Why Animal Seed Dispersal Matters in Plant Diversity and Ecosystems
Seed dispersal is the term coined for the movement of the seed from one place to another. This movement generally occurs with the help of dispersal vectors. Dispersal vectors can be biotic or abiotic components. Biotic dispersal vectors include animals, bees, and other insects whereas abiotic factors are wind, and water.
The need for dispersion arises because plants need to transport their seed far from the parental plant, this is done to reduce the intraspecific competition between the parent and progeny. The pattern of seed dispersion is greatly dependent on the dispersal vector that is being used. The following are the main types of dispersion mode - gravity, wind, water, animal, and animals. The dispersal unit of the plant is known as the diaspora, it includes seeds and additional tissue that assists in the seed dispersion.
Seed dispersal can be broadly classified into autochory and allochory. Autochory refers to the dispersion of the diaspore without any help from the external environment, whereas allochory refers to the dispersion of seed with the help of dispersal vectors. Ballistic and dispersion through gravity comes under the autochory mode of dispersion. Ballistic dispersion refers to the condition when a seed is ejected by the dehiscence of the fruit.
Seed Dispersal by Animals and Birds
Animals and birds disperse the seeds in different manners. Colourful fruits attract animals and birds. When they eat these fruits only the pulpy part is digested in their bodies and the seeds are excreted out by their systems in the form of their dropping, which eventually gives birth to new plants. Blackberry, cherry, tomato and apple seeds are usually dispersed in this way.
There are some species of squirrels that collect nuts from different plants like acorns and bury them under the ground as they store food for the winter season, these squirrels usually forget the place where they have previously buried the nuts and the seeds germinate into new plants.
Some plants bear seeds with hooks. A common example for such a type of plant is the Burdock plant. The seed of these plants gets attached on the fur of animals with the help of these hooks and are carried away to different places, far from their parent plants.
Examples of plants having seeds dispersed by animals are Dates, rambutan, sea grapes, sea holly, tamarind, raspberry, sunflower, tomatoes.
Dispersal by Animals
Seed dispersal by animals comes under the category of the allochory. Dispersal by the animal is termed as zoochory, zoochory. It can be further classified into saurochory, ornithochory, and, myrmecochory, which means dispersion by reptiles, birds, and ants respectively. Based on the way of carrying the diaspore zoochory can be classified as endozoochory, epizoochory, and synzoochory.
Endozoochory:
It is the condition when seeds are carried within the dispersal vectors, which are animals in this case. It is an example of mutualism between plants and animals. In this type seed is generally inside the fruit, animals that perform dispersion eat the fruit surrounding the seed and discard the seed at a distant location, thus performing dispersion. The plant provides fruit to meet the nutritional requirements of the animal whereas the animal helps in plant survival by allowing it to grow in various locations, thus both the species mutually benefited from the seed dispersal by animals.
Epizoochory:
It is the condition when an animal carries the diaspore outside its body. This condition arises in the case of accidental dispersion of the seed by animals. When animals or birds pass by the seed some of the seed might get attached to the body of the animal and move along with it, diaspore than generally falls at a distant location from the parental plant.
Synzoochory:
It is a condition when the diaspore seed has not completely entered the alimentary canal of animals rather they carry the seed in their mouth. Synzoochory is often seen in seed dispersal by squirrels. It is also done by rodents and birds. Synzoochory has a characteristic feature, the seed is generally a nut that is used as a food source by the animals.
Diplochory is also considered as a type of syn zoochory where it is indirect or two-step dispersion by an animal, this occurs when the nut is the food source of more than one species. It is a highly effective way of dispersion.
Seed Dispersal by Water
In the plant kingdom, water is natural and one of the fundamental sources of seed dispersal. In this mode of seed dispersal, seeds float away from their parent plant in rivers or ponds due to their light weight. Plants which grow near water bodies usually rely on water to transport their seeds to different places. The seeds of such plants are light in weight and thus float, or there may be fluff that helps buoyancy. This method of dispersal is mainly seen in those plants which are found in water or nearby the water bodies like sea shores, lakes, ponds etc. Some examples of plants whose seeds are dispersed by the water are Coconut, palm, mangroves, water lily, and water mint.
Characteristics of Seed Dispersal by Water:
There are some characteristic features of hydrochory they are as follows,
Diaspore must be lightweight and hollow
Diaspora can swim
They generally have long filaments
They generally do not produce nectar
They are not usually very colorful
They must have the property of hygrochasy, that is seed can germinate in moist weather.
Example of Seed Dispersal by Water
There are some of the following examples of water dispersal of seeds,
Mangrove plants are the best example of hydrochory.
Aster tripolium and Juncus species are some examples of hydrochory where seeds are transported to distant areas
Mesembryanthemum, Sedum, and other plants show characteristic features of hygrochasy.
Sea dispersal of the coconut palm is the best-studied common example of hydrochory.
Some other examples of plants that use water for seed dispersal are water plantain, yellow flag, sea kale, sea rocket, sea beet.
Seed Dispersal is an adaptive mechanism that can be seen in all seed-bearing plants, it can be defined as a method by which seeds move away from their parent plant in order to germinate and survive at a different place. There are many vectors/modes of transportation of the seed from one place to another.
Types of Seed Dispersal
Seed Dispersal by Wind
Seed Dispersal by Water
Seed Dispersal by Animals and Birds
Seed Dispersal by Gravity
Seed Dispersal by Explosion
FAQs on Seed Dispersal by Animals: How Animals Spread Seeds
1. What is the significance of seed dispersal by animals for a plant?
Seed dispersal by animals, also known as zoochory, is a vital mechanism for plant survival and reproduction. It helps plants in several ways:
- Reduces Competition: It moves seeds away from the parent plant, reducing competition for essential resources like sunlight, water, and soil nutrients.
- Colonisation of New Habitats: It allows plants to colonise new and potentially more suitable areas, expanding their geographical range.
- Increases Survival Rate: By moving away from the parent, seeds can escape predators and diseases that may be concentrated near the parent plant.
- Enhances Genetic Diversity: Spreading seeds over a wide area promotes cross-pollination and increases the genetic diversity of the plant population.
2. What are the main ways animals disperse seeds?
Animals disperse seeds through two primary methods:
- Endozoochory (Internal Transport): This occurs when an animal eats a fruit and the seeds pass through its digestive tract unharmed. The seeds are then deposited in a new location via the animal's droppings, often with a natural fertiliser. Birds eating berries is a classic example.
- Epizoochory (External Transport): This happens when seeds with specialised structures like hooks, barbs, or sticky surfaces attach themselves to an animal's fur, feathers, or skin. The animal carries the seed to a new location before it falls off or is groomed away. The burrs that stick to a dog's fur are a common example.
3. What are some examples of plants that rely on animals for seed dispersal?
Many plants have evolved to use animals for seed dispersal. Some common examples include:
- Guava and Berries: These have fleshy, sweet fruits that attract birds and mammals. The animals eat the fruit and excrete the seeds elsewhere.
- Acorns and Nuts: Squirrels and other rodents often collect and bury these seeds to store for later. They may forget some, allowing them to germinate in a new spot.
- Xanthium (Cocklebur): This plant produces seeds covered in stiff hooks that easily cling to the fur of passing animals.
- Mistletoe: Birds eat the sticky berries, and when they wipe their beaks on tree branches to clean them, they deposit the seeds, allowing the parasitic plant to grow on a new host.
4. How have fruits and seeds adapted for dispersal by animals?
Fruits and seeds show remarkable adaptations to attract animals. For endozoochory (dispersal via ingestion), fruits are often brightly coloured (e.g., red, blue, or black) to be visible, fleshy, and rich in sugars to be a tempting food source. The seeds themselves are tough enough to withstand digestive enzymes. For epizoochory (external dispersal), seeds or fruits develop specific external structures like hooks, spines, barbs, or sticky secretions that act like Velcro, allowing them to easily attach to fur or feathers.
5. What is the difference between seed dispersal by animals and by wind?
The key difference lies in the adaptations of the seeds and the nature of the dispersal. Seed dispersal by animals (zoochory) typically involves heavier seeds enclosed in attractive, fleshy fruits or seeds with hooks and spines. The dispersal is often targeted, as animals move between specific resources. In contrast, seed dispersal by wind (anemochory) involves seeds that are very light, small, and often have wing-like structures or feathery parachutes (like dandelions) to catch the wind. This method is more random and relies on air currents to carry seeds over long distances.
6. Do animals benefit from the process of seed dispersal?
Yes, the relationship between plants and seed-dispersing animals is often a form of mutualism, where both parties benefit. The animal receives a direct benefit in the form of a nutritious food source from the fleshy fruit surrounding the seed. For example, birds, bats, and monkeys get vital energy from eating fruits. In return, the plant gets its seeds transported to a new location, increasing its chances of successful reproduction. This mutual benefit is a key driver of co-evolution between many plant and animal species.
7. Are humans also considered agents of seed dispersal?
Absolutely. Humans are one of the most significant agents of seed dispersal, both intentionally and unintentionally. Intentional dispersal occurs through agriculture and gardening, where we plant seeds for crops and ornamental plants globally. Unintentional dispersal happens when seeds stick to our clothing or shoes (like burrs), or when we transport fruits and discard the seeds far from their origin. This process, known as anthropochory, has dramatically altered the distribution of plant species worldwide.
8. What happens to a seed inside an animal after it is eaten?
When an animal eats a fruit, the fleshy part is digested, but the hard-coated seed is designed to survive the journey through the digestive system. The acids and enzymes in the animal's stomach can actually help the seed by a process called scarification. This process weakens the tough outer seed coat, which can break dormancy and make it easier for the seed to germinate once it is excreted. The seed is then deposited in the animal's droppings, which provides a moist, nutrient-rich environment ideal for sprouting.
