Lecture 8 - Dispersal Vectors Flashcards
Defining short-distance (local) dispersal (SDD)
Local dispersal: distance >0.2z
Defining long-distance dispersal (LDD)
Long distance dispersal: distance >Vtg
V = is the early Holocene migrational velocity —the rate at which ice sheets retreated north. This rate is approximately 200m/year
tg = time to produce a “substantial” crop
Local-Dispersal Agents
- Ants (myrmecochory): These seeds are wind-abscissed, and dispersed. This is considered a secondary dispersal event.
- Small terrestrial verrtebrates: Can be primary (p) or secondary (s) dispersal events. Examples: mice, chipmunks, squirrels.
- Ballistic: Seeds are dehisced ballistically. A primary event. Most are herbs, but include a few tree species.
- Wind: Wind is considered a vector that moves seeds long distances. But, for large (heavy) seeds, it becomes a short distance dispersal vector.
Wind-abscised, ant dispersed
Seeds dispersed by ants are small and lack appendages. The dispersal event is therfore a secondary event.
Myrmecochory is ecologically significant because of its world-wide distribution. The forgaing ants drag the seeds to the subtarranean nest, trim off the elaiosome for later comsumption, and take the unwanted seed back up to the opening of the nest at ground level.
The seed is discarded on a mound of frass (ant feces) and ant corpses. In other words, nutrient-rich ground, perfect for germination.
Small terrestrial vertberates P or S
Seed-catching animals are both predators (they are granivores) and dispersers. This dual role complicates the plant-animal relationship. The losses (fraction of seeds that are eaten) come at a cost to the plant yet some seeds evolved to be attractive to the granivores. The massive maternal provisions in large seeds like acorns, appear to be the benefit of the granivore and less to the plant embryo.
The second dispersal by rodents has been studied using radioactive isotopes. A Geiger counter is then used to find the seeds several weeks later. The results are surprising: seeds are routinely moved 50 meters or more. What’s even more interesting is that there is always a fraction of tagged seeds that are never recovered.
Catching animals may be:
- Scatter hoarders - low catch density
- Larder hoarders - high catch density
Scatter hoarders - low catch density
Scatter hoarding, a strategy when animals disperse caches in many different locations, in many cases with one single food item in each one.
Scatter-hoarded food can usually not be guarded and defended. Instead, it is protected by concealment in combination with a low cache density.
Larder hoarders - high catch density
Older dominant individuals that are able to defend their caches against scrounging conspecifics are larder hoarders, whereas younger subordinate individuals are scatter hoarders.
Explosive dehisence
For trees distances are typically very short 20 to 30m.
Not surprisingly this strategy is **very rare for trees and uncommon for herbs. **
Long-distance dispersal vectors
- Wind for plumed and winged seeds (p)
- Wind as secondary agent for seeds on snow (s)
- Catching birds (p or s)
- Endozoochory - ingested seeds by animals - flying, arboreal or terrestrial (p or s)
- Epizoochory - e.g. hook, adhesive material (p)
- Epizoochory as a secondary event for seeds in mud
Wind as a primary LDD vector
TWO strategies to get seeds far away
- Place seeds as high up as possible (e.g. on the top of a vertical shoots rising well above the leaves)
- Rate of descent is an index of wind dispersal capability
Terminal velocity is minimized by the production of lift (winged seeds) or drag (plumed seeds).
Diaspores reach terminal velocity when drag or lift balance the acceleration due to gravity.
At the intersection of natural selection and aerodynamics
For intermediates seed size (0.1 to 10mg) the most efficient design is drag-promoting fibers-called plumed seeds.
Fibers of plumed seeds are about 1 cell thick.
Smaller plumed seeds have a planar array of straight fibers. e.g. dandelion and milkweed seed.
Larger plumed seeds have an unorganized mass of fibers.
e.g. Ceiba, cottonwood)
**
Descent is slowed by drag. **
At higher masses, the most efficient design is the asymmetric wing. (e.g. 60mg maple samara).
The descent of winged diaspores is moderated by the lift in generated by regular spinning around one or more axes.
Disadvantage of seeds with drag-promoting fibers: getting stuck !
Very big and very small seeds
As we approach seed masses of >1g, any lift or drag promoting appendage is leading to such a high terminal velocity as to be useless.
At the other extreme - very small seeds (<0.1 mg) (e.g. orchid seeds) - no dispersal appendage is necessary. But the very low maternal provision make such tiny seeds, uncommon.
Wind as a secondary event: dispersal on by on snow or sand.
If seeds fall on a snow surface they are, a least until the next snowfall buries them, available for re-entrainment by the wind - this time as a secondary event.
Two requirements for secondary re-entrainment:
- The primary event has to get the seed out into an open area (field, snow, covered river or lake).
- Very low seed mass to area ratio. For the wind to pick up the seed from the surface,** it must generate enough lift to counteract both gravity (pushing the seed downward) and friction (resisting a lateral push).**
