Case Study | Planet Earth | Episode 4: Caves

'Closed' Ecosystems

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Research ActivitiesDraw a food web of Borneo’s Deer Cave, based on the following information.

Bats and swiftlets – eat insects outside, excrete inside

Cockroaches – eat bat dung

Centipedes and whip scorpions – eat cockroaches

Cave cricket – eats bird eggs and nestlings

Cave racer snake – eats bats and swiftlets

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Research ActivitiesGather examples of the following types – halophiles, thermophiles, xerophytes, acidophiles.

Prepare a report, poster or PowerPoint presentation that describes the different types, their environments and lifestyles.

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'Closed' Ecosystems

The caves featured in this episode

are unique in many ways. They con-

tain entire and isolated ecosystems

in which many odd species have

adapted to suit the habitat niches

available. Some caves contain spe-

cies that are only found in that one

place and the ecosystem must have

been developing for millions of years

in that one isolated place for these

endemic species to develop.

Most ecosystems do not have

boundaries that are easy to define.

Animals and plants stray over any

border that we draw on a map. They

are Open Ecosystems. But in a few

cases, it is difficult for organisms to

enter or leave – an isolated pond or

oasis, for instance, or a cave. In one,

species are restricted by their need of

water. In the other, by their ability to

live in the dark. These odd examples

are almost Closed Ecosystems – no

system can be completely closed but

these come close.

In a normal ecosystem, the sun

powers the growth of green material

which is then eaten by herbivores

and so on. Photosynthesis is the driv-

ing force. However, in the cave the

sun does not penetrate and nothing

grows, so where does the energy

come from to start up the system?

The main source is the droppings

of the bats and birds. They leave

the cave to eat outside and excrete

inside the cave, providing food en-

ergy for small invertebrate omnivores

like cockroaches, the grazers of the

cave. They are detritivores (detritus

= waste). Larger invertebrates like

centipedes then eat the cockroaches

while micro-organisms break down

the cockroach wastes. It is estimated

that the bats eat up to 10,000kg of

insects every night, bringing that

energy into the cave. In more sealed

caves, water flowing through may

bring food sources like bacteria.

Troglobytes and Extremophiles

Troglobyte – animals that live in

and never leave the cave. They are

often specially adapted for life in the

dark and could not survive in a sunlit

habitat. In the Borneo area these

include blind, white crabs, millipedes

and pseudoscorpions.

Troglophile – animals that like caves

but can also live outside if neces-

sary. Here these include the cave

racer snake and the cockroaches,

drawn into the cave by the huge food

resource.

Trogloxene – animals that live in the

cave but leave it to feed such as the

bats and swiftlets.

The true troglobytes who never leave

the dark are often strangely adapted.

Animals like fish, salamanders and

crabs will often lose the pigment in

their skin and eyes as there is no

value to being coloured and their eyes

serve no purpose. Eventually, many

have lost their eyes altogether and

there are many example of blind cave

creatures. This means that they must

find their food by other means. Crabs

can try and eat anything they find and

some fish like the angel fish are filter

feeders, filtering nutrients out of the

water that flows over them. Salaman-

ders in the outside world have poor

eyesight and are sensitive to pres-

sure so it is no surprise to find cave

salamanders who have lost their eyes

and developed extreme sensitivity to

any movement around them through

sensory nodes in their skin.

Extremophiles – Extremophilic or-

ganisms are usually eukaryotes and

simple animals which have adapted

to live in extreme environments.

These include the snottite bacteria in

the film which get their food from the

toxic chemicals in the air of the cave

and exude strong sulphuric acid. It

seems impossible to believe that any

organism could tolerate such condi-

tions but little cave fish live in highly

acidic waters in huge numbers. In

Antarctica, fish live in sea water so

cold that they should freeze but they

have a substance in their blood,

an ‘anti-freeze’, which keeps them

unfrozen.

Check some of the extremophiles

here: http://www.astrobiology.com/

extreme.html

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