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weed seed dispersal mechanisms

Specialized mechanisms of some weed species to bury themselves: wild oat hygroscopic awns that twist themselves into the soil (self-seeding)

Ants play important role, especially in storage concentrations

Wind dispersal of weed seed is a function of:
-how fast seed falls: weight, density, ability to float in air
-height of release: may be most important factor in distance spread; mullein seed
-speed, direction and turbulence of wind between release point and ground
-very light, dust-like seed (poppies; fungal spores)
-species with specialized wind dispersal mechanisms generally don’t colonize as a horizon but as isolated individuals over a greater distance

Selection for time of seed release: adaptation for quick or slow release from parent plant?
-quick release: oversaturate predator demands of feeding and leave enough after that for new colonizers
-slow release: maximize chance some seeds land on ground during favorable time for quick germination or escape from predators

Differences in time:
-seed ripens on parent plant
-seed retained on parent before its dispersed (seed shattering)
-example: weeds with long flowering period and corresponding long period of ripening and release: seed shatters immediately after ripening on parent
-example: weeds flowering and seed production in narrower time period, often require harvesting activity to release seed: evolved with crop, maximize chance of dispersal with crop

-ones with hard gizzards that destroy seeds;
-soft gizzards that pass on viable seed

the part of the process by which an established and stabilized weed species in an area maintains itself within that area


Specialized mechanisms of some weed species to bury themselves: wild oat hygroscopic awns

Animal Dispersal:

many of our common, unspecialized weed seeds don’t move very far from mother plant without animals, humans

The dispersal of diaspores can be biotic (zoochorous) or abiotic (azoochorous) and their morphology is related to the method of dispersal. In biotic dispersal the vectors are numerous: invertebrates (flies, dung insects, ants (myrmecochory), earthworms, and snails), herbivorous fish (ichthyochory), marine turtles, lizards and desert iguanas (saurochory), birds (ornithochory), and mammals including human beings (mammaliochory). Abiotic dispersal is by wind (anemochory), water (hydrochory) or the tree itself (autochory). The last is achieved by active ballistics (tension generated by the dehydration of hygroscopic tissues), passive ballistics (movements of the seeds enclosed in the fruit), creeping diaspores, and barochory (dispersal by weight).

The spatial distribution of vertebrate-dispersed seedlings, saplings, and trees in regenerating forests may reflect a long-term legacy of remnant trees and their interactions with frugivorous vertebrates. In montane forests of Costa Rica, remnant trees influenced the composition of regeneration 3 and 14 years after abandonment; late-successional species were more abundant than early-successional species only beneath remnant trees. In the Caribbean lowlands of Costa Rica, tree and sapling density decreased with increasing distance from remnant trees 23 years after pasture abandonment ( Schlawin and Zahawi, 2008 ). Remnant trees also influenced the species composition of recruited trees, favoring the establishment of species with seeds >1 cm diameter.

Vertebrates are more likely to disperse seeds from consumed fruits than are insects, which (due to their small size) typically feed on portions of fruits and on or in seeds. However, dung beetles and ants may be important secondary dispersers, redistributing seeds from the dung of larger primary dispersers ( Andresen, 2002; Martínez-Mota et al., 2004 Andresen, 2002 Martínez-Mota et al., 2004 ). Insects, especially ants, are more likely to disperse small seeds, particularly of plant species adapted for dispersal by ants (myrmecochory) ( Leal et al., 2014 ).

Pollination, Seed Predation, and Seed Dispersal

(For features of ‘seed rain,’ the seed bank in the soil and the topics of seed dormancy and germination.)

Seed dispersal can be accomplished through both abiotic and biotic mechanisms. Abiotic dispersal involves wind and water; biotic dispersal involves autogenic mechanisms, such as explosive fruits, and various animal agents, including insects, fish, reptiles, birds, and mammals. Dispersal by animals typically is a consequence of frugivory or seed predation, but some species acquire seeds or spores through external attachment by various kinds of clinging devices, for example, adhesive material or barbed spines. Seeds of a majority of plant species are dispersed by animals in many ecosystems ( Howe and Smallwood, 1982 ).

Seed dispersal is a critical stage in the life cycle of the species. It transports physiologically independent individuals to the habitat occupied by their parents or to new suitable territories, where the seeds may colonize if environmental conditions are favorable. The unit of dispersal (diaspore, propagule, or diseminule) is determined by the embryo, the seed, the fruit, or the fruit and associated parts of the modified perianth, the receptacle, or combinations.

Seed dispersal is universally considered important for biodiversity conservation. The structure of the landscape has strong effects on the distances traveled by seeds, regardless of whether they are dispersed by abiotic factors (wind) or by animals. Therefore, any type of disturbance, such as habitat fragmentation or habitat modification by an invasive plant species for instance, is likely to change the patterns of seed movement, the patterns of seed recruitment, as well as the genetic structure of the plant populations. For wind-dispersed species, it is known that seeds travel much further distances in open landscapes than in dense forest, due to differences in the shape of the wind profile. On the other hand, plants depending on animals for seed movement are susceptible to dispersal failure when their seed vectors become rare or extinct. Disruption of the seed dispersal mutualism can have serious consequences for the maintenance of the plant populations. An increasing number of studies are showing how the populations of seed dispersers are being decimated, both in the tropics and in the temperate zones, and how this translates into a lower dispersal success of the plants depending upon them ( Figure 6 ). In tropical areas in particular, the widespread decimation of dispersers by overhunting and habitat loss are expected to have devastating long-term consequences for the maintenance of tree species diversity. An excessive LDD of elements alien to ecosystems also represents a threat to biodiversity, especially if it goes along an insufficient dispersal of native species.