PHOSPHORUS
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
again.
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.
CARBON
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.
3) Atmospheric
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.
Continental and
oceanic.
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:
1 The
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
3
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
Farne islands.
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
scientific theory.
Animals go to flowers for food. Some flowers offer rewards in the
form of nectar and pollen to attract animals, who in collecting the reward will
pollinate the flower. To make sure the correct shape or type of insect
pollinates them flowers and insects have coevolved to become increasingly
specialized. Some insects cheat by taking the reward without pollinating. Some
flowers cheat by pretending to be what they are not in order to get pollinated.
In the beginning.
Angiosperms evolved around 125 million years
ago. At that time there were already beetles and flies living off gymnosperms.
The new plants were just another source of food to them. They didn't have to
change in any way. The primitive flower looked rather like a magnolia, with
spirally arranged parts, a superior ovary, and separate petals. The mess and
soil method pollinated these flowers, i.e. the beetles and flies came along and
chewed off bits, and while they chewed pollen would stick to their bodies and
rub off on the next plant. It was a hit and miss affair. The pollen deposited
might be incompatible, the ovary might become part of the meal; but it needed
little investment from the plants.
Rewards.
Flowers evolved
nectaries. Nectar is high in carbohydrates and nitrogen. It also provides some
essential amino acids. Nectar was probably more nutritious than the other parts
of the plant, so may also have limited damage.
Specialization.
Around 60 million years ago there were many extinctions, and as usual
following this there was an explosion of new species. Flowering plants and
insect species numbers exploded. Bees, who see a different wavelength to us,
birds and other insects played an important part, as did butterflies and moths.
These insects coevolved with plants, their body shapes changed so that each
became increasingly more specialised. Flowers reduced pollen production, fused
parts, emitted special odours, had UV patterns, and even opened their flowers
at night. The ultimate form of this type of specialisation comes when one plant
depends on one species of insect to pollinate it, as is the case with the yucca
and yucca moth. The disadvantage to this is that it is very environmentally
sensitive.
Cheating by pollinators.
This happens when the reward
is taken but the plant is not pollinated. Each species of fig is dependent on
one or a few species of wasp to pollinate it. The relationship is very complex.
But there is a cuckoo-type wasp, which uses the fig to lay its eggs in without
pollinating it. Some flowers have long tubular corollas and only pollinators
with the correct length of mouthparts can reach the nectar. Some insects bypass
this obstacle by cutting a hole near the nectary and robbing it, again without
pollinating the flower.
Cheating by flowers.
This involves the
flower deceiving the pollinator into pollinating it without giving it a reward.
The hammer orchid blooms a few days before a certain species of female wasp
emerges as an adult. Part of the orchid looks just like a female wasp, and
consequently the hapless males try to mate with it. During the action of mating
pollen sticks to the wasp's back. As no females have emerged yet, when it goes
off to find another mate this will be another hammer orchid. Some plants give
off the smell of rotting meat. This entices flies to come and lay their eggs.
In the process pollen attaches itself to their bodies and they may go off to
pollinate another rotting plant, but the poor fly larvae will starve when they
hatch as there is no rotting meat for them to eat.
Non-specialization.
This appears to be a more recent trend, and
involves the formation of groups of showy flowers with nectar that is easy to
get at. For pollinators it must be the equivalent of going to a supermarket -
everything is on display and cheap (not a lot of energy expenditure) - it is
one-stop shopping. So from magnolias to daisies we seem to have come full
circle, except the plant is more organized and protected.
No-one really knows why the
angiosperms suddenly became so successful. They appeared around 135MYA, and
within a very short space of time became dominant in all habitats except tundra
and coniferous forests. Their origin is unknown as no common ancestor has been
found. It has even been proposed that they may have a polyphyletic origin. If
this is so then there will be enormous taxonomic problems as the division of
angiosperms will have to be broken up.
Gnetales are their closest living
relatives, and Benettitales are the closest fossil group to both Angiosperms
and Gnetales, so perhaps there is a common link to all of them in the Jurassic
or Triassic that is just waiting to be found. All of this has become known as
Darwin's "abominable mystery". What we do know is that the early Cretaceous
(the time when Angiosperms spread and became dominant) was a time of great and
rapid change in the climate and distribution of land.
Rapid change favours
species that can evolve and speciate quickly to take advantage of changing
situations. This is something that we do know Angiosperms are capable of. The
genetic variability and phenotypic plasticity of common weeds and the ability
of their seeds to lie dormant for years waiting for the right conditions to
germinate and quickly reproduce, can be seen in any garden.
The vast
continent of Pangaea broke up into Laurasia in the north and Gondwana in the
south, this changed the weather patterns from monsoonal to more arid and
warmer. Sea levels rose causing further isolation. Species that depended on
water for fertilization would have had extreme problems at a time like this, as
would those that could not cope with the desiccation. The fluctuation in
weather patterns and movement of land masses through different temperature
zones is still continuing- though at a reduced rate now that India has collided
with Asia.
Angios do not depend on water for pollination, some are animal,
some wind pollinated, and some can be pollinated by either method. The animal
pollinated seemed to come to prominence first. The pollinators were already in
existence - beetles, flies, small invertebrates. This method of pollination
would lead to rapid genetic variability, and with the changing climate etc.
this would probably lead to great diversification. With wave after wave of
success. Some species, e.g. magnolia-like, were once found right across
Laurasia, now however, they are found in S.E.Asia and the middle of the
American continent - what was once the 2 opposite edges of Laurasia. The
climate in these two areas has changed much less than that of the rest of
Laurasia, so presumably as conditions changed other species, e.g. Rhododendron
pushed the Magnolia out, then as things grew colder Rhododendron in its turn
was pushed out by other, better adapted species.
As the Angiosperms
diversified so did the pollinators, especially the insects. The newer insects
tended to become more specialised as did the plants; they became linked, it is
not known whether this coevolution is insect or plant driven. So from a general
open type of flower with a superior ovary pollinated by the "mess and soil"
method, we arrive at species specific pollination, and a great variety of shape
and form (fused petals, inferior ovary, special odours, UV patterns, honey
guides, nectaries etc.).
The grasses (wind pollinated) evolved later, on
the large dry praries of Asia and America. The climate would have been more
stable in those areas, and the topography was favourable for species that can
spread vegetatively being relatively flat.
So the success of the
Angioserms is probably due to their great ability to diversify and being around
at a time of great change, when that ability became their great advantage.