire-return intervals became longer when the simulations included harvesting and fire. Extinction probabilities under the current fire regimes (every 2–3 yr) suggest that populations will decline to lower densities. Our findings provide critical information for developing guidelines for sustainable use and management of NTFPs in Biligiri Rangan Hills, and demonstrate the need to incorporate various human-generated physical regimes in assessing sustainability of NTFPs.     

Culling Versus Density Effects in Management of a Deer Population

Abstract: Wildlife managers often manipulate hunting regulations to control deer populations. However, few empirical studies have examined the level of hunting effort (hunter-days) required to limit population growth and demographic effects through harvesting of females. Moreover, the relative importance of density effects on population growth has not been quantified. We reconstructed a sika deer [Cervus nippon] population over a period of 12 years (1990–2001) using age- and sex-specific harvest data. Using cohort analysis, we analyzed population dynamics, focusing on 1) the relationship between hunting effort and hunting-induced mortality rate, 2) relative contributions of hunting mortality and recruitment of yearlings to annual changes in population growth rate, and 3) annual variation in recruitment rate. Population size increased until 1998 and declined thereafter. The population growth rate changed more in response to annual changes in recruitment rate than hunting mortality rate. Temporal variation in recruitment rate was not controlled by birth rate alone; direct density dependence, intensities of hunting mortality for fawns, and for females (≥2 yr of age), which accounted for the fawn survival rate, were required as factors to explain temporal variation. Density effects on the recruitment rate were not strong enough to regulate the population within the study period; high hunting mortality, with intensive female harvesting, was necessary to prevent population growth. Hunting effort was a good predictor of the hunting mortality rate, and female harvest had a negative effect on the recruitment rate through fawn survival. We suggest that >3,500 hunter-days and prioritization of female harvesting are required to prevent increases in this deer population.     

Demography of the vegetable ivory palm Phytelephas seemannii in Colombia, and the impact of seed harvesting

1. The demography of the vegetable ivory palm Phytelephas seemannii was studied on the Pacific coast of Colombia and a female-based matrix model was used to determine the proportion of seed that can be sustainably harvested from the population.
2. The density of the palm stands ranged from 240 to 420 adult palms ha⁻¹. The sex ratio was 1:1 and palms of both sexes produced leaves at the same rate. Seedlings produced 1·2 leaves per year on average, juveniles 1·8, and adults 6·1–7·4.
3. Adult females had fewer leaves than males (18·5 vs. 21·3 on average). Leaves of females lasted about 2·7 years in the crown, those of males about 3·2 years.
4. Stems creep on the ground, growing at the apex and often dying behind, the extant portion not always reflecting the palm's total age. The longest stem of a female palm in the study plots was 2·5 m, corresponding to an 'extant' age of 85 years; the longest stem recorded was that of a male (outside the plots) 10·5 m long, corresponding to an 'extant' age of 184 years. These figures reflect differences in growth habit, not in longevity.
5. Reproduction began at about 24 years, when the palm still lacked an above-ground stem.
6. The population growth rate λ was 1·059, and was most sensitive to changes in survival of juveniles and adults, and relatively insensitive to changes in fecundity and growth.
7. River channel migration is the most important cause of adult mortality. Phytelephas seemannii is apparently an efficient colonizer of the understorey in the late phases of riverine forest succession.
8. The population can tolerate a harvest intensity of 86% of all seeds before λ decreases to the equilibrium level of 1·00. Monitoring of the populations under intensive harvesting is required.     

Consequences of sex‐selective harvesting and harvest refuges in experimental meta‐populations

Harvesting for food or sport is often non‐random with respect to demographic state, such as size or life stage. The population‐level consequences of such selective harvesting depend upon which states are harvested and how those states contribute to population dynamics. We focused on a form of selective harvesting that has not previously been investigated in an experimental context: sex‐selective harvesting, a common feature of exploited, dioecious populations. Using simple metapopulations (two patches connect by dispersal) of sexually dimorphic Bruchid beetles in the laboratory, we contrasted the effects of female‐selective, male‐selective, and non‐selective harvesting over six generation of population dynamics. We also tested the ability of a harvest refuge (one patch of the metapopulation free from harvesting) to mitigate the effects of harvesting, and whether refuge effects interacted with sex selectivity. Sex‐selective harvesting significantly perturbed operational sex ratios and harvest refuges dampened these perturbations. Metapopulations assigned to male‐selective and non‐selective treatments were able to fully compensate for harvesting, such that their dynamics did not differ from non‐harvested controls. Only female‐selective harvesting led to significant reductions in population size and this effect was completely offset by dispersal from a harvest refuge. A two‐sex model confirmed that population dynamics are more sensitive to female vs. male harvesting, but suggested that higher levels of male harvest than included in our experiment would cause population decline. We discuss the roles of density‐dependent competition and frequency‐dependent sexual processes in the population response to sex‐selective harvesting.     

Density-Dependent Harvest Modeling for the Eastern Wild Turkey

Abstract Many current wild turkey (Meleagris gallopavo) harvest models assume density-independent population dynamics. We developed an alternative model incorporating both nonlinear density-dependence and stochastic density-independent effects on wild turkey populations. We examined model sensitivity to parameter changes in 5% increments and determined mean spring and fall harvests and their variability in the short term (3 yr) and long term (10 yr) from proportional harvesting under these conditions. In the long term, population growth rates were most sensitive to poult:female ratios and the form of density dependence. The nonlinear density-dependent effect produced a population that maximized yield at 40% carrying capacity. The model indicated that a spring or fall proportional harvest could be maximized for fall harvest rates between 0% and 13% of the population, assuming a 15% spring male harvest and 5% spring illegal female kill. Combined spring and fall harvests could be maximized at a 9% fall harvest, under the same assumptions. Variability in population growth and harvest rates increased uncertainty in spring and fall harvests and the probability of overharvesting annual yield, with growth rate variation having the strongest effect. Model simulations suggested fall harvest rates should be conservative (≤9%) for most management strategies.     

Female Fecundity Is Dependent on Substrate, Rather Than Male Abundance, in the Wind-Pollinated, Dioecious Understory Palm Chamaedorea radicalis

We examined the impact of substrate, population density, and sexual composition on female reproductive success in harvested populations of the tropical understory palm Chamaedorea radicalis . Leaf harvest for the international floral greenery trade does not kill palms, but has been shown to reduce flowering frequency and has been projected to reduce population growth. Because C. radicalis is dioecious and wind-pollinated, a reduction in flowering density from leaf harvest or other anthropogenic factors may lower pollen movement between flowering conspecifics and lower fruit production. Such reduced fecundity in harvested populations can contribute to an Allee effect, where the per capita rate of population growth declines at low density. We tested for these effects by sampling C. radicalis along transects in ten populations, spanning a range of densities, during May (flowering peak) and August (fruiting peak) 2002. We applied path analysis using structural equation modeling (SEM) to test a set of hypothesized relationships between substrate, female size, several population parameters, and female fecundity. SEM models revealed that female fecundity was not dependent on any measure of population density or sexual composition, providing no evidence for an Allee effect. The strongest model ( R ²= 92.9%) related C. radicalis reproductive output only to substrate, female size, flower number, and proportional fruit set. This model revealed that palms on rock outcrops were larger, produced more flowers, and had higher proportional fruit set than palms on the forest floor. This finding suggests that protection of females on outcrops should enhance population growth and viability.     

Population viability analysis for Guarianthe aurantiaca, an ornamental epiphytic orchid harvested in Southeast México

The trade of wild epiphytic orchids has put many populations at risk of extinction; however, little is known about the impact of repeated harvesting of individuals on population dynamics. A population viability analysis (PVA) was developed to simulate the impact of harvesting on the risk of extinction of a population of an epiphytic orchid that is sold in Mexican markets. In January 2004, 388 individuals of Guarianthe aurantiaca were labeled and measured. The same individuals were measured again in December 2005 and December 2006. Capsule production and mortality were also recorded. Deterministic (d) and stochastic (s) matrix models were developed to estimate the population growth rate (λ). A PVA was developed to evaluate the probability of extinction over a 100-year period, given a threshold of 5% of the initial population and different harvest intensities of G. aurantiaca . The λ_d values were statistically indistinguishable from unity (0.989 ± 0.103 for 2004–2005; 0.990 ± 0.087 for 2005–2006); the λ_s values were below unity (0.988 ± 0.001). The reproductive stages had the highest elasticity values, whereas the persistence of individuals in their category was the demographic process with the highest elasticity. A life table response experiment showed that the difference in the λ_d values was accounted for by the positive contribution of the stasis of individuals in category j and the retrogression of individuals in category r₁ to category j. The extinction risk was 100% when more than 5% of the reproductive individuals were harvested. The results suggest that this G. aurantiaca population is in a precarious equilibrium and harvesting should be controlled and restricted to immature individuals.     

The Edible Plant Dichelostemma capitatum: Its Vegetative Reproduction Response to Different Indigenous Harvesting Regimes in California

To more thoroughly understand the ecological effects of harvesting geophytes for food by American Indians, an investigation of the ethnobotany and population dynamics of Dichelostemma capitatum (blue dicks), an attractive source of nutrition to many California Indian groups was undertaken. Some cultural groups harvest the corms and replant the cormlets, spare plants, and harvest after seeding to ensure replenishment of seed. Some Indian elders equate judicious harvesting with the maintenance and enhancement of field populations of this geophyte. A field experiment was conducted to determine the degree to which differences in intensity and timing of harvest, with and without replanting of cormlets, have any effect on corm and cormlet production. We found that harvesting at 100% intensity, through digging up all plants and corms, and without replanting cormlets at the seed stage, significantly reduces numbers of corms and cormlets compared to the controls (no harvest). However, harvesting at 50% intensity, through digging up half of all plants and corms at the flowering or seed stages, without replanting cormlets, was not significantly different from the controls (no harvest). The results suggest that harvesting blue dicks corms with a digging stick in the latter way could yield a sustainable level of harvest. Indigenous harvesting and management regimes may offer some of the best examples of long-term uses and management of the regional flora without detriment to its biodiversity. Restorationists are urged to study and experimentally mimic indigenous disturbance regimes and their ecological effects known to occur historically in various ecosystems. In some cases, areas will greatly benefit from the reintroduction of management and harvesting regimes that authentically simulate ancient indigenous interactions.     

Biomass Allocation in the Dioecious Tropical Palm Chamaedorea tepejilote and Its Life History Consequences

Abstract The patterns of resource allocation are described for a dioecious tropical palm, Chamaedorea tepejilote. Resource allocation was measured by harvesting fifteen plants of C. tepejilote. The relative allocation of biomass in the stem increased with the size of the plant; that in the leaves decreased and that in the other structures remained roughly constant. Female plants showed a greater total reproductive effort, though male plants produced more inflorescences during the flowering season. Both male and female plants allocated more resources to prop root than to hypogeal roots. The annual productivity of reproductive and vegetative parts of C. tepejilote was estimated using allometric relationships for different plant structures and from demographic data obtained from the field. Annually, female plants allocated significantly more resources to leaves than male plants. Yearly productivity of inflorescences was higher for male plants, while female plants had greater total reproductive productivity (inflorescences and fruits). Correlation analysis showed an increase in reproductive effort with plant size, and an inverse relationship between fecundity and probability of survival, fecundity and residual reproductive value, and reproductive effort and life expectancy; these relationships suggested a cost in reproduction. Additionally, mature plants with different growth rates exhibited differences in fecundity: tall plants (>2.5m height) that grew more than 40 cm in height in four years had lower values of fecundity in comparison to plants of slower growth. These data were discussed in the context of the implications in the life history of a dioecious tropical plant.     

Effects of size‐ and sex‐selective harvesting: An integral projection model approach

Harvesting is often size‐selective, and in species with sexual size dimorphism, it may also be sex‐selective. A powerful approach to investigate potential consequences of size‐ and/or sex‐selective harvesting is to simulate it in a demographic population model. We developed a population‐based integral projection model for a size‐ and sex‐structured species, the commonly exploited pike (Esox lucius). The model allows reproductive success to be proportional to body size and potentially limited by both sexes. We ran all harvest simulations with both lower size limits and slot limits, and to quantify the effects of selective harvesting, we calculated sex ratios and the long‐term population growth rate (λ). In addition, we quantified to what degree purely size‐selective harvesting was sex‐selective, and determined when λ shifted from being female to male limited under size‐ and sex‐selective harvesting. We found that purely size‐selective harvest can be sex‐selective, and that it depends on the harvest limits and the size distributions of the sexes. For the size‐ and sex‐selective harvest simulations, λ increased with harvest intensity up to a threshold as females limited reproduction. Beyond this threshold, males became the limiting sex, and λ decreased as more males were harvested. The peak in λ, and the corresponding sex ratio in harvest, varied with both the selectivity and the intensity of the harvest simulation. Our model represents a useful extension of size‐structured population models as it includes both sexes, relaxes the assumption of female dominance, and accounts for size‐dependent fecundity. The consequences of selective harvesting presented here are especially relevant for size‐ and sex‐structured exploited species, such as commercial fisheries. Thus, our model provides a useful contribution toward the development of more sustainable harvesting regimes.     

An assessment of the maximum sustainable yield of ivory from African elephant populations.

A general, logistic population model is used to explore the dynamics of harvested elephant populations. The model includes two features peculiar to elephant populations and the harvesting of ivory. First, because of the shape of the growth curve of tusks with age, the conversion factor that relates the number of elephants killed to the ivory yield in weight is not constant, but a function of the population size. Second, tusks from animals that die from natural causes can be retrieved and included in the total yield of ivory. The implications of the relationship between tusk size and age of an animal on the maximum sustainable yield in terms of ivory tonnage and in terms of the number of tusks are explored. The nonequilibrium implications of the tusk growth curve on the population dynamics under different harvesting strategies are also investigated. Results indicate that the maximum sustainable yield is achieved at very low harvest rates with population levels close to the pristine equilibrium. When tusks from animals that die of natural causes are included in the harvest, the maximum yield may, depending on the mortality and recruitment parameters, occur when there is no direct harvest.     

Matrix population models indicate that bark harvest of two medicinal plants in Uganda's Bwindi Impenetrable National Park is sustainable

Balancing forest conservation with resource extraction by local people is challenging. In the mountain forests of Bwindi Impenetrable National Park in Uganda, this was addressed by permitting regulated access to certain forest products in specific areas by authorized local people. However, it remained unclear whether harvest levels were biologically sustainable. Here, we used matrix population models and long‐term data to examine the impacts of bark harvesting on population dynamics of two important medicinal plants, Rytigynia kigeziensis and Ocotea usambarensis, in Bwindi. Only 4% of R. kigeziensis and 3% of O. usambarensis stems (>1.3 m height) showed signs of bark harvest, mostly mild harvesting. We found that the harvested populations of both species appeared stable or will moderately grow in the long run. Modelled population growth rates were mostly determined by survival probabilities. Similarity between the stable stage distributions predicted by the model and observed population structures suggests that our estimated vital rates (growth, recruitment and survival rates) are a reasonable representation of actual values in these populations. Thus, recent harvest levels of R. kigeziensis and O. usambarensis appear sustainable. Nonetheless, monitoring of harvested and unharvested populations by tagging, marking and remeasuring individuals should continue for both species.     

Discovery of the human homolog of sex-determining region (SRY) gene in dioecious plants

Sex determination in the early developmental stages of dioecious crops is economically-beneficial. During this study, a human homology of SRY gene was successfully identified in dioecious crops. SRY gene sequences of date palm and jojoba were submitted to GenBank under the accession numbers KC577225 and MK991776, respectively. This is the first report regarding the novel sex-determination methodology of four dioecious plants (jojoba, date palm, papaya, and pistachios). SRY sex gene was found in all the tested dioecious plant and human samples. This novel approach is simple and of significant importance for breeders. It facilitates the unambiguous selection of jojoba and date palm female plants at an early age and reduces the plantation cost of cultivating non-productive male plants. This is a rapid sex-determination technique for dioecious plants and mammals at an early stage. This technique specifically targets the SRY sequence that has been comprehensively investigated in humans. The kit development for the SRY-based sex determination of various crops is in progress.     

Demography and sustainable management of two fiber‐producing Astrocaryum palms in Colombia

The spear leaves of the palms Astrocaryum chambira and A. standleyanum have been traditionally used by Colombian indigenous communities as a source of fiber for handicraft production. Traditional management practices, including destructive harvest, have reduced population sizes of both species. We monitored a population of A. chambira in the Amazon, and one of A. standleyanum at the Pacific lowlands of Colombia. We then constructed integral projection models (IPM) to evaluate the transient population dynamics of populations under different exploitation regimes. Our results show that during the next 50 years the population of A. standleyanum will grow at an annual rate of 2.0 percent, and that of A. chambira at a rate of 0.8 percent. However, projected population growth is highly sensitive to harvest in both species: a destructive harvest of 5 percent of all usable individuals (subadults and adults) would cease population growth, while a 10 percent harvest intensity would cause populations to decrease by 0.5–0.6 percent annually. Our simulations further indicate that management practices associated with indigenous slash‐and‐burn agriculture would reduce fiber production, whereas caring for seedlings would increase population growth and fiber production in the coming decades. In order to sustain viable populations of both species and maintain a steady fiber supply, it is vital to prevent destructive harvest practices, and to leave some forest areas untouched, where populations can regenerate and act as a source of seedlings for intervened areas.     

Demography of Euterpe precatoria and Mauritia flexuosa in the Amazon: application of integral projection models for their harvest

Non‐Timber Forest Products like palm fruits are fundamental in the livelihood of Amerindian groups. For the last 10 years the fruits of wild palm species, like Euterpe precatoria and Mauritia flexuosa, have been entering into global markets. These species are mostly harvested felling the adults, a practice that may have a disproportionate impact in the conservation of the populations. We studied the demography of E. precatoria and M. flexuosa, two important palms of the Amazon, using integral projection models, to characterize the population dynamics and project impact of different fruit harvest methods: adult felling and direct fruit harvest. We followed over 1400 individuals from populations with low harvest intensities in the Tikuna community of Amacayacu, Amazonas, Colombia from 2011 to 2013, establishing four plots for E. precatoria in seasonally inundated forests and two for M. flexuosa in swamps. The population growth in E. precatoria was slightly declining, whereas M. flexuosa was stable. As expected, the simulations of adult felling diminished λ and the number of available adults under any scenario for either species. On the contrary, the simulations support existing information that climbing and other non‐destructive harvest methods would probably allow a sustainable management, satisfying the demand and preserving wild populations.     

The Beverton–Holt model with periodic and conditional harvesting

In this theoretical study, we investigate the effect of different harvesting strategies on the discrete Beverton–Holt model in a deterministic environment. In particular, we make a comparison between the constant, periodic and conditional harvesting strategies. We find that for large initial populations, constant harvest is more beneficial to both the population and the maximum sustainable yield. However, periodic harvest has a short-term advantage when the initial population is low, and conditional harvest has the advantage of lowering the risk of depletion or extinction. Also, we investigate the periodic character under each strategy and show that periodic harvesting drives population cycles to be multiples (period-wise) of the harvesting period.     

The Beverton-Holt model with periodic and conditional harvesting

In this theoretical study, we investigate the effect of different harvesting strategies on the discrete Beverton-Holt model in a deterministic environment. In particular, we make a comparison between the constant, periodic and conditional harvesting strategies. We find that for large initial populations, constant harvest is more beneficial to both the population and the maximum sustainable yield. However, periodic harvest has a short-term advantage when the initial population is low, and conditional harvest has the advantage of lowering the risk of depletion or extinction. Also, we investigate the periodic character under each strategy and show that periodic harvesting drives population cycles to be multiples (period-wise) of the harvesting period.     

Harvesting in seasonal environments

Most harvest theory is based on an assumption of a constant or stochastic environment, yet most populations experience some form of environmental seasonality. Assuming that a population follows logistic growth we investigate harvesting in seasonal environments, focusing on maximum annual yield (M.A.Y.) and population persistence under five commonly used harvest strategies. We show that the optimal strategy depends dramatically on the intrinsic growth rate of population and the magnitude of seasonality. The ordered effectiveness of these alternative harvest strategies is given for different combinations of intrinsic growth rate and seasonality. Also, for piecewise continuous-time harvest strategies (i.e., open / closed harvest, and pulse harvest) harvest timing is of crucial importance to annual yield. Optimal timing for harvests coincides with maximal rate of decline in the seasonally fluctuating carrying capacity. For large intrinsic growth rate and small environmental variability several strategies (i.e., constant exploitation rate, linear exploitation rate, and time-dependent harvest) are so effective that M.A.Y. is very close to maximum sustainable yield (M.S.Y.). M.A.Y. of pulse harvest can be even larger than M.S.Y. because in seasonal environments population size varies substantially during the course of the year and how it varies relative to the carrying capacity is what determines the value relative to optimal harvest rate. However, for populations with small intrinsic growth rate but subject to large seasonality none of these strategies is particularly effective with M.A.Y. much lower than M.S.Y. Finding an optimal harvest strategy for this case and to explore harvesting in populations that follow other growth models (e.g., involving predation or age structure) will be an interesting but challenging problem.     

Effects of Commercial Harvesting on Population Characteristics and Rhizome Yield of Anemone altaica

Effects of Commercial Harvesting on Population Characteristics and Rhizome Yield of Anemone altaica. Commercial harvesting constitutes a direct threat to numerous non–timber forest products (NTFPs), but its ecological effects have not been well documented. Anemone altaica Fisch. ex C. A. Mey, a spring ephemeral plant found in temperate forests of Eurasia, is a traditional Chinese herb. Owing to medicinal value, its rhizomes have been harvested for commercial purposes in northwestern China for many years. This paper addresses the ecological effects of commercial harvesting on A. altaica populations under different harvest intensities. The results show that size–selective harvesting of rhizomes can increase population densities by asexual propagation. Currently, two– to three–year–old individuals derived from asexual propagation are the main targets of commercial harvesting. The increased demand in recent years has resulted in earlier and more intensive harvesting activities largely impacting the natural recovery of the harvested populations. For sustainable use of this traditional medicinal species, we recommend that a periodic harvest strategy of three to four years be adopted.     

Hunting Bambi—evaluating the basis for selective harvesting of juveniles

Human harvesting is often a major mortality factor and, hence, an important proximate factor driving the population dynamics of large mammals. Several selective harvesting regimes focus on removing animals with low reproductive value, such as “antlered” harvests in North America and juvenile harvesting in many European countries. Despite its widespread use and assumed impact, the scientific basis of juvenile harvesting is scattered in the literature and not empirically well-documented. We give the first overview of demographic, evolutionary and practical management arguments for selective harvesting of juveniles. Furthermore, we empirically test two demographic arguments based on harvest statistics of Red Deer (Cervus elaphus) in seven European countries. P₁: Harvesting juveniles has little influence on harvest growth compared with harvesting adult females due to the lower reproductive value of juveniles than adult females; P₂: Harvesting of juveniles dampens variance in harvest due to lower and more variable natural survival rates of juveniles compared with adults. We found that harvesting juveniles has little effect on harvest growth rate, while harvesting adult females has a significant negative effect (consistent with P₁), but that increasing the proportion of juveniles in the harvest did not decrease the variability in harvest between years (P₂ not supported). Based on our empirical findings and overview of arguments, we discuss how the merits of juvenile harvesting may vary over time as populations move from a low density to a very high density state.