>Students will be able to give examples of how top predators demonstrate and maintain the health of their ecosystems
>Students discuss the inherent vulnerability of top predators in an ecosystem
>Students will explain how energy is lost as it moves through trophic
>Students will be able to give examples of how top predators demonstrate and maintain the health of their ecosystems
>Students discuss the inherent vulnerability of top predators in an ecosystem
>Students will explain how energy is lost as it moves through trophic levels
>Whiteboard or large writing tablet
>25 clickers (counters)
>Platypus Watch Packet (hyperlink available on Sustaining River Life cd and web versions)
>Food web badges
>Paper Bag Platypus instructions (see below)
Based on information from www.platypus.asn.au
Platypus are monotremes, as such, their urinary, digestive and reproductive systems all open into a single duct, the cloacae. This structure is very similar to the one found in reptiles and birds. Platypus lay eggs, however, the egg is retained for some time within the mother, who actively provides it with nutrients. Monotremes also lactate, but have no defined nipples, excreting the milk from their mammary glands via openings in their skin similar to sweat glands.
Platypus. Photograph: John Hawkins
The males possess hollow spurs linked to poison glands. It has been suggested that these are used primarily in fights between males during the breeding season. The female lays eggs in a different type of burrow, that she constructs herself. In this burrow the eggs are incubated and hatch. The young are tiny, blind and naked. They remain in the burrow for about four months during which time the mother provides them with milk. Females begin breeding at two or three years of age and typically lay 2-3 eggs at a time. In the wild platypuses have been recorded as living up to 13 years.
The best times to look for platypus are either the hour before sunset or the hour just after sunrise. They swim along the bottom of freshwater streams and lakes probing at mud and gravel beds with their highly sensitive bill. Research has shown that electro-receptors in the bill detect the muscle activity of prey. A typical diet consists of a variety of freshwater organisms including crayfish, shrimp, the larvae of water insects, snails, tadpoles, worms and small fish. The home ranges of males overlap those of several females. The male will maintain a burrow for both sexes during the breeding season.
The platypus was hunted intensively for its fur until the early 20th century. Until about 1950 it was also subject to accidental drowning in the nets used by inland fisheries. A problem that still persists today with some forms of yabby traps and illegally set nets.
Although the platypus is now considered to be common throughout its current distribution, its abundance is difficult to measure and therefore its future conservation status is not easily predicted. Several studies have reported fragmentation of platypus distribution within individual river systems. However, the total population and population trends of platypus are currently unknown and other threats are now presenting problems for the species.
Platypus populations are believed to have declined or disappeared in many catchments, particularly in urban and agricultural landscapes. In most cases, the specific underlying reasons for the reduction in numbers remain unknown.
It is also clear that platypus numbers have declined in the lower Murray-Darling Basin. There are no naturally occurring populations in South Australia any longer.
Habitat disruption caused by dams and weirs, agricultural activities and pollution are threatening Australian freshwater systems, and as a consequence, platypus. Poor land management practices lead to stream bank erosion, sedimentation of water bodies and the loss of vegetation in areas adjacent to waterways.
Entanglement in litter and fishing gear kills numerous platypus each year. In a survey recently conducted on platypus in urban waterways, 30% were found to be tangled in abandoned fishing line, six-pack rings and shopping bags. Many different kinds of litter have been found tangled around platypus, including elastic bands of various widths, rubber canning jar seals, loops of nylon fishing line, rubber gaskets, and loops of elastic from garments.
In addition, many platypus have scars on their bills and bodies which may have been caused by encounters with sharp objects in the water, such as broken glass, sharp pieces of metal or discarded wire.
Why top predators matter
Top predators, such as platypus are the bell-weather species of ecosystem health. By standing at the top of the food pyramid, all smaller, and often easily overlooked disturbances to the habitat are magnified and manifested. The top predator is most likely to be affected by habitat loss, as it requires the largest amount of it.
As the following activity demonstrates for the students, there are always more plants than herbivores, and more herbivores than predators. Thus predators are always in relatively small numbers, and often exert a strong influence over the communities in which they live.
Top aquatic predators, such as Murray Cod and aquatic mammals such as platypus, can exert a profound influence on the structure and function of aquatic ecosystems. Yet their numbers have fallen dramatically in the 20th century—by over 90 percent in the case of Murray cod, for instance.
Their loss has triggered a cascade of effects through succeeding layers of food webs. The loss of top predators not only has direct effects on the ecosystem (that is, a possible population boom in the species they eat as well as changes in population size of prey even lower in the food chain), but equally and in some places more significant behavioural effects (that is, changes in their 