Cover image: weather is just one process that can shape ecosystems. © Mathias Krumbholz, 2010.
Welcome to Part 5 of 100 Key Concepts, here’s a lighter issue of this series (in terms of size but perhaps not content). In the previous instalment we looked at populations and the interactions that control them. But populations are also controlled by abiotic processes, and that’s what we’ll be exploring this time.
43 NICHES
A niche is the specific set of resources, such as food, light, and space, that an organism is capable of exploiting. An organism’s niche determines its role in an ecosystem, such as apex predator or decomposer. There are two kinds of niches: the fundamental niche is the total area on the resource spectrum in which an organism could exist; the realised niche is the area in which is does exist, thanks to limits imposed by competition with other species. Think back to the barnacles we encountered a couple of instalments ago: the Chthamalus barnacles can grow at any depth on the shore, but are outcompeted by Balanus barnacles in deeper water, and so only exist in the shallows in places where both species occur together. The Chthamalus barnacle has a broad fundamental niche (it could exist anywhere in the intertidal range), but a narrow realised niche due to competition with Balanus.
44 SUCCESSION
Natural environments do not stand still: they develop, and tend to become more complex over time. This is the process of succession, by which one habitat gives way to another as conditions such as temperature, water availability, or soil fertility, change. Primary succession occurs on land completely devoid of life, like a new volcanic island or an area recently revealed by a melting glacier. Secondary succession occurs in places where organisms have been recently, like a burned field, and where plant seeds are likely to still lie dormant in the soil. This has been studied closely at Glacier Bay in Alaska, where the retreat of glaciers has been well-recorded for two hundred years. Land recently deglaciated is bare, scoured down to the bedrock by the grinding action of the ice, but is quickly colonised by mosses, lichens, and liverworts. These organisms release chemical compounds which break down rock, allowing them to access the nutrients locked inside, and in the process form a kind of thin, hard soil. This is colonised by so-called ‘pioneer plants’, which arrive first on the scene and can tolerate the thin, rocky soil and exposure to harsh climates. Over decades, the rock continues to break down, and organic matter from dead plants accumulates, forming a richer soil. This is colonised by willow shrubs, and then green alder trees, which form a woodland of relatively short trees. Alders are unusual in that they have nodules on their roots, which contain a kind of bacteria that pulls nitrogen out of the air and into chemicals in the soil. All plants need nitrogen to live – it’s crucial for producing proteins, including DNA – but none of them can get it by themselves. As the alders grow, their bacteria increase levels of nitrogen in the soil which, combined with the shelter provided by the alder trees, creates a more forgiving environment. These conditions allow Sitka spruce saplings to establish, and within two centuries of deglaciation, a mature spruce forest grows where before there was only rock.
45 CLIMAX COMMUNITIES
A climax community is what happens at the end of succession, a habitat stable enough that it will change very little over time. In Glacier Bay, the spruce forest won’t be a constant feature. As the glaciers retreat further from the coast, each successional habitat, from bare boulder fields to moss carpets, meadows, willow scrub, and alder forests, will follow the ice inland. The spruce forests will also follow, but on their southern, coastal edge they will give way to another habitat. As the soil becomes deeper and richer, and the climate warmer, the spruce will be outcompeted by pines, which themselves will give way to species like maples and oaks. It’s widely accepted that across much of the northern temperate zone (Europe, North America and North Asia), broadleaf forest is the dominant climax community, except in areas where it is, for example, too cold or wet for these trees to grow. In practice, however, a climax community is rarely reached, and even in the special circumstances when it is, it doesn’t last for long. Nature abhors stability, and the successional process is prevented from reaching its climax by disturbance.
46 INTERMEDIATE DISTURBANCE
Maybe it’s a powerful storm that blows down trees, or a landslide that buries them. Maybe it’s a wildfire that burns them up, or a flood that drowns their roots. Eventually, some form of disturbance will come along and reset the succession in that perfect climax community. Intermediate disturbance is generally regarded by ecologists as the ‘best’ kind, because it keeps things dynamic without destroying everything. Too little disturbance, and the community will be able to continue as usual: no new niches will open up, and no new species will be able to establish. Too much disturbance wipes the slate clean, destroying so much habitat that species are unable to survive. The savannas of East Africa are a perfect example of the diversity that can be achieved through intermediate disturbance. The patchwork of acacia-tree woodlands and open grassland provides opportunities for more species than either habitat would alone. If there were no large herbivores, like elephants or giraffes, acacia trees would grow unhindered, forming a vast forest good for monkeys, leopards, and not much else. Too much fire and there would be no trees at all, producing an expansive grassland swamped in wildebeest.
47 LATITUDINAL GRADIENTS
Life on Earth follows patterns, one of the most obvious of which is latitudinal gradients. Life at the North Pole and in the Amazon rainforest are very different, and not just because of the weather: the number of species living at the equator is much, much higher than at the poles. There’s a gradual decline in species richness as you move away from the equator. A quarter of a square kilometre of rainforest in Ecuador is home to more than 1,100 different species of tree. The entirety of Europe, an area forty million times greater, contains just 454. And it’s not just trees: almost all forms of life, except those specifically adapted to survive at the poles, are more speciose closer to the equator. There are numerous explanations for why this is. Maybe there’s more energy? Most of the energy for life on Earth comes from sunlight by way of photosynthesis, and there’s more sunlight at the equator than anywhere else. Maybe it’s to do with seasonality? The poles show massive climatic shifts between summer and winter, differences which become smaller as you move towards the equator, giving way to wet and dry seasons. Bang on the equator, it’s wet and warm all year round. Maybe this means there are more niches? Maybe it’s because habitats at the equator have escaped the worst effects of the ice ages that have plagued our planet for the past three million years? Nobody’s been able to pin it down to a single cause, but it’s likely a combination of all of these factors and more.
48 ISLAND BIOGEOGRAPHY
Islands are not like other places on Earth. In fact, they’re so different that their biology has its own class of study, island biogeography. Islands are hotspots of evolution, as their isolation means they often present many unfilled niches to the intrepid coloniser – just think of Darwin’s finches, descendants of a single species which split into fifteen upon finding the paradise of the Galapagos archipelago. This means they’re also hotspots of endemism, the phenomenon of a species existing only in a single place: all 100-plus species of lemur are endemic to Madagascar, having spread no further than the nearby Comoros archipelago in the past fifty million years. Islands are also, unfortunately, hotspots of extinction. New Zealand is a prime example: no terrestrial mammals reached the islands until the arrival of humans just 700 years ago. The birds which made it found an island where the only predators flew, meaning that over the course of evolutionary time many of them gave up the ability to fly altogether. The enormous moa, relatives of the South American tinamous but more closely resembling ostriches, were driven to extinction by hunting for their meat and eggs within 150 years of human arrival. The kakapo, the world’s largest parrot, is Critically Endangered thanks to the effects of introduced cats, rats, and stoats, and exists only on a few offshore islands.
49 CLIMATE CHANGE
It’s the word on everybody’s tongue these days: climate change. Generally we’re talking about the current dramatic, short-term warming of our planet caused by the release of human-made greenhouse gases, which poses an existential threat to humanity and many other species. At its baseline, climate change is a natural process: the Earth has passed through periods of extreme warm and extreme cold. Sixty million years ago, the whole planet was blanketed in lush rainforest; six hundred million years ago, the whole planet was encased in miles-deep ice. On a ‘normal’ timescale of tens of millions of years, climate change provides opportunities for new species, and even new habitats, to evolve as abiotic conditions open up new niches. But when it happens very quickly, over a period of one or two million years, it can be catastrophic. Rapid global cooling, caused by the evolution of land plants removing massive amounts of CO2 from the atmosphere, led to a mass extinction in the Devonian period, 370 million years ago. Increases in CO2, leading to global warming, contributed to the Triassic-Jurassic mass extinction 200 million years ago, and the Permian-Triassic mass extinction fifty million years prior, the most devastating extinction in Earth’s history, in which up to 70% of terrestrial vertebrate species and 96% of known marine life vanished. Such mass extinctions led to the freeing-up of ecological niches across the planet, and unanimously resulted in an explosion of new life forms: reptiles, dinosaurs, and birds all walked out of various mass extinctions. Little consolation for the species which did not make it. We may be standing on the brink of another mass extinction now, caused by the resource consumption of humans and propelled forwards by our impacts on the global climate. Lessons from the deep past tell us that things may not end well for our kind, should we fail to change our ways immediately.
All of these processes drive changes in the resources which are available to organisms, and by doing so control their populations. Next time, we’ll be examining these resources, and the different ways in which organisms exploit them.
Until then,
Jon
Field Notes 
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