Effectiveness of Mechanisms for Seed Dispersal

The dispersal of propagules is successful if they reach a site which allows germination and growth of a new plant. It is also important for survival of a species to exploit suitable growing sites further from the mother plant and to establish there, thus increasing the area of distribution, and the population's chances of survival. To a large degree this depends on the available mechanisms for dispersal and their effectiveness regarding a targeted (suitable site for growth) and broad (gain space) transport of propagules. However, such "targets" are not achieved by any of the forms of dispersal and therefore different vectors have proved particularly useful in certain regions. With the "choice" of vector, different strategies are developed during evolution in which safe sites and territorial expansion are balanced.

Howe and Smallwood (1982) studied seed dispersal in temperate forests in North America, as well as in neo- and palaeotropic forests. The data show that most trees are morphologically adapted to zoochory in the humid tropics as well as in the tropics with rainy and dry seasons. In temperate forests this also applies to

0 5 10 50 100 200

Years after cessation of use

Fig. 4.2.2. Relative frequency of four dispersal mechanisms in phanerogamic flora of fallow fields close to Montpellier (France) at different times after their abandonment. (After Lepart and Escarre 1983)

0 5 10 50 100 200

Years after cessation of use

Fig. 4.2.2. Relative frequency of four dispersal mechanisms in phanerogamic flora of fallow fields close to Montpellier (France) at different times after their abandonment. (After Lepart and Escarre 1983)

shrubs, but trees are predominantly anemochor-ic. This change in the form of dispersal is explained by regular winds occurring in temperate latitudes, but many attempts to explain linkage of certain species with certain vectors have so far not been successful. In dry areas, where obstacles hardly limit distribution by wind, ane-mochory predominates, but there are no recognisable trends that would indicate that autochor-ic forms are linked to particular biomes. In these forms, in addition to "primary" dispersal, "secondary" dispersal by a particular vector becomes important (Chambers 1994). Propagules reaching the ground by gravity (barochoric) or excreted by animals (endochoric) may drift in the wind or water, or be taken up by other animals. Engel (2000) reports this for dung beetles which take seeds from elephants' dung and transport them over short distances before burying them and thus acting as a secondary vector; he called this coprochory (Fig. 4.2.1 C). The importance of "secondary" distribution is often underestimated. The example of dung beetles shows that the most favourable micro-growth sites are often only found in the second transport phase. The spatial and temporal component is also important. A major vector in one year need not be so important in another when conditions have changed. Lepart and Escarre (1983; Fig. 4.2.2) showed how the proportion of the four mechanisms for seed dispersal changes in a fallow field. During the first years, anemochoric species establish predominantly via long-dis tance transport, but barochoric and endozoo-choric species become more important after the first two decades and finally, after a century, replace anemochoric species as the most important species.

Plants species often do not use one form of seed dispersal only. Different forms may supplement each other to better safeguard dispersal. Particularly under unfavourable external conditions, e.g. a short vegetation period in high mountains or long dry periods, reproductive dispersal is combined with vegetative propagation. Some plant species form different forms of propagules (heterospory) increasing their chances of dispersal.

Several hypotheses and models have been developed to explain the link between certain plant species and their chosen vector. The low investment model, for example, is based on the view that plants that produce many small seeds are linked to vectors for long-distance dispersal (anemochory, epichory). Thus they are able to disperse over large areas in a very short time; however, the fate of seeds distributed in such a way is fairly uncertain. According to the high investment model, some plants only form very few seeds but these are large and well equipped with energy and nutrients and are thus very attractive to birds and bats. They are only distributed over short distances, but usually find relatively safe conditions for germination. In zooch-ory both strategies are used, epichory (with low investment) and endochory (with high investment). The directed dispersal hypothesis of Howe and Smallwood (1982), of targeted, relatively safe dispersal, also corresponds to the high investment model. This contrasts with the colonisation hypothesis where opportunists exploit opportunities for rapid dispersal over large areas. The premise for the escape hypothesis is that the chances for establishment and germination depend on low density and therefore the proximity of the mother plant must be avoided.

There are very few empirical results confirming particular hypotheses. It is certain that many plants do not use only one vector, but possess morphological and other adaptations to exploit several vectors. Such polychory improves the chances of dispersal considerably.

The efficiency of seed dispersal has qualitative and quantitative aspects. The quality of a seed may decide whether or not a bird accepts it, how the seed is treated by the bird so that it is transported, and if what remains is able to ger-

| Table 4.2.3. Factors affecting efficiency of dispersal. (After Schupp 1993)

I. Quantity of seed dispersal

A. Number of visitors

1. Density of dispersal agent

2. Type of nutrition

3. Reliability of the visit

B. Number of seeds dispersed per visit

1. Number of seeds touched per visit

2. Probability of dispersal of a seed touched

II. Quality of seed dispersal

A. Quality of treatment

1. Seeds are transported intact or broken

2. Change in germination rate

B. Quality of seed deposition

1. Transport type a. Targeted choice of habitat b. Targeted transport

2. Deposition type a. Proportion of deposited seeds b. Mixing of different seeds minate. Of quantitative importance are the number of seeds and the number of visits of the distributor to the plant. Schupp (1993) tried to develop a hierarchical classification of the most important components for efficient seed dispersal (Table 4.2.3). The density of dispersal, as well as the type of seed treatment (intact or damaged), and patterns of dispersal (targeted or ran

In their mobile phase, plants move themselves (self-dispersal or autochory) by means of propagules (diaspores, e.g. spores, seeds, fruits, corms and runners); the most important form of autochory is barochory with the propagules falling under gravity and therefore not moving very far laterally. Alternatively, propagules are moved by various vectors (allochory), the most important of which are wind, water and animals and - particularly more recently - man. The propagules reach a location with favourable conditions allowing survival and establishment (safe site), and leading to further growth: this depends on the environment and specific requirements of the species and the physical parameters that hinder or aid dispersal.

Various hypotheses describe how effective the dispersal mechanisms are. The directed dispersal hypothesis - considers a mechanism where the propagules are specifically targeted, dom dispersal) are particularly important (see also Box 4.2.1).

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