Lecture Thirteen - Origins and evolution of Australian flora Flashcards
What are early views of Australian flora?
Early views are that Australian flora is primarily a result of migration from the Northern Hemisphere.
This view is based one:
1) Eurocentrism and,
2) Long belief that current continental positions similar to today (although tectonic they initially reinforced the concept of a recent intrusion from the north.
How has Australia’s position on the Earth changed over time?
Aus is drifting north about 506 cm/year.
Over the past 60Ma, Australia’s northward track has coincided with a globes trend of cooling - climate = more stable than elsewhere.
This is due to the lack of glaciers coupled with comparative flat topography.
What are the three major periods of plant evolution?
1) ~500Ma with evolution of ancestors of mosses (bryophytes).
2) 400Ma when vascular plants (ferns) first appear in the fossil record.
3) ~360Ma appearance of ‘seed’ plants (gymnosperms) - enclosure of plant embryo in protective covering (but not angiosperms, yet).
What occurred in the second period of plant evolution?
2) 400Ma when vascular plants (ferns) first appear in the fossil record.
Vascular plants are more advanced than bryophytes:
- Most have true roots and lignified vascular tissues and therefore get bigger.
- Have more complex leaves.
Late Silurian lycophytes ~ 420Ma:
Could represent Gondwana flora - much more advanced than contemporary northern hemisphere flora.
Club mosses, clase trees primitive vascular plants (reproduce via spores and have root systems).
Devonian club mosses ~ 380Ma:
First shrub and tree like plants.
Late Devonian - forest real trees.
Trees - vascular systems = water circulation and nutrient flow agains gravity.
End of Devonian - gymnosperms appeared.
Advanced root systems - soils, increased weathering, huge ecological feedback.
What occurred in the third period of plant evolution?
3) ~360Ma appearance of ‘seed’ plants (gymnosperms) - enclosure of plant embryo in protective covering (but not angiosperms, yet).
Gymnosperms had been around for ~130Ma.
Major diversification of gymnosperms.
Global impact of increase in plant biomass:
- reduction in atmospheric CO2 due to carbon ‘locking up’ in plant tissues and reactions between CO2 and soil minerals.
- Period of global cooling.
What are mesozoic gymnosperms?
Gymnosperms reach a peak in diversity and abundance in the Mesozoic.
In australia, remnant extant gymnosperms are dominated by:
- Cypresses = largely arid/semi-arid.
- Podocarpaceae = temperature and tropical.
Araucariaceae - woolemi, bunya, hoop and kauri pines.
Taxodiaceae - King Billy and pencil pines (only Tasmania).
What happened 130Ma and more recently?
Cira 130Ma, angiosperms appear = seeds in specialised chambers - ovaries.
Australian floal dominated by ferns, conifers (e.g. Wollemi Pine) and cycads.
Tectonically, Tethys Sea is closing, India northward bound, Australia still attached to Antarctica.
Rifting between Aus and Antarctiva underway. Circumpolar current commences once S. America separates from Antarctica.
beginning of tertiary - climate mark and moist, Aus covered by rainforest vegetation, including terms, gymnosperms and angiosperms.
Earliest angiosperm fossil material found ~circa 80Ma - Nothofagus and proteaceae pollen.
Eocene (~50-45Ma) replacement of ferm/conifer flora by other conifers and Nothofagus.
By 40Ma, cold adapted forests of Nothofagus (Southern beech) and gymnosperms predominated across southern Aus.
Cooler conditions during the late Tertiary (3-4Ma) particularly in SE Aus, increased seasonality and winter dominated rainfall. Cooling due to circumpolar current and global trends.
Explain in the evolution of arid and semi-arid taxa.
Retreat of rain forests - ecological vacuums.
New range of environment was probably incompletely exploited by available flora.
First source = species that evolved in situ.
Second source = shoreline species e.g. saltbushes.
Third source = ex-retreating rain forests e.g. eucalyptus.
What are ancient angiosperms?
High concentration of primitive angiosperms found in tropical rainforests - relics of earlier flora. e.g. species in the:
Austrobaileyaceae - pollen similar to oldest known pollen types. e.g Austrobaileya.
Winteraceae - ancient wood structure (lack of xylem vessels common to other angiosperms). e.g Drimys.
Aquifoliaceae - holly family - oldest (late Cretaceous) angiosperm pollen recorded to date in Aust. e.g. Ilex.
What evolution occurred in isolation to one another?
A consequence of sequential separation of Gondwanan components - strong phylogenetic links between:
- Australia and Africa
- Proteaceae - Banksia, Grevillea.
- Myrtaceae - e.g. Eucalyptus, dominant tree taxon in forest, woodland and mallee vegetation throughout Australia.
- Ericaceae (former Epacridaceae) - Australia and S. America. (early Tertiary land bridge)
e. g. Nothofagus
What is Sclerophylly?
Australia’s flora is unique as its dominated by species with rigid, small leaves - sclerophyll taxa.
Early botanists thought sclerophylly was a response to aridity.
More recently (e.g. Beadle) -> response to nutrient (esp. phosphorous) deficiency.
Two types of sclerophyll forest – wet and dry.
• Wet sclerophyll forest – canopy trees >30 m with an understorey of soft-leaved species (e.g. ferns).
• Dry sclerophyll forest – canopy trees 10-30 m, with an understorey of hard-leaved species (e.g. Epacris).
What is endemism?
• Long isolation - few immigrants from long-distance dispersal
• 85% of Aust. species are endemic - they occur here and nowhere else - 14 families endemic (higher than any other continent).
• Six families make up > 40% of the flora.
=> very high degree of endemism at family, genus and species levels.
• Principal avenue of ingress from tropical areas to the north, but only recently (geologically).
What is Wallace’s line?
Wallace’s line - biogeographical boundary between the Laurasian (to the north-west) and Gondwanan (to the south-east) floras (and faunas).
Wallace’s Line strongly delimits fauna:
WL not just zoogeographic – re plants, Gondwanan genera Casuarina, Eucalyptus, Acacia and Banksia naturally distributed to the south-east of this boundary, but not to the north-west.
Area now considered a zone of transition – Wallacea / Wallacia.
What is co-evolution and how has it occurred in Aus?
Great diversity and abundance of angiosperms inherently linked to co-evolution with animals, particularly insects. e.g. Figs and fig wasps
Australia’s extremely long isolation -> evolution of a range unique of mutualisms, including syndromes related to nutrition, seed dispersal, and pollination.
e.g. Seed dispersal by ants - ‘myrmecochory’ - seeds collected for oil-rich elaiosome, then discarded
- seed dispersed away from parent plant (reduced competition, possibly in better germination site).
What are pollination strategies in Australian plants?
Many Aust plants pollinated by vertebrates. e.g. Banksia, Eucalyptus.
Plants / flowers often brightly coloured, produce large amounts of nectar.
Birds - long, slender curved bills that probe flowers, mammals & birds with brush-tipped tongues.
