Seed Dispersal

Thinking like a plant….• Coast redwood

– Live 2,000 years– Make 1-10 billion seeds

(lifetime)

• Replacement?

• “Sweepstakes reproduction”

Seed dispersal• Move to “safe site:” suitable for germination

& establishment

• Recent importance: track “climate envelopes” during global climate change?

Why disperse?

Why disperse?• 1) Escape competition (parent/siblings)

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

Why disperse?• 3) Discover new habitats

• “Nothing lasts forever, even cold November rain”

Axl!

Why disperse?• 4) Gene flow/genetic diversity• 5) Maintain “sink” populations

(metapopulations: source vs sink populations)

Dispersal in seed plants• What’s a seed?

Dispersal in seed plants• Seed: mature ovule in ovary (of pistil)• Seed: “baby plant (embryo) in box (seed coat)

with lunch (endosperm in flowering plants)”

Fruits• What’s a fruit?

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

• Ovary wall becomes pericarp in fruit.

Fruits• Pericarp may develop layers.

• At most:– exocarp: outer layer– mesocarp: middle layer– endocarp: inner layer

Fruits vs seeds• Seed or fruit?

Fruits vs seeds• Dry indehiscent fruits

– achene: one seed, fused to pericarp at one point– Ex, dandelion, sunflower.

Fruits vs seeds• Dry indehiscent fruits

– grain (caryopsis): one seed, fused entirely to pericarp– Ex, corn, rice, wheat.

rice

Resolution• What’s dispersed: fruit or seed?

• Solution. Diaspore: Single dispersal unit (seed or fruit)

Fruit/Seed coat functions

Fruit/Seed coat functions• 1) Protect against:

– Seed predators

– Environmental conditions

Fruit/Seed coat functions

2) Seed dormancy– Hard endocarp/coat seals out water/oxygen

– Breaking layer: scarification

– Fruit/seed coat can contain germination inhibitors

Fruit/Seed coat functions• 3) Aid seed dispersal

Maple samaras Milkweed

Seed Dispersal• May be mutualism

Not!

Ballistic dispersal• NOT mutualism

– Ballistic dispersal=Ballistochory: fruit throws or squirts seeds

– Ex, touch-me-not (Impatiens, right), sorrel (Oxalis, left)

Wind/water dispersal• Other non-mutualist (physical forces)

• Anemochory=Wind dispersal (dandelion)• Hydrochory=Water dispersal (coconut)*

Dispersing dandelion fruits (achenes) *controversy!

Ectozoochory• NOT mutualism

– Most ectozoochory (or epizoochory): seed/fruit carried by animal outside body (hooks, barbs, glue)

Fruits of sandspur (Cenchrus)

Ectozoochory– Many plant species

– 17 top “hitchhiker plants”: ranked by SRDUs (Sock Removal Difficulty Units)

Uncarina #1!!

Dispersal Mutualisms• Ectozoochory mutualisms:

• Ex, scatter hoarding animals (squirrels, nutcrackers)

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

nutcracker– 1 bird can hide 90,000 seeds in 1 season

Dispersal Mutualisms• Endozoochory: fruit eaten & seeds thru gut

– Fleshy fruits (soft, sweet fruits)– Fruit pulp nutritive reward

Dispersal Mutualisms• Endozoochory: sometimes seed scarified

– Ex., dodo & Calvaria tree

Myrmecochory• Seed dispersal by ants (ectozoochory mutualism)

– Elaiosome: food body on outside seed coat/fruit– Only major mode using insect!

Stylophorum seeds Fremontodendron seeds

Myrmecochory• Common Eastern forest spring flowering plants.

Trillium

Trout lily

Bloodroot

Viola

Hexastylis

Biotic 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

Biotic pollination vs biotic seed dispersal• Seed “directed dispersal” (concept: seed carried directly

to safe site) unlikely

Syndromes?

Syndromes– Protein rare

reward

– Bird fruits no odor

– Ants only major insect group

– Less taxonomic specificity

– Guild: group species with similar ecological function

The (Plant) View• How measure dispersal? Metric?

• 1) Single species study: absolute distance

The Plant View• How measure dispersal? Metric?

• 1) Single species study: absolute distance

• 2) Comparative study: canopy diameters

• Ex, grasses vs oaks

• 3) Self-incompatible clonal plants: genetic neighborhood

Plant “choices”• As seed size increases:

– dispersal ___________– chance establishment ____________

Plant “choices”• Seeds yummy! High protein, high lipid• How protect?

Plant “choices”• Bribery!

• Large number seeds (scatter hoarders)

Plant “choices”• Bribery!

• Non-seed food reward. Fruit pulp, etc

Aril onnutmeg seed: aril is spice “mace”

Aril: outgrowth attachment ovule to ovary(funiculus)

Mace Windu…..

Plant “choices”• Poison!

– Defend seeds with toxins– Ex, castor beans (ricin)– Lethal dose: 1/5000 gram (twice as

deadly as cobra venom!)

Castor beans

Plant “choices”• Armor! Mechanical protection (stony seed coat,

endocarp)

Plant “choices”• Advertise: disperser gets message before

seed predators– Ripeness cues (color, smell, etc)– Pre-ripening flags: ripening begun– Foliar flags: leaf color indicates fruits ripening

(ex, poison ivy)