44.3A: What constitutes a biome? - Biology

44.3A: What constitutes a biome? - Biology

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A biome consists of all the habitats of a community that make up similar ecosystems in a particular region.

Learning Objectives

  • Differentiate biomes from other levels of ecological classification, including habitat

Key Points

  • The climate, including precipitation and temperature, and the geography control the type of biome found in a region.
  • There are two major classifications of biomes, which are terrestial and aquatic, and these include the types of biomes known as deserts, forests, grasslands, savannas, tundra, and freshwater environments.
  • A habitat is the location where a group of one type of organism (a population ) lives, while a biome is a community made of all the habitats in a given region and climate.
  • Different organisms inhabit different types of biomes.
  • Each type of biome can be found in multiple locations on Earth depending on its climate, geography, and organisms.

Key Terms

  • biome: any major regional biological community such as that of forest or desert
  • ecotone: a transition area between two adjacent ecosystems
  • habitat: a specific place or natural conditions in which a plant or animal lives
  • population: a collection of organisms of a particular species, sharing a particular characteristic of interest, most often that of living in a given area
  • ecosystem: a system formed by an ecological community and its environment that functions as a unit

What Constitutes a Biome?

A group of living organisms of the same kind that live in the same place simultaneously is known as a population. Populations live together in habitats, which together make up a community. An ecosystem is a community of living organisms interacting with the non-living components of that environment.

A biome is a community on a global scale, where habitats flank each other, and is usually defined by the temperature, precipitation, and types of plants and animals that inhabit it. The Earth’s biomes are categorized into two major groups: terrestrial and aquatic. Terrestrial biomes are based on land, while aquatic biomes include both ocean and freshwater biomes. The major types of biomes include: aquatic, desert, forest, grassland, savannas, and tundra.

Biome Attributes

Generally, biome classification is determined by the climate and geography of an area. Each biome consists of communities that have adapted to the different climate and environment inside the biome. Specifically, there are special vegetation adaptations as well as physical and behavioral adaptions made by animals in order to accommodate the environment. The eight major terrestrial biomes on Earth are each distinguished by characteristic temperatures and amount of precipitation. Comparing the annual totals of precipitation and fluctuations in precipitation from one biome to another provides clues as to the importance of abiotic factors in the distribution of biomes. Temperature variation on a daily and seasonal basis is also important for predicting the geographic distribution of the biome and the vegetation type in the biome.

The distribution of these biomes shows that the same biome can occur in geographically distinct areas with similar climates. Biomes have no distinct boundaries. Instead, there is a transition zone called an ecotone, which contains a variety of plants and animals. For example, an ecotone might be a transition region between a grassland and a desert, with species from both.

Top 11 Types of Biomes that Exist in India | Biology

This article throws light upon the top eleven types of biomes that exist in India. They are: 1. Tropical Rain Forest Biome 2. Tropical Deciduous Forest Biome 3. Temperate Forest Biome 4. Boreal Coniferous Forest Biome 5. Temperate Grassland Biome 6. Tundra Biomes 7. Temperate and Tropical Desert Biomes 8. Tropical Savanna and Grassland Biomes 9. Wetland Biomes 10. Freshwater Biomes 11. Marine Biome.

Type # 1. Tropical Rain Forest Biome:

In warm and wet climates of the tropics, this biome exits with most majestic and dense vegetation.

Species diversity and richness of life forms are maxi­mum in this biome.

The species diversity is so high that often difficult to find two individu­als of same species in close vicinity.

There are lots of full trees with epiphytic growth of mosses, ferns and orchids. The litter fall and their decomposition rate is very high in the forest. The forest is multitier with dense canopy cover.

Tropical rain forests are common in equatorial belt in Indonesia, Malaysia, Singapore, Hawaii, Amazonia, and also in Central Africa particu­larly in Zaire basin region. In India, the rainforests are confined to North-East in Assam, Meghalaya, Arunachal Pradesh, Mizoram and Manipur. Similarly rainforest also exits in South Western region of India, like Karnataka, Tamil Nadu and Kerala.

Over the years of extensive deforestation in tropics, the rainforest area rapidly disappeared and slowly converted into agricultural land. Rainforest is a natural forest which could not recover by man’s manipulation. Thus there is a great need for conservation of rainforest rather than creation of rainforest.

Type # 2. Tropical Deciduous Forest Biome:

In tro­pics, deciduous forest is very much prominent component, where forest shed their leaves be­fore winter onset. New flush of leaves appear after 2-3 months. These forests are not so dense as that of rainforest. There are places, where the deciduous forest may be dry and thorny.

In India, deciduous forest biome is very much predominant in various parts. Tropical sal forest or sal with other deciduous species is quite common. Ground cover vege­tation in deciduous forest is very significant.

Type # 3. Temperate Forest Biome:

In mid-altitude of mountains temperate forest biome exits. It extends up to tree line of upper elevation. In temperate region mostly broad leaved ever­green forest or needle leaved coniferous for­est or a mixture of species prevailed. The cli­mate have high humidity and thus having epi­phytes, mosses, ferns and other lower groups of plants.

In Himalayas, the rich biodiversity is prevailed in temperate biomes. There are a good num­ber of endemic species of various groups which prevail in this region. In many coun­tries, plantation cups were introduced in tem­perate climate by removing natural forest cover and there by destroying the rich endemic germplasm.

Type # 4. Boreal Coniferous Forest Biome:

This is a specialised temperate coniferous forest biome found in high mountains of Canada and Alaska. The plant community have low statured forests mostly coniferous species with ectomycorrhizae in the roots. Soil is somewhat acidic (4.5 to 5.0 pH) due to decomposition of needle leave litter. Often these forest also called Taiga. Forest floor have swamps and pit bogs.

Type # 5. Temperate Grassland Biome:

It is well known that grasslands are the most extensive formations of vegetation types found all over the world and in all ranges of climates from mesic to xeric and from cold to warm condi­tions. The temperate grasslands are however extensive in the North America and are called as prairies.

They may be tall grass prairie, mid grass prairie and short grass prairie depend­ing upon the height of the herbage portion. In many countries, the natural grassland is largely converted to grazing land or croplands. The temperate grassland is dominated by graminoids followed by sedges and forbs or the non-graminoids like dicot weeds.

Type # 6. Tundra Biomes:

These are extremely cold con­dition with alpine or subalpine habitats. Only herbs and dwarf shrubs are grown along with mosses, lichens and creepers. Because of ex­treme cold, the soil moisture is frozen at a depth of few centimeter from the top. This is called permafrost.

There are two kinds of tundra “Arctic tundra” in the extreme northern latitudes (north of 60 ON latitudes) and alpine tundra on mountain tops even at lesser latitudes. The re­gions is snow covered for sometime in a year. Soils are rich in organic matter because of very slow decomposition rate.

Type # 7. Temperate and Tropical Desert Biomes:

These are two kinds of desert habitats cold desert and warm or hot desert. Rainfall is very scanty (less than 500 mm per annum) in hot desert, while frost and snow are common in cold desert. Cold desert often noticed in temperate to subalpine region, with grasses and succulents, where as hot desert found in tropics where thorny forest, scrubs and succulents are grown.

West Indian desert which is a part of Thar desert is very well known hot desert, while in the Siberian region cold desert prevails.

Type # 8. Tropical Savanna and Grassland Biomes:

In an estimate it is reported that grass covered biomes constitute about 42.57% area in Africa, 6-12% in Asia, over 50% in Australia and about 80% in South America. In tropics, the grass­lands which is often called savanna are rich in grasses and sages interspersed by some shrubs and trees. But there are dried places where tall grasses dominate in ravine land with scattered trees.

Moreover the grassland of various coun­tries are named in different ways:

In Central India, the tropical grassland consti­tute about four distinct associations viz.

(i) Schima- Dichanthium type,

(ii) Dichanthium-Cenchrus— Lasiurus type,

(iii) Phagaguitics—Saccharum—lmperata type and

(iv) Themeda-Arundinella type.

Type # 9. Wetland Biomes:

Wetland habitat constitutes the transition zone between terrestrial habi­tats and deep water bodies. These includes, swamps, paddy fields, riverine flood plain, lakes, coastal swamps and so on. These habi­tats support specialized vegetation cover with characteristic fauna and serve as the breeding grounds of many migratory birds.

These habi­tats constitute wetland biomes. In tropics wet­land biome have rich flora and fauna. In many place such unique habitats are transformed for various man made activities and thus many species of flora and fauna became extinct from the native region.

Type # 10. Freshwater Biomes:

Freshwater biomes in­clude open water systems such as lakes and rivers and as water-logged regions known as bogs, marshes and swamps. Bogs consists of impervious substrates where rainfall is high. They are dominated by the growing plants able to tolerate waterlogged and nutrient-poor con­ditions such as Sphagnum moss and insectivo­rous sundews.

Swamps are tree-dominated wetlands occurring in tropical, subtropical and temperate regions.

Freshwater contains dissolved gases, nutrients, trace metals and organic compounds as well as organic and inorganic particles. These chemi­cal carbonate from rainwater which washes sub­stances out of the atmospheric dust deposi­tion and from the leaching of soils and rocks from the surrounding catchment areas.

Streams and rivers differ greatly, depending on their size. They also vary in their length from their source in upland areas to their mouth where the river meets the sea.

In gen­eral as the mouth of a river is approached:

(i) The speed of water flow decreases, the wa­ter becomes less turbulent and oxygen lev­els fall

(ii) The volume of water increases having accu­mulated as the river passes through its catch­ment

(iii) The energy of the river decreases, suspended material is deposited and the river bed be­comes composed of finer particles and eventually silts

(iv) The river bed becomes less steep because the larger volume of water erodes a broader channel

(v) Human influences increase many rivers flow through farmland and urban or industrial areas and receive agricultural run-off, treated sewage and other effluent which may raise the organic content of the river leading to eutrophication.

Streams are high in the catchment that are non-polluted, support caddis fly (Trichoptera) and blackfly (Simulium spp.) larvae feeding on fine organic particles. The water will be too turbulent and nuitrient poor for all but aquatic mosses, liverworts and algae.

Plankton com­munities, consisting of algae, photosynthetic bacteria, crustaceans and rotifers, can develop further downstream where the volume of moving water is increased and the current is reduced. Fish, reptiles, birds and mammals may be present.

As water flow continues to decrease, particu­larly at the edges of a growing channel, plank­ton communities become more complex and sediment is deposited, providing a roodng me­dium for larger aquatic plants (macrophytes) and a habitat for benthic organisms such as oligochaete worms, chironomid larvae and molluses.

Emergent plants, which grow up beyond the water’s surface provide physical habitat for invertebrates, fish and epiphytic algae, which in turn provide food for other orgamisms.

Type # 11. Marine Biome:

This is largest biomes of the world. It covers high saline coastal area to open sea area. In polar region it is mostly snow cov­ered. The coastal shallow marine biomes are highly productive and divisible into tidal neitric and the continental shelfs. Away from the coast are the oceanic belts with upper surface euphotic zone (up to 200 m) and lower battyal (200 m to 2000 m deep) and bottom abyssal dark deep zone.

Life is abundant in euphotic zone. Marine biome is the principal food source of mankind today and tomorrow.

In marine environment huge deposit of petro­leum, natural gases and minerals are recorded. But exploration of such resources leads to de­struction of marine biome. Over the year, ocean is used as a dumping ground of haz­ardous substances of diverse categories.

12th Class Biology Organism And Environments Biome

Definition : Each of the major terrestrial ecosystems or distinctive terrestrial areas with their group of climax plants and associated animals constitutes biomes. A biome is the largest terrestrial community. Rainfall, temperature range, nature of soil, barriers, latitude and altitude determine the nature and extent of biomes.

Major biomes of world : Biomes are often classified in seven categories :

(1) Tropical rain forests : The tropical rain forest, a biome occurs in regions of high temperature (average 25°C) and high rainfall ([200-450,,cm] per year). These tropical rain forests occur in Central America, around Amazon basin in South America, in Africa and in South-East Asia.

(i) This biome is characterized by multistoried vegetation (upto five distinct layers or storeys of vegetation). Further maximum biodiversity on land is shown by this biome and it is estimated that one half to two-thirds of all species of terrestrial plants and insects live in tropical forests.

(ii) Lianas (vascular plants rooted in soil and they only get support of trees for climbing to top) and epiphytes (air plants) are common in this biome due to excess of moisture. Further giant trees of the tropical forest support a rich and diverse community of animals on their branches.

(iii) No one species dominates in this biome.

(iv) The productivity of this biome is maximum.

(v) The trees of this biome possess buttressed trunks and phenomenon of cauliflory (presence of flowers and fruits on main trunk and main branches) is common in this biome.

(2) Savannahs : Like tropical forests, savannahs are found near the equator but in areas having less annual rainfall (90-150 cm/year). Some areas near the equator experience prolonged dry seasons. The heat, periodic dryness and poor soils cannot support a forest but have led to evolution of tropical open grasslands with scattered shrubs and trees.

(i) The vegetation of this biome support large grazing herbivores like buffalo, zebra, etc., which are food for carnivores like lions, tigers, etc. The savannah also supports a large number of plant eating invertebrates like mites, grasshoppers, ants, beetles and termites.

(ii) The termites are one of the most important soil organisms in savannahs.

(iii) Indian tropical grasslands are not true savannahs but these are the result of destruction and modification of tropical deciduous forests by cutting, grazing and fire.

(3) Deserts : These are the biomes that have 25 cm (10 inches) or less of precipitation annually.

(i) Sahara of North Africa, Thar of West Asia and Gobi of Asia are most important deserts.

(ii) Annual plants are abundant in deserts and tide over unfavourable dry season in the form of seeds. Succulent plants are characteristics of deserts. Trees and shrubs present in deserts have deep roots.

(iii) Desert animals have also fascinating adaptations that enable them to adjust with limited water supply.

(iv) Desert plants show phenomenon of Allelopathy, i.e., they secret some chemical substances which inhibit the growth of plants growing in their near vicinity.

(v) Deserts show poor biodiversity and their productivity is minimum.

(4) Temperate grasslands : Temperate grasslands experience a greater amount of rainfall than deserts but a lesser amount than savannahs. They occur at higher latitudes than savannahs but like savannahs are characterized by perennial grasses and herbs of grazing mammals.

Temperate grasslands have different names in different parts of the world, e.g., Prairies of North America, Steppes of Russia, Veldts of South Africa, Pampas of South America, Pusztas of Hungary and Tussocks of New Zealand.

(5) Temperate deciduous forests : Temperate deciduous forests occur in areas having warm summers, cold winters and moderate amount of precipitation ([75150,,cm]annually). The trees of this forest loose their leaves during autumn and remain dormant throughout winter (term &lsquodeciduous&rsquo derived from Latin word meaning &lsquoto fall&rsquo). These forests are present in Eastern United States, Canada and extensive region in Eurasia.

(i) In temperate forest biome, there is an upper canopy of dominant trees like beech, oak, birch, maple, etc. followed by lower tree canopy and then a layer of shrubs beneath.

(ii) Animal life in this biome is abundant on the ground as well as on the trees.

(6) Taiga : The taiga or northern coniferous forests or boreal forests consist of evergreen, cone bearing trees like spruce, hemlock and fir and extend across vast areas of Eurasia, and North America.

(i) The taiga is characterized by long, cold winters with little precipitation.

(ii) The harsh climate limits productivity of the taiga community. The cold temperatures, very wet soil during the growing season and acids produced by fallen conifers needles and Sphagnum inhibit full decay of organic matter, due to which thick layers of semidecayed organic material called peat is formed, which acts as energy source.

(7) Tundra : The tundra encircles the top of the world. This biome is characterised by desert like levels of precipitation (less than 25 cm annually), extremely long and cold winters and short warmer summers.

(i) Tundra is uniform in appearance and is dominated by scattered patches of grasses, sedges and lichens. Some small trees do grow but are confined to margins of streams and lakes (In general treeless).

(ii) Tundra is a biome of low diversity and low productivity.

(iii) The precipitation that falls remains unavailable to plants for most of the year because it freezes. During the brief arctic summer, some of the ice melts and permafrost (or permanent ice) found about a meter down from the surface, never melts and is impenetrable to both water and roots. However, the alpine tundra found at high elevation in temperate or tropical regions does not have this layer of permafrost.

Indian biomes : Indian forests are classified into three major types based on temperature are tropical, temperate, alpine.

(1) The marine environment : It is characterized by its high concentration of salt (about 3.5 percent in open sea) and mineral ions (mostly sodium and chloride followed by sulphur, magnesium and calcium).

(i) The vertical zones of the ocean are determined on the basis of availability of light for photosynthesis. The lighted upper 200 metres form the photic or euphotic zone. The next zone upto the depth [2002000] metres gets less light which is insufficient for photosynthesis form the aphotic zone. Below 2000 metre is the area of perpetual darkness, the abyssal zone.

(ii) Three major environments may be recognized in the ocean basin

(a) The littoral zone : The sea floor from the shore to the edge or the continental shelf.

(b) The benthonic zone : The sea floor along the continental slope and the aphotic and abyssal zone.

(c) The pelagic zone : Constituting the water of the ocean basin.

(i) Plankton : These are passively drifting or floating organisms. Most of these minute organisms, plankton includes photosynthesizing organisms like diatoms (phytoplankton) as well as heterotrophic organisms like small crustaceans (zooplanktons).

(ii) Nektons : These consist of actively moving organisms with well developed locomotory organs.

(iii) Benthonic organisms : These are found along the floor of the sea bed and include creeping, crawling or sessile organisms.

(2) Other (Lakes and Ponds) : Lakes and ponds are stagnant fresh water bodies and are found practically in every biome. Many lakes are direct or indirect result of glaciation. Others are natural or man made depression filled with water. The relatively shallow lakes, called eutrophic lakes, have a rich accumulation of organic products e.g., Dal lake of Kashmir.

Generally deep lakes, often with the steep and rocky sides, are poor in circulating nutrients like phosphates. These are called oligotrophic lakes. Some of the lakes contain a saline or brackish water (Sambhar lake of Rajasthan).

Biology Chapter 34 Review

Organisms rely on each other for nutrients
Nutrients cycle within an ecosystem
More nutrients = more organisms
Plants grow better in nutrient rich soil
Less nutrients caused by: pH, erosion, pollution, storm, soil structure, part of cycle gone extinct

Faster colder water has higher DOC
Stiller warmer water has lower DOC

Algae and other phytoplankton dine on the excess nitrates, then multiply at alarming rates. This process is known as a bloom. Two main things occur from this drastic increase in a population:

First, zooplankton eat the algae, and their population expands as well. Naturally, this creates more feces, which sinks to the bottom of the ocean.

Since there are more algae living, there will also be more algae dying. Lots of dead algae will sink to the bottom as well.

The previous two effects create lots of extra waste at the bottom (of benthic realm) of the ocean. Bacteria use lots more oxygen than they usually would otherwise attempting to decompose the waste. The oxygen levels in the lower part of the ocean decrease sharply, endangering many organisms that live there.

The bloom of algae creates extra oxygen due to photosynthesis. However, stratification prevents the extra oxygen from reaching the lower level of the Dead Zone through these steps.

1. The Mississippi river water flows into the Gulf. Since the river is shallower and does not come from an ocean, the new river water is much less dense than the ocean.
2. This extreme difference in densities essentially causes two distinct layers of ocean, both of which do not mix.
3. Since the algae float in the photic zone (the upper layer of water), all the oxygen created by them stays in the upper layer of the stratified water. However, the lower layer is where the waste is decomposed.
4. Because the Earth is warming due to climate change, the upper layer of the water and the shallow Mississippi will be even less dense than the ocean. This is because the ocean warms much less quickly than land because of both water's high specific heat and the cooling winds that flow over water's surface and remove the heat layer that air would usually provide. This supports the hypothesis that climate change is a factor in eutrophication.

Overall, the deeper ocean waters are losing oxygen, going from normal level of 4.8 mg/L to 2-3 mg/L (hypoxia) or to 0 mg/L (anoxia) in extreme cases. Though the process can be slowed by occasional weather events like hurricanes, the eutrophication still trundles along through its silent path of destruction,

What Makes A Biome?

Biomes are typically characterized by the resident biota within them. Currently, there is a disagreement in the scientific community about what exactly makes a biome.

Biology, Ecology, Conservation, Earth Science

Deciduous Forest Fall

Trees in a deciduous forest during the fall.

Photograph by Clarita Berger/National Geographic Creative

A biome is a community of plants and animals in a given climate, and each biome has life-forms that are characteristic to that place. For instance, the plants and animals that inhabit the Amazon rainforest are completely distinct from those in the Arctic tundra. However, not everyone agrees on exactly what constitutes a biome, and defining them presents a challenge.

Biomes are sometimes confused with habitats and ecosystems, but there are differences between them. Ecosystems focus on the way plants and animals, called biota, interact with the environment. The way nutrients and energy flow helps define ecosystems. A single biome can have multiple ecosystems within it. A habitat is specific to the area a population or species lives in. Biomes describe life on a much larger scale than either habitats or ecosystems.

Defining Biomes

Frederic E. Clements was an ecologist who studied the relationship between living things and their surroundings. He first used the term biome in 1916 and later worked with another ecologist, named Victor Shelford, to expand the definition of biome. By 1963, they were able to define the tundra, coniferous forest, deciduous forest, grassland, and desert as different biomes.

Biomes are different because of the organisms that live there and the climate of the area. The organisms within a biome also share adaptations for that particular environment. Adaptation is the process of change that a species goes through to become better suited to its environment. Climate is also a major factor in determining the types of life that reside in a particular biome. Several factors influence climate, such as latitude, geographic features, and how atmospheric conditions affect heat and moisture.

Not all scientists agree about the number of defined biomes. Most agree that climate and the organisms that live there are important. But some do not think factors like human activity and biodiversity, which is the variety of life forms that exist in a place, should be included in biome definitions. The main types of biomes that come out of the different definitions are tundra, desert, grassland, coniferous forest, deciduous forest, tropical rainforest, and aquatic biomes.

Tundra Biomes

The tundra is located at the northernmost parts of the globe. It is defined by long, cold winters and cool summers. The animals and plants that reside here have evolved adaptations, such as thick fur and the ability to hibernate, which allow them to survive in the frigid environment.

Desert Biomes

Deserts are defined by dryness, and can be located in both cold and warm climates. Life in these areas is adapted to a lack of water and nutrients.

Grassland Biomes

The grassland biome is found on every continent except Antarctica. It is characterized as being flat and grassy, with very little tree cover. Large mammals that graze, such as elephants or bison, inhabit these areas, along with small mammals, birds, and predators.

Coniferous Forest Biomes

Coniferous forests are also known as taigas or boreal forests. These areas experience long, cold winters, short summers, and heavy precipitation. The primary vegetation types are conifers and evergreen trees. Sometimes this category is split into another category known as the temperate forest, where temperatures are not as cold. One example of this warmer forest would be the western coast of North America, a humid forest system home to redwoods and cedars.

Deciduous Forest Biomes

Deciduous forests are located in eastern North America, Western Europe, and northeastern Asia. This biome is marked by broad-leafed trees, such as maple and oak, that lose their leaves seasonally as the temperatures begin to drop. Overall, these regions are temperate, that is, they have mild temperatures, but still have a distinct winter season.

Tropical Rainforest Biomes

Tropical rainforests in equatorial regions are warm and wet with diverse vegetation that forms a canopy. The uppermost trees and branches in a forest form a kind of roof&mdashthis is a canopy. Leaf litter on the ground and the humid conditions create a layer of nutrients above the low-quality soil, which allows for the growth of a wide variety of vegetation. Tropical rainforests are famous for hosting vast amounts of biodiversity.

Aquatic Biomes

There are numerous ways to classify aquatic biomes. Often freshwater and saltwater biomes are defined separately using factors, such as water depth, temperature, and salinity. Terrestrial biomes, or land biomes, are typically classified by vegetation types, but this method can be difficult to apply to aquatic environments. They do not have as much visible plant life.

Although biomes are often thought of as distinctly separate regions, in reality, they are not isolated from one another. Biomes do not typically have exact boundaries, but instead, there are frequently transition zones between biomes. These zones are referred to as ecotones, and they can be naturally occurring or created by humans.

Many biome definitions exclude humans. However, some scientists believe that human presence is an important part in defining biomes. They are of the opinion that most biomes are actually primarily influenced by humans. Scientists are also beginning to recognize how the results of human activities, such as habitat destruction and climate change, will change how biomes are defined in the future.


A useful way to understand potentially negative effects of landscape modification on native taxa is to consider the range of processes that may threaten a given individual species. Threatening processes associated with landscape modification may be broadly classified as exogenous (originating independently of the species’ biology) or endogenous (originating as part of the species’ biology), although this distinction may be blurred in some instances.

Exogenous threatening processes

Habitat may be broadly defined as the range of environments suitable for a given species. That is, it is a species-specific concept. Habitat loss is the dominant threat to species around the world ( Sala et al., 2000 ). Landscape modification for agriculture and urbanisation typically causes habitat loss for many species ( Kerr & Deguise, 2004 Luck et al., 2004 ). Because native vegetation is important for many species, numerous authors have equated ‘habitat’ with ‘native vegetation’ (e.g. Andrén, 1994 ). Although this classification may be appropriate in some situations ( Terborgh et al., 2001 ), in many situations it can be misleading. This is because a binary classification of land into habitat (native vegetation) and non-habitat (other land cover) ignores habitat suitability gradients and differences between species with respect to what constitutes suitable habitat for them (Fig. 3 Andrén et al., 1997 ) importantly, many native species can be conserved in well-managed production landscapes ( Daily, 2001 Lindenmayer & Franklin, 2002 ). On this basis, we suggest that the term ‘habitat’ and associated terms like ‘habitat fragmentation’ be used only in a single-species context (Table 1). The broader use of the term habitat (i.e. equating it with native vegetation) can result in the under-appreciation of differences between the unique habitat requirements of different species, and the under-appreciation of the potential habitat value of modified environments for some species. For a given species, habitat loss rarely occurs in isolation from other threats, but tends to coincide with habitat degradation, habitat sub-division and a range of additional threatening processes ( Liu et al., 2001 Fig. 4).

Threatening processes arising from landscape modification as experienced by a declining species. Threatening processes are broadly classified as deterministic versus stochastic, and exogenous versus endogenous. Deterministic threats predictably lead to declines, whereas stochastic threats are driven by chance events. Exogenous threatening processes are external to a species’ biology, whereas endogenous threats arise as part of a species’ biology (see text for details).

Habitat degradation is the gradual deterioration of habitat quality. In degraded habitat, a species may decline, occur at a lower density, or may be unable to breed ( Temple & Cary, 1988 Felton et al., 2003 Hazell et al., 2004 ). Degraded habitat may constitute an ‘ecological trap’ to which individuals of a species are attracted but in which they cannot reproduce ( Battin, 2004 ). Habitat degradation can be difficult to detect because: (1) some types of degradation take a long time to manifest (e.g. recruitment failure of cavity trees Saunders et al., 2003 ), and (2) some species with slow life cycles may continue to be present in an area even if they are unable to breed (e.g. cockatoos reliant on nesting hollows Saunders, 1979 ). Factors related to habitat degradation vary widely between species, and may include pressure from grazing ( Spooner et al., 2002 ), logging ( Recher & Serventy, 1991 ), or changed thermal regimes ( Jäggi & Baur, 1999 ). Detailed autoecological studies and monitoring are often required to detect and counteract habitat degradation ( Borghesio & Giannetti, 2005 ).

Habitat sub-division is the breaking apart of continuous habitat into multiple patches it is synonymous with what some authors have termed ‘fragmentation’ ( Fahrig, 2003 ). Smaller habitat patches can lead to population declines ( Bender et al., 1998 ), for example because resources in smaller patches may be more limited ( Zanette et al., 2000 ). In addition, habitat sub-division increases the isolation of remaining habitat areas. Habitat isolation can negatively affect day-to-day movements of a given species (e.g. between nesting and foraging resources Saunders, 1980 Luck & Daily, 2003 ). Habitat isolation also may negatively affect the dispersal of juveniles ( Cooper & Walters, 2002 ). Metapopulations, i.e. ‘set[s] of local populations which interact via individuals moving between local populations’ sometimes develop as a result of habitat isolation ( Hanski & Gilpin, 1991 ). Notably, patchy populations are true metapopulations only if movement between sub-populations is neither very uncommon nor very common ( Hanski & Simberloff, 1997 ). Finally, habitat isolation may negatively affect large-scale movements of species such as seasonal migration or range shifts in response to climate change ( Souléet al., 2004 ). The extent to which landscape modification results in habitat isolation depends on the interaction between a given species’ dispersal behaviour, mode and scale of movement, what constitutes suitable habitat for it, and how a given landscape has been modified. Habitat connectivity is the opposite of habitat isolation, and is contrasted against other connectivity concepts in the section entitled Connectivity below.

Endogenous threatening processes

In addition to direct negative impacts on a species’ habitat, declining species in modified landscapes often experience disruptions or changes to their biology, behaviour and interactions with other species. These changes are often triggered by exogenous threats, but may constitute threatening processes in their own right. Landscape modification can lead to altered breeding patterns and social systems. For example, birds may have shorter breeding seasons, lay fewer eggs, and rear fewer nestlings ( Hinsley et al., 1999 Zanette et al., 2000 ), or their mating systems may change ( Ims et al., 1993 ). Many other types of behavioural and biological changes have been observed for animals in modified landscapes, including disruptions to dispersal ( Brooker & Brooker, 2002 ), changed movement patterns over greater distances ( Recher et al., 1987 Norris & Stutchbury, 2001 ), altered home ranges ( Pope et al., 2004 ), higher incidences of fluctuating body asymmetry ( Sarre, 1996 ), changed vocalisation patterns ( Slabbekoorn & Peet, 2003 Lindenmayer et al., 2004 ), and disrupted group behaviours ( Gardner, 2004 ).

Changes to species interactions may affect competition, predation, parasitism and mutualisms. Increased competition can occur, for example, for insectivorous woodland birds in many Australian farming landscapes where the aggressive noisy miner (Manorina melanocephala) has increased in abundance ( Grey et al., 1997 ). Increased predation and parasitism have both been frequently reported in modified landscapes, especially for birds ( Robinson et al., 1995 Lahti, 2001 Zanette et al., 2005 ), but more recently also for complex insect–plant food webs ( Valladares et al., 2006 ). Increased pressure from competition and predation can be particularly severe when introduced species are involved. For example, competition by exotic snails has severely decimated Hawaii's native snail fauna ( Hadfield et al., 1980 ).

Landscape modification also may disrupt mutualisms. For example, Cordeiro & Howe (2003 ) demonstrated the disruption of the mutualism between the endemic tree Leptonychia usambarensis and fruit-dispersing birds in a modified landscape in Tanzania. Similarly, Kearns et al. (1998 ) argued that landscape modification may disrupt pollination throughout the world, as recently emphasized by Ricketts et al. (2004 ) for Costa Rican coffee farms.

Disruptions to species interactions have particularly severe effects when strongly interacting species are involved, which play a disproportionate role in maintaining ecosystem function ( Terborgh et al., 2001 Souléet al., 2005 ). Such species are sometimes also termed keystone species ( Paine, 1969 Power et al., 1996 ), and their importance is discussed in more detail below in the section entitled Extinction cascades.

Stochastic threatening processes

The exogenous and endogenous threatening processes discussed above are deterministic. Deterministic threatening processes are those which predictably lead to declines ( Gilpin & Soulé, 1986 Fig. 4). In addition to deterministic processes, stochastic processes may threaten species in modified landscapes. Exogenous stochastic threats are related to environmental variability, such as fluctuations in climate or natural catastrophes like hurricanes or wildfires ( Simberloff, 1988 ). Endogenous stochastic threats occur as part of a species’ life cycle, and include demographic stochasticity (e.g. year-to-year variability in reproductive success) and genetic stochasticity (e.g. genetic drift). Endogenous stochastic threats are more pronounced in small populations ( Roughgarden, 1975 Keller & Waller, 2002 ).

Interactions among threatening processes and extinction proneness

Species declining as a result of landscape modification are typically affected by both deterministic and stochastic threats. Exogenous threats often lead to the initial decline of a species. The resulting smaller populations, in turn, are more susceptible to endogenous threats that reinforce the decline of the species ( Clark et al., 1990 Fig. 4).

Many factors have been suggested to be related to the extinction proneness of species in modified landscapes, including habitat or niche specialisation, home range size, mobility, extent of geographic distribution, population density or rarity, edge sensitivity, body size and dietary specialisation ( Johns & Skorupa, 1987 Brashares, 2003 Koh et al., 2004 Cardillo et al., 2005 Kotiaho et al., 2005 ). Although these factors are often considered as equally plausible in studies on extinction proneness, it is important to note that some of them are more directly related to extinction proneness in a causal sense than others. For example, body size is a proxy for a range of other ecological attributes, including area requirements, mobility and dietary specialisation ( Laurance, 1991 ), whereas habitat specialisation is directly linked to the threatening process of habitat loss. Table 2 outlines some of the most direct links between known threatening processes and likely factors contributing to (or ameliorating) extinction proneness. Notably, some of these links are complex, and potentially contentious. For example, although high mobility can help species to move between habitat patches (Table 2), it may also lead to an increased number of individuals dispersing into unsuitable habitat, thereby threatening population persistence ( Gibbs 1998 Casagrandi & Gatto 1999 ). Further investigation of the links between threatening processes and extinction proneness in the future may lead to an increasingly robust, process-based understanding of species extinctions. A likely emergent pattern is that a given species’ ability to withstand human landscape modification is related to the extent to which landscape modification causes habitat loss and isolation, and the disruption of biological and interspecific processes for that individual species. Small population size (natural or human-induced) will further exacerbate a species’ risk of extinction due to stochastic events (Fig. 4 Table 2).

Threatening process Ameliorating biological attribute Explanation
Habitat loss and habitat degradation Low habitat specialisation Specialised species are more likely to lose their habitats as a result of landscape change
Disturbance tolerance Disturbance-tolerant species are more likely to find suitable habitat in modified landscapes
Ability to live in the matrix Species that can live in the matrix experience no habitat loss as a result of landscape modification
Habitat isolation and sub-division Ability to move through the matrix Species that can move through the matrix are less likely to suffer the negative consequences of habitat isolation
Dispersal ability Strong dispersers may be more likely to maintain viable metapopulations (but note this is contentious — see text)
Disrupted species interactions Limited dependence on particular prey or mutualist species Species that can switch prey or mutualists are more likely to withstand landscape change
Competitive ability Species that are strong competitors are less likely to be outcompeted by species whose habitat expands as a result of landscape change
Disrupted biology Low biological and behavioural complexity Species with a complex biology (e.g. social or breeding systems) are more likely to have their biological processes disrupted as a result of landscape change than species with simpler biological systems
Stochastic events Population density High density populations contain many individuals even in a small area, and hence are more resilient to stochastic threats

Lakes and Ponds Biome

Lakes and Ponds represent a freshwater biome type that is generally referred to in the scientific community as a lentic ecosystem (still or standing waters). Scientists that study lakes and ponds are known as limnologists. In this overview we hope to describe a few of the biotic (plant, animal and micro-organism) interactions as well as the abiotic interactions (physical and chemical).

Lakes and Ponds Video

In this Untamed Science video we explore the lakes and ponds biome. While the rest of the crew enjoys the lake, Haley takes off canoeing in an effort to describe this amazing biome.

Lake Zonation

A lake can be divided into different zones: the littoral zone, the limnetic zone, the euphotic zone and the benthic zone. These zones are illustrated bellow.

Lake Stratification

Intense heating of the surface waters of a lake help create a strong stratification of lake waters. The upper layer is known as the epilimnion. This layer is affected by winds and stays fairly well mixed. Just below the epilimnion is the thermocline where the water stops mixing. It serves as a boundary layer for the cold, deep water below. The hypolimnion is the cold, deep-water, stagnant layer. This layer often has very low oxygen in the water. This is often accompanied by dissolved hydrogen sulphide and other sulfurous gases.

Lake Cycling and Turnover: Temperate Lake Model

In temperate lakes, the changing of the seasons help move water in the lake. Tropical lakes often stay stratified because warm water always stays on the top. In temperate lakes the winter months chill the surface water so that it gets colder than the water underneath, causing it to sink. This happens in the spring and fall as shown in the diagram bellow.

How do lakes form?

There are many different ways that a lake can form. One common way that lakes have formed in northern North America is through the recession of glaciers. Many of the lakes in Minnesota were formed in this way. The rift lakes in Africa are formed through tectonic activity as two plates separate from one another. Lake Tanganyika was formed in this way. Another great example of lake formation is the creation of oxbow lakes. These lakes are formed when a meandering river bend is pinched off from the main channel.

How do lakes die?

Lakes by nature are ephemeral. They all receive sediment input from rivers and streams that lead into the system. Given that the lake is not expanding through tectonic activity, a lake has a limited existence. The exact time that the lake may survive depends on the rate of sedimentation and the depth of the lake.

Common Invasive Species in Lakes and Ponds

In the US, as in many countries, rivers and streams have been damned up creating lakes that did not exist before the dam construction. This creates a lot of new habitat for both animals and aquatic plants that can begin to colonize the area. It also provides a great place for invasive species to take hold, especially when there are little to no native plants in the area that are already filling available niches.

Some of the common invasive species in lakes and ponds include zebra mussels, sea lampreys, hydrilla, water hyacinth and Eurasian water milfoil.

An Example Species: Alligator Snapping Turtle

The Alligator Snapping Turtle is found throughout the southeastern United States and can be a formidable sit and wait predator in this habitat. It can grow to be over 100 years old and can be over 3 feet in length. For this species profile video we traveled to Mississippi and the Vicksburg Army Corp of Engineer Research Facility.

9. The Evolution of Biological Individuality

Finally, the evolution of biological individuality continues to be a lively topic (Okasha 2011 Calcott & Sterelny 2011 Bourrat 2015 Clarke 2016b O&rsquoMalley & Powell 2016 Queller & Strassmann 2016 Herron 2017 Sterner 2017). The starting point here is the idea that the history of life is the history of the construction of more complicated biological individuals from simpler individuals, with natural selection (operating at one or more levels) facilitating the transitions between these individuals. Underlying these ideas is the assumption that many or all biological individuals are hierarchically organized: earlier individuals provide the material basis for later individuals. For example, prokaryotes, which are single-celled organisms without a nucleus, form the material basis for single-celled eukaryotes, which do have a nucleus in turn, single-celled eukaryotes serve as the material basis for multicellular eukaryotes.

The evolution of biological individuals from prokaryotes to single-celled eukaryotes around 2 billion years ago, and from those to multicellular eukaryotes in the last 600&ndash800 million years, are established facts. In addition, there appear to be no counter-examples to this evolutionary trend. Yet speculation and controversy surround almost everything else that has been said about these evolutionary transitions. Consider three such issues on which there is a sort of default position in the literature that remains subject to ongoing philosophical and empirical interrogation.

First, it is common to view the evolution of individuality itself as the evolution of complexity. There are, however, questions both about how complexity itself should be measured or conceived and about what empirical evidence there is for viewing the complexity of individuals as increasing over evolutionary time (McShea 1991). Are the number of cell types that an individual has considered (Bonner 1988), the types of hierarchical organization it manifests (Maynard Smith 1988), or some more taxa-specific criterion, such as the information required to specify the diversity of limb-pair types (Cisne 1974)? Fossils constitute a principal source for the criteria that have been proposed here. Yet different kinds of organisms leave fossils with distinct kinds of features, and some kinds of organisms are more likely to leave fossils than are others.

One natural suggestion is that there may well be different kinds of hierarchies for the evolution of individuality, since kinds of individuals can differ from one another in more than one way. Daniel McShea (2001a, 2001b McShea & Changizi 2003) has proposed a structural hierarchy that is based on two components, the number of levels of nestedness and the degree to which the highest individual in the nesting is individuated or developed. McShea provides an overarching framework in which eukaryotic cells can be viewed as evolving from differentiated aggregations of prokaryotic cells that have intermediate parts multicellular eukaryotes as evolving from differentiated aggregations of single-celled eukaryotes and colonial eukaryotes as evolving from differentiated aggregations of multicellular eukaryotes.

By contrast, Maynard Smith and Szathmáry (1995) focus on differences in how genetic information is transmitted across generations, proposing eight major transitions in the history of life. These start with the transition from replicating molecules to compartmentalized populations of such molecules, and end with the transition from primate societies to human societies. While Maynard Smith and Szathmáry are interested in individuality and complexity, their eight transitions do not form a continuous, non-overlapping hierarchy. Their discussion is focused primarily on exploring the processes governing each of the particular transitions they propose in terms of changes in replicative control. O&rsquoMalley and Powell (2016) have recently argued that not only does this perspective omit critical events&mdashsuch as the acquisition of mitochondria and plastids, in what those authors prefer to think of as turns rather than transitions in the evolution of living things&mdashbut also that what is needed is a

supplementary perspective that is less hierarchical, less focused on multicellular events, less replication oriented, and in particular, more metabolic. (O&rsquoMalley and Powell 2016: 175)

Second, it is common to view the trend from prokaryotes to multicellular eukaryotes as resulting from some type of directional bias, one that makes the trend a tendency supported by underlying mechanisms and constraints. Perhaps the tendency is underwritten by thermodynamic, energetic considerations, by facts about the generative entrenchment of developmental systems (Griffiths & Gray 2001), or by evolutionary advantages of increases in size (McShea 1998). But in supposing that there is some type of directional bias, each of these hypotheses might be thought committed to the sort of Panglossianism about adaptation that Gould and Lewontin (1979) are famous for critiquing, or (more subtly) to a view of evolutionary change as progressive or inevitable in some way. Gould has used his discussion of the Burgess Shale (Gould 1989) to challenge such views of evolution, arguing that the disparity of the fossils in that shale indicates that living things are significantly less different from one another than they once were. Gould argues that the range of biological individuals now on the planet is largely the result of highly contingent extinction events, and there should be wariness of immediately assuming that observed trends or patterns are adaptive (or other) tendencies.


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