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.
In the Mediterranean ecosystem a principle coordinate analysis of reproductive characteristics showed that there were three major trends in shrubs.
1) Small flowered, fleshy fruited with one or few seeds, pollinated by small insects.
2) Large flowered, dry fruited, many seeded, hermaphrodites pollinated by large, efficient insects carrying large loads.
3) Small flowered, rewardless, mono- or dioecious, pollinated by the wind.
Dispersal is by bird and mammal (mainly the fleshy fruits), ants, wind and perhaps even reptile.
In Mediterranean ecosystems water and nutrient stress are the main environmental influences on shrubs. The different breeding systems may be different strategies for maximizing reproductive output.
1) The entomophilous dioecious species. In this type of plant it is common to find lower nectar production in the female plants as nectar production has been shown to reduce seed production. So there is a division of labour with the female devoting resources to fruit and seed formation, and the male to pollinator reward. These species are predominantly relics from tropical Tertiary forest environments, and are pollinated mainly by nectar-seeking insects.
2) The hermaphrodite entomophilous group. This is the largest group, they have small-seeded fruits some of which need fire to break dormancy. Fire is a common occurrence in the Mediterranean type ecosystems, so this group may have evolved under the existing Mediterranean conditions. Many do not produce nectar, but copious pollen, in one, Cerinthe major, many individuals produce little or no nectar while a few produce a lot. Some of the orchids are completely rewardless with neither nectar or insect-available pollen. This has been termed "pollination on the cheap".
So in all cases it would seem that nutrient and water stress are influencing the reproductive strategies of these plants. The three main trends show the differing ways of allocating resources towards reproduction and consequently this resource allocation affects the method of pollination and dispersal.
3) The wind pollinated group have been found to produce pollen of lower calorific value than animal pollinated species. This may be because wind pollination is a less efficient method, and so requires the production of greater quantities of pollen. It was also found that in Phillyrea latifolia (hermaphrodite and male plants) there was greater pollen production in male plants than in hermaphrodites, which may indicate an evolutionary trend towards dioecy enabling the hermaphrodites to concentrate on the female function.
Discuss the origin and evolution of sclerophyllous vegetation, with emphasis on plants of the Mediterranean.
Mediterranean climate. The history of a separate Mediterranean climate really started 6 million years ago. The collision of India into Asia caused the rise of Tibet, and Africa's movement north caused mountain building in Spain and North Africa. The Tethys sea dried up leaving a huge salt pan. Presumably most vegetation in the area that was subject to wind blown salt dies, also the temperature was very high and the area arid. Then the Mediterranean opened and a fairly lush vegetation grew with invasions from the surrounding areas. The onset of galciation brought climatic fluctuations which gradually settled to resemble more or less the climate we see today. This caused a gradual disappearance od plant unable to withstand periods of summer drought - these would either die out or retreat north - the south was even drier. So there was a gradual change favouring plants that could withstand quite long droughts on a soil that had never been very nutrient rich and may still have contained salt which could rise to the surface by capillary action. Added to this were the frequent volcanic eruptions causing local extinctions, but more widespread depositions of pumice etc. During this time such plants as remained would have to evolve mechanisms to cope with the increasingly harsh and unpredictable climate.
Sclerophyllous leaves are found on plants from dry, hot and nutrient poor habitats. They are usually relatively small, with small thick-walled cells, and often both faces of the leaf are the same. They are usually covered by a thick cuticle and epidermis. Some exude wax or resin, and the stomata are often sunk into the leaf and may be protected by hairs. The leaf may also roll up to protect the stomata from excessive evaporation; this is common in plants from salty areas. Most of these modification appear to have arisen to conserve water, but they are also useful in nutrient poor habitats. For instance smaller leaves require less nutrients as do leaves that last for more than one year.
Sclerophyllous vegetation. This type of vegetation takes its name from the leaf which is the most visibly adapted part, but the vegetation share some other common adaptations to drought and poor soil. Many of the species have protected roots to maximise nutrient uptake in areas of local enrichment. Many are legumes or other species that have symbiotic relationships with nitrogen fixing bacteria. Many make use of mycorrhizal associations again enhancing nutrient as well as water uptake. A few are even carnivorous, so extending their sources of nutrients. And some are parasitic on other species, or their own species, or even their own roots. Sclerophyllous vegetation usually have small flowers which may be wind pollinated or pollinated by small insects, but they do not invest heavily in nectar and other pollination rewards. However there is fairly heavy investment in producing a single, or few seeded fleshy fruit. Most have the ability to spread vegetatively, which is useful as most are dioecious. So as far as reproduction goes there is a distinct pattern of investment in pollen as opposed to nectar, and reproduction without sex. This is taken to extremes by some plants, e.g. Osyris alba, where the female flowers are rewardless but mimic the male so pollination can occur. Also the female flowers occur less often than the male.
Origins. It is postulated that the sclerophyllous vegetation of the Mediterranean regions has evolved from the lusher tropical vegetation that may have existed before the onset of the climatic changes. And the various adaptations we see today came about in order to cope with the increasingly harsh conditions, and those that couldn't cope simply went extinct.
The future. For a plant so limited in nutrients and water the investment in the production of fleshy fruits seems excessive. But these fruits are dispersed by birds, and birds like fleshy fruits. This is another common link to the lush tropical flora that is believed to have existed previously. Recently it has been noticed that ants play a greater part in dispersal of seed than had previously supposed. If a plant relies on ants entirely it will have no need for fleshy fruits, as the ants are interested in the lipid-rich elaiosome around or in a groove of the seed. So there may be a general trend in the sclerophyllous Mediterranean vegetation towards dry fruit dispersed by ants. This is common in other Mediterranean climates, but there may be dangers ahead as in some places the Argentine ant, Iridomyrmex humilis, has displaced the native ants. The Argentine ant is not interested in elaiosomes, so will not disperse seed. These plants may also be able to reproduce vegetatively, but after fire, seed dispersal is important. And unless the seeds are cached by harvester ants they are unprotected. So regeneration after fire is severely limited. This may lead to soil erosion and desertification.