Cover image: the black rhinoceros is one of the world’s most endangered mammals, though the population of ~5,500 has begun to increase. © Swati Sani
Welcome to the eleventh (and penultimate!) instalment of 100 Key Concepts. Last time we looked at which ecological functions or conservation priorities make species important. Today, we explore the reason we need conservation in the first place: what happens when things go wrong in the natural world?
84 TROPHIC CASCADES
Many people are familiar with the concept of a food chain: plants get energy from sunlight, herbivores eat the plants, and carnivores eat the herbivores. While this is a very simplified version of what happens in nature, it does the job here. When a species is added to or removed from a food chain, it has effects on the species around it. When the species is a top predator, this starts a trophic cascade, where consequences ripple down the food chain. This can have serious effects on an ecosystem, as herbivores become more common and the plants they eat less so, and can even switch the ecosystem to a completely different habitat. The best-recorded example of a trophic cascade occurred in Yellowstone National Park, in the northwestern USA, in the early 21st century. Grey wolves were extirpated in the Greater Yellowstone Ecosystem in 1925, and remained absent for decades until a pack was reintroduced in 1995/96. At the time, the park had a massive population of wapiti elk, which soon became the wolves’ favourite prey. Predation by wolves reduced the elk population, but had the added effect of making the surviving elk more afraid – since there could be a wolf anywhere, at any time, the elk never stayed in one place for long (this effect is called a landscape of fear). Once the elk’s behaviour changed, the tree seedlings they used to eat were given a chance to grow, and woodlands of aspen and willow trees regrew along the banks of Yellowstone’s rivers. Even species which rarely came into contact with the wolves benefited indirectly from their presence: the regenerating scrub provided the perfect food for bison, and the new woodlands boosted beaver populations.
87 MESOPREDATOR RELEASE
Herbivores aren’t the only species affected by apex predators. Mesopredators occur in the middle of the food chain, above herbivores but below apex predators. While top predators do sometimes hunt mesopredators, their impacts on prey populations impact other predators which feed on the same animals, and mesopredators are also susceptible to the landscape of fear. But when apex predators disappear, the mesopredators suddenly find themselves at the top of the food chain, and experience mesopredator release. Smaller than apex predators, mesopredators often breed faster and produce more offspring, leading to populations which grow larger and faster than their prey can tolerate. In the pine woods of Scotland, springtime brings with it a chorus of echoing, clicking clucks. These are the mating calls of the male capercaillie, a large black grouse, and an ice age relic. In the 1780s, the capercaillie was extirpated from Britain by hunting for food and sport, but was re-established in the early 1800s using birds imported from Scandinavia. Having previously done well, the capercaillie is now once again on the brink of extinction in Scotland, and one of the main reasons for its decline is mesopredators. Without the bears, wolves and lynx that used to inhabit the Caledonian pine forests, and proved no threat to the capercaillies, red foxes – which raid capercaillie nests – have become a serious problem. By driving foxes out of their territories, these larger predators would have kept the fox population low, allowing capercaillies to nest in peace. Now, combined with other threats – the Caledonian pine forests occupy just 1% of their former range, and capercaillie frequently die flying head-first into wire fences – this charismatic bird may well suffer a second extinction in the near future.
Humans have relied on the natural world for food, fuel, and material resources for hundreds of thousands of years. Over the course of history, innovations in technology and techniques made the acquisition of these resources increasingly efficient, but over the past century, this process has gone into overdrive. From chainsaws and engine-driven ploughs, to mechanical harpoons and overpowered firearms, technological advancements in the 1900s have placed many species at risk of overexploitation. Most organisms can tolerate a low level of exploitation, but when they are removed from an ecosystem faster than they can be replaced through reproduction, populations begin to decline. The more overexploited they are, the faster their populations decline, and eventually they may even become extinct, affecting the other species that depend on them. In 1992, after decades of changing fishing practices, the population of cod in the northwest Atlantic completely collapsed to just 1% of its pre-1950 levels. Increasing demand for cod, ever-expanding fishing fleets, and the introduction of equipment that could fish ever-deeper waters, led to large numbers of young cod being removed from the population before they could breed. Taking these immature fish was like chopping halfway through the trunk of a tree: once enough of the mature adults were gone, the whole thing fell apart. This had huge social and economic impacts on settlements in northeastern North America, many of which relied almost exclusively on the cod fishery. Humans weren’t the only species to be affected: cod are apex predators, and once gone, the populations of species they predated, including lobsters, crabs, and shrimp, exploded. Fortunately, this problem became part of the solution: fisheries in northeastern North America switched to harvesting shellfish, which reproduce much faster than cod; and with the implementation of strict quotas and detailed monitoring, the cod population has begun to recover.
Substances which animals can’t break down, like heavy metals and certain pesticides, remain in their tissues. When a contaminated animal is eaten by another, the predator also ingests the dangerous substances. Since the predator is usually eating many contaminated animals, the concentration of contaminants in the predator is much higher than in any of its prey items. This concentration up the food chain is called bioaccumulation, and can lead to damaging, even lethal concentrations of chemicals in the bodies of apex predators. In 1939, chemist Paul Hermann Müller discovered the insecticidal properties of the chemical DDT (dichlorodiphenyltrichloroethane). It was widely used as an agricultural and domestic pesticide for the next three decades, until its impacts on the natural world were revealed. DDT clings to the bodies of insects, and the crops on which it is sprayed, so it is ingested by insectivorous and seed-eating birds, like thrushes and sparrows. In these birds it’s at too low a concentration to do much damage, but they in turn are consumed by birds of prey, including hawks, falcons, and eagles. In these birds, DDT reaches such a high concentration that it affects their reproduction. Contaminated birds were found to lay eggs with significantly thinner shells than in the past, meaning that when the parents tried to incubate their clutches, the eggs were crushed. Across Europe and North America, populations of nearly all birds of prey declined as a result. Fortunately, DDT was banned in the USA in 1972, and completely prohibited worldwide by 2004. As a result, species that were brought perilously close to extinction by the actions of this insecticide, including peregrine falcons and bald eagles, are now starting to recover.
88 HABITAT FRAGMENTATION
Habitat fragmentation occurs when a large patch of habitat is split up, by areas of different habitat or by structures like roads, which are often less hospitable to the original habitat’s residents. This reduces the habitat area available for organisms to use, making their populations smaller. It can also make it more difficult for individuals to move between patches, so that struggling populations cannot be saved by organisms moving in from neighbouring patches. The Yangtze River in southern China is the third longest in the world, and also one of the most fragmented. For thousands of years the Yangtze was famous for the freshwater dolphins, baiji, that inhabited its lower course. Starting in the 1950s, as China underwent rapid development as a nation, the baiji faced increasing pressure from humans. Housing and industrial development, logging, shipping, hunting, pollution, and drought all pushed the baiji closer to extinction. The final nail in the baiji’s coffin came in 2003, with the construction of the Three Gorges Dam across the Yangtze’s course in Sandouping. At 181 metres tall, more than two kilometres from end-to-end, and with no way around, the dam cut the baiji’s habitat in two. With individuals unable to move between them, the populations above and below the dam both became much more threatened, and much less stable. The last definitive sighting of the baiji was in 2002, when the individual known as Qiqi died. Even if a few individuals remain today, they are likely too old to breed, rendering this unique species essentially extinct.
89 EDGE EFFECTS
At the boundary between two habitats, population structures and ecosystem processes change. Where woodland meets grassland, there might be dappled shade, perfect for plants which need both shade and sunlight. Where marsh meets open water, oxygen levels might be higher, perfect for damselfly nymphs. These are edge effects, and while they can be damaging to organisms which rely on ‘pure’ habitats, they can be good news for species which need a range of conditions to thrive. In the UK, some of our rarest butterflies are those which live in woodlands. However, these aren’t the birdwing butterflies of Papua New Guinea, or the morpho and owl butterflies of the Amazon, which spend their lives in hot dense forests. Here, temperatures are almost always too low for butterflies, and they need direct sunlight in order to survive. In the hearts of ancient woodlands, where centuries-old oaks make a blanket-like canopy, such sunlight is hard to come by, but it’s much more available at the edges of woodlands. Common butterflies, like the speckled wood and holly blue, can often be seen patrolling strips of grass along the edges of woodland rides, where paths and tracks cut through the trees. Some of our rarer butterflies, like the silver-washed fritillary, purple emperor, wood white, and white-letter hairstreak, are still going strong in places where coppicing is practiced – the process of cutting hazel trees at the base every few years, letting them grow back bushy to produce long canes for fencing, also lets light flood into temporary woodland glades.
90 EXTINCTION DEBT
Extinction occurs when the last living individual of a species dies. Sometimes, extinction happens rapidly: the extinction of the San Benedicto rock wren can be pinned down to the exact hour that its island home erupted in a volcanic apocalypse. Most of the time, however, extinction is a slow process, which can take years or decades to complete. Extinction debt refers to future extinctions that are ‘locked in’ due to events in the past, and which are unavoidable unless humans intervene. Extinction debt happens because many species are slow to respond to changes in their environment, and may even survive for several generations before finally dying out. Across Africa, up to 30% of primates – some sixty-plus species – are predicted to become extinct in the near future due to loss of key forest habitat that has already occurred. If extinctions occurred at the same time as habitat loss, many of these species would already be gone, but species’ ability to survive in sub-optimal habitats has delayed the inevitable. Protection of small areas of pristine forest allows these species to hold on, but to recover their populations, new areas of forest need to be planted, and damaged patches restored.
91 ALLEE EFFECTS
In some species, the survival of individuals depends on being part of a large or dense population. When population size or density is reduced, Allee effects (named for American ecologist Warder Clyde Allee) reduce individual survivorship. This means that even for species where declines have been stopped, small populations may have them committed to extinction. The African wild dog used to be widely distributed across the savannas and woodlands of southern and eastern Africa, and the Sahel belt that lies south of the Sahara Desert. Today, their distribution has become seriously fragmented, with some populations isolated hundreds of miles from the next closest wild dogs. Like wolves, African wild dogs live in large packs, which allows them to share out responsibilities. While some dogs leave the den to hunt prey – a task which requires a large group, when your antelope prey can be twice as fast as you – others remain behind to care for the pups, and defend them against lions, leopards, and hyenas. But when packs are too small, the dogs face a lose-lose situation: either too few go hunting, and the pack fails to find enough prey; or they leave the pups unguarded at the den, and fail to reproduce successfully. Either way, if there aren’t enough dogs, the pack dies out.
92 INBREEDING DEPRESSION
Inbreeding occurs when closely-related individuals reproduce together. This is a widespread taboo in many human societies, and is strongly avoided by many other species too, for good reason. Siblings are likely to share a high proportion of their gene variants, known as alleles, which can be either dominant (only one copy needs to be present in an organism’s DNA for the trait to be expressed) or recessive (two copies are required). Recessive alleles sometimes reduce an organism’s ability to survive, but because it needs two copies to have an effect, natural selection has trouble weeding it out of the gene pool. Inbreeding causes recessive alleles to build up within a population, until the organisms become riddled with diseases and other traits that negatively impact their survival. Over generations, this causes inbreeding depression, and in isolated populations where fresh genes cannot be brought in by organisms migrating from other populations, the build-up of negative traits can lead to extinction. A small population of grey wolves living on the border of Norway and Sweden is completely isolated from other wolves – the nearest ones live in Finland, over 900 km to the east. This population is descended from just three individuals, so all of the wolves are highly related to each other. Due to high levels of inbreeding, many of the cubs born in this population are born with physiological disabilities, which often prove fatal: most cubs die in their first year. Unless wolves are imported to save the population, or the Finnish population is allowed to expand into Scandinavia, this population will likely go extinct within a few generations.
Fortunately, it’s not all doom and gloom. Over the last century, humans have got pretty good at recognising our impact on the natural world, and making sure it’s a good one. Next time, we’ll be looking at the ways we can produce conservation successes.