Getting Away from Your Parents I Seed Dispersal Dispersal in Space

Seeds are dispersed away from parents for several reasons. First, it avoids seedlings competing with their parents or siblings. Competition among siblings or between parent and offspring is more intense because they tend to have similar resource requirements and strategies to acquire these resources. Being dispersed away from the maternal plant also decreases the likelihood of mating with a sibling, which could lead to inbreeding depression.

Second, dispersal increases colonization opportunities. If all seeds fall directly around their parent plant, then the species has little chance of colonizing new habitats

Fig. 6.3. Examples of three morphs (peripheral, intermediate and central) of achenes from two populations of wingpetal (Heterosperma pinnatum). The Tula population received heavy rain while the Mirador population was frequently disturbed (Venable et al., 1995; with permission of the Botanical Society of America and the authors).

or expanding its range. Also, seedling establishment is sometimes higher in habitats away from the parent plant, e.g. where a parent plant creates too much shade for the seedling.

Finally, dispersal also reduces the chance of all seeds from an individual being eaten by seed herbivores or otherwise destroyed all at once by parasites or pathogens. This could occur if all seeds were dispersed to the same small area in high densities of closely related individuals. This would increase the intensity of attack by parasites or pathogens attracted to specific genotypes of seeds.

The cost of dispersal is that energy is allocated to dispersal structures (e.g. wings or fleshy fruits) and away from seed production itself; the benefits of seed dispersal must outweigh this cost (Howe and Smallwood, 1982; Willson, 1992; Eriksson and Kiviniemi, 1999). If a plant produces dispersal structures, then less resources are available to produce seeds. Consequently, there is a trade-off between dispersal and seed production, and plants have developed

Table 6.1. Dispersal methods (based on van der Pijl, 1982, and Willson et al., 1990).

Dispersal method Morphological features

Weed examples

Wind

Vertebrate Ant

Ballistic External None

Wings or plumes that slow rate of fall

Fleshy fruit, aril or seeds/fruit with high nutritional content Elaiosome (food body) to attract ants

Seed is ejected from fruit by specialized structure External barbs and hooks or hairs that adhere to fir and feathers No evident structures

Dandelion, Taraxacum officinalis Bull thistle, Cirsium vulgare European buckthorn Rhamnus cathartica Common barberry Berberis vulgaris Bull thistle, Cirsium vulgare Mile-a-minute weed, Polygonum perfoliatum Bur-cucumber, Sicyos angulatus Touch-me-not, Impatiens glandulifera Bluebur, Lappula echinata Common burdock, Arctium minus Lambsquarters, Chenopodium albam different dispersal mechanisms or 'syndromes' to maximize dispersal and minimize energy losses.

Dispersal syndromes

The main types of dispersal syndromes are: wind, vertebrate, ant, ballistic and external. Others have no special dispersal device (van der Pijl, 1982; Willson et al, 1990) (Table 6.1). Wind-dispersed seeds are small and light, and tend to have wings or plumes that slow their rate of fall and this increases their dispersal distance. Seeds can be dispersed by animals, either on the animal's exterior or else internally. Seeds dispersed on the outside of animals have features such as barbs, hooks or are sticky causing them to attach to fur, hair or feathers. Seeds that are dispersed internally tend to have fleshy fruit that attracts animals but also have hard seed coats to protect the seed while it passes through the animal's gut. The behaviour of an animal determines the fate and the distribution of a seed; for example, hoarding animals may produce seed caches - clumps of seeds (and seedlings) (Howe and Westley, 1997). A special type of animal dispersal is by ants. These seeds offer external fat tissues (elaisomes) to entice ants. Ballistically dispersed seeds are usually housed in fruit that has a trigger mechanism to propel the seed away from the parent plant. Seeds with no special devices are usually round and have no external morphological features.

This classification system is based on morphological traits of the seeds. Thus, while small light seeds are easily dispersed by wind, they are not considered 'wind-dispersed' unless they have specific structures (Willson et al., 1990). Seed size is associated with dispersal type. Large seeds (>100 mg) tend to be vertebrate dispersed whereas small seeds (<0.1 mg) tend to be unassisted: seeds with other types of dispersal syndromes are intermediate in size (Hughes et al., 1994). Seeds and fruits with specific dispersal syndromes tend to have predictable characteristics (colour, odour, form) and may provide rewards to dispersal agents. Some species may have two or more modes of dispersal.

Humans have become an excellent agent of seed dispersal. While weed seeds do stick to our hair and skin, we are an important seed dispersal agent more because of our technology and mobility across the planet. There are numerous examples of vehicles and native, exotic and domesticated animals dispersing weed seeds, although much of it is rather passive or at least accidental in terms of the motives of the dispersal agent (McCanny and Cavers, 1988; Schmidt, 1989; Mack, 1991; Lonsdale and Lane, 1994). Examples include:

• seeds from burdock (Arctium minus) and many other species sticking to clothes and fur;

• seeds from stinging nettle (Urtica dioica) and many other species stick to mud affixed to motor vehicles;

• seeds from many agricultural weeds (e.g. proso millet) being combined along with the crops;

• weeds that have been a part of commercial seed mixes for both agriculture and ecological restoration purposes (e.g. dame's rocket, Hesperis matronalis).

The last bullet point above is also an example of deliberate dispersal, i.e. species were introduced and used before it was realized that they were prone to being weeds. We have intentionally introduced seeds for landscaping, farming or erosion control.

Weed dispersal may be said to be more 'deliberate' in the sense that native seed dispersers may preferentially consume fruits from exotic weeds. This occurs when weeds have larger, more palatable seeds or fruit than native species; for example, weedy English hawthorn (Crataegus monogyna) has seeds that are more attractive to birds than native Crataegus species (Sallabanks, 1993). An odder example is that of the seeds of weedy pine species (Pinus) introduced to Australia are now dispersed by cockatoos instead of by wind as is normal in native pine habitats (Richardson and Higgins, 1998).

Primary and secondary dispersal

Seeds will not be evenly dispersed throughout a habitat because they are subject to myriad abiotic and biotic factors. These may be categorized as primary dispersal (seed is dispersed from the parent plant to the ground) and secondary dispersal (seed is

Seeds on parent plant

Seeds on parent plant

Seed loss

Seed loss

Seed germination

Fig. 6.4. Movement and fates of seeds (based on Chambers and MacMahon, 1994).

Seed loss

Seed germination

Fig. 6.4. Movement and fates of seeds (based on Chambers and MacMahon, 1994).

subsequently moved to other sites) (Fig. 6.4).

The extent of primary dispersal is determined by the morphological characteristic of the seed. For example, small winged or plumed seeds (e.g. fleabanes, Erigeron spp.) travel further in wind than unadorned and relatively heavy seeds (e.g. pigweeds). Secondary dispersal is an important factor because it moves seeds horizontally away from or closer to the parent, or vertically deeper or shallower in the soil. The extent of secondary dispersal is dependent on seed characteristics and how they interact with abiotic factors (gravity, wind, rain and frost heaving) and the physical structure of the community (topography, vegetation and soil) (Chambers and MacMahon, 1994). Even slight changes in the physical structure can change secondary dispersal patterns. A slight change in soil particle size can determine whether a seed moves in response to wind. Secondary dispersal is further influenced by biotic factors. Many types of seed movement are the result of animals (digging, burrowing, scatter-hoarding). For example, the presence of earthworms, increases the depth at which annual bluegrass (Poa annua), common chickweed (Stellaria media) and shepherd's purse (Capsella bursa-pastoris) seeds are buried (van der Reest and Rogaar, 1988; Fig. 6.5). Animals show specific preferences for weed seed types, but their level of consumption is dependent on seed density (e.g. Cromar et al., 1999). Animals also have microsite pref-

Fig. 6.5. The effect of earthworms on the vertical distribution of annual bluegrass (Poa annua), common chickweed (Stellaria media), and shepherd's purse (Capsella bursa-pastoris) seed (redrawn from van der Reest and Rogaar, 1988).

favourable

'Non-dormant

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