Lecture 14 Notes (intregrated and refocused)

Species interactions:
Our own health and well-being are influenced by a variety of ecological interactions.

Ecological interactions are the “balance of nature,” Nature services/ecological services = how we benefit from this. Balanced systems are more resistant to invasions. How species affect one another is at the base of this.
- What is an ecological community? There are interactions that bind it together – “network of influences” (vs. assemblage)

Food Webs: Many ways for species to interact with one another
- includes bottom up (lower to higher trophic levels) and top down (higher to lower trophic levels) interactions.
- Changes at bottom or top will have ramifications upwards or downwards.

Mutualism: +/+ both species benefit (pollinators and flowers)
- Food and fiber growth depends on this (pollination, mycorrhizae, nitrogen fixation)
- Conservation efforts to protect from catastrophic decline that could be associated with mutualists (example: pollinated plant population decreasing with decline in honeybees)
Facultative- beneficial but not essential to either species' survival
(Example: Boran-Honeyguide - Indicator birds help humans find honey and humans break open hive for birds. Note: here's a link which shows more about these interactions—that mentions more about the idea that the honeyguide might lead somebody somewhere dangerous… It also says that it's a nest parasite, kind of like a cuckoo or a cowbird…Greater_Honeyguide)
Obligate: essential to the survival of one or both species
(Example: Ant-Acacia - Ants defend plant and clear away leaf litter. Thorns provide housing and leaves provide nectar and beltian bodies (food) for ants.)

Competition: -/- each is negatively affected, reduce ability for others to retain (Two animals use same resources)
- acquired by one reduces availability of resource to the other
- Interspecific (different species) and intraspecific (between same species)
- linked to struggle for existence and uniqueness of a species' niche
- Widespread, linked to evolution and to niche and equilibrium number of species in an area.
Predation: +/- one is positively affected, on is negatively affected. Includes herbivory, parasitism (lynx and bunny)
Commensalism: +/0 some benefits and one is unaffected(neutral) – difficult to demonstrate/find examples
Example: Cattle egrets, walk behind cattle and eat insects stirred up; clown fish and anemones

Niche = everything about abiotic and biotic environment that determines ability of organism to survive – how it makes a living, where it’s found. Species will be limited by shortage of any of these resources. All the places could occupy if all competitors were gone (fundamental niche). "every point in which corresponds to a state of the environment that would permit a species to exist indefinitely."
- Realized niche is smaller than fundamental niche due to competition with other species (Hutchinson).

Gause's Principle: Competition exclusion principle
- one species per niche – species that have identical niches can’t coexist (for long) = principle of competitive exclusion
Example: bottle experiments - put two similar species in simple isolated environment. Only one will make it
*Study by field observations: five species of warblers in spruce forest (very uniform) – how do they coexist? Answer = niche partitioning (divvy up resources – parts of trees foraged)
- Enough differences could be found to explain coexistence arguing that each species' niche was sufficiently different

*Resource utilization spectra = Galapagos finches seed size example (each specializes in a certain size of seeds). Can measure width, distance, how tightly packed niches are, look at overlap and competition – but no simple relationship between these two.

Species that appear similar are actually different in resource use (different niches).
Niche overlap could infer that competition is operating.
Overlap and competition:
There is no simple relationship between overlap in resource use and competition.
• No overlap could mean no competition
• No overlap could mean intense competition
• High overlap could mean intense competition
• High overlap could mean negligible competition
- Overlap in one dimension may be ameliorated by resource use in other dimensions.
- Overlap is irrelevant if the resource is not limiting.

Saturation- appears to be unused niches (“unused niches”), not changes in density
- Changes in species richness are not accompanied by changes in average density or resource use in those that remain
(Example: 27 different bugs on bracken fern-probably still more places/feeding practices that could be used)

Often needs experimenting to discern the relationships
*Barnacles and Competition:
The distribution of Balanus and Chthamalus only partly overlapped. Both species settled throughout the inter-tidal zone. Larvae of Balanus died out in the upper regions. Larvae of Chthamalus died out in lower regions. When larvae were transplanted (rocks moved), other species prevented them from colonizing. Still, Chthamalus thrived in lower inter-tidal zone but Balanus die out in upper.

-Indirect competition: Individuals in population compete for resources: nesting sites, food, soil minerals.
-Direct: One individual from species interferes with or directly harm an individual from a different species.
• Allelopathy – sometimes called chemical competition
• *Sage species inhibiting growth of others

Modeling – using differential equation again.
Lower case alpha is the competition coefficient.
When = 1, individual in species 2 has same effect on per capita growth as an individual of species 1
When = 0, species 2 has no effect on the per capita growth rate of species 1
If between 0 and 1, intraspecific competition is stronger than interspecific competition.
If greater than 1, interspecific competition is greater than intraspecific competition

Lotka-Volterra Competition Models: shows how different species influence one another
- Intraspecific Competition: dN/dt=rN(1-N/K)

- Interspecific competition: dN1/dt=r1N1(1-(N1+α12N2)/K1)
- a12 = the competition coefficient and N2 is the density of Species 2
- Competition coefficient "converts" individuals of species 2 into equivalents of species 1
- The Competition Coefficient α
• When the coefficient = 1, then an individual of species 2 has the same effect on per capita growth rate as an individual of species 1
• When the coefficient = 0, then species 2 has no effect on the per capita growth rate of species 1
• If 0 < coefficient < 1, then intraspecific competition is stronger than interspecific competition
• If the coefficient is > 1, then interspecific competition is greater than intraspecific competition

Equilibrium: The net zero growth isoclines for each species represent combinations of species 1 and species 2 at which the populations are at equilibrium.
- will only occur if isoclines cross
- For coexistence: Interspecific competition must be less than intraspecific competition
- species will limit their own population growth before they reach densities that exclude their competitors

Equilibrium point depends on the competition coefficients and carrying capacities
- For stable coexistence of both species, the product of the competition coefficients must be less than 1

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