Keystone Species

What is a Keystone Species?

A keystone species is like the keystone in an arch - remove it and the entire structure collapses. Though they may not be the most abundant, their ecological role is irreplaceable.

Concept Origin

Robert Paine's Discovery (1969)

The Starfish Experiment:

• Biologist removed starfish from Pacific tidal pools
• Without starfish predation, mussels took over
• Mussel dominance excluded 15+ other species
• Biodiversity collapsed from removal of ONE species
• Coined term "keystone species"

Key Insight: Not all species are equal - some hold ecosystems together through:

• Critical resource provision
• Population control
• Habitat creation
• Facilitation of other species

Types of Keystone Species

Keystone Predators

Function: Control prey populations, prevent competitive exclusion

Examples Worldwide:

• Sea otters (control sea urchins, protect kelp forests)
• Wolves (control deer, allow forest regeneration)
• Big cats (maintain herbivore balance)

Costa Rican Example:

• Jaguars control peccary and deer populations
• Prevent overgrazing of forest understory
• Allow tree seedling establishment

Keystone Mutualists

Function: Provide irreplaceable ecosystem service

Fig Trees (Ficus spp.) in Costa Rica:

Why Keystone:

1. Year-round fruiting: Produce figs when other trees don't fruit
2. Feeding 100+ species: Birds, bats, monkeys, insects depend on figs
3. Dry season resource: Critical food during scarcity
4. Obligate mutualism: Fig wasps can't survive without figs
5. Seed dispersal: Animals eating figs spread seeds of other trees

Impact if Removed:

• Fruit-eating animals starve during lean months
• Populations of toucans, monkeys, bats crash
• Seed dispersal network collapses
• Forest regeneration fails
• Cascade affects entire ecosystem

Keystone Engineers

Function: Create or modify habitat for many other species

Ceiba Trees (Ceiba pentandra):

Why Keystone:

1. Emergent crown: Towers above canopy, creates unique habitat
2. Nesting platforms: Raptors, parrots nest in massive branches
3. Epiphyte habitat: Hundreds of orchids, bromeliads per tree
4. Cavity formation: Woodpeckers create holes used by 20+ species
5. Drought resistance: Retains leaves when others drop, feeding wildlife
6. Sacred status: Cultural protection preserves surrounding forest

Keystone Pollinators

Function: Pollinate multiple species, maintain plant diversity

Bats in Costa Rica:

• Pollinate 50+ night-flowering tree species
• Many trees evolved bat-specific flowers
• Includes commercial species (balsa, guava)
• Also disperse seeds of 100+ plant species
• Remove bats = reproductive failure of many trees

Hummingbirds:

• Pollinate 1000+ Neotropical plant species
• Many plants exclusively hummingbird-pollinated
• High energy needs = visit many flowers per day
• Critical for maintaining plant genetic diversity

Keystone Modifiers

Nitrogen-Fixing Trees:

Guanacaste (Enterolobium cyclocarpum):

Why Important:

1. Nitrogen addition: Fixes atmospheric N₂ into soil
2. Fertility islands: Creates nutrient-rich patches under canopy
3. Facilitates succession: Allows other trees to establish
4. Reduces competition: Suppresses grass that excludes tree seedlings
5. Wildlife magnet: Large pods feed numerous animals

Impact:

• Transforms degraded pastures back to forest
• Without nitrogen-fixers, reforestation much slower
• Catalyzes ecosystem recovery

Costa Rican Keystone Trees

Fig Species (Ficus spp.)

Strangler Figs:

• Create standing dead trees (snags)
• Snags provide nesting cavities
• Support woodpeckers, owls, parrots
• Cavities used by 40+ bird/mammal species

Free-Standing Figs:

• Massive canopies shelter understory
• Root systems stabilize streambanks
• Prevent erosion in steep terrain

Almendro (Dipteryx panamensis)

Why Keystone in Caribbean Lowlands:

1. Macaw nesting: Only tree scarlet macaws nest in
2. Large cavities: Size and strength required
3. Timber value: Logging threatens macaw populations
4. Critical population: Removal = macaw extinction locally
5. Cascade: Macaws disperse seeds of other trees

Palm Species

Coyol (Acrocomia aculeata):

• Dense spines create predator-free refuges
• Small birds nest within spine protection
• Fruits feed many mammals
• Creates microhabitat in open areas

Identifying Keystone Species

Criteria

Ask these questions:

1. Does it provide unique resource? (year-round fruit, nesting sites)
2. Do many species depend on it? (50+ associates)
3. Is the resource irreplaceable? (no substitute available)
4. Would removal cause cascade? (multiple species decline)
5. Does it modify environment? (creates habitat, fixes nitrogen)

Not Every Important Species is Keystone:

• Common species can be removed without collapse
• Keystone = disproportionate impact relative to abundance
• Rare species can be keystone if highly specialized role

Conservation Implications

Protection Priorities

Focus on Keystone Species:

• Protecting fig trees = protecting 100+ dependent species
• More cost-effective than species-by-species approach
• "Umbrella species" concept - protect one, save many

Restoration Ecology:

• Plant keystone trees first
• Nitrogen-fixers prepare soil for other species
• Fig trees attract seed-dispersing animals
• Accelerates succession and biodiversity recovery

Threat Assessment:

• Loss of keystone = ecosystem collapse risk
• Almendro logging threatens macaws, peccaries, agoutis
• Climate change may disrupt fig phenology
• Pesticides threaten pollinator keystones

Why It Matters

Understanding keystone species helps with:

• Conservation planning: Prioritize protection efforts
• Restoration: Choose species for reforestation
• Impact assessment: Predict logging/development effects
• Management: Recognize critical species to protect
• Education: Illustrate ecosystem interconnections
• Climate adaptation: Protect species that build resilience

Examples of Removal Impacts

Historical Cases

Wolf Removal in Yellowstone:

• Deer overpopulated
• Overgrazed riverbanks
• Aspens/willows couldn't regenerate
• Songbirds declined (no nesting habitat)
• Beavers declined (no willows for dams)
• Rivers changed course (erosion)
• Reintroduction restored entire ecosystem

Sea Otter Hunting:

• Urchins exploded
• Urchins ate all kelp
• Kelp forests disappeared
• 100+ species lost habitat
• Fisheries collapsed

Message: One species removal = cascading ecosystem failure