8.4 Sustainable harvest of renewable natural resources

All living species are able to reproduce faster than needed to sustain constant population sizes. If they could not do so, they would not be able to recover from periods of increased mortality. The fact that populations can bounce back from disturbances demonstrates their ability to grow their populations.

Because all species can grow in numbers, it is, theoretically, possible for humans to harvest individuals without threatening the existence of the species involved. So long as the harvest leaves enough individuals to sustain the harvested populations, harvest can be sustainable.

It’s not a bad theory, and it holds true for a variety of plants and animals in a variety of settings. For example, waterfowl hunting in North America has supported both subsistence and recreational hunting across three countries fairly successfully for many decades.^[1] But humans have been involved in unsustainable harvest since at least the last Ice Age, when they were a driver of extinction of the large mammals of that period. We have not needed huge numbers of humans or advanced technology to develop unsustainable practices.

For any species in any area, there is a limit to the number of individuals the area can support sustainably, called the carrying capacity. Carrying capacity is not constant through time, but depends on the productivity of the area – usually affected by climate and nutrients available to plants – the nature and number of food species, predators, and competitors, and any disturbances, natural or anthropogenic.

It’s natural to think that management of harvested species would seek to maintain them at carrying capacity – it’s the maximum number of individuals one should manage for – but a population at carrying capacity is a population close to its limits. Food is just enough to sustain the population, leading to higher mortality of young and old individuals, and lower productivity of reproducing individuals. Instead, for harvested species, the goal of harvest regulations is typically to maintain the population well below carrying capacity, where the population is still large enough to withstand unexpected disturbances, but small enough that resources are relatively abundant and reproduction can be quite high, producing lots of “excess” individuals that would increase the population under natural conditions but can be harvested under managed conditions that seek to maintain the base population size.

This section discusses both terrestrial and marine species because many of the issues associated with harvest are similar.

Understanding sustainable harvest – fish and wildlife

We need detailed information about population characteristics in order to determine a sustainable level of harvest, regardless of the species involved. The larger the proportion of “extra” individuals we want to harvest, the greater the need for accurate information and care in managing the harvest.

For wildlife populations, starting information includes a good estimate of total population size, rate of increase of the population, and the age classes of reproductive individuals and their age-specific reproduction rate. If animals do not reproduce before they are 3 years old, we need to be sure that a good number of individuals reach that age. If only a few three-year-olds reproduce at all, and they all have a single offspring, but 4-year olds often have twins, and animals 5 years old or older might have three or more offspring, then we may want many individuals to reach at least 5 years of age.

But how are trappers, hunters, or fishers to know how old the individuals they target are? Few species are easy to age, accurately. In some mammals, males can be aged to some extent by size of horns or antlers, and size is correlated to age in some species, but not always closely. As a result, even if we have good information about age and reproduction, we may not be able to use it when harvesting.

Harvest limits on male mammals with horns and antlers may specify recognizable categories of those to try to approximate age classes. But for many species, it may be hard to do more than separate the very young from the not-very-young.  Harvest limits can be set under such conditions, but they have to factor in uncertainty about age and the chances of harvesting reproductively important individuals. Birds and mammals are typically caught one at a time, and are not subject to commercial harvest as fish are. Under these circumstances, it’s easier to document harvest, or, at least, legal harvest.

Harvest limits on fish that are caught on single-hooks lines often specify a required minimum size because size is easy to measure and determining age requires killing the fish. For fish that are caught in nets, the mesh size of the net can allow small individuals to escape, but those caught in the net are likely to be seriously injured or killed when the net is hauled in. It’s still possible to throw too-small individuals back overboard, if the catch is being handled at the level of individual fish while at sea, but this may only serve to feed sharks and other species that accompany fishing vessels.

An additional complication in understanding sustainable harvest in fisheries is the role of technology in assisting harvest. Advanced sonar systems on fishing boats not only locate schools of fish but can, in some instances, identify species. Gear to handle miles of nets and lines of baited hooks and “mother ships” that can receive the harvest of many fishing ships extend the reach of fishing effort and enable it to continue nonstop. This level of effort requires well-informed management to understand the potential for large harvests.

Impacts on non-target species

Nets are examples of unspecific harvesting gear – they catch whatever enters them that cannot escape through the mesh – jellyfish, sea turtles, sea birds, marine mammals, and many species of non-target fish. Drift nets, which are typically miles long, are now illegal in the territorial waters of many nations, but are still used on the high seas. When they become old and tattered, they are often cut loose from the ship and discarded into the ocean where they can continue to catch anything in their path, as “ghost nets.”^[2] Exclusionary devices are available for some kinds of nets and some kinds of non-target species, to allow escape of unwanted catch. They increase the cost of the nets and are not always popular, but in some areas, they are required.

Baited hooks, particularly if they are on longlines – kilometer-long fishing lines lined with hooks – also catch many non-target species. Non-target or wrong-size fish caught on longlines are unlikely to survive if released, due to exhaustion and injury. Birds, sea turtles, and marine mammals drown.

Nontarget fisheries catch is referred to as bycatch, and in some fisheries, it comprises the majority of the catch. A recent review of bycatch assessments reported bycatch rates ranging from <1% to >90% depending on the target species. Shrimp fisheries are consistently the worst for bycatch.^[3]

Among harvest methods for birds and mammals, nets (birds) and traps and snares (mammals) may catch and kill non-target animals. Nets are illegal as a means of trapping birds in most countries. Legal use of traps and snares is often for furs or to eliminate pest species, rather than for food. Nontarget species typically do not survive.

Note that the occurrence of bycatch does not necessarily compromise the sustainability of a particular method if sustainability is judged only by the status of the target species. Judgements of sustainability in such cases should properly consider the full range of impacts of the harvest, not only impacts to the target species.

Planning for disturbance

All species are subject to disturbances that may cause gradual or sudden reductions in population sizes – wildfire, drought, flood, disease, changes in predator or prey populations, etc. Such disturbances are not always detectable – for example in fish from the deep ocean or high seas, or terrestrial species that are secretive – yet such disturbances may significantly reduce populations of target species. When this occurs, previously safe harvest levels may threaten population viability.

Monitoring of harvested populations and harvest calculations that factor in the possibility of changes in population size after harvest levels are set can help to limit harm following unexpected mortality, but these require the additional expense of monitoring and conservative harvesting approaches that may be unpopular with stakeholder harvesting groups. Managers sometimes differentiate between maximum sustained yield that may not leave much margin for safety and optimum sustained yield that explicitly acknowledges possibilities of error in the understanding of population processes and the possibility of surprises. However, in some cases, calculations of maximum sustained yield are conservative and account for similar issues as optimum sustained yield – one has to examine the assumptions and calculations to know for sure.

Requirements for sustainable harvest

Sustainable harvest is more likely to be successful with species that reproduce at least somewhat quickly. Wildlife and fisheries experts differentiate between species that reproduce quickly and at fairly early ages, called r-selected species, and species that reproduce slowly and later in life, called K-selected species; of course, many species fall somewhere between these categories. As an extreme example of a K-selected species, Greenland sharks mature sexually around age 150 and have a gestation period of 8 to 18 years, after which they give birth to perhaps 10 young. Although a female may give birth to hundreds of offspring in her life, the slow rate of reproduction makes the species very vulnerable to overfishing – population recovery could take centuries, and any overestimate in the population size could easily lead to dangerous overfishing. Unfortunately, Greenland sharks are not the target of a species-specific fishery, but are more likely to be taken as bycatch, so no fisheries management accompanies their harvest.

The problem of nontarget harvest highlights another important aspect of sustainable harvest – it should be planned, and the vast majority of harvest should be legal and reported. Nontarget take is unmanaged and its sustainability cannot be guaranteed. Illegal take – poaching of plants and wildlife and illegal and unreported landings of fish – can reduce viability of populations but also threaten entire species with extinction. In the case of wildlife, poaching often targets individual species or groups of species, however illegal capture techniques such as wire snares can catch many species. Illegal and unreported landings of fish often involve nontarget fishing techniques which reduce fish available for subsistence and commercial harvest, render population estimates for legal fishing untrustworthy, and increase bycatch of fish, birds, marine mammals, sea turtles, and other groups.

Obviously, we cannot accurately estimate the level of illegal harvest. The most trafficked mammal in the world is the pangolin – a group of 8 species of medium-sized mammals that look like a cross between an anteater and a pinecone (Fig 1). Estimates of illegal harvest range from hundreds of thousands to over a million, annually. They are taken for food in Africa and as a delicacy and for their scales, for medicinal purposes, in Asia. Trade in illegal natural-resource harvest is considered the fourth largest illegal market in the world.^[4] For fishing, a US agency report puts illegal and unreported fishing at 20% of global take, with national levels as high as 50%,^[5] but these are estimates with little data behind them.

Understanding sustainable harvest – trees

In general, tree populations are more easily understood than animal populations – trees stand still, and as a result, population monitoring is more straightforward. Ownership of plants and plant products can also be more straightforward. For harvest purposes, we typically deal with trees on the basis of their diameter, which is fairly easily measured, and their height, which can usually be estimated with some reasonable degree of accuracy.  

Just as we harvest wild fish and also have aquaculture, we harvest trees from natural forests and also have plantations. Silviculture, the tree equivalent of agriculture, is the process of managing trees for forest products, and involves practices for both natural forests and plantations.

We divided trees into conifers and broad-leaved trees when we studied terrestrial biomes. Timber producers make the same division, but refer to softwoods (conifers) and hardwoods (broad-leaved trees), although some hardwoods are noticeably harder than others. Softwoods are used for paper, cardboard, dimension lumber used in construction (2x4s and so forth), and engineered wood products such as plywood and strand board. Many of these products can be made from small-diameter trees, but large lumber beams require older trees. Hardwoods, in contrast, are used for furniture, flooring, veneer, and the pallets used in shipping. These tend to require larger trees. Both softwood and hardwood trees may be used to create wood pellets and other forms of biomass for energy. Wood of any size, including branches, and even shrubs, can be used for pellets.

Harvest from natural forests can involve harvest of selected individual trees or clusters of trees, leaving a thinned but recognizable forest; harvest of most of the trees, leaving some to provide a bit of shade and seeds to stock the next generation; or clearcutting of all the trees. If only certain species or sizes are desired, then so-called select cutting approaches will work, and leave behind a forest that will still provide habitat to many (but not all) forest species. However, if only the highest-quality trees are being taken out without any plan for continuing to increase the value of the forest – sometimes called highgrading – then forest health and economic value is likely to decrease over time. Select cutting is standard in natural hardwood stands. Hardwoods are also grown in plantations for commercial purposes and on private land, often for investment value.

Clearcutting – the removal of all trees on a site – can be controversial. Visually, the impact is overwhelming – one day a forest is there, and then there is a forest of stumps, with piles of removed branches and the marks of forest equipment on what was the forest floor. Clearcutting can increase erosion and impair water quality of nearby streams, in some cases for years. It completely removes forest habitat, reducing biodiversity significantly. But so can intense wildfires, ice storms, and windstorms.

The primary stated purpose of clearcutting is to regenerate a more desired forest stand. For purposes of timber production, natural forests contain unwanted tree species, and older trees may grow more slowly and are more likely to have diseases that reduce their value.

Clearcutting offers managers an opportunity to reset the forest to the desired species (one or more, but usually just one) and to take advantage of the faster growth rates of young trees. In addition, some tree species have young stages that need sunlight for growth, and those species grow poorly in the shade of a mature forest. However, arguments that clearcutting is not about profit but only about site preparation for the desired forest are disingenuous and ignore the fact that a clearcut of a natural mature or old-growth forest may produce the most valuable harvest the land will ever produce – a harvest more valuable than any subsequent, managed harvest, which will almost certainly involve younger, smaller trees.

On a site with relatively intact soil, a new forest will begin to grow almost immediately, or, in drier places, as soon as the rains begin. Wildlife species adapted to open areas will soon arrive, but the suite of species will differ substantially from the species that occupied the original forest. As more and more older forests are cut, the species adapted to mature and old-growth forests have declined, some of them precipitously, and the profit motives that accompany commercial timber harvest do not permit such forests to return. Managed forests may grow to the smaller end of the mature size classes, but often do not get even that old.

Timber managers use the term rotation length to describe the period of time between successive clear cuts. Forests grown for wood pellets (energy), paper pulp, and lower-value timber products – often conifer stands such as pine – may be managed on rotations as short as 10 years. These stands can be thought of as a slow version of corn. Sites are prepared, stands are fertilized, treated with pesticides and harvested. Value of short-rotation forests to wildlife is low but not absent. Some of the environmental impacts of agriculture (including fertilizer and pesticide runoff) occur, although usually at lower levels than for commercial agriculture.

Longer rotation lengths allow trees to grow larger, and may increase biodiversity, depending on how the stand is managed^[6]. Managed stands on public lands typically receive less intensive management and may support more biodiversity. However, such stands are still likely to be monocultures or near monocultures when they are replanted after harvest.

Some energy tree crops are grown as coppices – fairly dense, planted stands of trees that are harvested by cutting back the stems every 2-5 years (Fig 2). Unlike pines and firs, trees grown in this way are usually broadleaf trees such as willow that are able to sprout back after their stems are cut to the ground. Some species can be cut 7 or more times before they must be dug up and replanted.^[7]

Plantations of all types are often sited on land that was originally natural forest, so they represent an overall long-term loss of natural forests and of biodiversity. Because plantations are heavily managed, many species of plants and animals are excluded. Plantations may also attract pest species and disease organisms, to the detriment of nearby natural forest.

Genetically modified trees are increasingly used for plantation stock. Such genetic engineering now involves a host of characteristics including growth rate (which affects rates of carbon sequestration), resistance to disease, resistance to herbicides (much as in agricultural crops, to allow herbicides to be used freely in plantations to suppress any competing plant growth), etc. Recently, genome editing was used to reduce the amount of lignin (a natural “woody” polymer) in poplar trees, which in turn reduced the chemicals needed to process the wood into engineered wood for structural building material. The resulting engineered product was stronger and longer lasting than previous products.^[8]

Use of engineered varies widely, with some countries having almost all plantations planted in engineered species (mostly poplar, eucalyptus, pines) and other countries banning them. The distinction between genome-editing, which only involves changes to the existing genome, and genetic modifications in which genes from other organisms are inserted into the tree genome, is sometimes important in defining whether the new varieties are legal.

Certifying sustainability of harvested natural resources

Sustainability certification programs seek to ensure that natural resources are harvested ethically and responsibly, in ways that protect biodiversity, environmental quality, and social values. In order to be viable, such programs must meet a social or economic demand, so that producers and consumers will support certification. Transparency and oversight are important to maintain trust in the system. Small-scale producers may not have access to such programs, may be unaware of them, or may not be able to afford to meet their requirements The Forest Sustainability Council, the most widely used forest-products certification organization, makes specific efforts to include smallholders in its program.^[9]

In addition to certification programs such as the Forest Sustainability Council (FSC) or the Marine Stewardship Council (MSC), nonprofit organizations provide some sustainability recommendations such as the US Monterey Bay Aquarium’s Seafood Watch guides that help consumers to make informed choices about seafood.^[10]

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.