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.     

On the optimal path in the dynamic pool model for a fishery

Ecosystem-based fishery management (EBFM) is a new direction for fishery management, essentially reversing the order of management priorities to start with the ecosystem rather than the target species. This concept of management is a direct extension of the concept of a holistic approach incorporating interspecific interactions and physical environmental influences. However, because of the limited understanding of the complexity of marine ecosystems, few fisheries are actually managed on a multispecies basis. Even now, in order to specify a practical fishing policy we need a single-species model and utilize it by partially taking account of the effects of other factors mentioned above on the target species biomass. In fact, it is contended that in systems with moderate amounts of data, EBFM could be characterized by effective single-species management with the addition of precautionary set-asides for unknown ecosystem components. Hence, it is still necessary to examine a single-species model so as to clarify the extent of its applicability. The model investigated in this paper is what is called the dynamic pool model, which was proposed by C.W. Clark in the mid-1970s as a dynamic optimization of the classic Beverton and Holt static model for a fishery, in an attempt to make the process of growth and aging inherent in each of the creature resources reflect directly into the economic process. This dynamic model has been applied to a wide variety of commercial fish species. However, the applications have been largely confined to computer simulations using the discrete-time stand-by of the original Clark continuous-time model. This situation is caused mainly by the complexity of the mathematical structure of the Clark model. In this paper, we first specify the material related to the complexity. Subsequently, we provide a rigorous proof for the long-standing conjecture due to Clark concerning the optimal path or harvesting schedule. In addition, two derivative cases are examined: one is the case in which a year-class of fish leaves a given fishing sea area permanently before its natural biomass peaks, the other is the case in which the escapement of a year-class is required to be more than a given minimum level.     

The Beverton–Holt q-difference equation

The Beverton–Holt model is a classical population model which has been considered in the literature for the discrete-time case. Its continuous-time analogue is the well-known logistic model. In this paper, we consider a quantum calculus analogue of the Beverton–Holt equation. We use a recently introduced concept of periodic functions in quantum calculus in order to study the existence of periodic solutions of the Beverton–Holt q-difference equation. Moreover, we present proofs of quantum calculus versions of two so-called Cushing–Henson conjectures.     

A bioeconomic analysis of bushmeat hunting

Unsustainable bushmeat hunting is a major threat to mammal species, particularly in West/Central Africa. We developed a multispecies dynamic simulation model of hunter behaviour, parameterized using data from the Ashanti region, Ghana. The model distinguishes between two hunting techniques, snaring and gun hunting. We analyse the impact of key economic parameters on off-takes. Economic incentives determine the effort devoted to hunting, the choice of hunting technique, and the species that are consumed domestically or traded in markets. These factors, together with the growth rates and catchabilities of hunted species, determine the ecological impact of hunting. The results suggest that increased bushmeat prices are likely to lead to a switch from snaring, which is cheaper but less efficient, to gun hunting, with a consequent impact on vulnerable species. Increases in agricultural prices have an ambiguous effect on hunter behaviour, depending on the balance between incentives to invest in agriculture and increased consumption as incomes improve. Penalties are more effective if they target bushmeat sales, rather than the act of hunting. This model represents a step forward because it explicitly considers bushmeat as a component of the household economy. This has important implications as regards the development of policies to conserve species hunted for bushmeat.     

Public health significance of zoonotic bacterial pathogens from bushmeat sold in urban markets of Gabon, Central Africa

Wild animal meat represents an important source of protein for many people in central Africa. Also known as bushmeat, this meat commodity is derived from wild animals hunted under uncontrolled conditions, transported to distant markets under rudimentary or no hygienic methods, and often eviscerated >24 hr after death. Considering the plausible role of wildlife as a reservoir for bacterial zoonotic pathogens, bushmeat may be an important public health risk in Central Africa. This cross-sectional survey served to evaluate the presence of Campylobacter, Salmonella, and Shigella in the muscle tissue of 128 wild animal carcasses from several hunted wildlife species (guenons [Cercopithecus spp.], collared mangabeys [Cercocebus torquatus], gray-cheeked mangabeys [Lophocebus albigena], African crested porcupines [Atherurus africanus], duikers [Cephalophus spp.], and red river hogs [Potamocherus porcus]) sold in two markets of Port-Gentil, Gabon, in July and August 2010. Salmonella was detected from one carcass; no Campylobacter or Shigella was detected. If Campylobacter and Shigella were present, the maximum expected prevalence was estimated at 6% and 1%, respectively. In light of such very low apparent muscle contamination levels, bushmeat likely does not represent a health risk per se with respect to Campylobacter, Salmonella, or Shigella. However, because carcass evisceration and skinning can take place within households prior to consumption, consumers should follow strict hygiene and food safety practices to avoid potential health hazards associated with the handling, preparation, or consumption of bushmeat.     

Eating and conserving bushmeat in Africa

In Africa, overhunting of tropical wildlife for food remains an intractable issue. Donors and governments remain committed to invest in efforts to both conserve and allow the sustainable use of wildlife. Four principal barriers need to be overcome: (i) communities are not motivated to conserve wildlife long‐term because they have no formal rights to benefit from wildlife, or to exclude others from taking it on their land; (ii) multispecies harvests, typical of bushmeat hunting scenarios, place large‐bodied species at risk of extinction; (iii) wildlife production cannot expand, in the same way that livestock farming can, to meet the expected growth in consumer demand; and (iv) wildlife habitat is lost through conversion to agriculture, housing, transportation networks and extractive industries. In this review, we examine the actors involved in the use of wildlife as food and discuss the possible solutions required to address urban and rural bushmeat consumption. Interventions must tackle use and conservation of wildlife through the application of context‐relevant interventions in a variety of geographies across Africa. That said, for any bushmeat solution to work, there needs to be concurrent and comparable investment in strengthening the effectiveness of protected area management and enforcement of wildlife conservation laws.     

Bushmeat genetics: setting up a reference framework for the DNA typing of African forest bushmeat

The bushmeat trade in tropical Africa represents illegal, unsustainable off‐takes of millions of tons of wild game – mostly mammals – per year. We sequenced four mitochondrial gene fragments (cyt b, COI, 12S, 16S) in >300 bushmeat items representing nine mammalian orders and 59 morphological species from five western and central African countries (Guinea, Ghana, Nigeria, Cameroon and Equatorial Guinea). Our objectives were to assess the efficiency of cross‐species PCR amplification and to evaluate the usefulness of our multilocus approach for reliable bushmeat species identification. We provide a straightforward amplification protocol using a single ‘universal’ primer pair per gene that generally yielded >90% PCR success rates across orders and was robust to different types of meat preprocessing and DNA extraction protocols. For taxonomic identification, we set up a decision pipeline combining similarity‐ and tree‐based approaches with an assessment of taxonomic expertise and coverage of the GENBANK database. Our multilocus approach permitted us to: (i) adjust for existing taxonomic gaps in GENBANK databases, (ii) assign to the species level 67% of the morphological species hypotheses and (iii) successfully identify samples with uncertain taxonomic attribution (preprocessed carcasses and cryptic lineages). High levels of genetic polymorphism across genes and taxa, together with the excellent resolution observed among species‐level clusters (neighbour‐joining trees and Klee diagrams) advocate the usefulness of our markers for bushmeat DNA typing. We formalize our DNA typing decision pipeline through an expert‐curated query database – DNAbushmeat – that shall permit the automated identification of African forest bushmeat items.     

Extinction and invasion do not add up in noisy dynamic ecological networks

Species extinction and invasion concurrently affect the composition and properties of ecological communities, yet their effects have largely been studied separately, and with more focus on species and ecological functional groups than the whole-community level. We adopted a dynamic ecological network approach to compare the effects of simultaneous single-species primary extinction and invasion on a set of ecosystem metrics to the effects of extinction and invasion in isolation. We also investigated the relationship between the impact and reversibility of extinction or invasion through reintroduction or eradication, respectively. We used Monte Carlo simulations of bioenergetic ecological network models that combined trophic and mutualistic interactions, contained either prey-dependent or ratio-dependent trophic functional responses, and incorporated either white or pink environmental stochasticity. As the separate extinction or invasion impact increased, the simultaneous extinction–invasion impact increased but was decreasingly additive of the two separate impacts, across all ecosystem metrics. Greater extinction or invasion impact was associated with lower reversibility for most model types and ecosystem metrics. There were also systematic differences between models with prey- and ratio-dependent functional responses. These results highlight the importance of considering the combined effects of extinction and invasion in ecological studies, management and restoration.     

Evolution in metacommunities

A metacommunity can be defined as a set of communities that are linked by migration, and extinction and recolonization. In metacommunities, evolution can occur not only by processes that occur within communities such as drift and individual selection, but also by among-community processes, such as divergent selection owing to random differences among communities in species composition, and group and community-level selection. The effect of these among-community-level processes depends on the pattern of migration among communities. Migrating units may be individuals (migrant pool model), groups of individuals (single-species propagule pool model) or multi-species associations (multi-species propagule pool model). The most interesting case is the multi-species propagule pool model. Although this pattern of migration may a priori seem rare, it becomes more plausible in small well-defined 'communities' such as symbiotic associations between two or a few species. Theoretical models and experimental studies show that community selection is potentially an effective evolutionary force. Such evolution can occur either through genetic changes within species or through changes in the species composition of the communities. Although laboratory studies show that community selection can be important, little is known about how important it is in natural populations.     

Non-Additive Increases in Sediment Stability Are Generated by Macroinvertebrate Species Interactions in Laboratory Streams

Previous studies have shown that biological structures such as plant roots can have large impacts on landscape morphodynamics, and that physical models that do not incorporate biology can generate qualitatively incorrect predictions of sediment transport. However, work to date has focused almost entirely on the impacts of single, usually dominant, species. Here we ask whether multiple, coexisting species of hydropsychid caddisfly larvae have different impacts on sediment mobility compared to single-species systems due to competitive interactions and niche differences. We manipulated the presence of two common species of net-spinning caddisfly (Ceratopsyche oslari, Arctopsyche californica) in laboratory mesocosms and measured how their silk filtration nets influence the critical shear stress required to initiate sediment grain motion when they were in monoculture versus polyculture. We found that critical shear stress increases non-additively in polycultures where species were allowed to interact. Critical shear stress was 26% higher in multi-species assemblages compared to the average single-species monoculture, and 21% greater than levels of stability achieved by the species having the largest impact on sediment motion in monoculture. Supplementary behavioral experiments suggest the non-additive increase in critical shear stress may have occurred as competition among species led to shifts in the spatial distribution of the two populations and complementary habitat use. To explore the implications of these results for field conditions, we used results from the laboratory study to parameterize a common model of sediment transport. We then used this model to estimate potential bed movement in a natural stream for which we had measurements of channel geometry, grain size, and daily discharge. Although this extrapolation is speculative, it illustrates that multi-species impacts could be sufficiently large to reduce bedload sediment flux over annual time scales in streams where multiple species of caddisfly are present.     

The impacts of different management strategies and environmental forcing in ecological communities

Understanding the effects of population management on the community a target species belongs to is of key importance for successful management. It is known that the removal or extinction of a single species in a community may lead to extinctions of other community members. In our study, we assess the impacts of population management on competitive communities, studying the response of both locally stable and unstable communities of varying size (between four and 10 species) to three different management strategies; harvesting of a target species, harvesting with non-targeted catch, and stocking of the target species. We also studied the consequences of selecting target species with different relative abundances, as well as the effects of varying environmental conditions.We show here how the effects of management in competitive communities extend far beyond the target population. A crucial role is played by the underlying stability properties of the community under management. In general, locally unstable communities are more vulnerable to perturbation through management. Furthermore, the community response is shown to be sensitive to the relative density of the target species. Of considerable interest is the result that even a small (2.5%) increase in the population size of the target species through stocking may lead to extinction of other community members. These results emphasize the importance of considering and understanding multi-species interactions in population management.     

Regime, phase and paradigm shifts: making community ecology the basic science for fisheries

Modern fishery science, which began in 1957 with Beverton and Holt, is ca. 50 years old. At its inception, fishery science was limited by a nineteenth century mechanistic worldview and by computational technology; thus, the relatively simple equations of population ecology became the fundamental ecological science underlying fisheries. The time has come for this to change and for community ecology to become the fundamental ecological science underlying fisheries. This point will be illustrated with two examples. First, when viewed from a community perspective, excess production must be considered in the context of biomass left for predators. We argue that this is a better measure of the effects of fisheries than spawning biomass per recruit. Second, we shall analyse a simple, but still multi-species, model for fishery management that considers the alternatives of harvest regulations, inshore marine protected areas and offshore marine protected areas. Population or community perspectives lead to very different predictions about the efficacy of reserves.     

Bushmeat Consumption and Preferences of Two Ethnic Groups in Bioko Island, West Africa

We studied consumption and preference of meats of wild species (bushmeat) by inhabitants of Bioko Island, Equatorial Guinea. The aim of the study was to quantify frequency of consumption and stated preferences of the two main ethnic groups (Bubi and Fang) in the island. Although members of both ethnic groups lived on the island, the Fang originated from the continent and maintained strong links with this area. Thus, preference and consumption of the Fang reflected exposure to animals found in the continent as well as on Bioko. A sample of 196 subjects (115 Bubi and 81 Fang) was interviewed using semistructured questionnaires. A total of 55 different bushmeat species was identified as preferred or consumed by interviewees. Principal component analyses of stated consumption and preference indicated differences between ethnic groups in their general responses. Further analyses of the effects of preference and other factors on consumption of the three main species mentioned (blue duiker (Cephalophus monticola), Emin's rat (Cricetomys emini), and brush-tailed porcupine (Atherurus africanus) were undertaken. Proportional odds logistic regression models for ordered categorical response data were employed. Results indicated that age and sex of the respondent did not affect consumption, but ethnic group was statistically significant for the three-study species. Consumption and preference of the different meats (N = 11 species) in relation to their availability in the market and price was studied using multiple linear regressions. Consumption is driven predominantly by availability but there is some influence of preference; price of the meat did not have a significant influence.     

Revisiting Beverton–Holt recruitment in the presence of variation in food availability

Understanding density-dependent changes in juvenile survival and growth rates is of great importance because these rates determine recovery rates for imperiled populations and/or sustainable harvest rates. Unfortunately, the mechanisms leading to density dependent survival and growth are among the least understood process in biology and fisheries. Previous work has shown that small fish may vary foraging times to achieve a target growth rate, resulting in the well-known Beverton–Holt recruitment function with variation in food availability affected the initial slope of the recruitment curve. We amend their derivation to show that incorporating fish growth under a variety of evolutionary strategies for balancing foraging time and predation risk still leads to recruitment approximately as expected under the Beverton–Holt recruitment model but that changing food availability affects both the initial slope and maximum recruitment level. We demonstrate that when food availability is known to vary over time, these models often result in a more parsimonious alternative than the standard Beverton–Holt function. Further, Beverton–Holt recruitment is expected when foraging times are adjusted to balance fitness gains from growth against mortality risk. Finally, linking recruitment success to food availability warns that species with high scope for density dependent survival (high compensation ratio or steepness) may be extremely sensitive to changes in available food densities. This work emphasizes the sensitivity of stock-recruitment parameters to food availability and strongly suggests a need to carefully monitor lower trophic levels to better understand and predict dramatic changes in juvenile recruitment and carrying capacity.     

Persistence and permanence in a metapopulation model with space-limited recruitment

In this paper we analyze a metapopulation model with space-limited recruitment. The model describes the population dynamics of sessile adult and planktonic larvae in a common larval pool. We introduce the basic reproduction number of each species which is the expected number of future larvae reproduced by one larva. We consider the conditions for the persistence of the multi-species and multi-habitats model and the permanence of the single-species model. Subsequently, we consider the conditions for the existence of the non-trivial steady state of the single-species model and its global stability, and the permanence of the two species and two habitats model.     

The Seedling Ratio Method for determining ungulate impacts on forest understoreys: Utility in an Australian ecosystem

The Seedling Ratio Method was devised to assess the impacts of introduced ungulates on plant species richness in forest understoreys. The method has successfully assessed ungulate impacts on species richness in forests in New Zealand and Hawaii, which do not have native herbivores. We tested the three critical assumptions that underlie this method to investigate its potential for use in an Australian ecosystem. This study was conducted in the Yarra Ranges National Park (YRNP), Victoria, which had a high‐density population of an introduced deer species, Sambar (Cervus unicolor), in addition to several native herbivore species. One of the three key assumptions of the Seedling Ratio Method was not supported, whereas conventional differential exclosures showed clear and separate impacts by Sambar and native herbivores. We conclude that the Seedling Ratio Method could not provide a clear indication of browsing impact on forest understoreys in YRNP.     

The ecological drivers and consequences of wildlife trade

Wildlife trade is a key driver of extinction risk, affecting at least 24% of terrestrial vertebrates. The persistent removal of species can have profound impacts on species extinction risk and selection within populations. We draw together the first review of characteristics known to drive species use – identifying species with larger body sizes, greater abundance, increased rarity or certain morphological traits valued by consumers as being particularly prevalent in trade. We then review the ecological implications of this trade-driven selection, revealing direct effects of trade on natural selection and populations for traded species, which includes selection against desirable traits. Additionally, there exists a positive feedback loop between rarity and trade and depleted populations tend to have easy human access points, which can result in species being harvested to extinction and has the potential to alter source–sink dynamics. Wider cascading ecosystem repercussions from trade-induced declines include altered seed dispersal networks, trophic cascades, long-term compositional changes in plant communities, altered forest carbon stocks, and the introduction of harmful invasive species. Because it occurs across multiple scales with diverse drivers, wildlife trade requires multi-faceted conservation actions to maintain biodiversity and ecological function, including regulatory and enforcement approaches, bottom-up and community-based interventions, captive breeding or wildlife farming, and conservation translocations and trophic rewilding. We highlight three emergent research themes at the intersection of trade and community ecology: (1) functional impacts of trade; (2) altered provisioning of ecosystem services; and (3) prevalence of trade-dispersed diseases. Outside of the primary objective that exploitation is sustainable for traded species, we must urgently incorporate consideration of the broader consequences for other species and ecosystem processes when quantifying sustainability.     

Effects of invasive seaweeds on feeding preference and performance of a keystone Mediterranean herbivore

The consequences of invasive species on ecosystem processes and ecological interactions remain poorly understood. Predator–prey interactions are fundamental in shaping species evolution and community structure and can be strongly modified by species introductions. To fully understand the ecological effects of invasive species on trophic linkages it is important to characterize novel interactions between native predators and exotic prey and to identify the impacts of invasive species on the performance of native predators. Although seaweed invasions are a growing global concern, our understanding of invasive algae—herbivore interactions is still very limited. We used a series of feeding experiments between a native herbivore and four invasive algae in the Mediterranean Sea to examine the potential of native sea urchins to consume invasive seaweeds and the impacts of invasive seaweed on herbivore performance. We found that three of the four invasive species examined are avoided by native herbivores, and that feeding behaviour in sea urchins is not driven by plant nutritional quality. On the other hand, Caulerpa racemosa is readily consumed by sea urchins, but may escape enemy control by reducing their performance. Recognizing the negative impacts of C. racemosa on herbivore performance has highlighted an enemy escape mechanism that contributes to explaining how this widespread invasive alga, which is preferred and consumed by herbivores, is not eradicated by grazing in the field. Furthermore, given the ecological and economic importance of sea urchins, negative impacts of invasive seaweeds on their performance could have dramatic effects on ecosystem function and services, and should be accounted for in sea urchin population management strategies.     

The bushmeat trade in southwestern Nigeria: A case study

Hunters and bushmeat retailers in Bendel State, Nigeria were interviewed from May through August 1982. Hunting was found to take place in all months of the year, but became more intense during the dry season. Twentyseven species of mammal were reported to be hunted, while 22 species of mammal and five species of reptile were found on sale. Retailers reported that the grasscutter (Thryonomys swinderianuswas the most popular species among their customers, followed by Maxwell's duiker, the brushtailed porcupine, and the bushpig. The grasscutter was also the most widely sold species, followed by the giant rat, monkeys, and Maxwell's duiker. Game animals were found to be shot and sold with little regard to existing laws, with the result that the larger mammals have become rare in Bendel and populations of the smaller ones have come under severe pressure. The bushmeat trade is highly commercialized, and substantial profits accrue to middlemen. Recommendations are made for the conservation of a natural resource, wildlife, that not only provides a significant amount of animal protein but also supports a large rural economy.     

Distinct impacts of different mammalian herbivore assemblages on arctic tundra CO2 exchange during the peak of the growing season

Herbivores play a key role in the carbon (C) cycle of arctic ecosystems, but these effects are currently poorly represented within models predicting land–atmosphere interactions under future climate change. Although some studies have examined the influence of various individual species of herbivores on tundra C sequestration, few studies have directly compared the effects of different herbivore assemblages. We measured peak growing season instantaneous ecosystem carbon dioxide (CO₂) exchange (photosynthesis, respiration and net ecosystem exchange) on replicated plots in arctic tundra which, for 14 years, have excluded different portions of the herbivore population (grazed controls, large mammals excluded, both small and large mammals excluded). Herbivory suppressed photosynthetic CO₂ uptake, but caused little change in ecosystem respiration. Despite evidence that small mammals consume a greater portion of plant biomass in these ecosystems, the effect of excluding only large herbivores was indistinguishable from that of excluding both large and small mammals. The herbivory‐induced decline in photosynthesis was not entirely attributable to a decline in leaf area but also likely reflects shifts in plant community composition and/or species physiology. One shrub species – Betula nana – accounted for only around 13% of total aboveground vascular plant biomass but played a central role in controlling ecosystem CO₂ uptake and release, and was suppressed by herbivory. We conclude that herbivores can have large effects on ecosystem C cycling due to shifts in plant aboveground biomass and community composition. An improved understanding of the mechanisms underlying the distinct ecosystem impacts of different herbivore groups will help to more accurately predict the net impacts of diverse herbivore communities on arctic C fluxes.