Lecture 14 notes

Species 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.

Two-way interactions:
Mutualism (both benefit)
Competition (both affected negatively)
Predation (one is positively affected)
Commensalisms (one benefits, other unaffected)

What is an ecological community? There are interactions that bind it together – “network of influences” (vs assemblage)

Food web: producers, first consumer (different kinds, by resources consumed), levels of predators. Changes at bottom or top will have ramifications upwards or downwards. More complicated than just two-way interactions.

Mutualisms: both benefit. Facultative (not essential) and obligate (essential to one or both). An example is pollination, conservation concern because documented decline in pollinators. Food and fiber production depend on this.
Examples:

Boran-honeyguide mutualism. Facultative. African tribal people and bird – people findd bee colonies more quickly when guided by bird. Search time reduced by third. Humans use smoke to subdue bees – bird gets larvae, wax, less likely to be stung.

Ant-Acacia mutualism
Acacia provides shelter (hollow thorns) and food (nectar, beltian bodies – carbohydrates and proteins) to the ants. Ants remove herbivorous insects, organic material that could be a fire risk.

Commensalism – one benefits, other neutral. Hard to actually demonstrate.
Examples:
Cattle egrets. Walk behind cattle, eat insects stirred up.
Anemones and clown fish – argued that mutualism because fish would drop morsels of food, but not demonstrated in research.

Competition:
Two require same resource, if acquired by one reduces availability of resource to the other. Both intraspecific and interspecific. Widespread, linked to evolution and to niche and equilibrium number of species in an area.

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). Realised is when competitors are around.

Gause’s principle: one species per niche – species that have identical niches can’t coexist (for long) = principle of competitive exclusion.

Bottle/garden 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)

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.

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

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