|The general ecology of the winter moth, Operophtera brumata.|
|How can we tell whether insect herbivory is genuinely detrimental to trees?|
|Discuss the possible roles of "maternal dominance" and "haplodiploidy" in the evolution of social behaviour in Hymenoptera.|
|Some insects are sedentary and some are habitually "migratory". Discuss the relative advantages and disadvantages of these two types of behaviour.|
|How is the excretory system of insects involved in the control of water balance?|
|Discuss the role of true flies (Diptera) in the health of humans and their domestic animals.|
|Outline the wide variety of mechanisms of pollen and seed dispersal which can be found in plants of the Mediterranean ecosystem. Discuss possible explanations for this diversity.|
|Outline pollination mechanisms|
|Insect hormones in moulting and reproduction|
|Contrast the lifecycles of hemimetabolous and homometabolous insects. Discuss why there are so many more holometabolous species.|
|Why is the cultivation of crops inevitably associated with pests?|
|Describe the role of the insect gut and its symbionts in overcoming the biochemical barriers to herbivory.|
|The planning of buffer zones around tropical forest reserves.|
|The ecology of islands|
|To what do you attribute the success of the angiosperms?|
|Symbiosis in plants|
|Methymercury pollution in Minamata Bay and the Agano River in Japan|
|Discuss the origin and evolution of sclerophyllous vegetation, with emphasis on plants of the Mediterranean.|
|An account of the biogeochemical cycling of phosperous and carbon|
|The pollination of flowers by animals|
|Discuss the factors predisposing British forests to pest outbreaks.|
|The role of field margins in conservation and as habitats for species useful to the farmer|
|RECONCILING THE NEEDS OF NATURE CONSERVATION AND THE BENEFICIAL USE OF A NATURE RESERVE BY PEOPLE|
|THE IMPACT OF TSETSE FLY CONTROL PROGRAMMES|
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.
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.
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.
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.
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.
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.