Seed Dispersal Thinking like a plant….

•  Coast redwood tree

•  May live 2,000 years. May make 1-10 billion seeds in lifetime

•  Sedentary, mindless

•  Often long lifespan, Often high fecundity

•  “Sweepstakes reproduction”

•  Takes just one seed to replace itself!

Seed dispersal

•  How seeds move from parent plant to “safe sites:” places suitable for germination and seedling establishment

•  Importance: may determine if species can track “climate envelopes” as they change due to global climate change

Why disperse?

•  1) Escape competition from parent/siblings

•  2) Escape from predation/disease (if density-dependent)

•  3) Discover new suitable habitats

•  4) Contribute to gene flow/genetic diversity

•  5) Maintain populations in “sink” habitats (metapopulations: source vs sink populations)

Seed plants: Seed is fertilized ovule in ovary of pistil

•  Seed: baby plant (embryo) in box (seed coat) with lunch (endosperm in flowering plants)

•  Fruit: Mature ovary of flower (contains one or more seeds)

•  Ovary wall becomes pericarp in fruit.

•  Pericarp may develop specialized layers. At most these are:

–  ectocarp: outer layer

–  mesocarp: middle layer

–  endocarp: inner layer

Fruits vs seeds

•  Dry indehiscent fruits

–  achene: one seed, fused to pericarp at one point

–  Ex, dandelion, sunflower.

–  grain (caryopsis): one seed, fused entirely to pericarp

–  Ex, corn, rice, wheat.

Resolution

•  Solution: diaspore. Single dispersal unit of plant (seed or fruit)

Fruit functions

•  1) Fruit can be protective against:

–  Seed predators

–  Environmental conditions

•  2) Fruit can promote seed dormancy

–  Hard endocarp can seal out water/oxygen

–  Breaking layer called scarification

–  Fleshy fruits can contain germination inhibitors (prevent seed from germinating in fruit)

–  Rotting or digestion by animal gut needed to remove inhibitors so seed will germinate

•  3) Fruit can aid seed dispersal

•  Wings, barbs, fleshy reward

Seed coat functions

•  1) Protect embryo (like pericarp)

•  2) Promote seed dormancy (like pericarp)

•  3) Aid in seed dispersal

–  Hairs on some wind-dispersed seeds

Ballistic dispersal

•  Many types of seed dispersal are NOT mutualisms

–  Ex, Ballistic dispersal=Ballistochory: fruit throws or squirts seeds

–  Example, fruits of touch-me-not (Impatiens, right), sorrel (Oxalis, left)

•  Other non-mutualist examples (physical forces)

•  Anemochory=Wind dispersal (dandelion)

•  Hydrochory=Water dispersal (coconut)

Ectozoochory

•  Some types of biotic seed dispersal are NOT mutualisms

–  Ex, most cases of ectozoochory. Seed or fruit carried by animal outside body (hooks, barbs, glue)

–  Ex, Devil’s claw (Proboscidea)

–  Makes fruit with large hooked claws (elongated top of capsule).

– 

–  Many plant species use this technique

–  Table shows the (unofficial) 17 top “hitchhiker plants,” ranked by SRDUs (Sock Removal Difficulty Units)

• 

•  Ectozoochory: Some cases are mutualisms. Example, scatter hoarding animals (squirrels, nutcrackers)

–  Collect and hide seeds or fruits in caches

–  Some escape and germinate

–  Pinus albicaulis (white bark pine) and Clark’s nutcracker

–  1 bird can hide as many as 90,000 seeds in one season

• 

•  Endozoochory: fruit eaten and seeds travel thru gut

–  Common with fleshy fruits (soft, sweet fruits)

–  Fruit pulp is nutritive reward to disperser

• 

•  Endozoochory: sometimes seed scarified

–  Example, dodo

–  Flightless “pigeon” found only on Mauritius in Indian Ocean

–  Discovered by European visitors around 1600

•  Had gizzard with stone against which food (seeds) ground

•  Seeds of Calvaria tree have thick endosperm

•  Apparently need scarifying to germinate, and dodo gizzard may have done this

•  Problem: dodo extinct by 1680s!

•  Tree seeds may not have germinated in nature since then

Myrmecochory

•  Seed dispersal by ants (mutualism)

–  Elaiosome: food body on outside of seed coat/fruit

–  Ants collect seed, remove elaiosome, discard seed underground or aboveground

–  Only major seed dispersal mode using an insect!

•  Common for Eastern forest spring flowering plants.

Pollination vs biotic seed dispersal

•  Trait Pollen dispersal Seed dispersal

•  Animals Often insects Mammals/birds

•  “Target” Stigma Safe site

•  Motivation Floral reward None

•  to target

•  Cues to target Floral traits None

•  Seeds: “Directed dispersal” unlikely

Syndromes

•  Protein rare reward (most plants N limited)

•  Bird fruits with no odor

•  Ants only major insect group

•  Additional syndrome: mud dispersal (“sole botany”)

•  43 species, no specific seed characteristics

•  Includes 5 species of most common weeds:

•  Polygonum (knotweed)

•  Capsella (shepherd’s purse)

•  Stellaria (chickweed)

•  Chenopodium (lamb’s quarter)

•  Poa annua (bluegrass)

• 

•  Less taxonomic specificity

•  Guilds: groups of species with similar ecological function

•  Ex, frugivore birds and summer-fruiting fleshy-fruited plants

The Plant View

•  How measure dispersal? What’s the metric?

•  1) Single species study: absolute distance

•  2) Comparative study: canopy diameters

•  Ex, grasses vs oaks

•  3) Self-incompatible clonal plants: genetic neighborhood

Trade-offs

•  Generally, as seed size increases dispersal decreases

•  But chance of successful establishment increases

• 

•  Danger for Plant:

•  Seeds are yummy! High protein, high lipid to supply embryo

•  Solution 1: bribery!

•  Use large numbers of seeds (scatter hoarders)

•  Solution 2: bribery!

•  Use non-seed food reward. Fruit pulp, etc

•  Solution 3: poison!

•  Defend seeds with toxins

•  Ex, castor beans contain ricin

•  Lethal dose: 1/5000 gram (twice as deadly as cobra venom!

•  Solution 4: armor!

•  Defend seeds with mechanical protection (stony seed coat, endocarp)

•  Solution 5: advertise so correct animal gets message before seed predators arrive!

•  Cues to fruit ripeness (color, smell, etc)

•  Pre-ripening flags: signals ripening has begun (red color in ripening blueberries)

•  Foliar flags: leaf color change indicates fruits ripening in fall (ex, poison ivy)