An ecosystem is a geographic area where living things such as plants, animals and micro-organisms interact with each other as well as with non-living things like sunlight, water, soil, weather, climate, atmospheric gases and nutrients. These living (biotic) and non-living (abiotic) components are important for the functioning of the whole system.
An ecosystem can be very large covering thousands of hectares, like a rainforest, or very small like a tide pool. Tide pools are tiny pools of sea water that are formed on rocky beaches during low tide. They have plant and animal life living inside them. Ecosystems can be very complex; for example, the rainforests, or relatively simple in areas that experience harsh climates like the North and South poles.
The different categories of ecosystems are terrestrial, freshwater, and marine.
Each of these can be further sub-divided into different types based on their physical environment and its biodiversity.
Examples
Terrestrial ecosystems - tropical rainforests, deserts, grasslands, etc.
Freshwater ecosystems - rivers, lakes, ponds, etc.
Marine ecosystems - coral reefs, mangroves, deep sea thermal vents, etc.
Biomes - A biome is an area of the planet that can be classified according to the plants and animals that live in it. Temperature, soil, and the amount of light and water help determine what life exists in a biome.
For example, tropical deserts across the globe will have similar kinds of vegetation. This is mainly determined by the climate of the place, among other factors. This means that even though the types of trees or plants found in the Sahara desert of Africa are different from the Thar desert of India and Pakistan, they will have similar features that help them adapt to the hot and dry climate of the desert. The difference between a biome and an ecosystem is that a biome is made up of many ecosystems. Within a desert, an oasis is an ecosystem.
Dynamics
The interaction between living and non-living elements in an ecosystem is much more complex than just a simple food web. An ecosystem can be impacted by internal and external factors. For example, in a terrestrial ecosystem, plants use sunlight and take in carbon dioxide (CO2) from the atmosphere to make food by photosynthesis. They also take nutrients from the soil to grow. Plants give out oxygen which is used by animals. Primary consumers like deer and rabbits also eat plants. Animal excreta and dead matter are decomposed by bacteria and fungi, releasing CO2 into the atmosphere and also breaking them down into soil nutrients that can be used by that plants.
These internal processes strive to maintain balance within an ecosystem. They are self-regulating and will work together to form a state of equilibrium (see Feedback Loops for more info). For example, when the population of herbivores such as the deer, in an ecosystem, increases, the amount of grass available for the deer decreases as more deer eat more grass. As the availability of grass declines, the number of deer population also falls because now there is less food available. This gives the grass a chance to bounce back.
External Factors
External factors also affect an ecosystem. One constant external force that affects ecosystems is the Sun. The amount of energy it provides enables plants to make their food. Some of this energy is distributed through the entire
food chain. However, the amount of sunlight reaching the earth is different in different seasons.
Other external factors include storms, earthquakes, floods, droughts and other natural phenomena. It could also be in the form of invasive species. Introduction of an animal or a plant that does not belong to that particular ecosystem, can use up food sources and nutrients meant for the local plants and animals, thereby disrupting the balance of the ecosystem.
But one of the main external factors that ecosystems are being affected by, is the human factor. Changes in the ecosystems due to deforestation, destruction of habitats, pollution, are all responsible for disrupting the balance of ecosystems. Humans have also introduced non-native species to ecosystems which have become invasive species.
Resistance and Resilience
Resistance and Resilience - Some ecosystems are stronger than others and can withstand changes even due to external factors. The ability of an ecosystem to withstand changes and maintain stability is called resistance.
For example, studies have shown that the annual flooding in Kaziranga is good for the forest as the Brahmaputra brings with it fertile alluvial soil that helps the vegetation in the forest. Similarly, the dry deciduous forests (grasslands) of Bandipur are fire resistant.
Some ecosystems have the ability to return to their balanced state either by adapting to changes or by absorbing the impacts after having undergone disruption due to external factors. This is known as resilience.
For example, Coral reefs are important ecosystems which are under serious threat due to climate change. However, studies are showing that some coral colonies are adapting to the changes and bouncing back.
It is, however, important to remember that there is a threshold to the resistance or resilience of an ecosystem. Changes to a fragile ecosystem will damage it irreversibly.
Biodiversity
Biodiversity can play an important role in the resistance or resilience of an ecosystem. An ecosystem where there is more than one plant or animal species performing the same function, is more likely to be resistant or resilient versus an ecosystem where only one species is performing a specific function. The loss of that one species in the latter example will result in the collapse of the entire ecosystem.
Human Factor
As learned earlier, fragile ecosystems have the potential to be damaged permanently. Deforestation, destruction of habitats, urbanisation, pollution, ocean acidification, over fishing etc are all human activities that are causing irreparable changes to our ecosystems sometimes even damaging them permanently. Human induced climate change is also influencing the external factors like temperatures, weather, etc.
We are now aware of the of the interconnections between humans and ecosystems (See Ecosystem Services) and how they impact our lives and livelihoods. With more awareness the relationship between humans and natural ecosystems is changing and can continue to change for better.
Facilitator Cues
What are ecosystems? and How are they classified?
What is a dynamic ecosystem? and Why is it important to understand the complex dynamics of an ecosystem?
The Food Chain
Definition
All living beings need energy to grow and function. They get this energy in the form of food. All animals eat other living beings and receive nutrients that are necessary for their survival on this planet.
In a food chain, plants are the producers that make their own food through photosynthesis with the help of sunlight, water, carbon dioxide, and nutrients from the soil. Animals that eat plants like deer, rabbits, elephants, or caterpillars that feed on leaves are called the primary consumers or herbivores. And animals that feed on the primary consumers (or herbivores), are called secondary consumers or carnivores. In turn, secondary consumers are eaten by tertiary consumers.
For example, plants (producers) are eaten by caterpillars (primary consumers); smaller birds (secondary consumers) eat caterpillars, and the smaller birds are then eaten by large birds like the eagles.
Plants are always at the base of any food chain. And animals that are at the top of the food chain are called apex predators. Example - tigers, lions, orcas, etc. These are animals that don't have natural predators. However, when plants and animals die, including the apex predators, they are eaten by smaller microscopic organisms like bacteria or organisms like fungi. These are known as decomposers. The decomposers break down dead plant and animal matter into nutrients in the soil which are then absorbed by the plants, thus completing the chain.
Animals like humans that feed on both plants and animals are called omnivores.
Examples
Marine ecosystem - single celled organisms called phytoplankton are food for fish and other marine animal larvae called zooplankton - zooplankton, in turn, are eaten by smaller fish, which are eaten by fish like mackerels which are in turn eaten by whales, dolphins, and sharks.
Forest ecosystem - In a forest ecosystem, the deer eats the grass and the tiger eats the deer.
Pond Ecosystem - Blue green algae, like plants, make their own food through photosynthesis. The algae is food for mosquito larvae. These larvae are food for dragonfly larvae, which is food for small fish. And birds like kingfishers or cormorants eat the small fish in the pond.
Food Web
As our understanding of the food chains grew, we realized that animals are not just a part of a simple food chain but a complex food web. Food chains interconnect to form a food web. One animal can eat different species of plants or one animal species in an ecosystem can have multiple predators.
For example in a forest ecosystem -
Producers are of different species, grass, shrubs, trees, etc. These are consumed by many different types of herbivores (primary consumers) like deer, elephants, rabbits, monkeys. These herbivores are preyed upon by different carnivores (secondary consumers) like the dholes (wild dogs), jackals, and leopards. And then there are tertiary consumers or apex predators like tigers or eagles that eat primary consumers or sometimes even secondary consumers.
Trophic Levels and Energy Flow
Trophic levels are the positions of organisms in a food chain. Living organisms in an ecosystem are grouped into trophic levels.
Producers are the first trophic level of every food chain.
This is followed by primary consumers or herbivores at the second trophic level.
Then the secondary consumers or carnivores at the third trophic level
Tertiary consumers at the fourth trophic level
And the decomposers are at the last trophic level of a food chain.
At every level there is an exchange of energy. This energy in a food web is known as Biomass. Plants, while producing food through photosynthesis, store energy inside them. When herbivores at the first trophic level consume plants, they take in only a part of that energy and the rest is lost in the form of waste, or used up to perform functions like respiration, digestion, movement, etc. Thus the energy or the biomass decreases at each trophic level.
In a healthy food web, the producers are in abundance followed by many herbivores and relatively few carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass.
When one link in the food web is threatened, some or all of the links are weakened or stressed.
In a food web where even one link is threatened (by decline or increase in number of species at any trophic level), it can result in the weakening of some or all the links.
What 1-2 key ideas should the facilitator focus on while communicating the topic with their local communities?
The idea of primary, secondary, and tertiary consumers
What is a food chain and how is it connected to a food web?
What are trophic levels and how does energy flow through a food chain?
Trophic cascade is a phenomenon in which there are occasional changes in the number or behaviour of a species in one part of the food web. This alters the numbers, not just of the species it feeds upon, but also of species with which it has no direct interaction. This indirect interaction that happens across the trophic levels is described as a trophic cascade.
Trophic Levels
Trophic levels are the positions of organisms in a food chain. Living organisms in an ecosystem are grouped into trophic levels.
Producers are the first trophic level of every food chain.
This is followed by primary consumers or herbivores at the second trophic level.
Then the secondary consumers or carnivores at the third trophic level
Tertiary consumers at the fourth trophic level
And the decomposers are at the last trophic level of a food chain.
Examples
Top-down Cascade
Predators help to keep balance in an ecosystem. By feeding on their prey, they directly impact the number and behaviour of their prey. For example, their prey might avoid certain parts of the forests where the predators roam. In a top-down trophic cascade, changes in the number or behaviour of predators impact not just the number and behaviour of the prey but also one level below and eventually across multiple trophic levels in an ecosystem.
For example, a change in the number or behaviour of wolves in an ecosystem will impact the deer population, which in turn will also impact the plants they eat. If wolves were to go extinct from an ecosystem, the deer population will increase and more deer will feed on more grass and plants, thus leading to a decline in the plant population. This will also impact other animals in the ecosystem that feed on the plants.
Bottom-up Cascade
In a bottom-up cascade, the impacts happen at producer (such as plants or algae) levels. Changes in primary producers at the bottom of the food chain will not just impact the herbivores that feed on them but also impact the carnivores feeding on the herbivores as well as the apex predators.
For e.g. - If a fungal disease or a pest attacks the grass in an ecosystem, the number of rabbits and other herbivores that feed on them will start to decline due to starvation or they may be forced to move out to find food. This decline in the number of herbivores will lead to an impact on the carnivores that feed on them.
Trophic Cascades and Climate Mitigation
Human interventions like habitat destruction, game hunting, overfishing, pollution of land, water and soil has led to changes in the number or behaviour of predator species resulting in changes in the entire ecosystems.
Trophic cascades can, in certain circumstances, help to combat and mitigate the impacts of climate change by restoring healthy ecosystem services. For example, where carnivores have been hunted or are being impacted due to climate change related causes, herbivores may exist at extremely high densities. The overgrazing by the herbivores will limit the ability of woodlands to regenerate, thereby also inhibiting their ability to sequester/store carbon. Returning or reintroducing predators to the area can, therefore, help woodlands regenerate and lead to more carbon being captured in the trees and soils.
Similarly, an absence of marine predators - specifically whales - could lead to increased greenhouse gases. Whales play a crucial role in the ocean carbon sequestration (capture and removal) from the atmosphere by circulating essential nutrients from the bottom of the ocean to the surface, which is food for the plant plankton and other carbon capturing flora. By protecting and conserving these species, we secure their vital place in the ocean's food web and their role in capturing carbon and fighting global climate change.
Some researchers argue that the world is kept green by a three trophic level interaction, where predators control grazers (herbivores) that would otherwise overgraze and eliminate vegetation. Some others argue that not all plants are edible, and that plants also have inbuilt defenses against herbivores. Therefore complete elimination of vegetation may not happen. This debate is ongoing, but the dominant view remains that herbivores can impact many aspects of plant ecology like their distribution and population, etc. but are not key drivers of the productivity of entire autotrophic ecosystems.
What 1-2 key ideas should the facilitator focus on while communicating the topic with their local communities?
Facilitators can help their communities learn about trophic levels by watching the 2 short films shared below under Resources. This will help the learners understand the idea of different trophic levels and the key interactions that are constantly occurring between them and that lead to a state of dynamic balance in those ecosystems.
How does it connect systemically with the other topics?
We've explored the importance of Greenhouse Gases and how they can either keep the planet at a healthy average temperature or help accelerate its warming. Trophic Cascades can help regulate these temperature changes, as in the example of marine predators such as whales that help balance carbon sequestration in the oceans.
Trophic Cascades also play an important role in restoring ecosystem services by keeping herbivore populations in check, that otherwise may lead to overgrazing which in turn would limit the ability of woodlands and grasslands to store carbon.
Examples of success stories from around the world
The reintroduction of wolves into Yellowstone National Park is an excellent example of how trophic cascades play a vital part in maintaining ecological balance across the Earth's ecosystems.
Some species play a crucial role in the functioning of the entire ecosystem even when they are a small part of the ecosystem. These are called Keystone species.
Their presence enables other species to survive. Losing a keystone species doesn't just affect the next species in the chain, but can affect the entire ecosystem. Without them, the ecosystems that they are a part of, would be dramatically different or even cease to exist.
We've learned that food webs are complex and interconnected in an ecosystem. The multiple interconnections in a food web tell us that if one species is affected, it will impact not just the next species in the chain, but also a level lower. This is known as a Trophic cascade.
However, scientists have identified species in some ecosystems that play a more important role than others in maintaining balance and diversity in this complex ecosystem. These species are not just connected to other species in a food web but also provide habitat, resources, and food to many other species in an ecosystem, just by virtue of their natural behaviour.
Background
Biologist Robert Paine came up with the concept of keystone species while studying the intertidal ecosystem on the Pacific coast. Have you ever noticed an architectural arch made up of stone or bricks? In the arch, the central stone is called the keystone. This stone holds all the other stones in place. If we remove the keystone in the arch, the entire structure will fall apart.
Just like the keystone in the arch, the keystone species in an ecosystem holds all the interconnections together. The fall of the keystone species will result in the falling apart of the entire ecosystem.
In his study, Robert Paine studied the interactions of different species in a food web in the rocky intertidal zone. And he observed that removal of a predator species in this food web led to the decline of diversity in the ecosystem. The intertidal zone consisted of mussels, barnacles, and starfish. The starfish was the predator species that fed on other species like the mussels and barnacles.
In one region of the intertidal zone, he experimented by removing the starfish completely. This led to an increase in the mussels species that took over the entire region, removing all the other species of barnacles and other animals. In the region where the starfish population was untouched, the diversity of organisms remained intact. This showed that the starfish kept the mussels population in check and thus helped in maintaining the diversity and health of that ecosystem.
Examples
Other Keystone Species
Keystone species are not always predators. They could be herbivores like elephants, primary producers like trees, or scavengers like vultures.
Asian or African elephants, for example, are the keystone species in the forest or grassland ecosystems. They help other species in the ecosystem by creating pathways in densely forested parts. They often knock down or uproot trees, but this action helps in providing sunlight to younger plants on the forest floor that are otherwise struggling to grow under the dark forest canopy. During hotter months, they are known to dig for watering holes using their trunks and feet. These small sources of water help in providing water for other animals in the hot summers. These watering holes also become microhabitats where frogs and smaller plants and animals live.
Elephants also help with seed dispersal. The seeds of some trees that grow in the ecosystems that elephants are a part of, need to pass through the digestive tract of elephants, in order to germinate! Elephant dung is highly nutritious for smaller organisms like the dung beetle and is an excellent source of manure for the forests. Some types of algae and fungi also grow from the elephant dung, which is a source of food for animals like monitor lizards and star tortoises.
That's why elephants are also known as ecosystem engineers. They create, alter, and transform habitats for other animals to survive.
Another keystone species that is not a predator is the Fig Tree. Studies by researchers show that over 10% of the world's bird species and over 6% of the world's mammals are known to eat figs as well as a small number of reptiles and fish. Fig trees bear fruits throughout the year thus providing a source of food for many birds and animals in seasons when other fruits are scarce. Without the fig trees, all the birds and animals depending on them may decline or disappear.
Mangroves are also considered keystone species. Mangroves not only provide food and oxygen but also offer us protection from coastal storms, cyclones and tsunamis. They also act as nurseries for young fish and provide shelter and food for many other species including birds, reptiles and mammals.
Scientists are still learning about keystone species in different ecosystems; however the rapid changes taking place in these ecosystems due to climate change, can affect species even before we can learn about them and can cause irreparable damage. Learning about keystone species helps us understand how complex the interconnections are of just one species in an ecosystem and the need to preserve and conserve biodiversity in an ecosystem.
What 1-2 key ideas should the facilitator focus on while communicating the topic with their local communities?
A key idea to focus on would be, a keystone species need not be the largest animal in the ecosystem to have an outsize impact. It can be anything really, including an insect. This means, we cannot ignore smaller flora and fauna just because of their small size or because we believe they could not have much of an impact.
How does it connect systemically with the other topics?
It connects (and impacts) directly and indirectly with ecosystems, the services they offer, environmental feedback loops, greenhouse gases, and global warming among others. Without keystone species, all of these would be impacted leading to unprecedented changes on the planet.
Examples of success stories from around the world.
The American Prairie grasslands, historically, was home to between 30-60 million Plains Bison. The local Native American communities depended heavily on these animals for everything from their food to their clothing. The sudden influx of newer settlers from Europe nearly wiped out this massive bison population until only a couple of dozen individuals remained in Yellowstone National Park by the 19th century. Until then, the Bison, through their massive consumption of grasses, had sustained the Great Plains, fostering vegetation and habitats for other keystone species like the black-tailed prairie dogs. They also provided water sources during droughts as their wallowing would create shallow pools that quickly became watering holes for smaller animals and insects. In 2012, biologists and conservationists initiated a successful recovery effort, boosting the bison population to several thousand through the introduction of breeding programs. Their efforts were so successful that the effects on the American Prairie could even be seen from space.
Activity
Watch the movie and have a discussion with participants to deepen their understanding of the keystone species.
An ecological niche is defined as the role of an organism in a community and its interactions with other species in the community. A species usually carves out a niche for itself by being able to adapt to specific physical and environmental conditions such as terrain, availability of nutrients, temperatures, etc. and its interactions with other species including competition, predation, etc.
The competitive exclusion principle says that when two species compete for the same resources in a place, they can't both stay there. To deal with this, they try to find different ways to survive without fighting. If one species figures it out, they'll stop competing. But if neither does, then the one that's better at using the resources they want will win, and the other will disappear. Hence a specific niche is helpful for an animal or plant as it reduces competition with other species.
Examples of Ecological Niche
Dung beetles
Dung beetles feed on dung. They are found on all the continents except Antarctica. These beetles have carved themselves a niche where they roll balls of dung and transport them underground through the help of tunnels that they build. These dung balls are then stored in the underground tunnels as food reserves. Female dung beetles also lay eggs in these balls so that when the larvae hatch, they can feed on the dung.
By creating tunnels in the soil, these insects help in aeration of the soil and the buried dung releases nutrients into the soil. Also, by feeding on dung these beetles help reduce the dung available for flies to breed, thereby reducing some of the population of flies. In this way, dung beetles serve several important functions in their habitat.
Xerophytic plants
such as cacti and aloe vera, have developed smart ways to survive in dry places with very little water. They store water in their thick, fleshy leaves and grow long roots to find water deep underground. When i’s dry, these plants can also drop their leaves or even fold them up to stop water from escaping. They sometimes also have a protective layer - called a cuticle - on their leaves, like a waterproof jacket, that helps to do the same. Their hairy leaf coverings also help keep moisture in.
The surface of plant leaves have tiny mouth-like structures called stomata that take in CO2 and release oxygen and water during the day. However, succulents open their stomata at night to save water during the hot day. So, they've figured a few tricks to stay hydrated and thrive in water-scarce environments.
The importance of niches
It is important for ecologists to have complete knowledge of an animal’s niche to make informed interventions and work on appropriate conservation actions for the environmental changes caused by human action like climate change and habitat destruction.
Stable habitats are required for ecological niches to exist. Even a small disturbance or change can wipe off niches.
For example, dragonfly larvae can only develop in a waterbody within a certain range of acidity, chemical composition, temperature, prey population, and a limited number of predators. Adult females require the right kind of vegetation to lay eggs. So do the larvae, to survive and metamorphose into adults. The adult dragonfly also impacts its surroundings in the following ways. The eggs become food for other insects; the larvae act as both, predators and prey, and help in adding nutrients to the water; and adult dragonflies prey on other insects. If the conditions were to undergo changes that were different from the niche requirements, the species, in this case the dragonfly could face extinction.
Facilitators Cues
What is ecological niche?
Why is it important to understand ecological niche?
