Species Interactions Competition: (-,-) interaction Mutualism: (+, +) interaction Commensalism: (+, 0) interaction Exploitation: (+, -) interaction

Species Interactions• Competition: (-,-) interaction• Mutualism: (+, +) interaction• Commensalism: (+, 0) interaction• Exploitation: (+, -) interaction

Competition• Def’n: Mutually adverse interaction between organisms that use a shared resource that is in limiting supply

• Interaction: ( -,- )Effect on Sp. 1

Effect on Sp. 2

Plant Competition• Light very important: explains trees!

Competition: (-,-) interaction• Interference Competition:

– Individuals interact directly to limit one another’s access to resource

• Resource Competition: – Individuals interact with resource in effort to obtain more. Once gained, resource unavailable to competitor.

Resource competition• Example: creosote bush (Larrea tridentata)

• Important desert shrub

Resource competition

• Avoidance of root overlap

Interference Competition: Allelopathy• Allelopathy: chemical released by one plant species negatively impacts another

• Differs from resource competition, where resources removed from environment

• In allelopathy chemicals are added to environment

Examples• Some of these chemicals inhibit other plants

• Example, Salvia (sage) in coastal sage scrub (California)

Examples• Releases cineole and camphor from leaves

• Inhibit germination & growth of annual plants

• In grassland, “bare zones” around shrub borders

camphor

Weeds and allelopathy• Example, Centaurea

maculosa (spotted knapweed)

• Major invasive nonnative weed

Weed example• Example, Centaurea maculosa (spotted knapweed)

• Releases catechin into soil• When absorbed by roots of other plant species, causes wave of cell death to move up roots into stems

• Aids invasiveness

catechin

Pattern• Pattern: placement of individuals of a species relative to each other or members of another species

• Important for sessile organisms (ex, plants)

• May reveal species interactions– Positive may result in clumping– Negative may result in wide spacing

Pattern• Patterns:

– Clumped distribution (positive interaction)

– Distances less than expected compared to random


– Uniform distribution (negative interaction)

– Distances greater than expected compared to random


– Random (no association)– Individuals dispersed without influence by another’s location

Prediction?• What pattern predicted for creosote bushes?

• Clumped, uniform, random?


Mutualism: (+, +) interaction

• Example:• Lichens: partnership between fungi and unicellular photosynthesizer (green alga or cyanobacteria)

Lichen• Fungus forms body, obtains photosynthate from partner

• Together, can colonize harsh environments.

Mycorrhizae• Mycorrhizae: Association of fungus with plant root

• Common: 90% of plants do this!• Exceptions:

– Aquatic vascular plants– Cabbage family

Mycorrhizae• Fungus extends into soil (up to 8m away!) and aids in uptake of nutrients (P, Zn, Cu in particular) and possibly water for plant

• Fungus obtains sugars from plant

Mycorrhizae• Important in revegetation/reclamation of disturbed areas: if fungi not present, plants don’t do well!.

Plant on left grownwithout mycorrhizal fungi

Revegetating a mine

Nitrogen Fixation• Nitrogen: lots in atmosphere (79% N2)• But plants can’t use that• Nitrogen fixation: bacteria use N2 to form NH3 (ammonia) or NO3- (nitrate)

Nitrogen Fixation• Special ways to get N:

– Symbiotic bacteria: many legume plants form nodules on roots.

– Rhizobium (N-fixing bacterium) inhabit, obtain sugars, provide plant with N

Defensive mutualisms• Typically between plants and insects• Insect gets home/food, plant gets protection from herbivory/competitors/hazards

Defensive mutualisms• Example, Acacia and acacia ants• Acacia, trees/shrubs in legume family. Some of those in Central America have large hollow stipular spines

• Ants nest in spines

Defensive mutualisms• Example, Acacia and acacia ants• Leaves have extrafloral nectary at base (makes nectar but not produced on a flower)

• Young leaflets have Beltian Bodies (protein rich) at tips

Defensive mutualisms• Benefits to plant:• Ants eat insect herbivores• Ants are territorial and defensive and attack herbivores not useful for food (even humans)

• Ants clear area below shrub: removes competitors, protects shrub (and ants!) from fire


Commensalism: (+, 0) interaction• Interaction that benefits one

species but has no effect on other• Plant/plant examples:

– some epiphytes– “nurse plant effect”

Example: some epiphytes• Epiphyte: Plant that grows on surface of other plants

• Example:– Spanish moss

Example: some epiphytes• Epiphyte: Plant that grows on surface of other plants

• Example:– Lots in wet tropics

Commensalism• Nurse plant effect (+, 0)• Larger plant provides favorable microhabitat for seedlings of another species (+)

• Seedling so small it has no effect on larger plant (0)

Commensalism• Example,• Saguaro cactus (Carnegiea gigantea)

• Large individuals hundreds of years old

Commensalism• Seedlings found only under shrubs/trees

• Particularly Palo Verde trees


Documents

Species Interactions Competition: (-,-) interaction Mutualism: (+, +) interaction Commensalism: (+, 0) interaction Exploitation: (+, -) interaction