Seed dispersal, the movement of seeds away from their parent plant, is fundamental for plant survival and spread. It increases the chances seeds find suitable locations for germination and growth, while reducing competition with the parent plant. This also helps plants escape localized predators and pathogens. Ultimately, dispersal allows plants to colonize new areas, contributing to biodiversity and ecosystem health.
Seeds Travel Inside Animals
Many plants rely on animals to consume their fruits, allowing seeds to travel through the animals’ digestive systems, a process known as endozoochory. This mutualistic relationship provides animals with a nutritious food source, while the seeds are typically passed unharmed, often with a deposit of nutrient-rich feces that aids germination. Birds and mammals are key dispersers, including rodents, elephants, monkeys, and bats. Some fish and reptiles also disperse seeds in specific ecosystems. For example, three-wattled bellbirds disperse Ocotea endresiana tree seeds in Latin America, depositing them under perches where conditions are favorable. Cherries, figs, mangoes, guavas, and carob are other examples of fruits adapted for this internal dispersal method.
The journey through an animal’s gut can benefit seeds, as digestive acids and enzymes can scarify the hard outer seed coat. This scarification can enhance the speed and amount of germination by making it easier for water to penetrate the seed and initiate growth. Fruits adapted for endozoochory are often fleshy, colorful, and palatable, attracting animals with their visual appeal and nutritional rewards. Unripe fruits may be green to hide among leaves and are often sour or bitter, discouraging consumption until the seeds are mature and ready for dispersal.
Seeds Hitch a Ride on Animals
Another method of dispersal, known as epizoochory, involves seeds or fruits attaching to the exterior of animals and being inadvertently transported to new locations. This often happens as animals move through vegetation, with seeds clinging to fur, feathers, or even skin. These seeds have physical adaptations like hooks, barbs, spines, burrs, or sticky coatings. For instance, burdock and cleavers (goosegrass) have burr-like fruits with hooks that readily cling to animal fur or clothing.
Bidens pilosa (blackjack) also has hooked seeds that stick to passing animals. The unicorn plant features spines that can catch around the feet of grazing animals like deer or antelope. This external attachment allows for transport over varying distances, from short local movements to potentially long-distance dispersal, particularly with migratory birds. While the animal receives no direct benefit from carrying these seeds, this passive transport is an effective strategy for plants to reach new habitats.
Seeds Stored by Animals
Some animals actively collect and transport seeds, often with the intention of consuming them later, a behavior known as caching or hoarding, which is a form of synzoochory. This dispersal occurs when animals bury or store seeds in various locations but then forget about them or abandon their caches. A common example is squirrels burying acorns or jays caching nuts for future consumption. These forgotten seeds then have the opportunity to germinate away from the parent plant, often in protected, nutrient-rich environments.
A specialized form of this dispersal is myrmecochory, which involves ants. Plants dispersed by ants produce seeds with a lipid-rich appendage called an elaiosome, which is attractive to ants. Ants carry the seeds back to their nests to feed the elaiosomes to their larvae. Once the elaiosome is consumed, the seed is discarded in underground middens or ejected from the nest, where it can germinate in favorable conditions, often benefiting from the nutrient-rich ant nest environment. This method offers benefits such as protection from predators and pathogens, and escape from adverse environmental factors like fire.
Plant Adaptations for Animal Dispersal
Plants have evolved diverse features to facilitate animal dispersal. For internal dispersal, adaptations focus on fruit palatability and seed protection. Fruits are often fleshy, sweet, and brightly colored to attract animals, while seeds have hard coats to survive digestion. Aromas can also play a role. External attachment relies on physical structures that enable seeds to cling to animal surfaces, such as hooks, barbs, or sticky coatings. Seeds dispersed through caching often possess high nutritional content, like fats and proteins, encouraging animals to collect and store them. For myrmecochory, the elaiosome provides lipids and other nutrients, attracting ants to carry seeds away.