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Scientific background

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xxxxx An alpine tundra ecosystem comprises many species. In this picture, two reindeer
xxxxx represent the herbivores and a rough-legged buzzard represents the predators.

Scientific background

An ecosystem consists of living organisms, such as plants and animals, and the environment surrounding them (the water, the air, the soil etc).

The organisms in an ecosystem can be divided into trophic levels according to their way of living and their function in the ecosystem. Plants, herbivores and predators constitute different trophic levels. Within the ecological science, we investigate how the environment, different species and trophic levels affect each other. This determines how the ecosystem is structured, i.e. how abundant different species are, and how large parts of the biomass that is found at different trophic levels.


HHHHHSome predator species favour others by leaving carcasses behind (blue arrows).
HHHHHHowever, they can also have a negative impact on the other predators through
HHHHHcompetition and/or predation (red arrows). Predators also kill and eat different
HHHHHprey species, and this can have a negative impact on prey abundance. But at
HHHHHthe same time, the predators depend on their prey species for food (green arrows).

The organisms in an ecosystem interact with each other in several ways. Predators kill and eat their prey, whilst herbivores graze and browse on their food plants. Individuals of the same species may compete with each other over food, territories and partners, but individuals of different species may also compete. Such interspecific competition is usually most intense between species that share similar ways of life.

Predators do not only interact with their prey, but also with other predators. Larger predator tends to be dominant over smaller predators in direct interactions. This implies that large predators may impact the abundance of mesopredators (medium-sized predators) through interspecific killing and/or competition. Mesopredators may in turn suppress the abundance of even smaller predators. Hence, when one predator species declines or increases, this may affect the abundance of other predator species. Predators can also have positive effects on each other, for example when one predator leave prey remains behind which scavenging predators can feed upon.


HHHHHThe arctic tundra has a cold climate with long winters. The reindeer is the only
HHHHHdeer species on the tundra, but the reindeer migrate to forested regions in
HHHHHwinter. This implies that there is very little left to eat for large predators,
such as the wolf, which need large prey. As a consequence there are relatively
HHHHHfew wolves in tundra regions. North American research has shown that since there
HHHHHare so few wolves, the number of reindeer is primarily determined by the climate
HHHHHand the productivity of the ecosystem. Hence, reindeer can limit the abundance
HHHHHof their favourite food plants through grazing.

An important ecological question is how species, trophic levels and ecosystems are limited, i.e. what determines how many individuals there are of each species or in each trophic level.

We distinguish between ‘top-down’ and ‘bottom-up’ limitation. Plants can be limited bottom-up by the availability of nutrients, water and radiation from the sun; herbivores by the availability of food plants, and predators by the availability of suitable prey. Top-down limitation can be carried out by predators through predation and by herbivores through grazing. The structure of an ecosystem is generally affected by both forms of limitation. Which form that is most important varies between ecosystems, for example due to species composition, the environment and climatic conditions.

In ecosystems with harsh climate and low primary productivity, e.g. tundra and deserts, plants grow slowly and feed relatively few herbivores. Hence predators so rare that they have little limiting impact on the herbivores.



HHHHHSouth of the tundra, there are forests with a milder climate and resident (non-
HHHHHmigrant) ungulates. Hence, wolves do well and wolf numbers are related to the
HHHHHproductivity of the ecosystem (if wolves are not regulated by hunting). Research
HHHHHcarried out in North America suggests that wolf predation thereby may limit
HHHHHungulate numbers top-down. This implies that grazing has relatively weak impact
HHHHHon food plants, which instead may be limited bottom-up to a larger extent.


In contrast, where the climate is favourable for plants, e.g. where it is warm and sunny but not dry, there can be lush vegetation which grows rapidly. Such productive ecosystems allows rapid growth of herbivore populations, which in turn implies that the ecosystem can feed relatively large numberos of predators. Hence, predators may exert strong top-down limitation of herbivores.

Ecosystems are affected by man in several ways. We limit species through hunting and fishing. We exterminate some species but introduce others. Human land use (e.g. agriculture and forestry) changes the environment, and this may favour some species but be unfavourable to others. Sometimes the human impact can cause ecosystem functions to strengthen or weaken, which in turn may change how the entire ecosystems is limited and structured. Examples of ecosystem functions are photosynthesis, grazing and predation.

In many ecosystems, man has eradicated large predators. These top predators may have a key function in ecosystems, because predation may regulate and suppress large herbivores. Thus, when large predators go extinct, large herbivores may increase in abundance, and the increased grazing pressure may have a negative impact on the vegetation.


Hairston NGF, Smith FE, Slobodkin LB. 1960. Community structure, population control, and competition. American Naturalist 94: 421-424.

Fretwell SD. 1977. The regulation of plant communities and the food chains exploiting them. Perspectives in Biology and Medicine 20: 169-185.

Oksanen L, Fretwell SD, Arruda J, Niemelä P. 1981. Exploitation ecosystems in gradients of primary productivity American Naturalist 118: 240- 261.

Crête M, Manseau M. 1996. Natural regulation of cervidae along a 1000 km latitudinal gradient: change in trophic dominance. Evolutionary Ecology. 10: 51-62.

Crête M. 1999. The distribution of deer biomass in North America supports the hypothesis of exploitation ecosystems. Ecology Letters 2: 223-227.

Crooks KR, Soulé ME. 1999. Mesopredator release and avifaunal extinctions in a fragmented system. Nature 400: 563-566.

Oksanen L, Oksanen T. 2000. The logic and realism of the hypothesis of exploitation ecosystems. American Naturalist 155: 703-723.