Plant responses to the biotic
environment
• The resources that are important for plants
are light, water, minerals, oxygen, carbon
dioxide and space. Plants also need to
defend themselves against herbivores.
Plant-plant relationships
• Many plants secrete a poisonous
substance into the soil to prevent other
plants growing near them = allelopathy.
• Seed dispersal means spread over a wide
area to ensure survival.
• Plants in reduced light grow larger leaves
• Stratification means plants are adapted to
different light intensities to grow in different
conditions.
• Epiphytes are plants that perch on tall
trees to get light at higher intensities.
Lianas are plants that climb tall trees for
the same reason.
• Plant parasites: true suck the sap and do
not photosynthesise, hemi-parasites suck
the water but do own photosynthesis.
• Seaweeds are adapted to different depths
– green at surface, brown mid depth and
red deepest.
• Cluster growing – different species grow
close together to support each other
(mainly grasses).
• Fruit trees rely on others to pollinate
(pears, feijoas)
• Clover benefits other plants growing near
due to the bacteria in their roots.
Plant-fungi relationships
• Mycorrhizal fungi forms mutulistic
relationships with many plant roots. Very
important in pine forests in NZ. They help
plants absorb water and get nutrients in
return.
• A lichen is an obligate mutulistic
relationship. It is an algae and a fungi that
have to live together to survive.
Plant-animal relationship
• Herbivores eat plants in many ways – graze,
browse, sapsuckers, nectar feeders, stem
borers, chewers, pollen feeders, fruit eaters,
seed eaters, gum eaters.
• Plant defences against herbivores
– Thorns: modified side branches, spines modified
leaves and stings which are modified hairs.
– Divarication: plants that branch at wide angles with
closely interlaced branches.
– Chemicals: toxic or just bad tasting, some
plants produce antibiotics when attacked
(caffeine, chilli), chemicals produced when
chewed (cyanide, pyrethins, tannins, nicotine,
strychnine etc., or smells released to stop
animals landing on them (onion, garlic etc).
– Sticky sap or seeds to trap would be eaters
– Low growing point to recover quickly
– Enclosing seeds in hard shell or prickles
– Seed masting
– Hiding
– Regeneration
– Waxy cuticle and epidermis
Co-operative relationships between
plants
• The presence of one species of plant
growing near another may protect it from
grazers – marigold roots secrete a
nematode killing chemical that helps other
plants nearby, willows and alders can
communicate by airborne chemical cues to
develop insect resistant chemicals.
Co-operative relationships between
plant and animals
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Pollination
Ant guarding
Living among thorns
Eating succulent fruit