Communities: Nature and Change in Communities

Question 1

Define biodiversity and list the three levels.

Answer 1

describes the variety of biota at a particular biotic level.
1) genetic
2) species
3) community (ecosystem)

Question 2

What is a rank abundance curve?

Answer 2

plots the proportional abundance of each species (pi) relative to others in rank order
pi = number of individuals of one species / total number of individuals in the community
lower number = more common
higher number = less common

Question 3

What is a species accumulation curve?

Answer 3

species richness plotted as a function of total number of individuals counted
point of no significant return - when increased sampling effort stops revealing new species, meaning most or all of the species in a community have been observed - may not be able to be calculated (i.e. bacterial communities)

Question 4

Define direct and indirect interactions. Provide examples of each.

Answer 4

direct interactions - occur between two species (i.e. competition, predation, facilitation)
indirect interactions - relationship between two species is mediated by a third or more species
trophic cascade - an indirect interaction that occurs when a change in the rate of consumption at one trophic level results in a change in species abundance or composition at lower trophic levels (i.e. the reintroduction of gray wolves to Yellowstone reduced the number and changed the behavior of elk)
trophic facilitation - an indirect interaction that occurs when a consumer is indirectly facilitate by a positive interaction between its prey and another species (i.e. black needle rush shades soil which decreases transpiration and decreases soil salinity; therefore marsh elder shrubs have a decreased fitness due to presence of rush because they thrive in higher salinity soils)

Question 5

Define foundation species. List, describe, and provide examples of the two types.

Answer 5

have large, community-wide effects by virtue of their size or abundance, strong competitive ability, or provision of habitat or food to other species.
ecosystem engineers - a type of foundation species that influences its community by creating, modifying, or maintaining physical habitat for itself or other species (i.e. beaver, coral, trees)
keystone species - have strong effects because of their role in a community, usually though indirect interactions; their effects is large in proportion to either their biomass or abundance (i.e. sharks are a key predator that keep marine ecosystems in balance and avoids over populations and out competition of other species)

Question 6

Define succession. Provide examples.

Answer 6

the process of change in species composition over time as a result of abiotic and biotic agents of change
i.e. the Great Barrier Reef (coral bleaching as due to changes in water temperature and chemistry such as dissolved oxygen levels)
i.e. the eruption of Mt. St. Helens (the devastation created new habitats devoid of any living organisms, and there was a gradual replacement of species over time)
i.e. mass extinction events

Question 7

Define primary and secondary succession. Describe the factors that determine the magnitude of succession.

Answer 7

magnitude dependent on tempo and area extent of the agent of change
primary succession - involves the colonization of habitats devoid of life; follows a disturbance event (i.e. new islands formed by volcanoes)
secondary succession - involves the reestablishment of a community in which most, but not all, of the organisms or organic constituents have not been destroyed (i.e. seasonal droughts, seasonal wildfires, mild tropical storms)
climax stage - the point where a community becomes mostly stable until another disturbance even occurs - no real-world examples because communities are dynamic

Question 8

Name and describe the three different models of succession.

Answer 8

facilitation model - the earliest species in a community will modify the environment in ways that will overall benefit later species
tolerance model - the earliest species in a community do not make environmental changes that will significantly impact the establishment of later species
inhibition model - the earliest species in a community are very strong competitors due to their ability to withstand harsh conditions, making it more difficult for the establishment of later species

Question 9

Describe four major stages of succession. Which model does this support? What factors increase across the stages of succession?

Answer 9

Pioneer - the first few years after which a glacier retreats and habitat is exposed; initial colonization where a community is dominated by species such as lichen
Dryas - after appx 30 years, a shrub named Dryas became dominant; woody plants were introduced along with original pioneer species
Alder - after appx 50 years
Spruce - after appx 100 years, a climax community with a mature forest is created; the community will continue to undergo changes, but none as drastic as these four stages of succession
supports the facilitation model of succession
germination, nitrogen levels, soil organic matter, and growth increase across the stages of succession

Question 10

Describe alternative stable states.

Answer 10

different communities can develop in the same area under similar environmental conditions; communities are considered stable when they return to their original states after a disturbance; the trajectory of succession is not always repeatable or predictable