Seed dispersal Flashcards
Distinction between seed dispersal and pollination
Pollinators are “paid on delivery” but seed dispersal agents mus be “paid in advance”, making it impossible for plants to direct seed dispersal agents to particular target
Wind dispersal
Since seeds cannot be directed to a precise target, wind makes more sense as a seed than a pollen vector. But size matters
Small, light, seeds can disperse a long way (> 1 km), but have no reserves.
Large seeds cannot go far, but contain reserves for establishment in deep, forest shade.
Which groups of plants use wind to disperse seeds?
Generally restricted to:
1.Orchids and other tiny-seeded plants
2.Large-seeded trees and climbers in the upper canopy, where winds are strongest, e.g. the Dipterocarpaceae
3. Open habitats, e.g. many grasses
Water dispersal
Water dispersal: About 72% of Earth’s surface is covered with water, so it makes sense that plants will exploit this reliable vector. Currents dictate long-distance propagule dispersal via ocean
Coastal species:
Nypa fruticans
Barringtonia asiatica
Cocos nucifera (most commonly known water dispersed seed)
Mangroves: a tree community whose constituent species have independently evolved water-dispersed propagules
Animal dispersal
Birds are the most important seed dispersal agents in tropical forests
‘Gut passage’ takes around 15 - 40 minutes in most frugivorous birds (and bats)
Size matters with birds, since they have no teeth and usually swallow fruits whle. In general, bigger birds have bigger maximum gape widths and can swallow larger fruits
e.g.
Flowerpeckers (c. 6 g) can only swallow fruits < 8 mm in diameter
Imperial pigeons and large hornbills (< 2000 g) can swallow fruits >30 mm diameter.
Bulbuls are important medium-sized dispersal agents for fruits <13-15mm diameter throughout TEA (+ Africa) in both forest and non-forest habitats
Fruit bats
Disperse fewer seeds overall than birds, but some plant species produce fruit targeted at animals with no colour vision, but an excellent sense of smell.
Tiny-seeded fruits, like those of figs, are crushed in the mouth, and the juice and many of the seeds are swallowed. These seeds are defecated in flight.
Large-seeded fruits (Inocarpus) are carried in the mouth 10-100 m to a ‘feeding roost’ (usually a tree branch), where the bat hangs upside down. The fruits are then peeled, if necessary, crushed in the mouth, and only the juice swallowed. The seeds are dropped under the feeding roost.
If dispersal is so crucial, why are so few seeds getting dispersed long distances by bats (and birds too)?
Consider this: fruits that are attractive to dispersers may even serve to keep the disperser near the mother tree, leading to no dispersal of most seeds!
Also: Dispersal needs only a few successful events to make a huge difference, both in terms of reproduction success and gene flow
Primate dispersal
Primates are the most important mammalian seed dispersal agents in most tropical forests. In TEA the most important primate groups for seed dispersal are:
Gibbons, which eat lots of fruits, swallow most seeds whole, and defecate them 100-1000 m away.
Macaques eat lots of fruits, but spit most seeds, sometimes after transport in cheek pouches for < 200 m
Rats and rodents
Mostly seed predators, but they also store large, non-fleshy fruits and seeds for later consumption.
Scatter hoarding: caches of one or a few seeds buried in the soil. Effective means of dispersal, even if most are later retrieved and eaten.
Larder hoarding: deep underground large deposits. Not effective seed dispersal.
Modelling seed dispersal by animals
(A) Seed ingestion is constrained by the gut capacity of the frugivore, here illustrated by a giant sloth (Megatheriidae).
(B) Seed retention is modeled as a fat-tailed distribution representing the relative frequency of seeds eliminated at different times after ingestion.
(C) Animal movement is simulated under models where individuals can move in any direction after ingesting the seeds of a plant or plant aggregate.
(D) While moving, the frugivore deposits the seeds at different distances from the source.
(E) Seed dispersal kernels – probability density functions of dispersal distances – are estimated by combining information on retention time of ingested seeds and movement.
Adhesion dispersal by the coastal tree Pisonia grandis
Mass adhesion of Pisonia can kill potential seed dispersers: White terns in Polynesia
However, apparently enough birds do not get killed so that the tree is not negatively impacted and seeds get dispersed long distances
What happens when animal vectors are extirpated?
Many dispersal agents for large fruits and/or long distances have been eliminated by hunting from uch of the tropics
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