Lecture 23

Question 1

what is a trophic level

Answer 1

a specific position or level in a food chain or food web - representing the organism’s role in the flow of energy and nutrients
absolute bottom = decomposers, fungi
bottom = primary producers = plants
middle = primary consumers = herbivores
top = tertiary consumers = carnivores who eat secondary consumers who are also predators

Question 2

what are secondary consumers

Answer 2

carnivores - top of the tropic level (level 4) who eat primary consumers such as herbivores

Question 3

T/F there is energy loss as a trophic level moves up

Answer 3

true
- losing energy each time sets a limit to the top trophic level
- not enough energy to pass on to the next trophic level
- only 10% of the energy is given to the next tropic level as 90% is used for heat and metabolic processes
- the shape of the pyramid indicates this

Question 4

What is the difference between a food web and a food chain

Answer 4

• food chain = single, linear sequence that shows how energy flows from one organism to another in an ecosystem - straightforward consumption
  energy flows from producer –> primary consumer –> secondary consumer –> tertiary consumer –> quaternary consumer
• a food web is more complex and interconnected to show different organisms in an ecosystem and how they have multiple feeding relationships = variety of energy flows between one species

Question 5

what is a parasitoid

Answer 5

an insect that lays eggs in preferably another insect body which acts as a parasite

Question 6

indirect effects

Answer 6

species 1 alters the effect that another species has on a third species
- a chain of interactions
when 2 predators are fighting over 1 prey, if species 1 eats more there is less for the other predator species
- this is related to exploitative/scramble competition when fighting over a species

Question 7

what is the HSS, trophic cascades

Answer 7

also known as the green world hypothesis
- interactions between 2 trophic levels cascade to a third trophic level
- herbivores have a negative effect on the plants, while predators have a negative effect on the herbivores bc they eat them
- although predators whom are secondary consumers have a positive indirect effect on the plants bc they eat most of the herbivores so it protects the plants from being hunted and eaten

Question 8

green world hypothesis

Answer 8

why is the world green? it shows that there is a lot of plant population but why don’t all the herbivores eat them all
- bc carnivores keep down herbivores so herbivores don’t set plants to extinction and limit their growth
- one trophic level (secondary) exerts influence on a second (herbivores) by affecting a third (plants)
- predators have positive effects 2 levels down

Question 9

top-down control

Answer 9

the abundance of organisms at higher trophic levels like predators regulates the populations at lower trophic levels such as the herbivores and primary producers
- u can test for this effect by removing predators to see if the herbivores increase in abundance - or if there is changes to the entire ecosystems structure

Question 10

bottom-up control

Answer 10

the abundance of organisms at lower trophic levels like plants regulate the populations of higher trophic levels like predators and herbivores
- abundance of the higher trophic levels is bc there are limited resources
- to test for this add a resource to the ecosystem and see how the above levels change in abundance

Question 11

explain the lizard, plants, beetles and spiders interactions

Answer 11

lizards eat spiders and beetles
spiders eat beetles
beetles eat plants
Are the lizards indirectly benefitting the plants?
- lizards do benefit plants bc of unequal interaction strengths
- the effect that lizards have on their prey, spiders is weak but strong for beetles (herbivores)
- so the presence of lizards removes herbivores from the ecosystem, allowing plants to thrive without many beetles eating them
- but it can depend and go the other way depending on the impact the predators prey has

Question 12

give an example of trophic cascades between aquatic and terrestrial animals

Answer 12

fish eats dragonfly larvae, larvae turn into dragonflies, dragonflies eat pollinators like bees
the fish as the secondary consumer indirectly benefits the plants bc they kill the dragonflies that kill herbivores - this allows plants to thrive without having issues with pollination
- the presence of no fish has an increase in prey
- plants near a pond do better bc they get better pollinations because there are fewer dragonflies

Question 13

T/F indirect effects are much weaker than direct effects

Answer 13

false
they can be equally as strong

Question 14

T/F we can always predict the outcome of indirect effects

Answer 14

false
they are predictable and depend on interaction strengths between trophic levels

Question 15

T/F there is an unevenly distributed number of species with animals being the most abundant

Answer 15

false
insects and plants are the most abundant - they have 400k of different species

Question 16

why is it difficult to be a herbivore

Answer 16

plant tissues are hard to convert into useful animal tissues when animals eat them
- plants = cellulose and lignin which is indigestible without microbial symbiosis
- plant tissues are also very defensive, induced, chemical, and secondary defences bc they are sessile and need ways to protect themselves
- plants and insects’ coevolutionary arms race = why we have so much abundance - they are under lots of selection

Question 17

what is an example of a plant that as defence against herbivory

Answer 17

milkweed - not tasty white sap that is excreted if it is damaged or disturbed
- more general insects can not eat milkweed but only insects that have become specialists to milkweed and adapted can evade the defense
- monarch butterfly larva = feed on milkweed and store toxin in tissues of body to become poisonous to others - warning colours of bright orange

Question 18

why are plants and herbivores interacting at an arms race

Answer 18

since plants = lots of defences, herbivores are under lots of selection to detoxify other mechanisms to overcome plant defences
- we think this is a very common process bc no plant species is toxic enough to escape from a specialist herbivore = arms race

Question 19

how are defence compounds that plants use important to humans?

Answer 19

1. plants’ secondary chemicals have lots of biological activities
2. some alkaloids are important to humans - caffeine, cocaine, nicotine, morphine

Question 20

how are vertebrate herbivores different from invertebrate herbivores

Answer 20

invertebrates usually develop completely on a single well-defended plant
while vertebrate grazers often eat some plant tissue and move on to the next plant = mixed diets to avoid high concentrations of one toxin

Question 21

Why isn’t the pressure of the physical environment enough to create lots of adaptations?

Answer 21

because the physical environment isnt complex enough to produce extraordinary species diversity
- it takes interactions with other complex organisms to produce unlimited diversification