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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 Bloodroot Hexastylis Trout lily Viola Biotic pollination vs biotic seed dispersal • Trait • Animals Pollen dispersal Often insects Seed dispersal Mammals/birds • “Target” Stigma Safe site • Motivation to target Floral reward None • 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 on nutmeg 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)