Cover image: the giant panda is a worldwide symbol of nature conservation. © Joachim S. Müller
Welcome to the tenth instalment of 100 Key Concepts. Last time we looked at how organisms reproduce, and the measures they take to ensure their offspring’s survival. This segment marks a change of tack from the rest of the series: we’re moving on from pure ecology, and looking more at conservation. To start, let’s find out what makes certain species particularly ‘important’.
77 KEYSTONE SPECIES
The keystone has played an important role in architecture for centuries. It sits at the apex of an arch, where it stops the whole thing from falling apart. Keystone species play a similar role in the natural world: they are species which have an unusually large impact on their environments, relative to their abundance – a classic case of a little going a long way. Some keystone species, like trees, physically form a habitat; others, like elephants, maintain it through their actions. Keystone species play a critical role in maintaining an ecological community’s structure, affecting many other species, and their absence often has a huge impact. One of the best-recorded examples plays out in the cool Pacific waters off the coast of California, where forests of kelp (a giant seaweed) fill the sunlit shallows. Kelp grows from a holdfast, a root-like structure that attaches the fronds to the rocky seabed. The tangled holdfasts are the perfect hiding place for fish, supporting a huge variety of species. Unfortunately for kelp, these holdfasts are the preferred food of sea urchins, and if they’re severed, the kelp dies. Luckily, the kelp has an ally: this ecosystem’s keystone, the adorable sea otter. Sea otters are experts at eating urchins, which keeps the urchin populations low, allowing kelp forests to recover from urchin damage. But in areas where the sea otters have vanished – often due to hunting, pollution, or human recreation – urchin populations explode. The kelp forests vanish, replaced by barrens which are good for the urchins, but inhospitable to most other life. Without hiding places, the fish leave, hurting any other species (like bears, eagles, and humans) that depend on them. Fortunately, when the sea otters are allowed to return, they set about decimating the urchin population. The kelp groves re-establish, and the diversity of life returns to these nutrient-rich coastal waters.
78 INVASIVE SPECIES
Humans have been moving animals and plants around for millennia, and today is no exception. An invasive species is one that has been introduced outside its native range, deliberately or otherwise, and become problematic in the places it has established itself. Invasive species are one of the biggest challenges to conservation, posing risks to other species, whole ecosystems, and human health and economies. In the tropical forests and grasslands of Central and South America, the cane toad is a voracious predator of insects and small vertebrates. In the early 1900s, the cane toad was introduced to Puerto Rico to handle outbreaks of pests in sugarcane plantations. When this was successful, it was taken to Australia in the 1930s to do the same job there. Unfortunately, the target pest here was the grey-backed cane beetle, which presented the toads with a big problem: they couldn’t climb, and the beetles lived at the tops of sugarcane plants. Naturally, the toads gave up immediately, spreading across northeastern Australia in search of easier prey. This presented Australia’s native fauna with a twofold problem: first, the toads were eating all the invertebrates forming the base of the food chain. Secondly, whenever anything tried to eat the toads, it would die: cane toads secrete toxins from glands just beneath their skin, against which Australia’s native amphibian-eating animals have no defence. The cane toad is currently one of the world’s worst invasive species. Fortunately, there is some hope. Recently, the rakali, Australia’s native water-rat, has learned how to eat cane toads while avoiding the toxic skin, by slitting the toad open, eating the flesh, and sucking out the liver and heart. The cost of eradicating cane toads would be enormous, but hopefully the rakali can help.
79 INDICATOR SPECIES
Some species, like the cane toad, are generalists that can survive in a range of environments. Others are specialists, and need specific conditions to thrive. These specialists can often be indicator species, whose presence in an ecosystem can give scientists an idea of how healthy the environment is. This makes monitoring certain environments much easier, quicker, and cheaper than it might otherwise be. In the UK, a variety of insects spend their childhoods in fast-flowing streams. Some, like dragonfly nymphs and rat-tail maggots (which breathe from the surface using a long tube), can tolerate silty or polluted water. Others, like mayfly nymphs and caddisfly larvae, breathe with a complex set of delicate, feathery gills. These gills can easily absorb toxic pollutants, or get clogged with silt, so the presence of these species indicates a clean, high-quality stream. Other species can indicate other factors about a habitat. For instance, dog’s mercury and bluebells are both very sensitive to disturbance, and take a long time to spread once they colonise an area, so large numbers of these plants indicate ancient woodlands that have been standing for centuries.
80 UMBRELLA SPECIES
When it comes to conservation, one question trumps all others: what should we save? Resources such as money, time, and labour are often in short supply. One way of getting around this is by targeting umbrella species. These are often large species which occupy large areas, or rely on a variety of high-quality habitats. Protecting them by preserving these habitats indirectly protects the other species in their community, producing large conservation benefits from relatively small inputs. In India, many areas of forest and woodland are protected for the benefit of tigers, which hold high status as the country’s national animal. Areas protected for tigers support a variety of other threatened species, including geckos, bats, and rusty-spotted cats. Tigers have huge territories, so protecting them also means protecting the corridors of habitat linking larger forest patches. Tigers aren’t the only species to use these corridors, meaning that populations of many species avoid becoming fragmented and isolated.
81 FLAGSHIP SPECIES
Since ye olden days, the flagship has usually been the best or most important ship in a fleet. In ecology, a flagship species serves as the icon or ambassador for a particular conservation initiative, representing it to the world. Flagship species are easily recognisable, and often aesthetically attractive, so they can help generate public support for a cause. Some of the world’s largest conservation organisations are associated with charismatic flagship species, such as the giant panda (WWF), the African elephant (Space for Giants), the pied avocet (RSPB), and the white rhinoceros (Save the Rhino). Closer to home, the bittern – a relative of herons that is a specialist species of reedbeds – serves as the flagship for the Arnside and Silverdale Area of Outstanding Natural Beauty, which has long been a stronghold for this rare species in northern England.
82 ECOSYSTEM ENGINEERS
Ecosystems don’t just spring up overnight: they are constantly changing, and need maintenance to stay functional. An ecosystem engineer is a species that creates, modifies, maintains, or destroys a habitat through the activities it undertakes to survive. Ecosystem engineers have big impacts on the species richness and community composition of the world around them, disproportionate to their own abundance, which makes them a subset of keystone species. In northern Eurasia, many waterways are home to beavers, which are famous for damming streams to slow the flow of water. This creates large pools behind the dam, flooding into the surrounding forest, which allows the beavers to seek out trees to eat without leaving the water. As well as letting beavers move around, these ponds are home to many species not found in fast-flowing streams, including a range of fish and many insects, like damselflies. Out of the water, the wet woodlands provide a wider range of resources for a wider range of terrestrial species, providing tender grazing for bison and nesting habitat for willow tits.
83 FUNCTIONAL GROUPS
All species carry out particular functions in their ecosystems: fungi are decomposers, vultures are scavengers, lions are apex predators. Groups of species carrying out similar functions make functional groups. The presence of specific functional groups, and the diversity of species within them, can have big impacts on how an ecosystem works. In temperate zones across the world, many grassland ecosystems would collapse without pollinators helping flowering plants reproduce. In rainforests, many different bird species feed on fruits, and disperse the seeds they eat much further than the trees could manage alone, maintaining a mix of tree species across the forest.
All species are important, but some play particularly crucial roles in ecology and conservation. When these species are threatened with extinction, or wiped out altogether, things start to go wrong for ecosystems, and that’s what we’ll be looking at next time.