Principles of Ecology: Biomes I

Zoogeographical Realms

  • Originally based on the distribution of large mammals across the world.

  • First described by Wallace (1876).

  • Updates to the realms:

    • We now know a lot more than we previously did!

    • Refer to Holt et al. (2013) for updated realms: NEARCTIC, NEOTROPICAL, AFROTROPICAL, PALEARCTIC, ORIENTAL, AUSTRALIAN, etc.

  • Map of evolutionary uniqueness:

    • Darker the red = more unique. Mean pB values: 0.68, 0.51, 0.42 (Holt et al., 2013).

  • Considering different taxa:

    • Amphibians, Birds, Mammals (Holt et al., 2013).

Biomes

  • Definition: Organised realms distinguished primarily by their predominant vegetation.

  • Mainly determined by temperature and precipitation.

  • Collections of species share adaptations to the climate of each region.

  • Examples:

    • Tropical forest

    • Savanna

    • Temperate deciduous woodland

    • Temperate grassland

    • Boreal forest

    • Tundra

    • Desert

    • Mountains

    • Chaparral

    • Polar ice

Tropical Forests

  • Global distribution:

    • Central America - ~50%

    • Africa - ~20%

    • South East Asia and Malaysia - ~30%

  • Characteristics:

    • Occur near the equator.

    • High yearly rainfall (exceeds 200cm per year).

    • Mean annual temperature 21 – 36 degrees Celsius.

    • Continuous warmth + continuous wet = continuous plant growth.

  • Importance:

    • Provide ecosystem services (a positive benefit that ecosystems provide to humans).

    • Generate much of the oxygen generation for Earth.

    • Absorbs carbon dioxide (carbon sink).

    • Amazon rainforest alone estimated to harbour 76 billion tonnes of carbon.

    • As the forest is lost, more carbon is emitted.

    • More emitting more each year than it is absorbing.

  • Species diversity:

    • Among the most species-rich and productive of all biomes.

    • May be as many as 300 tree species in just 2-3km^2!

    • Tropical rainforests estimated to contain more than half of the world’s species.

    • High biodiversity reflected in the large number of biotic interactions.

  • Tropical forest structure:

    • Intense competition for light on the forest floor.

    • Plant community stratified into layers.

    • Each group of species adapted to different levels of shade.

  • Tropical forest layers:

    • Emergent layer: Tops of the tallest trees – much higher than the average trees in the canopy.

    • Canopy layer: The upper parts of most trees (about 20 – 40 m tall).

    • Understory layer: Also known as ‘understory’. Tall shrubs and subcanopy trees.

    • Herb layer: Herbaceous vegetation.

    • Forest floor: teeming with animal life, especially inverts.

  • Epiphytes:

    • Grow on other plants for physical support and to obtain adequate light.

    • Rely on nutrients in rainfall and water that drains of their host.

    • No roots in the soil - energy gained from photosynthesis.

    • Commensalism, not parasitism. Many orchids, ferns and cacti are epiphytic.

  • Stranglers:

    • Some being as epiphytes, but send their roots downwards to the forest floor.

    • These roots eventually envelope tree and can kill it.

    • Known as stranglers.

    • These species are parasitic.

    • The fig family (Ficus spp.) is well represented among stranglers.

  • Climbers:

    • Start life on the ground and climb to reach light.

    • Lianas – start life as shrubs & rely on other plants to climb on to obtain light. Lianas are woody and reach the canopy.

    • Vines/creepers – herbaceous plants. Change leaf structure the further up they go. Herbaceous climbers rarely reach the canopy.

  • Plant adaptations:

    • Broad leaves to maximize energy intake.

    • Plants have arrested growth patterns and wait for a gap in canopy to grow upwards towards light.

    • Many plants have slick, water repellant coatings on leaves to allow rain to run off easily.

    • Drip tips, drain water from leaf surface.

  • Animal life:

    • Majority of plants rely on pollination or seed dispersal.

    • Plant structure creates a variety of niches for fauna.

    • Form distinct assemblages associated with each layer.

    • Emergent layer: Mobile animals can live in the emergent layer; birds, mammals, insects.

    • Canopy layer: Many insects and several vertebrate species are confined to the canopy or sub-canopy – rarely descend to the ground. e.g. Sloths, monkeys, insects, amphibians.

    • Forest floor: Equally diverse animal, fungal and microbial communities on the forest floor.

Savanna

  • Global distribution: Cover ~60% of land area in Africa.

  • Temperature & rainfall:

    • Rainfall varies a lot throughout the year.

    • Distinct wet and dry seasons.

  • Characteristics:

    • Also known as ‘tropical grasslands’.

    • Tend to be slightly more north and south to the equator than tropical forests.

    • Rainfall is more seasonal – distinct wet and dry seasons.

    • Tropical forests replaced with thorn forests, widely spaced trees and grassland.

  • Plant life adaptations:

    • Dry conditions favour plants with small leaves.

    • Often have thorny branches as defence against browsers.

    • Evolved due to the evolutionary pressure from diversity of herbivores.

    • Plants adapted to dry conditions known as xerophytes.

  • Large herbivores:

    • Important part of savanna biomes - limit spread of shrub and woodland.

    • Grazers: Feed on above-ground parts of non-woody plants. Low-lying vegetation (e.g. grass).

    • Browsers: Feed on leaves, fruits of high-growing woody plants, soft shoots and shrubs.

  • Large herbivores = large carnivores

    • Big cats, hyenas.

    • Scavengers that follow them – vultures, jackals.

  • Other life:

    • Insects consume much more plant material than mammalian herbivores.

    • Grasshoppers, locusts, ants.

    • Hot, open habitats ideal for reptiles.

    • Other have adapted to feeding on this abundant food source – e.g. secretary bird.

  • Fire in savanna:

    • Electrical storms set fire to the dry grasslands.

    • Sweep through in quick, short bursts.

  • Adaptations to fire:

    • Seeds of many plant species adapted to surviving fire. Stay in ground until conditions are right. i.e. the rains come.

    • Many animals (particularly birds) take advantage of frequent fires by following the moving edge of fire and pick out insects as they try to escape.

Temperate Deciduous Woodland

  • Global distribution:

    • Refer to the map for distribution.

  • Characteristics:

    • Mild winters, warm wet summers.

    • Precipitation relatively high all year round.

    • Large range (600mm to 2000mm per year).

  • Seasons in woodlands:

    • Four distinct seasons.

    • Most broad-leaved trees drop their leaves in winter.

    • Coniferous species can tolerate lower temperatures.

    • Spring fauna take advantage of lack of leaves.

  • Layers of deciduous woodland:

    • Tree stratum, the tallest layer, 20-30 m high

    • Small tree or sapling layer - short tree species and young trees.

    • Shrub layer - shrubs like rhododendrons, azaleas, mountain laurels, and brambles.

    • Herb layer – bracken and ferns

    • Ground layer - lichens and bryophytes

  • Plant strategies:

    • Most temperate woodlands naturally cycle between K-strategists and R-strategists.

    • K-strategists: Fewer larger seeds, poor dispersal, slow growing, longevity. Main tree species forming the tree stratum. e.g. Oak (Quercus spp.), beech (Fagus spp.), chestnut (Castanea spp.) & elm (Ulmus spp.)

    • R-strategists: Rapid growth, intense competition, re-colonize in gaps from tree felling/disturbance. Smaller trees, shade tolerant species, shrub, herb and ground layer.

  • Animal adaptations:

    • Animals match their life cycles to times of plenty.

    • Hibernate: Reduce metabolic costs when food is scarce.

    • Migrate: Move to exploit more abundant resources elsewhere. e.g. store food for hard times.

  • Arboreal animals:

    • Hazel dormouse (Muscardinus avellanarius): Nests & feeds in canopy. Hibernates underground.

    • Pine marten (Martes martes): Nests in tree holes or old squirrel or bird nests. Can take prey from woodland floor. Some species adapted to living high in the tree layers.

  • Ground-dwelling animals:

    • Deer species: Browse and graze.

    • European badger (Meles meles): Forage in woodland and nearby. Less active in winter – setts.

Temperate Grasslands

  • Global distribution:

    • Known as 'prairies'

    • Known as 'pampas'

    • Known as 'steppes'

  • Characteristics:

    • Annual precipitation 400-600mm.

    • Strong summer peak.

    • Hot summers, cold winters.

    • These conditions have led to communities dominated by grasses, with a mixture of other herbaceous plants.

  • Fire and grazers:

    • As with tropical grasslands (savanna), fire and grazing maintain dominance of grasses.

    • Bison, horse, antelope historical the principal large grazers.

    • Insects major herbivores in all temperate grasslands.

    • Most grasslands now have few wild grazers – intensively farmed for meat or cereals.

Summary

  • Earth can be grouped into different biomes, categorised by temperature, rainfall and altitude.

  • Tropical forests are found near the equator and characterised by high rainfall and warmth.

  • Tropical forests have a huge biodiversity and animals and plants have special adaptations in order to reduce intense competition.

  • Tropical grasslands, known as savanna, have high temperatures but the rainfall varies throughout the year.

  • Savannas are dominated by grass species, and have thorny and shrub trees.

  • Grazing and browsing animals, and fire, are important to prevent woodland growth.

  • Temperate deciduous woodlands tend to have warm and wet summers, and mild winters.

  • Like with tropical forests, there are different layers.

  • Temperate grasslands are dominated by grass species, with occasional herbaceous plants.

  • They have hot summers and cold winters.

  • Like with tropical grasslands, grazing and fires are important to maintain dominance of grass species.