Community Assembly

Question 1

How are communities structured?

Answer 1

via both direct and indirect species interactions

Question 2

What is trophic cascade?

Answer 2

indirect interaction in which predator of a predator (or herbivore) benefits the producer indirectly
- direct interactions in trophic cascade are the negative predator-prey interactions
- indirect interaction is positive impact of carnivore on producer by consuming sea urchin

Question 3

What is trophic facilitation?

Answer 3

indirect effect in which one species benefits another via a positive interaction (mutualism)

Question 4

Ecological networks are…

Answer 4

a set of ecological interactions within a community

Question 5

What do food webs summarize?

Answer 5

feeding relationships in a community

Question 6

Levels of food webs

Answer 6

• level 1: primary producers (plants and algae)
• level 2: herbivores (heterotrophes - insects and mites)
• level 3 and above: predators (heterotrophes - birds, mammals, spiders etc.)

Question 7

What are Guilds?

Answer 7

• are a group of species that exploit the same resources in related ways
• feed on organisms in across trophic levels

Question 8

Limitation of food webs?

Answer 8

while a food web illustrates the complexity of interactions among species in different trophic levels it does not reveal the abundance of specific trophic levels

Question 9

What do Biomass Pyramids represent?

Answer 9

illustration of abundance at various trophic levels

Question 10

What is a Keystone?

Answer 10

• an organism that defines the entire ecosystem
• these species have low functional redundancy
• if they disappear, no other species are able to fill their ecological niche

Question 11

Keystone predator species keep the numbers of prey populations below K, and thus…

Answer 11

• decrease competition
• create ecological niches
• increase diversity within communities

Question 12

Difference between Keystone and Dominant species?

Answer 12

keystone species have a high impact DESPITE having a low abundance
- aka a disproportionate impact relative to their population size

Question 13

How do Predators increase diversity?

Answer 13

• as the number of species in his inter intertidal food webs increased, the proportion of the web represented by predators also increased
• as the number of predator species in a community increased, the local species diversity increased

Question 14

Do predators directly influence species diversity (Paine experiment)

Answer 14

• paine experimentally removed top predatory from temperate intertidal food web and monitored response over 2 years
• removal of starfish (top predator) caused decline in diversity from 15-8 species
• appears competition is for space

Question 15

Example of Keystones in Conservation Efforts

Answer 15

• after wolves disappeared from Yellowstone National part in the mid-1900’s, elk populations exploded
• elk eat a lot of plants, including young seedlings and willows which grow along riverbanks
• the roots of these plants hold soil in place and their disappearance led to massive erosion, destroying habitats for beavers and birds among others

Question 16

What are the requirements for keystone status is

Answer 16

• have relatively low biomass in the community
• high impact on community structure

Question 17

Example of Non-predator keystone species

Answer 17

• argentine ants, a species that does not disperse seeds, invaded shrublands of the fynbos of south africa
• they displaced native ant species that were known to disperse large seeds, but small- seed-disperser ants were largely unaffected

Question 18

Umbrella Species

Answer 18

• species that are selected as representatives of their ecosystem when conservation plans are being made
• protection of umbrella species will protect a large amount of co-occurring species, as their home ranges overlap
• this is a way to made as large of an impact as possible, using the least amount of resources
• these can be keystones, but do not have to be

Question 19

Ecosystem Engineers

Answer 19

• differ in type of impact, are not essential due to their trophic effect
• their abundance does not need to be high to have an impact on the ecosystem

Question 20

What are the two types of ecosystem engineers?

Answer 20

1. allogenic - modify the environment by mechanically changing the living/non living material from one form to another
2. autogenic - modify the environment by modifying themselves

Question 21

Beavers as ecosystem engineers

Answer 21

• beavers cut down trees near streams, forming dams
• dams cause flooding, formation of ponds and increased riparian area
• beavers build lodges in resulting ponds
• major impacts on both stream and surrounding terrestrial area

Question 22

What is the key difference between a dominant, foundational species vs. a keystone species?

Answer 22

dominant species are the most abundant species, whereas keystones exert control over a community via important roles/niches