Trophic Relationships and plant-animal interactions

Question 1

Trophic levels

Answer 1

Primary producers =
plants
* Primary consumers =
herbivores
* Secondary consumers =
predators = carnivores
who eat herbivores
* Tertiary consumers =
carnivores who eat
secondary consumers
(not shown in figure to
the right; also
“predators”)
* Decomposers = eat
dead organic matter

Question 2

Indirect Effects

Answer 2

One species alters the effect that another
species has on a third
* Already talked about one example:
Exploitative or
scramble
competition, if
the contested
resource is
a species

Question 3

Trophic Cascades: “HSS”

Answer 3

Interactions between
two trophic levels
‘cascade’ to a third
trophic level

Question 4

Why is the world green

Answer 4

“The world is green” because carnivores
keep down herbivores so herbivores don’t
limit plant growth
* Example of an indirect effect: one trophic
level exerts influence on a second by
affecting a third
* Trophic cascades involve effects that
alternate across trophic levels
* Can drastically affect communities

Question 5

Top-down versus bottom-up
control

Answer 5

Top-down control: abundances kept low
because of predation
(Experimental test = predator removal)
* Bottom-up control: abundances kept low
because of resource limitation
(Experimental test = resource addition)

Question 6

Will lizards benefit plants or not?

Answer 6

Lizards eat spiders
* Lizards eat herbivores
* Spiders eat herbivores
* Herbivores eat plants
* Trophic cascades likely,
but can’t answer from
topology of the food web
alone …
* … so must do
experiments!

Question 7

Lizards do benefit plants because
of unequal interaction strengths

Answer 7

The effect of lizards on
spiders is weak
* But the effect of lizards on
herbivores is strong
* So lizards reinforce, rather
than counteract, the
effects of spiders …
* … but it could have gone
the other way!

Question 8

Trophic
cascades across
ecosystems:

Answer 8

far-reaching
effects
of fish in ponds
Solid lines =
Direct effects
Dashed lines =
Indirect effects

Question 9

Special difficulties of herbivory

Answer 9

Easy to be a carnivore: animal tissues easy to
convert into animal tissues
* Plant tissues hard to convert into animal tissues
21
Special difficulties of herbivory
* Cellulose and lignin tough,
indigestible without
microbial symbionts
* Plant tissues heavily
defended against
herbivores
* Coevolutionary race
between plants and insect
herbivores is responsible
for much of biodiversity:
specialization is common

Question 10

Plant-herbivore interactions as
an arms race

Answer 10

Plants evolve toxins to reduce herbivory; insects
evolve detoxification or other mechanisms to
overcome plant defenses
* Very common (we think plants taste OK because our
food crops have been artificially selected for low
toxicity)
* Many types of secondary chemicals; alkaloids
especially potent and prominent
* Chemicals often deter generalist herbivores
* But no plant species is toxic enough to escape from
specialist herbivores
* And specialist insects may evolve to use “defense”
chemicals as (1) feeding stimulants or (2) defence
compounds
* Result: escalation, arms race!

Question 11

Challenges and solutions are
different for vertebrate herbivores

Answer 11

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