Lecture 23

Question 1

primary producers

Answer 1

plants

Question 2

primary consumers

Answer 2

herbivores

Question 3

secondary consumers

Answer 3

carnivores who eat herbivores

Question 4

tertiary consumers

Answer 4

carnivores who eat secondary consumers (predators)

Question 5

decomposers

Answer 5

eat dead organic matter

Question 6

what does the pyramid shape represent?

Answer 6

decreasing biomass in higher trophic levels

Question 7

describe indirect effects in food webs/chains and give an example

Answer 7

• one species alters the effect that another species has on a third
• eg exploitative or scramble competition, if the contested resource is a species

Question 8

trophic cascades: HSS

Answer 8

interactions between two trophic levels cascade to a third trophic level

Question 9

why is the world green?

Answer 9

• Hairston, Smith, Slobodkin (1960) proposed the green world hypothesis
• states that carnivores keep down herbivores so herbivores don’t limit plant growth

Question 10

how is the green world hypothesis an example of an indirect effect?

Answer 10

one trophic level exerts influence on a second by affecting a third

Question 11

top down control

Answer 11

abundances kept low because of predation
- experimental test = predator removal

Question 12

bottom-up control

Answer 12

abundances kept low because of resource limitation
- experimental test = resource addition

Question 13

solid lines for trophic cascades

Answer 13

direct effects

Question 14

dashed lines for trophic cascades

Answer 14

indirect effects

Question 15

compare indirect and direct effects

Answer 15

• indirect effects can be as strong as direct effects
• outcomes are not fundamentally predictable; this depends on interaction strengths
• experiments are needed (perhaps long term)

Question 16

where does much biodiversity reside?

Answer 16

plants and insects

Question 17

why are there so many species of insects and plants?

Answer 17

• Coevolution
• Niche specialization
• Rapid reproductive cycles
• Habitat diversity
• Polyploidy in plants
• Metamorphosis in insects
• Geographic and climatic stability
• High mutation and adaptation rates

Question 18

difficulties of herbivory as opposed to carnivore

Answer 18

• animal tissues are easy to convert into animal tissues
• plant tissues are hard to convert into animal tissues

Question 19

3 difficulties of plant tissues

Answer 19

• cellulose and lignin are tough and indigestible without microbial symbionts
• plant tissues are heavily defended against herbivores
• coevolutionary race between plants and insect herbivores is responsible for much of biodiversity: specialisation is common

Question 20

example of plant defences against herbivores

Answer 20

milkweeds exude distasteful white sap if damaged
- most generalist insects can’t eat milkweeds, but specialists can evade defences
- milkweed-feeding specialist monarch butterfly larva cut leaf midrib to reduce sap pressure before eating
- caterpillars don’t detoxify the poison, but sequester it in their cuticle, making themselves poisonous and distasteful

Question 21

brightly coloured insects

Answer 21

• frequently toxic
• warning coloration

Question 22

plant-herbivore interactions as an arms race

Answer 22

• plants evolve toxins to reduce herbivory; insects evolve detoxification or other mechanisms to overcome plant defences
• many types of secondary chemicals (esp alkaloids) often deter generalist herbivores
• no plant species is toxic enough to escape from specialist herbivores
• specialist insects may evolve to use defence chemicals as feeding stimulants or defence compounds

result: escalation, arms race!

Question 23

why do we think plants taste ok?

Answer 23

our food crops have been selected for low toxicity

Question 24

how are plant defensive compounds important to humans?

Answer 24

• many plant secondary chemicals have diverse, potent biological activities
• some alkaloids are important to us

Question 25

examples of useful alkaloids for humans

Answer 25

coffee - caffeine
coca - cocaine
tobacco - nicotine
opium poppy - morphine

Question 26

in what ways are challenges and solutions different for vertebrate herbivores?

Answer 26

• Many insects complete development on a single, often well-defended plant; they must overcome plant defences
• Vertebrate grazers often eat some plant tissue, and then move on to another plant
• Vertebrate herbivores often select mixed diets containing foods processed by different
  detoxification pathways, thereby avoiding high doses of any one toxin
• Some detoxification by microbes in fermenting chambers

Question 27

rumen

Answer 27

foregut

Question 28

cecum

Answer 28

hindgut

Question 29

what produces unlimited diversification?

Answer 29

physical environment isn’t complex enough to produce extraordinary species diversity
interactions with other organisms do this

Question 30

why do plants around a pond with fish get better pollination and bees?

Answer 30

there are fewer dragonflies

Question 31

two examples of complex networks of species interactions in ecological communities

Answer 31

• parasite-herbivore-plant network (Costa Rica)
• plant-pollinator network (Greenland)

Question 32

why do lizards benefit plants?

Answer 32

because of unequal interaction strengths:
- effect of lizards on spiders is week
- effect of lizards on herbivores is strong
- lizards reinforce the effect of spiders

Question 33

lizards eat

Answer 33

spiders and herbivores

Question 34

spiders eat

Answer 34

herbivores

Question 35

what do Anolis lizards eat?

Answer 35

spiders and beetles

Question 36

Spiller and Schoener 1992

Answer 36

replicate Caribbean islands; remove Anolis lizards