Biology 12

452 MHR • Unit 5 Population Dynamics

solar

energy transport over the land

precipitation

over the land

(95)

percolation

in soil

run-off and ground water (36)

net movement

of water vapour

by wind (36)

evaporation

from the sea

(319)

evapotranspiration

(59)

precipitation

over the sea

(283)

Figure 13.26The water or hydrologic cycle. The width of the arrows indicates the

relative amounts of water being carried in each route. The numbers in parentheses

represent estimates of global water flow in billion billion ( 1018 ) grams per year.

into oil, peat, or coal deposits, where they are

unavailable to living organisms. When we burn

these fossilized remains as fuel (or when they are

eroded), the elements move from this biotic “vault”

into the accessible abiotic bank formed by soil,

water, and atmosphere.

Figure 13.25The four nutrient reservoirs are categorized

with respect to (a) whether they involve biotic or abiotic

components of the ecosystem and (b) whether the nutrients

they contain are directly available to living things or not.

The abiotic vault in which nutrients are locked

consists of rock. When soil is converted to rock, the

chemicals contained in the rock are held very tightly

and become inaccessible to living things. However,

these chemicals can be released back into the soil,

water, or atmosphere by weathering or erosion.

Having looked at some of the features shared by

all biogeochemical cycles, it is useful to examine

a few specific cycles in more detail. Because

ecosystems are complex and can exchange materials

with each other, it is difficult to trace the route

taken by any of the cycling elements. For example,

an ecosystem with finite boundaries (such as

a pond) exchanges nutrients with many other

ecosystems. Water running off adjacent land

introduces chemicals to the pond from distant

sources, and visiting birds carry away nutrients in

the bodies of the fish or insects they have eaten.

Following the inputs and outputs of ecosystems is

challenging, but researchers have worked out the

basic routes followed by materials in several

biogeochemical cycles. One of the most familiar is

the water (hydrologic) cycle, in which oxygen and

hydrogen cycle together, as shown in Figure 13.26.

fossilization

photosynthesis

respiration,

decomposition,

excretion erosion, burningof fossil fuels

weathering,

erosion

formation of

sedimentary

rock

Biotic component

available as nutrients

living or recently

living organisms

Biotic component

unavailable as nutrients

coal, oil,

peat

Abiotic component

available as nutrients

atmosphere,

soil, water

Abiotic component

unavailable as nutrients

minerals in rocks