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Directed dispersal describes nonrandom movement of seeds and reproductive units toward favorable locations by an abiotic or biotic vector. Along with colonization and escape, directed dispersal is one of three main theories explaining the advantages of dispersal for plants. According to the colonization hypothesis, plants disperse randomly throughout the environment. At a subset of those landing sites, conditions are favorable, and plants can germinate. In some cases, plants may persist in the seed bank until conditions become favorable. According to the escape hypothesis, plants disperse to avoid increased competition, predation, and pathogens associated with proximity to the parent (Janzen-Connell effects). Of the three theories, directed dispersal confers the greatest fitness advantage to plants. Depending on the degree to which this fitness advantage determines plant demography or community assembly, directed dispersal can influence ecological processes. Directed dispersal influences ecological processes in different ecosystems around the world using a variety of plant species and vectors. The coevolved adaptations and unique traits that promote directed dispersal in those varied ecosystems exemplify the complex feedback between organisms and the environment.

Background

Although most commonly associated with plant-animal relationships, other vectors can generate directed dispersal. For example, water-dispersed wetland plants selectively arrive at favorable hydrologic conditions depending upon whether they sink or float. Similarly, air currents pull wind-dispersed seeds toward gaps where colonization can occur . Water and wind are common abiotic vectors of plants, so more examples may exist than are currently demonstrated.

Most of the established examples in the scientific literature are plant-animal mutualisms. In a dispersal mutualism plants reward animals nutritionally and energetically in exchange for moving propagules or seeds to new locations. Some plants disperse by attaching to an animal's body, but this strategy is rare compared to others. Other plants disperse by producing a highly-nutritious seed that is attractive to vertebrates. A subset of animals, such as squirrels and crows, collect and hide those seeds around the landscape. Seeds that are not retrieved can germinate. However, in most plant-animal dispersal relationships, animals consume and pass the seeds in excrement. Although seed consumption and passage is often associated with fleshy fruit, herbivores can consume seeds while consuming the foliage of plants that do not produce fleshy fruit. These unique types of dispersal relationships can each provide different pathways that generate directed dispersal.

Classic examples of directed dispersal by animals

Mistletoe

Mistletoe are parasitic plants that grow in trees in many ecosystems. Arboreal mammals and birds consume mistletoe berries, which contain seeds encased in a sticky substance. When arboreal mammals and birds defecate the seeds, the sticky substance prevents the seeds from falling off the host tree. That seed eventually germinates and the plant produces a root that penetrates the host tree. Interestingly, the dispersing animals in this relationship often excrete the seeds on twigs that are appropriately sized for mistletoe to parasitize.

Nurse plants

Birds are attracted to nurse plants that provide perches in arid systems, grasslands, and early successional areas. The soil beneath nurse plants is often more fertile, moist, and subjected to less extreme sunlight than the rest of the surrounding landscape. Because of these conditions, seeds excreted by birds beneath perches may fare better than other plants not directed toward nurse plants.

Myrmecochory

Some plant species produce seeds with specialized fat bodies that attract ant species. Ants collect those seeds and return to the nest, where they remove and feed upon the fat body. The remaining seed is discarded in the nutrient rich refuse pile associated with the ant colony. Plants growing from these refuse piles may benefit from the fertilizing effect of the nutrients.

Seed caching

Pine and nutcracker species have a coevolved relationship resulting in directed dispersal. Corvids collect seeds from pine cones, store the seeds in specialized pouches, and cache the seeds in open clearings. Similarly, other caching corvids and squirrels move and hide seeds. In both examples, the interactions in this relationship is complex because the animals can destroy the seeds by eating them. However, in some cases, the seeds are forgotten and the cached in a location that is favorable to the plant.

Cadaver islands

When animals die, decomposition creates cadaver islands that are fertile and open areas for plants to establish. While some scavengers, such as vultures, exclusively feed on carrion, other scavengers are omnivorous. Omnivorous scavengers visiting these cadaver islands can disperse seeds from plants in the surrounding landscape. Alternatively, animals can perish with seeds still in their digestive system, and those seeds can later germinate.

Aquatic plants

Ducks can transfer aquatic plants between water bodies through defecation and plant parts stuck to their bodies. This movement between otherwise isolate water bodies promotes genetic diversity for aquatic plants and provides ducks with food resources.

Forest gaps

Many plant species thrive in the openings of forest canopies created by disturbance or mortality. Some fruit-eating birds prefer the edges of these gaps and deposit seeds in the sunlight rich environment. Similarly, herbivores that may disperse seeds frequent gaps.