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Bill Tulloch Memorial Lecture - 1998

Plants, Animals, Sex or.... Don't Kill the Messengers

Betsy R. Jackes

James Cook University and CRC-TREM

Forty-three years ago Canadian foresters who were using pesticides close to alfalfa crops, lost a court case because it was found that the pollinators were being killed and hence seeds were no longer being produced.

Recently a number of studies have shown that the level of seed set has been decreasing both in plants growing in the wild and in the garden. However the absence of many native birds and insects have either not been noticed or it has been assumed that they weren't there. Yet most of us in the Society for Growing Australian Plants assume that we are doing our bit by saving vulnerable species. I would suggest that in many cases we are only buying time until more research has been done, and more knowledge of the plant's reproductive biology and the life history of the associated pollination vectors is known.

So, in this article I would like to talk about Australian plants. How to recognise probable reproductive techniques, as well as who are the flowers designed to attract.

The dilemma facing plants wishing to reproduce, is how do you find a mate when rooted to the spot? Plants have used a lot of lateral thinking to solve this problem. Some of the breeding systems encountered are:

• Bisexual, where both males and females are combined in the one flower and no messenger is required to effect fertilisation and hence seed set (autogamy - inbreeding).
• Bisexual, where both are present but for one reason or another are unable to make contact and need a messenger (xenogamy - outbreeding). An example would be where the pollen is shed before the stigma is receptive, or it may not be able to germinate on that particular stigma or if it does it may not be able to effect fertilization.
• Unisexual, where males and females are separated and occur in different flowers. If both sexes are on the one flower then this is monoecy. If the sexes are on different plants then this is dioecy. In both these examples a messenger is absolutely necessary.

Within these basic systems many variations are encountered, as well it is not uncommon to find that outbreeding is the normal pattern at the beginning of the season and that selfing either within the same flower (autogamy) or between flowers with an identical genetic makeup (geitonogamy) is the pattern at the end of the flowering season. Only autogamous flowers do not require pollinators. Although pollination may be effected by wind, water and gravity the most common method is via animal vectors, although many plants have fall back positions. Thus breeding systems range from those that require no mediators, to those that don't care but prefer to outcross, to those that have no choice if they are to set seed. It is the latter that we are likely to lose from our flora, if we do not understand the life history and behaviour of their pollinators and protect them.

Insect pollinators on Buckinghamia celsissima (top) and Senecio magnificus (bottom). Select the thumbnail image or plant name for a higher resolution image (38k and 34k).

Ever since the Cretaceous, flowers have been co-evolving with their pollinators, some of whom are regarded as generalists, to others which are highly specialised. Flowers pollinated by wind tend to be relatively unspecialised and unspectacular, such as the grasses. Flowers requiring animal vectors show enormous variation, because the non-sexual parts of the flower are their method of advertising - I'm here! The size and nature of the inflorescence, floral structure, shape, colour and floral rewards all influence the pollinator choice. The life-history of potential pollinators must also be considered, what type of food is required and for what period, thus they need a variety of plant species flowering over a period of time if they are get to sufficient energy to complete their life-cycle. Don't forget they also have to find somewhere to live.

Native bees and possums like such sites as hollow branches and holes in tree trunks, the very things that are lost when paddocks are cleared and/or areas are regularly burnt.

Native bee nests in the Atherton-Mareeba area of North Queensland, have probably decreased by more than 50% in the past 20 years (T. Bartareau, pers. comm.).

In England, a survey by Corbet et al., (1995) for "Flora for Fauna", showed that the "improved flowers" and hybrids commonly grown in modern gardens offer little or no rewards, so potential pollinators are not attracted. The loss of hedgerows and use of agricultural chemicals has also decreased the potential breeding sites and resulted in a loss of individuals. The higher the number of specialised pollinators, the greater the effect of pesticides and feral animals on the native populations and hence obligate outcrossing species will soon be heading towards extinction.

A study in Argentina (Aizen and Feinsinger, 1994) found that a decline in the number of solitary bees coincided with increasing habitat fragmentation. This decline reduced the reproductive success of some 16 different tree species. It was also found that 5 acres or less could not sustain sufficient numbers of wild pollinators to maintain full reproductive fitness.

What happens when the number of host plants is too low to maintain viable populations of pollinators? The vector populations will gradually die because of lack of food and the dependent plants will eventually become senescent. Unless they are partially inbreeding there will be no seed set and the plant species or population will die out. Is this not often the case in a garden? Then more fresh seed is collected from the wild thus depleting the reserves.

Feral honeybees (Apis mellifera) will also contribute to the loss of native bees by competing for resources. Gross and Mackay (1998) have shown that they also reduce the overall seed set and fitness of the plants normally pollinated by native fauna.

Much research is needed to understand the reproductive biology of our flora and the behaviour, life-history and home range of our pollinators. What can the non-professional do to minimise this impact of loss of habitat? Fortunately it is not hard to make an educated guess as to the probable breeding system and possible pollination vectors. All it takes is a little knowledge, patience and good observation. So let's start.

1. The Breeding System

Most flowering plants are sedentary, and only the pollen and fruits/seeds are able to move, thus they have resorted to a great variety of methods to ensure pollination, fertilization and fruit set occurs. Outbreeding is preferred over selfing which leads to inbreeding and in the long term, loss of variability and an inability to cope with change.

The following key will help you to understand the basic patterns.

1. Male and female function on different plants ------ obligate outcrossing (dioecy)
1. Male(M) and female(F) function on same plant

2. M and F in different flowers ------ obligate outcrossing, (monoecy)
2. M and F in same flower

3. One sex matures before the other, but could be a chance of overlap ------ outcrossing normal (xenogamy)
3. Both sexes mature at the same time

4. If stigma below the anthers, then selfing (autogamy) highly likely
4. If stigma above the anthers, then may self but probably facultative, xenogamy, i.e. it has a choice but outcrossing is the most likely.

To prove if selfing is occurring, then carefully enclose the flower in a bag preferably before the flower opens so as to minimise any chance of foreign pollen entering. Then if fruit and seeds mature, the plant does not require an external pollination vector. If the anthers are removed before the pollen is released and seeds form, then these have probably arisen from unfertilised cells and hence the offspring have the characteristics of the parent. Such adventitious embryos are found in a number of plants such as some species of Syzygium.

If a flower requires pollinators then it usually advertises in some way. The majority of plants using animal vectors also offer rewards but some are deceitful.

2. Advertisement

What is the function of a flower? The sepals and petals not only protect the sexual organs but also advertise their presence. They can also say to a vector, don't bother to visit, you are too late or that there is nothing here for you.

Hence, flowers come in many different colours, shapes and sizes. They may be arranged in a great variety of inflorescences, all designed to attract pollinators so that ‘sex’ may result. Some flowers are quite promiscuous and don't really mind who their pollinators are as long as pollination occurs, these are regarded as generalists. Others, such as many orchids and figs are highly specialised and are restricted to a single pollinator such as the fig wasp.

The following diagram modified from Faegri and van der Pijl (1979), shows some of the common, flower shapes and colours that are preferred by common pollinators. Please note there are many exceptions, such as parrots don't reject gum blossoms just because they are white or cream.

Some relationships between flower shape, pollination vector and flower colour modified from Faegri and van der Pijl (1979)

3. What are the Rewards?

A. Pollen

A highly nutritious food often used by bees, flies, beetles and thrips. A convenient source of food.

B. Nectar

This is the prime reward offered to many pollinators. The amount, concentration and timing of production e.g. continuous or diurnal, will affect the type of pollinator as does the floral morphology. A butterfly can reach the bottom of a tubular flower but a bee cannot unless it acts as a “robber’ and bites a hole in the side. Birds like watery nectar as do butterflies but bees like it thicker. Thus in the early morning birds may be the floral visitors and later in the day as the water evaporates and the nectar becomes more viscous then bees will be the main visitors. Nectar composition may also affect the nature of the floral visitors. Sucrose-rich nectar tends to be associated with long-tongued vectors such as bees, birds and butterflies, but bats prefer hexose-rich nectar (Proctor et al., 1996).

	The Myrtle and Protea families include many species which supply nectar to pollinators, particularly birds and small mammals. Examples are (top) Calothamnus quadrifidus, Myrtaceae, and (bottom) Dryandra praemorsa, Proteaceae. Select the thumbnail image or plant name for a higher resolution image (47k and 39k).

C. Floral Tissue

Chiefly eaten by beetles, bees and bats. In Idiospermum, the petals are chewed, by beetles, in Senna the staminodes are eaten by bees. Figs provide gall flowers for larval growth.

D. Oils, Resins, Gums and Perfumes

The first three are usually collected by bees, in some cases to attract females and in others for nest construction. Perfumes are more commonly used as an attractant than as a reward, e.g most ‘bat’ flowers are heavily scented.

Thus a combination of flower shape, colour and the nature of the rewards forms what is called a pollination syndrome. Although there are many types of syndromes and many examples, I would now like to present some examples of these syndromes. Please remember that these are generalisations and many exceptions do exist.

Some plants will have a range of pollinators some may be more effective than others. Some species of Banksia may be visited by birds, possums and bats. To be an effective pollinator then the portion of the body where the pollen is deposited must come in contact with the stigma at some stage so the pollen can be transferred.

In Australian rainforests assorted insects and particularly beetles are common pollinators. Large beetles tend to be clumsy so need large robust flowers. Small beetles are relatively agile, in which case the flowers may be quite small and grouped together. Beetles are attacted by a strong often fruity scent and sometimes by colour. In Idiospermum australiense which Stuart Warboys (1997), has been studying the tiny beetles are attracted by both scent and colour. They squeeze though the petal gaps into the centre, where protected from predators they go about reproducing and eating, meanwhile the flower has a chance of being serviced simultaneously.

In conclusion, I'd like to stress again the importance of saving the messengers if we are to conserve areas, look after our crops, and reduce the chance of species becoming extinct. Even the honeybee is in trouble in America and to a lesser extent in Europe from varroa mites and this is threatening the yields of some of our major crops. Where there is a specialised relationship then if lose one may lose the other. Loss of hummingbirds in South America affects many species. Loss of nectar feeding bats could lead to a cascade of events, no pollination then no fruits, no seeds, no seedlings etc. Then what about all the other fauna and flora that depend on them?

So learn to recognise the pollinators before it is too late and help save not just our unique native flowers but also our fauna and our crops. Are you providing Fauna for your Flora or are both slowly suffering? Are their genes screaming “Pass me On”? Think about it.

Selected References

This article was presented to the Society for Growing Australian Plants' Queensland Region Conference, Townsville, 28 June-5 July, 1998.

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Australian Plants online - September 1999
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