Ecosystem

I

Introduction

Ecosystem, a relatively self-contained, dynamic system composed of a natural community along with its physical environment. The concept, first developed in the 1920s and 1930s, takes into account the complex interactions between the organisms—plants, animals, bacteria, and fungi—that make up the community and the flows of energy and matter through it.

Ecosystems can be classified in many different ways and the term itself is used in a variety of contexts. Areas as small as intertidal rock pools and as large as entire rainforests can both be described as ecosystems. However, it is generally not possible to state with precision where one ecosystem ends and another begins. The notion of rigidly separable ecosystems is thus a largely artificial one.

II

Major Ecosystems

Nevertheless, the ecosystem concept has proved very useful in ecology. One way in which it is applied is to describe the world’s major different habitat types. Terrestrial ecosystems include: Arctic and alpine ecosystems, dominated by cold areas and lacking trees; forest ecosystems, which can be subdivided into a whole range of types including tropical rainforests, Mediterranean evergreen forests, temperate and boreal forests, and temperate deciduous forests; grasslands and savannahs; and deserts and semi-arid ecosystems. Freshwater ecosystems include lakes, rivers, and marshland. “Hybrid” terrestrial and freshwater systems include swamp forests and seasonal floodplains. Marine ecosystems comprise an enormous range, from coral reefs, mangroves, sea-grass beds, and other coastal and shallow water ecosystems to open-water ecosystems and the mysterious, little-known systems of the abyssal plains and trenches of the world’s oceans.

The term ecosystem may also be used to describe geographical areas which contain a wide range of habitat types which are linked by ecological processes. Thus the Serengeti-Seronera region in east Africa, one of the most spectacular wildlife areas in the world, is often regarded as a single ecosystem whose different constituent habitats include open grassy plains, woody savannah, dense thickets, and patches of forest, as well as rocky outcrops (kopjes), rivers, streams, and seasonal pools. Similarly, the most productive parts of the world’s oceans have been divided into a series of Large Marine Ecosystems which contain many different habitats. Examples of Large Marine Ecosystems with widely different characteristics include the whole of the Black Sea, the Benguela Current system off the coast of south-western Africa, and the Gulf of Mexico.

III

Natural Changes in Ecosystems

The natural world is in a constant state of flux. Changes can be seen at all time-scales, from the shortest to the longest. Short-term changes, which are observable by people, are often cyclical and predictable: night and day, the monthly cycle of the tides, the annual change of the seasons, and the growth, reproduction, and death of individuals. Viewed at this level many ecosystems, when not disturbed by humans, appear superficially to be stable and unchanging, maintained in equilibrium by the “balance of nature”.

It is becoming increasingly apparent that this is not the case. However, longer-term changes—those running over decades, centuries, millennia, and ultimately over tens of millions of years—are far less easy for us to track. The science of ecology itself is less than a century old—the merest blink of an eye in the history of most natural ecosystems. Moreover, it is evident that most of these longer-term changes are not regular and predictable at all.

Globally, climate is undoubtedly the most important factor in the short to medium term. On land, temperature, rainfall, and seasonality are the three factors which are most important in determining the distribution of ecosystems. Changes in any one of these can have a lasting effect. In recent geological time the most dramatic example of this is undoubtedly the series of Ice Ages which characterized much of the Pleistocene epoch. These protracted periods of global cooling had a major effect on the world’s ecosystems, causing the spread of ice cover in temperate areas and the shrinking of moist forest habitats in parts of the tropics.

Climatic change with a wide geographical influence can also occur over shorter timescales. One of the most striking examples is El Niño, a cold-water current which periodically sweeps across the Pacific. This has a major impact on marine ecosystems, leading, for example, to the death of reef corals in many parts of the Pacific, and the lowering of fisheries production in the Humboldt Current ecosystem off Chile and Peru. El Niños occur irregularly and vary in intensity and impact; it is rare for more than 20 years to pass without one and occasionally they may be separated by only 1 or 2 years. El Niños also affect terrestrial ecosystems through changing rainfall patterns, particularly in the Americas.

Local events can also have major effects on ecosystems: fires, floods, and landslides are all natural events which can have catastrophic local impact. This impact is not necessarily negative: many ecosystems actually require periodic disturbance to maintain themselves. Fire-climax ecosystems, in which periodic burning is an essential part of the growth cycle, are widespread in semi-arid areas such as much of Australia.

At longer timescales geological processes and evolution itself play a crucial role in changing the functioning of ecosystems. Continental Drift literally changes the face of the planet, creating new landscapes and destroying others, while evolution leads to new life-forms which may themselves create new ecosystems at the same time as leading to the extinction of other species and the loss or transformation of the ecosystems of which they were a part.

However, this is not to mean that there is no continuity in natural ecosystems. Many show great resilience and persistence through periods of many millions of years. Examples of ecosystems which have apparently remained stable for a very long time include those on the vast deep-sea plains, the Mediterranean-type ecosystems of southern Africa and western Australia, and some areas of tropical rainforest, such as those in parts of the south-east Asian mainland and in the mountains of eastern Africa.

IV

Human Influence on Ecosystems

All natural environments and ecosystems now have an unprecedented problem to deal with: humanity. Humans appear to be squeezing into a few short centuries changes on a scale which would otherwise be expected over thousands or millions of years. The full impact of these remains to be guessed at. Major human impacts on ecosystems include the following: