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Patterns of Biodiversity: Latitudinal and Altitudinal Gradients, Climate Zones of Earth, Species - Area Relationships, Practice Problems and FAQs

Patterns of Biodiversity: Latitudinal and Altitudinal Gradients, Climate Zones of Earth, Species - Area Relationships, Practice Problems and FAQs

You are living in a world which possesses a variety of organisms. But you might have noticed that all organisms are not present everywhere in the world. For example, Kangaroos are indigenous (native to a given region or ecosystem) to New Guinea and Australia.

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Fig: Kangaroo (Macropodidae)

You also might have noticed that some areas have more biodiversity than others. Biodiversity refers to the totality of genes, species and ecosystems of a region. It is not uniform throughout the Earth. It varies with a change in climate zones of Earth, latitude and altitude. For example the distribution of organisms in a forest of tropical region and desert defers.

Do you have any idea why there is less diversity in deserts or freezing mountain peaks? Yes, you are correct. The availability of less sunlight is the major reason for less biodiversity in freezing mountains. The regions which receive less sunlight show less rate of photosynthesis and ultimately results in less productivity. Now you understand that there is a difference in the patterns of diversity. Let’s discuss more about the patterns of biodiversity in this article.

Table of contents

Season

The rotation of the Earth around the Sun and the tilt of its axis cause annual variations in the intensity and duration of solar radiation and temperature on Earth, resulting in distinct seasons.

A season is a period of the year that is distinguished by special climatic conditions. The four seasons are spring, summer, fall, and winter. They follow one another regularly. Each season has its own light, temperature, and weather patterns. Such constant environments promote niche specialisation and lead to a greater biodiversity.

Fig: Seasons

Niche specialisation

The process by which a species becomes better adapted to the specific characteristics of a particular habitat is called niche specialisation.

Patterns of biodiversity

Patterns of biodiversity can occur within various habitats, communities, species, regions, biomes, ecosystems, and on the Earth as a whole. The patterns of biodiversity differ locally or globally over time. The diversity of plants and animals is not uniform across the globe. It shows an uneven distribution. Species diversity decreases from the equator towards the poles. It is because the tropical region has favourable conditions for growth throughout the year. Tropics with the latitudinal range between 23.5°N to 23°S harbour more species.

For example, Columbia, located near the equator has about 1400 species of birds, New York (41°N) has about 105 species of birds and Greenland (71°N) has only 56 species. India, having much of its land in the tropical latitudes and has more than 1200 species of birds.

Fig: Examples for latitudinal gradients

Criteria for studying patterns of biodiversity

Ecologists identified a regular pattern in the biodiversity in the environment. They studied patterns of diversity by considering the following criteria:

  • Latitudinal gradients
  • Altitudinal gradients
  • Climate zones of Earth
  • Species-Area relationships

Latitudinal gradients

Horizontal mapping lines on Earth are lines of latitude. The equator is an imaginary horizontal line which divides the Earth into two equal hemispheres. These are the Northern Hemisphere and Southern hemisphere. Parallel to this equator are other 180 lines in total. These are called latitudes. There are 90 latitudes in the Northern hemisphere and 90 latitudes in the Southern hemisphere. Some of the important latitudes are named.

Fig: Major latitudes

The availability of sunlight varies with latitude, hence there is change in temperature. Latitudinal gradients of biodiversity are referred to the biogeographic patterns that tell the way in which the components of phylogenetic, taxonomic, genetic, phenetic or functional dimensions change with latitudinal position on the surface of the earth. For example, species diversity decreases from the equator (Africa) towards the poles (Antarctica). It is because the tropical region has favourable conditions for the growth throughout the year, whereas temperate and arctic regions have severe climates with a short growing period for plants.

Fig: More sunlight available at tropical regions

Altitudinal gradients

Altitudinal gradients refers to the difference between the minimum and maximum elevations in which a species is found within the range of survey sites. The species diversity decreases from lower to higher altitude on a mountain due to drop in temperature and greater seasonal variability at higher altitudes.

Climate zones of Earth

The Earth has three main climate zones. These are the tropical, temperate, and polar zones.

Figure : Tropic, Temperate and polar zones

Tropical zone

The climate region near the equator with warm air masses is known as the tropical (latitudinal range 23.5°N to 23.5°S) zone. In the tropical zone, the average temperature in the coldest month is 18°C. Tropical environments are less seasonal, relatively more constant and predictable. More sunlight energy is available in the tropics because of its latitudinal range as shown in the image above. The rays from the sun strike Earth's surface most directly at the equator. As the rays hit more directly, the area is heated more and this causes more evaporation resulting in precipitation in the form of rain. This in turn contributes to higher productivity and biodiversity in the tropical regions. Thus, there is a constant weather pattern in tropics like hot humid air, dry but misty mornings and late afternoon downpours and convectional storms. Tropical environments are less seasonal, relatively more constant and predictable too. Such constant environments promote niche specialisation and lead to greater species diversity. The forest in the tropical region has more diversity as compared to the forest in temperate region of the same area size. For example, the Ecuadorian forest of the tropical region has 10 times more species as compared to the Midwest forest of temperate region.

Fig: Latitudinal gradient shown by Ecuadorian forest

Examples for tropical regions with more biodiversity

Examples for tropical regions with more biodiversity are given below:

Amazonian rainforest

The Amazonian rainforest is a tropical rainforest in Brazil that covers most of the Amazon basin of South America. The Amazon river passes through this forest. It is also known as the ‘lungs of the planet’. A rainforest is an area of tall, mostly evergreen trees and a high amount of rainfall. It is home to more than 40,000 species of plants, 3,000 fishes, 1,300 species of birds, 427 mammals, 427 amphibians, 378 reptiles, and more than 1,25,000 species of invertebrates. Scientists estimated that in these rain forests there might be at least two million insect species, which are not yet discovered.

Fig: Amazonian rain forest

Congo basin

The Congo basin is the second largest rainforest in the world. It is located in Africa. The Congo river passes through this forest. 10,000 plant species can be found in and around this forest. Endangered wildlife, including forest elephants, chimpanzees, bonobos, and gorillas inhabit these lush forests. 400 other species of mammals, 1,000 species of birds and 700 species of fish can also be found here.

Fig: Congo Basin rain forest

Polar zone

The North (60°N) and South (90°S) poles have climates that are polar because of nearby cold air masses. The north polar zone is called the arctic region and the south polar region is called the antarctic region. In this climate zone, the warmest month's average temperature is less than 10°C. This temperature is not favourable for most of the organisms here and therefore it shows less biodiversity. For example, Greenland at 71° N has only 56 species.

Fig: Polar zone

Temperate zone

The climate zone known as temperate is located between the equator and the North and South poles. The average temperature of the coldest months in the temperate zone is lower than that of the tropical zones. In tropical and polar climates, the weather is consistent throughout the year. In temperate zones, the weather is affected by both warm and cold air masses at different times during the year, so the weather changes with the seasons. Temperate deciduous or temperate broad-leaf forests are a variety of forests present in this region, which are dominated by trees that lose their leaves each year. Temperate deciduous forests are home to a variety of trees like oak, birch, maple etc. Temperate regions have been subjected to frequent glaciation in the past whereas the tropical latitudes have remained relatively undisturbed for millions of years.

Fig: Temperate deciduous forest

Types of temperate zones

There are two temperate zones, one is present in the northern hemisphere and one is present in the southern hemisphere.

North temperate zone

The north temperate zone extends from the Tropic of Cancer (approximately 23.5°N) to the Arctic Circle (66.5°N).

South temperate zone

The south temperate zone extends from the Tropic of Capricorn (23.5°S) to the Antarctic Circle (66.5°S).

Torrid region

The torrid region is the hottest region of the Earth which is bounded by the tropics of Cancer and Capricorn. The torrid zone receives the maximum amount of heat throughout the year because the rays of the sun fall vertically on this region.

Fig: Common fauna in the torrid region

Frigid zone

The two areas of the Earth respectively north of the Arctic Circle and south of the Antarctic Circle are called the frigid zone (very cold in temperature). The rays of the sun have to cover a larger distance and due to this their intensity decreases as the regions are far away from the equator. So they get less direct sunlight than other areas. Some or all of the land is covered with permanent ice here. This zone shows less biodiversity.

Fig: Frigid zone

Reasons for more biodiversity in tropics

Scientists proposed the following reasons for more biodiversity in tropical regions:

Speciation

The term speciation refers to the formation of new and distinct species. Speciation is a function of time because it is an evolutionary process and requires billions or millions of years. The temperate regions underwent frequent glaciations in the past and therefore, cannot evolve. Unlikely, the tropical regions remain undisturbed for millions of years and receive a long evolutionary time for species diversification.

Fig: Tropical regions remain undisturbed

Tropical environments

The tropical environments are less seasonal and therefore, constant and predictable. Extreme temperature variations in seasons are not faced by most of the tropical regions. In the tropical zone, the average temperature in the coldest month is 18°C. This constant environment promotes niche specialisation and as a result, greater species diversity occurred. On the contrary, the temperate regions are more seasonal and possess less species diversity.

Solar energy

More sunlight energy is available in the tropical regions, which contributes to higher productivity, this in turn contributes to greater diversity. More producers (forest), herbivores and other members of the food chain will be present here. As a result, this contributes to greater biodiversity in this region.

Fig: More sunlight energy leads to more productivity

Species - Area relationships

During the extensive investigation in the South American rainforests, Alexander von Humboldt proposed the Species - Area relationships. He was a geographer and naturalist from Germany.

Fig: Alexander von Humbolt

According to him within a region, species richness increases with increasing the area of observation or exploration, but up to a certain limit. For example, wide varieties of species will be present deep in the forest than near to the peripheral area, because the explored area is more.

Fig: Varieties of species present deep in the forest

Graph of Species - Area Relationships

The relationship between species richness and area for a wide variety of taxa like freshwater fishes, birds, and angiosperms (flowering plants) gives a rectangular hyperbola. This is because the rate at which the variety of plant and animal species are spotted in any area initially is very large in comparison to the area covered. So the graph is showing a rectangular hyperbola.

Rectangular hyperbola is a special type of hyperbola in which its asymptotes (X - axis and Y - axis) are perpendicular to each other. An asymptote is a straight line that constantly approaches a given curve but does not meet at any infinite distance as shown in the graph

Fig: Species - Area Relationships

Expression of Species - Area Relationships

When exploring a forest, many different types of vegetation can be found, as well as many animal species such as birds, insects, and rodents. As more area is explored, more species of animals and plants are observed. As more of the forest's interior and core parts are explored, the chances of seeing different kinds of flora and animals rise. The variation in flora and fauna species is limited because if more areas are covered, the species remain the same as spotted initially. As a result, up to a certain level, species diversity is exactly proportional to area covered.

Species diversity ∝ Area

S ∝ A

For a wide variety of taxa, such as angiosperm plants, bats, birds and freshwater fishes, the relation between species richness and area gives a rectangular hyperbola. This is because initially the species spotted are very large to the area covered. However, as the more area covered, the species of flora and fauna do not increase at the same rate and this results in rectangular hyperbola.

Species diversity ∝ Area

S ∝ A

S = CAZ

Where,

C = Y-intercept (Proportionality constant)

Z = Regression coefficient (slope)

S = Species richness

A = Area

By applying log on both sides, the equation will change into log scale. When a log equation is plotted on a graph, this will give a straight line or linear graph.

S = CAZ

Applying log on both sides

log S = log (CAZ)

log S = log C + Z log A

The mathematical equation of a straight line is as follows:

y = mx + c

Here,

y = log of species richness (S)

x = log of Area

m = slope of line, i.e. Z

C = constant i.e. log C

Fig: Species - Area Relationships in log scale

Z or value of slope

Regardless of the taxonomic group or locality, ecologists have established that the value of Z lies in the range of 0.1 to 0.2. The slope of the regression line remains similar whether it is taken for plants in Britain, birds in California or molluscs in New York.

Fig: Z lies in the range of 0.1 to 0.2

When the Species - Area correlations are examined over very vast areas, such as for entire continents, the slope of the line becomes substantially steeper (Z values are in the range of 0.6 to 1.2). The slope is 1.15 for frugivorous (fruit-eating) birds and mammals in tropical woods on several continents.

Fig: Z values are in the range of 0.6 to 1.2

Z or value of slope indicates the species richness of a given area. In the linear graph, the slope which is the Z value is directly visible and hence it can be interpreted easily.

Z > 1

If the value of Z is more than 1, the slope will be steeper. In this case, the increase in species is more as compared to the area covered.

Fig: Steeper slope

Z = 1

If the value of Z equals one, it shows that the increase in the variety of species is equal to the area covered.

Fig: Linear slope

Z < 1

If the value of Z is less than one, the slope will be gentle. In this case, the increase in species is less than the area covered.

Fig: Gentle slope

Significance of Species-Area Relationships

The following are the significances of Species-Area Relationships:

  • This curve can help in understanding the structure and inner workings of a habitat.
  • It can relate the factors that will have an impact on the survival of a species.
  • Knowing the factors can help in creating more effective conservation strategies.

Practice Problems

Q1. Determine the correct statement regarding latitudinal gradient.

A. Biodiversity remains constant with latitude
B. Species diversity increases from the equator towards the poles
C. Tropical regions harbour more species than temperate and polar regions
D. Tropical regions were subjected to many glaciations in the past

Solution: Horizontal mapping lines on Earth are lines of latitude. The availability of sunlight varies with latitude, hence there is change in temperature. Latitudinal gradients of biodiversity are referred to the biogeographic patterns that tell the way in which the components of phylogenetic, taxonomic, genetic, phenetic or functional dimensions change with latitudinal position on the surface of the earth. The forest in the tropical region has more diversity as compared to the forest in temperate region of the same area size. This is because tropical regions receive more sunlight and have constant weather due to which speciation occurs. These constant environments promote niche specialisation and as a result, greater species diversity occurs. Hence, the correct option is c.

Q2. Rahul was researching the diversity of species in a particular location. He obtained a (Z) value of between (0.6 - 1.2). Determine the type of place he was studying based on the facts provided.

A. State
B. Continent
C. Tropical rainforest
D. Country

Solution: Scientists estimated that when the species-area correlations are examined over very vast areas, such as for entire continents, the slope of the line becomes substantially steeper (Z values are in the range of 0.6 to 1.2). Based on this, we can conclude that Rahul was studying about the continent. Hence, the correct option is b.

Fig: Z values are in the range of 0.6 to 1.2

Q3. When a Species - Area Relationships plots on a log scale, it shows:

A. Rectangular hyperbola
B. Hyperbola
C. Straight line
D. S-shaped curve

Solution: When a log equation of Species - Area Relationships is plotted on a graph, this will give a straight line or linear graph.

S = CAZ

Applying log on both sides

log S = log (CAZ)

log S = log C + Z log A

Hence, the correct option is c.

Fig: Species - Area Relationships in log scale

Q4. Which type of slope is obtained if the value of Z is greater than one?

A. Steeper
B. Gentle
C. Straight
D. Hyperbola

Solution: If the value of Z is more than 1, the slope will be steeper. In this case, the increase in species is more as compared to the area covered. Hence, the correct option is a.

Fig: Steeper slope

FAQs

Q1. Why is the Amazonian rainforest known as the lungs of a planet?
Answer:
The Amazonian rainforest is a tropical rainforest in Brazil that covers most of the Amazon basin of South America. The Amazon river passes through this forest. The Amazon rainforest is known as the lungs of the planet because it absorbs huge amounts of carbon dioxide and produces 80% of the total oxygen of the Earth. This happens because of the higher diversity of plants present there.

Fig: Amazonian rain forest

Q2. Why species diversity decreases when moving from equator to poles?
Answer:
The species diversity decreases when moving from equator to poles because the equator region receives straight sunlight and this results in more productivity. On the contrary, poles receive less amount of sunlight in the slanting rays and this decreases biodiversity.

Q3. What is the pattern of species biodiversity?
Answer:
The species diversity increases as the latitude decreases. For example, the forest in the tropical region has more diversity as compared to the forest in temperate region of the same area size. For example, Ecuadorian forest in the tropical region has 10 times more species as compared to the Midwest forest of temperate region.

Q4. How can one find the relation between species richness and area?
Answer:
The relation between species richness and area can be find from the given equation:

log S = log C + Z log A

Where,

C = Y-intercept

Z = Regression coefficient

S = Species richness

A = Area

 

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