An account of the biogeochemical
cycling of phosphorus and carbon
The time for
one atom of phosphorus weathered from rock to go round the cycle from soil,
water, ocean, sediment, then rock uplifted by tectonic activity takes 100s of
millions of years.
An atom is weathered from rock (the weathering rate can
be increased by erosion due to deforestation etc.) and goes into the soil. This
can start a mini-cycle between plant-herbivore-plant, with detrivores and
carnivores sometimes playing a part too. Eventually the atom is washed into a
stream, river or lake, and heads towards the ocean. But before reaching the
ocean the atom will probably pass through many mini-cycles (nutrient
spiralling). As when in the soil the atom is taken up by plants, which are
eaten by animals, and the animal waste is again taken up by plants. It can take
hundreds, or only a few years for the atom to reach the ocean. Once in the
ocean another mini-cycle starts, this is between plankton and herbivores, the
atom cycles between the top sediment layer and the ocean surface, and can stay
in this cycle for a million years before being incorporated into sediments
which will form sedimentary rock. After a few hundreds of millions of years
this rock will be uplifted by tectonic activity (very little ocean crust is
over 2 x 109 years old) and the process of weathering can start
Man has been short-circuiting this cycle by fishing, removing atoms
from the ocean mini-cycle and returning them to the soil as fertilizer then to
the water as sewage. And there has been increased erosion caused by a
combination of deforestation and bad farming methods, which speeds up the soil
to water mini-cycle. The effects of this short-circuiting is to increase the
phosphorus in lakes and streams etc. which may cause algal blooms.
There are four main sources/sinks for carbon: 1) Terrestrial, the source is
CO2 uptake from the atmosphere by plants. This starts a mini-cycle
between plants, herbivores, predators, microbes, etc. Most of the carbon is
held in the plants and is released back to the atmosphere as respiration (from
plants as well as other organisms) or when the biomass is burnt. Man will not
disturb this mini-cycle as long as the amount of biomass remains the same. So
you can burn as many trees as you like as long as this is compensated by the
uptake in carbon from new trees. The other output from terrestrial communities
is when dead organic matter is buried in swamps or bogs and after millions of
years metamorphoses into fossil fuels.
2) Aquatic, is similar to the
terrestrial community in that it has a mini-cycle of plants, herbivores and
predators, however the biomass content is much smaller. There is a cycle of
diffusion between the ocean and the atmosphere. And it is believed that the
deep ocean acts as a sink for excess CO2, there is also a lot of
carbon locked up in limestone. There is a constant rain of CaCO3 from the surface layers, this comes from shellfish, foraminifera and algae. Emiliania huxleyii (a planktonic alga) converts CO2 to scales
of CaCO3. If it can be grown on a massive scale it might help turn
some of the increased amounts of CO2 in the atmosphere to chalk. As
in the terrestrial community there is an output of buried dead organic matter
to fossil fuels, though usually only in shallower waters.
CO2 content has increased around 25% over the last 250 years or so.
This may lead to a general warming up of the oceans and a change in weather
patterns, which may proceed at a rate much faster than man can cope with. This
has happened because man has short-circuited the system by removing and burning
vast amounts of fossil fuels quicker than they are replaced from land and
water. So more CO2 is being put into the atmosphere than is being
removed. The amount is small compared to the total amount of carbon in the
system, but it may be enough to start off a chain of events that could be
catastrophic for mankind.
All biogeochemical cycles can continue without
man, and all change imperceptively over millions of years, e.g. originally
there was no ozone layer and no oxygen in the atmosphere. The difference that
man has made in the last few hundred years is his technological ability to
break into cycles and speed things up. This has brought immense benefits for
mankind, but the changes have been all one way, i.e. releasing locked up
nutrients/fuels. Only now are we able to see that this can lead to conditions
beyond our control that are adverse to our way of life. All we need to do is
find a way of locking up the excess chemicals we release in the amounts
required to keep things "stable" - in fact things are never stable - but they
seem so to us, as we are here for such a short time.
|The ecology of islands
In considering island ecology one of the most important
distinctions is the difference between continental and oceanic islands.
Continental islands will have been joined at one time to a greater land mass
and only became islands as the sea level rose. This may have happened a few
times or only once in the island's history. Java was once part of the East
Asian mainland and the UK was once joined to Europe, consequently they are
surrounded by water usually less than 200 m deep. Oceanic islands are usually
surrounded by much deeper water, and have never been connected to any mainland.
Some of the islands may be of quite recent origin and are usually formed by
volcanic action. Some are of these volcanoes are still active. The Hawaiian
Islands are a good example.
The flora and fauna of oceanic and continental
islands usually have differences. Continental islands start off with the same
flora and fauna as the mainland; they are said to be super saturated. As time
passes there are some extinctions especially among the larger mammals and
predators. So a continental island of a given area will usually support fewer
species than the equivalent area of mainland. This is because there is less
chance of recovery from natural pertubations, and movement out of and into the
area is not possible.
Oceanic islands rely entirely on the ability of
species to cross the vast expanses of ocean. Therefore although oceanic islands
may contain many species found on the mainland, it will be that subset which
has a good dispersal ability. Seeds of seaside plants that are small and wind
dispersed, or can withstand salt water, migratory birds which may bring seeds
with them, insects blown off course or onto pieces of floating vegetation are
all common colonizers. So chance plays a great part in the colonization of
oceanic islands, and they usually support fewer species for their size than the
equivalent continental island.
The species number is also related to the
distance from the mainland, with the furthest islands having the fewest
species. Because of the difficulty in reaching far islands there is a large
time lag between arrivals (averaging 350 000 years per bird species in Hawaii),
so speciation can occur. Therefore oceanic islands may be species poor in
comparison to continental islands, but they have a much greater number of
endemics. Another difference is that once a species has reached an oceanic
island it often loses its powers of dispersal. Perhaps this is because
dispersal is no longer an advantage when you are surrounded by miles of open
water. Speciation may also result in size differences with mainland varieties,
e.g. giant tortoises in the Galapagos.
Many of these ideas are brought
together in Macarthur and Wilson's Equilibrium theory, which states:
number of species will increase with island area
2 For an island of a given
size the number of species decreases with distance from the source area
There will be a continual turnover of species where colonizations will be
balanced by extinctions.
Statement 1 can be illustrated by many examples of
log species/log area graphs, e.g. Mediterranean Island Lepidoptera, species in
Caribbean islands. The slope of the graph steepens as you move from mainland to
continental to oceanic islands.
Statement 2. A few mangrove islands off
Florida were surveyed for arthropods, then bits of the island were chopped off
and the islands surveyed again, this was repeated once more. It showed that
though habitat diversity stayed the same, species number decreased with
decreasing island size.
Statement 3. Another mangrove island was surveyed
then defaunated and surveyed at regular intervals. In about a year or so the
species number had reached the pre defaunated number, but was very different in
composition. The final species number was around 25 but around 90 different
species had colonized the island; some stayed, some vanished. Similar things
(without defaunation) have happened in Californian Channel Islands and the
Immigration is high and extinction low at first because most
new arrivals will be new species. This changes as species numbers increase,
with fewer niches and competition there is a greater chance of extinction.
Extinction is always a threat on smaller islands because populations are small
and natural pertubations have a greater effect.
Some have argued that the
turnover in the Californian islands was due to man introducing species and
poisoning others. It has also been said that the model is too simplistic,
reducing everything to species numbers and extinction rates. It is obvious that
not all species are equally likely to survive, and that they all have different
attributes. However the theory has generated lots of interest and consequently
lots of research. So even if it is proved entirely wrong it will have served a
purpose in spurring on others to find out more - surely the whole idea of any