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)