Interspecific Interactions

Species interactions

When two organisms interact, there are three possible impacts on each: positive, negative, or neutral

+,-, 0

What do you need for competition to happen

Key requirement: at least one shared, limiting resource

ex: habitat

two modes of competition

Exploitative vs. Interference
Intraspecific vs. Interspecific

Modes of competition (1)

Exploitative - two species use the same resource, and each depletes that resource for the other, without directly interacting VS. Interference - one individual tries to prevent another individual from interacting with that resource. (depletes energy, instead of spending it reproducing)

Modes of competition (2)

Intraspecific Vs. Interspecific

Effects of Competition on Competitors

• More individuals in a habitat = lower survivorship, increased development time (takes longer to adulthood), reduced body size

Competitive Exclusion Principle

two species competing for the same limited resource cannot coexist at constant population values; one will outcompete the other.

How can species avoid competition

Niche partitioning and character displacement

Niche partitioning

the division of resources among species to minimize competition by utilizing different habitats or resources.

“restrict species to realized niches that are smaller than their fundamental niches”

fundamental niche

the physical conditions under which a species might live, in the absence of interactions with other species.

realized niche

the actual niche of a species whose distribution is restricted by biotic interactions such as competition, predation, disease, and parasitism.

character displacement

when two competing species coexist in the same environment they form traits that are different which reduces competition and niche overlap

if competition is strong and pervasive enough?

cause evolutionary response (change fundamental niche)

another saying for character displacement

directional selection on the competitors causes morphological changes that can lead to decreased niche overlap.

allopatric

condition in which population or species have non overlapping geographic ranges

sympatric

condition where populations or species have overlapping geographic ranges

character displacement criteria

morphological differences between sympatric species > differences between allopatric populations

character displacement criteria

Differences between sympatric (live in same geographic area) and allopatric populations have genetic basis.

character displacement criteria

differences between sympatric and allopatric populations evolved in place (not derived from founder groups that already differed)

character displacement criteria

• Variation in the character must affect use of resources.

• Must be demonstrated competition for the resource and competition must be directly correlated with character similarity

• Differences in character cannot be explained by differences in resources available to each of the populations.

exploitation (+/-)

one organism feeds on another

predation

parasitism (parasitoids, pathogens)

predation

one organism kills and consumes another

parasitism

one organism lives on/in host and feeds on its tissue

Parasitoids

insect larvae that consume hosts

Disadvantages of parasites

• complex life cycles/several stages

• trouble getting from one host to another

• interactions result in complex adaptations

complex interactions of parasties

altering host behavior

Ex; make it so the host is more susceptible to predation or enhances parasite transmission.

Predator & prey graph

Decline in prey, reduces predators who then reduces prey even further until they starve

exploitation and abundance

predators, parasites, pathogens influence distribution abundance and structure of prey and host populations

Ex. invasive species can alter ecosystems and outcompete native species.

how can prey survive with invasive species/exploitation

need refuge to escape predation and reduce competition. (living in large group)

Types of defense prey use

• physical

• chemical

• mimicry

physical defense

camouflage, horns, spikes

chemical defense

aposematic coloring - warning colors (bright/contrasting patterns)

Mimicry

a defensive strategy where prey resemble other organisms or objects to avoid predation.

Mullerian mimicry

a form of mimicry where two or more noxious species (organisms that are harmful/cant eat) will evolve to look similar to predators

(ex; Monarch and viceroy butterfly harmful from chemicals in diet have same wing pattern)

Batesian mimicry

a harmless species evolves to resemble a noxious species to avoid predation to gain protection from predators.

(ex: yellow jacket and clearwing moths)

Major types of mutualism (beneficial)

Endosymbiosis
• Dispersive (pollination / seed dispersal)
• Defensive
• Resource-based

endosymbiosis

symbiotic relationship where one lives inside the other

dispersive (pollination/seed dispersal)

a mutualistic interaction where one organism assists in the reproduction of another by transferring pollen or seeds.

How do plants benefit from seed dispersal (5)

1. Reduced competition

2. Reduced seed predation

3. Colonization (patch dynamics)

4. Directed dispersal

5. Increased germination rate

Defensive mutualism

ones leaves food and shelter in return for protection against predators, parasites, herbivores

(ex: aphids produce honeydew for ants, they protect against ladybugs)

resource-based

two species exchange resources

ex: mycorrhizal fungi and plants

cheating in mutualism

the act of one species taking advantage of a mutualistic relationship without providing any benefit in return.

amensalism

one species is harmed while the other is unaffected.

ex - large tree shades smaller plants, limiting their growth.

takeway to cheating

can destabilize mutualistic relationships and lead to a decline in population for the species providing benefits.

continuous/unchecked cheating can lead to interactions that resemble amensalism

Food webs

summary of feeding interactions within a community

indirect food web interactions

One species affects another through third, intermediary species.

(ex: wolf eats deer, plants grow)

direct interactions (food web main focus)

pos. and neg. effect of one species on another without involvement of an intermediary species

(ex; snake eating a mouse)

keystone species

species that exert strong effects on community structure, despite low biomass/abundance

ex: sea star