What is Symbiosis?
Symbiosis describes intimate relationships between different species that live in close association. These partnerships have evolved over millions of years and are essential to tropical forest ecology.
Types of Symbiosis
Mutualism (Both Benefit) ✅✅
Both species gain advantages and the relationship is mutually beneficial.
Commensalism (One Benefits, Other Neutral) ✅◯
One species benefits while the other is neither helped nor harmed.
Parasitism (One Benefits, Other Harmed) ✅❌
One organism benefits at the expense of the other, causing harm but usually not death.
Costa Rican Tree Symbioses
Mutualistic Examples
Guanacaste & Rhizobium Bacteria:
- Enterolobium cyclocarpum + nitrogen-fixing bacteria
- Tree provides sugars to bacteria in root nodules
- Bacteria convert atmospheric nitrogen to usable form
- Both benefit: tree gets nitrogen, bacteria get food/shelter
- Result: Guanacaste thrives in poor soils
Guarumo & Azteca Ants:
- Cecropia spp. + Azteca ants
- Tree has hollow stems, produces food bodies
- Ants live inside, patrol for threats
- Ants attack herbivores, clear competing vines
- Classic defensive mutualism
Fig Trees & Fig Wasps:
- Ficus species + species-specific wasps
- Wasp pollinates fig, lays eggs inside
- Fig provides food and nursery for wasp larvae
- Neither can reproduce without the other
- Co-evolved obligate mutualism
Trees & Mycorrhizal Fungi:
- Nearly all trees + soil fungi
- Fungi colonize roots, extend mycelial networks
- Fungi deliver phosphorus, water, minerals
- Trees provide carbohydrates from photosynthesis
- Critical for tree establishment and growth
Commensal Examples
Epiphytes on Trees:
- Orchids, bromeliads, ferns on branches
- Epiphytes gain high-light position without soil
- Trees generally unaffected (unless overloaded)
- Epiphytes don't take nutrients from tree
- Common in Costa Rican cloud forests
Birds Nesting in Tree Cavities:
- Woodpeckers, parrots use tree holes
- Birds gain shelter
- Trees neither helped nor harmed significantly
- Old cavity trees important for wildlife
Parasitic Examples
Strangler Figs:
- Ficus spp. start as epiphytes
- Roots grow down trunk, eventually surround host
- Fig competes for light, nutrients, water
- Host tree often dies, fig becomes free-standing
- Parasite becomes independent tree
Mistletoe:
- Various mistletoe species on branches
- Mistletoe roots penetrate host bark
- Steals water and minerals from host
- Can weaken or kill small trees
- Birds spread seeds, continuing cycle
Fungal Pathogens:
- Various fungi attack living tissue
- Cause diseases like leaf spots, cankers
- Harm or kill host trees
- Natural population control
Evolutionary Perspective
How Symbioses Evolve
- Initial contact: Species encounter each other
- Repeated interaction: Contact becomes regular
- Natural selection: Favors beneficial interactions
- Co-evolution: Both species adapt to each other
- Obligate dependency: May become unable to survive apart
Specificity
- Generalist symbioses: Many partners possible (most mycorrhizae)
- Specialist symbioses: One specific partner only (fig wasps)
Ecological Importance
Forest Function
Nutrient Cycling:
- Mycorrhizae move nutrients between trees
- Nitrogen fixation adds fertility to ecosystem
- Decomposers break down dead material
Structural Diversity:
- Epiphytes create microhabitats
- Ant-plants provide insect housing
- Strangler figs create nesting cavities
Pollination & Seed Dispersal:
- Specialized relationships ensure reproduction
- Fig-wasp system supports wildlife
- Birds, bats, insects serve as partners
Population Control:
- Pathogens prevent species dominance
- Herbivores controlled by ant guards
- Balance maintained through competition
Climate Change Impacts
Threats to Symbioses
Temperature shifts:
- May desynchronize partners (flowers/pollinators)
- Fungi and bacteria sensitive to heat
- Could break apart evolved relationships
Rainfall changes:
- Mycorrhizae need specific moisture
- Drought stress weakens mutualistic benefits
- May favor parasites over mutualists
Species range shifts:
- Partners may migrate at different rates
- Could separate obligate relationships
- Creates ecological mismatches
Why It Matters
Understanding symbiosis helps with:
- Reforestation: Must inoculate seedlings with mycorrhizae
- Conservation: Protecting one species requires protecting partners
- Pest management: Ant-plant systems provide natural pest control
- Ecosystem restoration: Symbioses must be restored, not just trees
- Climate adaptation: Maintaining symbiotic networks builds resilience
- Biodiversity: Symbioses multiply species diversity