23 Flashcards
primary producers
plants
primary consumers
herbivores
secondary consumers
carnivores who eat herbivores
tertiary consumers
carnivores who eat secondary consumers (predators)
decomposers
eat dead organic matter
what does the pyramid shape represent?
decreasing biomass in higher trophic levels
describe indirect effects in food webs/chains and give an example
- one species alters the effect that another species has on a third
- eg exploitative or scramble competition, if the contested resource is a species
trophic cascades: HSS
interactions between two trophic levels cascade to a third trophic level
why is the world green?
- Hairston, Smith, Slobodkin (1960) proposed the green world hypothesis
- states that carnivores keep down herbivores so herbivores don’t limit plant growth
how is the green world hypothesis an example of an indirect effect?
one trophic level exerts influence on a second by affecting a third
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
solid lines for trophic cascades
direct effects
dashed lines for trophic cascades
indirect effects
compare indirect and direct effects
- 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)
where does much biodiversity reside?
plants and insects
why are there so many species of insects and plants?
- Coevolution
- Niche specialization
- Rapid reproductive cycles
- Habitat diversity
- Polyploidy in plants
- Metamorphosis in insects
- Geographic and climatic stability
- High mutation and adaptation rates
difficulties of herbivory as opposed to carnivore
- animal tissues are easy to convert into animal tissues
- plant tissues are hard to convert into animal tissues
3 difficulties of plant tissues
- 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
example of plant defences against herbivores
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
brightly coloured insects
- frequently toxic
- warning coloration
plant-herbivore interactions as an arms race
- 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!
why do we think plants taste ok?
our food crops have been selected for low toxicity
how are plant defensive compounds important to humans?
- many plant secondary chemicals have diverse, potent biological activities
- some alkaloids are important to us
examples of useful alkaloids for humans
coffee - caffeine
coca - cocaine
tobacco - nicotine
opium poppy - morphine
in what ways are challenges and solutions different for vertebrate herbivores?
- 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
rumen
foregut
cecum
hindgut
what produces unlimited diversification?
physical environment isn’t complex enough to produce extraordinary species diversity
interactions with other organisms do this
why do plants around a pond with fish get better pollination and bees?
there are fewer dragonflies
two examples of complex networks of species interactions in ecological communities
- parasite-herbivore-plant network (Costa Rica)
- plant-pollinator network (Greenland)
why do lizards benefit plants?
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
lizards eat
spiders and herbivores
spiders eat
herbivores
what do Anolis lizards eat?
spiders and beetles
Spiller and Schoener 1992
replicate Caribbean islands; remove Anolis lizards
