Species Interactions

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

Pattern • Patterns: • Uniform distribution (negative interaction) • Distances greater than expected compared to random

Pattern • Patterns: • 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!. Revegetating a mine Plant on left grown without mycorrhizal fungi

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

Species Interactions