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• Pollination

• Ant guarding

• Living among thorns

• Eating succulent fruit