Measurement of Seed Predation and Dispersal

A number of factors influence rates of seed predation and dispersal. The extent of seed mortality, mechanism of seed transport, distance moved from the parent plant, attraction of particular dispersal agents, and thermodynamic constraints determine the probability that seeds will survive and be moved to suitable or distant locations. Pollinators and seed predators can have opposing effects on seed production. Steffen-Dewenter et al. (2001) reported that pollinator activity decreased, but seed predation increased, on experimental Centaurea jacea plants, with distance from seminatural habitats in an agricultural landscape in Germany.

Several methods have been used to measure seed predation and dispersal. Predispersal seed predation can be measured by marking fruits or seeds on the plant and observing their fate, using a life table approach (see Chapter 5). Mature fruits and seeds can be collected for emergence of seed predators (Steffan-Dewenter et al. 2001) or dissected or radiographed for identity and number of internal seed predators or evidence of endosperm digestion by heteropterans (e.g., Schowalter 1993). Seed-piercing Heteroptera may leave detectable pecti-nases or stylet sheaths on the seed coat of consumed seeds (Campbell and Shea 1990). Postdispersal seed predation can be measured by placing marked seeds on the ground and measuring rate of disappearance (C. Chapman and Chapman 1996, Heithaus 1981, O'Dowd and Hay 1980, Schupp 1988). Seeds marked with tracers can be identified in caches or fecal material for assessment of seed dispersal rate (e.g., O'Dowd and Hay 1980).

Seed predators are capable of consuming or destroying virtually the entire production of viable seed of a given plant species in some years (Coe and Coe 1987, Ehrlen 1996, Robertson et al. 1990, Schowalter 1993,Turgeon et al. 1994). The intensity of seed predation depends to a large extent on seed availability. Seed predators focus on the largest or most concentrated seed resources (Ehrlen 1996). During years of poor seed production, most or all seeds may be consumed, whereas during years of abundant seed production, predator satiation enables many seeds to survive (Schowalter 1993, Turgeon et al. 1994). Long-lived plant species need produce few offspring over time to balance mortality. Hence, many tree species produce abundant seed only once every several years. Years of abundant seed production are known as mast years. Poor seed production during intervening years reduces seed predator populations and increases efficiency of seed production during mast years (Fig. 13.6).

Insects generally are more important predispersal seed predators than are vertebrates, but vertebrates are more important postdispersal seed predators (Crawley 1989, Davidson et al. 1984, Louda et al. 1990b, Schupp 1988). Predispersal seed predators greatly reduce seed production efficiency and reduce the number of seeds available for postdispersal seed predators and dispersal. K. Christensen and Whitham (1991) reported that seed-dispersing birds avoided foraging in pinyon pine trees in which the stem- and cone-boring moth, Dioryctria albovitella, had inhibited cone development and increased cone mortality. At the same time, frugivores and postdispersal seed predators consume colonized seeds and can significantly reduce populations of predispersal seed predators (Coe and Coe 1987, Herrera 1989). Sallabanks and Courtney

80 H

r 100

1989

1990

1991

1992

Year

□ Seeds damaged by insects

| Relationship between total seed produced, seed loss to insects, and seed yield in a Douglas fir seed orchard in western Oregon. Data from Schowalter (1993).

(1992) suggested that seed predators and dispersers often may exert opposing selection pressures on temporal and spatial patterns of fruit and seed production.

Seed dispersal is an important mechanism for plant colonization of new sites. However, dispersal also may increase seed and seedling survival. Schupp (1988) reported that vertebrate seed predators limited seed survival under the parent tree to 15% of marked seeds but that dispersal distances of only 5 m significantly increased seed survival to nearly 40% over a 7-month period (Fig. 13.7). C. Chapman and Chapman (1996) compared fruit and seed disappearance and survival of seeds remaining under the parent canopy for six tree species in a tropical forest in Uganda. Three of the six species showed higher rates of seed removal at locations away from the parent canopy compared to locations under the parent canopy, whereas the other three species showed no difference in seed removal between locations. However, for two of the latter species, survival of transplanted seedlings was much higher under conspecific canopies than at locations away from conspecifics, but subsequent herbivory tended to be higher on seedlings under conspecific trees. Fruits not harvested by dispersers usually rot on the ground, destroying the seeds within (Asquith et al. 1999, Janzen and Martin 1982). Ants often play a critical role in seedling survival and germination under parent trees by foraging on fruit, cleaning seeds, and dispersing seeds to ant nests (Oliveira et al. 1995, Passos and Oliveira 2003). Seeds not cleaned by ants succumb to decay. These results indicate that seed dispersal to suitable sites represents various tradeoffs. Nevertheless, the efficiency with which seeds reach favorable sites is critical to plant population dynamics.

Seeds transported by wind or water often have low dispersal efficiency, for which plants must compensate by producing large numbers of seeds. Animals are u œ œ CO

100 n

Beneath

Gap T

Away Gap C

Canopy treatment

Survival of Faramea occidentalis seeds beneath fruiting parent trees (Beneath), away from parent trees (Away; 5 m from crown perimeter of nearest fruiting adult), and within the canopy (Gap C) and trunk (Gap T) zones of treefall gaps on Barro Colorado, Panama. Survival of seed was significantly (P < 0.05) higher 5 m from parent trees than beneath parent trees or in treefall gaps. Data from Schupp (1988).

presumed to be more efficient dispersal agents, but this may not always be accurate. Seeds drop from animal vectors with no more likelihood of landing on suitable germination sites than do seeds deposited by wind or water, unless animal dens or habitats provide suitable germination sites. However, the direction of animal movement is more variable than that of wind or water. Birds, in particular, quickly cover large areas, but local seed redistribution by ants also can significantly affect plant demographies (Gorb and Gorb 2003, O'Dowd and Hay 1980). A number of plant species are specifically adapted for seed dispersal by animals. Myrmecochorous species produce a lipid-rich elaiosome to attract ants, which move seeds variable distances, depending on whether the elaiosome is removed prior to or during transport or at the nest (Fig. 13.8; Gorb and Gorb 2003). Some species with large seeds or thick seed coats may show reduced dispersal or germination ability where movement by animals or seed scarification is prevented (Culver and Beattie 1980, Oberrath and Bohning-Gaese 2002, Temple 1977). However, many seeds are dispersed more passively by various animals, including secondary dispersers such as dung beetles that redistribute frugivore dung (Fig. 13.9).

Seed storage underground by ants and rodents may move seeds to sites of better soil conditions or reduce vulnerability to further predation. A number of studies have demonstrated that seedlings germinating in ant nests are larger and have higher survival rates than do seedlings emerging elsewhere (A. Andersen 1988, Bennett and Krebs 1987, Culver and Beattie 1980, Rissing 1986, D.Wagner 1997). Ant nests may or may not enrich surrounding soils (Horvitz and Schemske 1986, Westoby et al. 1991; see Chapter 14). Soil from ant nests often has significantly higher concentrations of nitrate, ammonium, phosphorus, and water and higher nitrogen mineralization rates than does soil away from nests (A. Andersen 1988, Culver and Beattie 1983, Herzog et al. 1976, Holdo and McDowell 2004, Lesica and Konnowski 1998, Mahaney et al. 1999, D. Wagner 1997, D. Wagner et al. 1997). However, Rice and Westoby (1986), Hughes (1990), and Gorb and Gorb (2003) found that myrmecochorous plants do not necessarily show distribution patterns associated with soil fertility or with ant nests. Gorb and Gorb (2003) found that foraging Formica polyctena transported myrmecochorous seeds to territorial borders after removing the elaiosome, thereby distributing seeds widely, but non-myrmecochorous seeds were transported to nests, where they remained, leading to increased competition between plants that grew on the mound.

Plants may benefit from seed deposition at suitable depths for germination or protected from intense predation by vertebrates (Cowling et al. 1994). Shea et al. (1979) found that germination of serotinous seeds of several legume species, in Western Australia, was enhanced by seed redistribution by ants to depths that were heated sufficiently but protected from higher surface temperatures during high-intensity autumn fires. O'Dowd and Hay (1980) reported that transport of diaspores of Datura discolor by ants, to nests averaging only 2.3 m from the nearest plant, reduced seed predation by desert rodents from 25-43% of seeds in dishes under parent plants to <1% of seeds in dishes near ant nests. Heithaus (1981) found that when seed dispersal by ants was experimentally prevented, rodents removed 70-84% of Asarum canadense and Sanguinaria canadensis seeds, compared to 13-43% of seeds lost when ants were present. Fur-

o CS

Distance from parent plant to the nest (D), m

Distance from parent plant to the nest (D), m

Relationship between seed number transported to ant nests and distance from the parent plant to the nest for given diaspore dropping rates (A) and relationship between seed number transported to nests and dropping rate of diaspores for given distances from the nest (B). From Gorb and Gorb (2003) with permission from Kluwer Academic Publishers. Please see extended permission list pg 572.

Relationship between seed number transported to ant nests and distance from the parent plant to the nest for given diaspore dropping rates (A) and relationship between seed number transported to nests and dropping rate of diaspores for given distances from the nest (B). From Gorb and Gorb (2003) with permission from Kluwer Academic Publishers. Please see extended permission list pg 572.

thermore, laboratory experiments demonstrated that rodents located buried seeds less frequently than seeds on the surface and consumed buried seeds less often when elaiosomes were removed, as done by ants. Hughes (1990) reported that changes in nest structure, indicated by relocation of nest entrances, may provide refuges for seeds remaining in abandoned portions of nests and reduce seedling competition by preventing long-term concentration of seeds in localized sections of nests.

Seed Dispersal Pictures Water

| Dung beetles represent secondary dispersers of seeds in vertebrate dung.

| Dung beetles represent secondary dispersers of seeds in vertebrate dung.

C. Spatial and Temporal Patterns of Seed Predation and Dispersal

Few studies have compared seed predation and dispersal among ecosystems. Different agents dominate these processes in different ecosystems (Moll and McKenzie 1994). For example, dominant plant species in temperate, especially arid, ecosystems frequently have wind-dispersed seed, whereas plant species on oceanic islands often are water-dispersed (Howe and Smallwood 1982). Howe and Smallwood (1982) concluded that consistently windy ecosystems promote wind-driven dispersal, whereas more mesic conditions promote animal-driven dispersal. Old World deserts have relatively few (<5%) animal-dispersed plant species (Howe and Smallwood 1982). More than 60% of temperate and tropical forest plant species are dispersed by animals (Howe and Smallwood 1982). A variety of large vertebrate herbivores are important frugivores and seed dispersers in temperate and tropical ecosystems (e.g., Janzen and Martin 1982). Fruits and seeds in seasonally flooded tropical forests often are dispersed by fish during periods of inundation (de Souza-Stevaux et al. 1994, M. Horn 1997, Howe and Smallwood 1982). Bats and primates are more important frugivores and seed dispersers in tropical forests than in temperate ecosystems. Insects are ubiquitous frugivores and seed predators but may be more important dispersers in grassland and desert ecosystems, where transport to ant nests may be critical to protection of seeds from vertebrate seed predators, from competition, and from fire (e.g., Louda et al. 1990b, Rice and Westoby 1986).

Rice and Westoby (1986), Rissing (1986), and Westoby et al. (1991) discussed a number of potential factors affecting differences in the incidence of ant-dispersed seeds among biogeographic regions. Myrmecochory appears to be more prevalent in Australia and South Africa than in other regions. One hypothesis is that smaller plants (characteristic of arid biomes) generally are more likely to be ant-dispersed than are larger plants. A second hypothesis is that the relatively infertile soils of Australia and South Africa preclude nutrient allocation to fruit production, forcing plants to adapt to seed dispersal by ants rather than vertebrates. Finally, Australia and South Africa lack the large harvester ants (e.g., Pogonomyrmex spp., Messor spp., and Veromessor spp.) common in arid regions of North America and Eurasia. These ants consume relatively large seeds, limiting the value of an elaiosome as a food reward for seed dispersal.

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Responses

  • anke
    How do humans affect the dispersal of plants?
    6 years ago
  • flavus
    What are the factors influence the seed dispersal?
    5 years ago

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