Objectives
>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
Duration
2 hours
Materials required
>Whiteboard or large writing tablet
>Markers
>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)
Background
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 prey’s predator-avoiding behaviour once they are no longer at risk of being eaten).
All large predators perform a very important function in their habitat: helping to regulate and control the populations of other species. When an animal group loses its natural predators, overpopulation, sickness and mass die-offs can result.
The secretive platypus, an icon of our rivers. Photograph: Kathyrn Black
Watching for platypus
Platypus are most often observed near dawn or dusk, although the animals are also sometimes seen in the middle of the day, particularly along waterways where the animals are relatively abundant.
Males and females are both dark brown in colour, with lighter underparts and a small white spot just in front of the eye. They are surprisingly small - approximately 40 to 60 centimetres in length. They float very low in the water, with a slightly higher profile marking the location of their head and rump. When viewed at a distance from the bank, they can have quite a strong resemblance to a floating stick - and hence are often first recognised by the conspicuous bow wave created as they paddle along the surface.
While platypus sometimes swim strongly in one direction, more often they dive and resurface in a leisurely manner as they feed - generally popping up again within 25 metres of the point where they dived. When foraging, platypus typically remain underwater for less than a minute before returning to the surface to chew their food and sometimes to groom (usually by scratching with a hind foot). Platypus dives usually create a very distinctive circular ripple pattern. This sometimes has a small number of bubbles in the centre, caused by pockets of air being forced from their double layer of fur. When alarmed, a platypus will “splash-dive”, making a relatively loud single or double splashing noise. Normal “duck-dives” for food are much quieter, sometimes (but not always) producing an audible “plop”.
For local platypus ‘hot spots’ contact your local river management authorities, Catchment Management Authority or Waterwatch coordinator.
Australian water rats can easily be mistaken for platypus, especially if an animal is seen only briefly. At a distance of more than 20 metres it can be very difficult to tell the two species apart in the water. The thin, white-tipped tail of the water rat is the most obvious way to distinguish it from a platypus, which has a flat, paddle-like tail. Water rats are also much more likely to be seen out of the water (feeding on a rock or log or running along the bank) than platypus are.
Diving ducks (especially Musk ducks) and even swimming tortoises can be mistaken for platypus momentarily, but are usually readily distinguished upon longer observation.
Vocabulary
Carrion
Detritus
Ecosystem
Environment
Food chain
Food web
Habitat
Macroinvertebrates
Photosynthesis
Trophic level
Energy pyramid
Top predator
Producers
Primary consumers
Herbivores
Secondary consumers
Carnivores
Tertiary consumers
Lesson plan
Fun for younger students – try making a platypus from a paper bag. See instructions below.
1. In a suitable place in the school yard (a native garden for example) or nearby bushland, review with students the basic components of an energy pyramid, starting with the sun, working through plants (producers), herbivores (primary consumers), small carnivores (secondary consumers), up to larger carnivores (tertiary consumers).
2. Hand out counters and instruct the students to take two minutes to record every individual plant that they see in a defined area (which has been divided up for groups to focus on one part each).
3. Record the total number of plants all the students recorded with tally marks (if possible) on the lower portion of a butcher’s paper or board (see example above).
4. Clear the counters and give students two minutes again to record herbivores and signs of herbivores. (Chewed leaves and scats). Remind students that most herbivores are very small, grasshoppers and most beetles are definitely on the list!
5. Record these counts with tally marks above the producers.
6. Repeat looking for secondary consumers and their signs (small carnivores, such as spiders, dragonflies and mosquitoes).
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