Herbivory and Plant Defences Flashcards
Rainforests are typified by complex networks of interactions between species
Herbivore - Plant
Ant - Plant
Nutrient cycling (mycorrhizas and soil microbes)
Plant pollination by insects, birds and bats
Seed dispersed by birds and mammals
Why are interactions between herbivores and young leaves important in regulating trophic dynamics?
Interaction of herbivores and young leaves of tree seedlings is important in regulating trophic dynamics, as it determines herbivore population sizes and / or seedling persistence.
What are the most important consumers of leaf material?
Insects are the most important consumers of leaf material in tropical forests although vertebrates, fungi and other pathogens have an effect.
What are the different ways plants attack, poison, starve or avoid herbivores?
Plants have evolved an array of biotic, chemical, mechanical and phenological ways to attack, poison, starve or avoid herbivores.
What are annual herbivory rates in temperate and tropical forests?
Annual herbivory rates in temperate forests have been estimated to average 7% of leaf area and in tropical humid forests this is 11% for shade tolerant plants and 48% for gap specialists.
Insect herbivores are
very diverse
Physical defences in plants
e.g. spines on the stems of rattans, bamboos and acacias protect against vertebrate herbivores.
Stinging plants occur throughout the tropics and protect against vertebrate herbivores.
Chemical defences in plants
Some trees in Ugandan forests have toxic bark to deter elephants.
The majority of herbivore damage in the tropics
occurs during the short window when leaves are young and expanding.
Invertebrates are the most important
herbivores in tropical forests.
Plants invest in a range of leaf defences:
Leaf toughness, high fibre content and other physical defences are effective in mature leaves but are not compatible with young expanding leaves.
Leaves may invest in diverse chemical defences including, alkaloids, tannins and phenols
Rapid leaf expansion to minimise damage
Synchronous leaf production to satiate herbivores
Delayed greening of young leaves
Extra-floral nectaries and ant defence
Why do many young leaves of tropical species exhibit delayed greening?
Energy losses are reduced by delaying the development of chloroplasts thus young expanding leaves can look almost white or varying shades of red-purple.
The red/purple colour is due to the presence of other pigments such as anthocyanins (which are masked by chlorophyll).
Leaves with delayed greening are not as nutritious and not as attractive to herbivores as young green leaves.
The cost of reducing herbivory by delayed greening is forfeited photosynthate.
Delayed greening is prevalent among (but not exclusive to) understory plants where photosynthesis is limited by shade.
Ant-plants (myrmecophytes)
The leaves of some plant species are protected by ants (ant-plant mutualism).
In exchange for a home and food ants will aggressively attack both invertebrate and vertebrate herbivores.
Plants provide ants with extra floral nectaries, food bodies and nesting sites called domatia.
Many Cecropia in the Neotropics and Macaranga in Asia are well defended by Azteca and Crematogaster spp. ants respectively.
Macaranga bancana has developed an obligatory relationship with the Heart Gaster Ants (Crematogaster spp).
Plants are usually colonized as saplings.
The queen ant enters the young plant by chewing her way through a shoot.
She then seals herself from inside and lays her eggs.
She will have to care for the larvae until they turn into worker ants.
These workers will then look after subsequent batches of eggs that the queen will lay.
The plant provides food for the ants in the form of the many small, white, starchy bodies found on the undersurface of the stipules
The ants farm scale insects of the genus Coccus in the hollows of the stem and feed on the honeydew that these sapsuckers secrete.
The plant benefits from the protection these ants provide against herbivores, insects and pathogenic fungi.
Whenever insects or other large organisms land on the plant, the resulting vibrations will alert the ants.
They will then swarm out from their galleries inside the stems by way of the tiny holes that are found on the surface of the stems.
It has been reported that the ants will then raise their rear portion, the gaster, in agitation and squirt formic acid on to the intruders.
Kursar and Coley (2003) carried out a survey of the defensive traits of young leaves from over 200 unrelated species in Africa, Southeast Asia, and the Neotropics.
Traits are classified into two “syndromes”:
Those that have well-defended young leaves – (defense syndrome)
Those that rely on rapid expansion to minimize the period when leaves are young and most vulnerable to herbivores (escape syndrome).
However these are not always discrete classes, but a continuum.
Coley et al., (2005) investigated the defence strategies of two closely related Inga species which co-exist in the same habitat.
They showed that each had evolved a different suite of adaptations in response to herbivore pressure.
Leaves from both species suffered similar levels of herbivore damage.
However, the damage was done by different communities of herbivores
Three herbivore species were restricted to I. goldmanii and six to I. umbellifera.
Homoptera, Orthoptera, and leaf-cutting ants were found on I. umbellifera, but were never recorded from I. goldmanii.
There were clear differences in the classes of secondary defence compounds (non protein amino acids and flavanoids) in the leaves of the two species.
Feeding trials showed that flavanoids found in I. goldmanii were more bioactive against herbivores
I. goldmanii had higher levels of biotic defences with more extra floral nectaries per leaflets attracting more ants that I. umbellifera.
Young leaves of I. umbellifera were produced in fewer but more synchronous flushes and expanded more quickly than was the case with I. goldmanii.
Young leaves of I. umbellifera had low levels of chlorophyll and delayed greening.
