Effects Of Seed Predation And Dispersal

Seed predators and dispersers influence plant population dynamics and community structure by affecting both seed survival and seedling recruitment. Robertson et al. (1990) reported that predispersal seed predation rates varied widely among mangrove species at study sites in northeastern Australia. Three species (Ceriops australis, C. tagal, and Rhizophora apiculata) had fewer than 10% of seeds damaged by insects, whereas six species (Avicennia marina, Bruguiera gymnorrhiza, B. parviflora, Heritiera littoralis, Xylocarpus australasi-cus, and X. granatum) consistently had >40% of seeds damaged. These mangrove species also showed variation in survival and growth rates (height and diameter) of seedlings from insect-damaged seeds. Ehrlen (1996) reported a significant positive correlation between the change in population growth rate and the reproductive value of seeds, as reduced by seed predation, indicating that survival of seeds and seedlings is the most important aspect of seed predator effects on plant population growth.

Postdispersal seed predators similarly affect the survival and growth of seeds and seedlings. Seeds selected for storage in ant nests or refuse piles often show increased survival and seedling growth, relative to seeds in control sites (A.Andersen 1988, Culver and Beattie 1980, Hughes 1990, Rissing 1986). Enhanced seedling growth on ant nests may reflect the higher nutrient concentrations (A. Andersen 1988, Culver and Beattie 1983, Herzog et al. 1976, Holdo and McDowell 2004, Mahaney et al. 1999, D.Wagner 1997, D.Wagner et al. 1997), greater water-holding capacity (Jonkman 1978, D. Wagner 1997) of ant nests, or protection from vertebrate seed predators or fire (Louda et al. 1990b, Rice and Westoby 1986).

The composition of the granivore community affects plant community development. R. Inouye et al. (1980) reported that exclusion of granivorous rodents or ants altered densities and community composition of annual plant species (Table 13.1). Rodents preyed selectively on large-seeded species (e.g., Erodium spp. and Lotus humistratus). In plots from which rodents were excluded, these species increased to dominate vegetative biomass and replace small-seeded plant

TABLE 13.1

Effects of removal of ants, rodents, or both on densities of certain annual plant species, all plants, plant biomass, and two measures of species diversity. Values given are ratios of treatment to control (+Rodents +Ants) means. Numbers in parentheses are mean values for unthinned plots except for plant biomass and the two measures of diversity, which are for control plots. Statistical analysis was by ANOVA.

Effects of removal of ants, rodents, or both on densities of certain annual plant species, all plants, plant biomass, and two measures of species diversity. Values given are ratios of treatment to control (+Rodents +Ants) means. Numbers in parentheses are mean values for unthinned plots except for plant biomass and the two measures of diversity, which are for control plots. Statistical analysis was by ANOVA.

+Rodents +Ants

+Rodents -Ants

-Rodents +Ants

-Rodents -Ants

Effects of removal of Rodents Ants

Initial Census 29 January 1977

1. Large plants

1.00 (35.8)

0.98

2.08

2.35

Increaseb

NS

2. Small plants

1.00 (292.5)

3.30

3.32

3.17

NS

Increaseb

Final Census 2 April 1977

3. Erodium cicutarium

1.00 (1.8)

1.83

7.03

16.11

Increaseb

NS

(seed mass = 1.6 mg)

4. E. texanum

1.00 (0.6)

0.88

2.07

0.78

Increasea

NS

(seed mass = 1.6 mg)

5. Euphorbia polycarpa

1.00 (0.6)

2.00

0.14

0.29

Decrease2

NS

(seed mass = 0.2 mg)

6. Filago californica

1.00 (142.1)

1.90

1.43

2.59

NS

Increasea

(seed mass = 0.04 mg)

7. Lotus humistratus

1.00 (11.4)

1.14

2.43

5.22

Increaseb

NS

(seed mass = 1.5 mg)

8. All plants

1.00 (209.6)

1.35

1.34

1.94

Increasea

Increaseb

9. Dry mass (all species)

1.00 (5.8)

1.07

2.09

2.17

Increaseb

NS

10. Species diversity (H')

1.00 (2.78)

0.73

0.99

0.89

NS

Decreasea

11. Species evenness (E)

1.00 (0.53)

0.77

1.99

1.04

NS

"O

II O

m 70

ANOVA, analysis of variance; NS, not significant. "Significant at P < 0.05. 'Significant at P < 0.01.

Reproduced from R. Inouye et al. (1980) with permission from the Ecological Society of America.

species, especially Euphorbia polycarpa. Ants preyed most intensively on the most abundant plant species (Filago californica) .When ants were excluded, this small-seeded composite became numerically dominant and reduced species diversity.

Many plant species have become dependent on animal mutualists for seed dispersal. Seed and seedling survival for some species depends on distance from parent plants, under which seed predation may be concentrated (O'Dowd and Hay 1980, Schupp 1988). As found by Powell and Powell (1987) and Steffan-Dewenter and Tscharntke (1999) for pollinators (see earlier in this chapter), decline in abundance of seed dispersal agents may threaten persistence of some plant species.

Plant species adapted for dispersal by vertebrates often have hardened seed coats to survive gut passage and may require scarification during passage through the digestive systems before germination is possible. Temple (1977) noted the coincidence between the age (300-400 years) of the last naturally regenerated tambalacoque trees, Sideroxylon sessiliflorum (= Calvaria major) and the disappearance of the dodo in 1680 on the South Pacific island of Mauritanius. When S. sessiliflorum seeds were force fed to turkeys (approximately the size of the dodo), the seed coats were sufficiently abraded during gut passage to permit germination, demonstrating a potential role of the dodo in dispersal and survival of this once-dominant tree. Although the primacy of the dodo's role in S. sessi-liflorum survival has been challenged (e.g., Witmer 1991), it appears that S. sessiliflorum and other plant species have suffered from disappearance of seed-dispersing animals from Mauritanius. Janzen and Martin (1982) suggested that a number of tropical plants may show reduced seed dispersal as a result of the Pleistocene extinction of the large mammalian fauna that likely fed on their fruits and dispersed seeds. In any event, many large-fruited species experience high seed mortality in fruits rotting under trees in the absence of effective dispersal (Asquith et al. 1999, Janzen and Martin 1982, Oliveira et al. 1995). Disappearance of native ant seed dispersers as a result of habitat fragmentation or competition from invasive ant species (e.g., A. Suarez et al. 1998) similarly may threaten the survival of ant-dispersed plant species. However, seed dispersers also have been shown to facilitate the spread of exotic plant species (J. M. B. Smith 1989).

The effects of seed predation and dispersal on nutrient cycling or other ecosystem processes have not been studied. However, these organisms affect the movement of nutrients in fruits and seeds. By dispersing fruits and seeds, frugivores in particular remove the large energy and nutrient pools in fruits from under parent trees and distribute these over a large area. Furthermore, as for herbivores and pollinators, seed predators and dispersers affect the spatial distribution of various plant species that differentially control nutrient fluxes.

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