MECHANISMS CONTROLLING SEED FLUX

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MECHANISMS CONTROLLING SEED FLUX by Charles H. Walker

Research is however needed to establish which of the possible mechanisms effectively control seed flux in particular communities, and thus to evaluate the effects of seed eating animals on plant dynamics (Marone et al. 2000).

Mechanisms that can control seed flux are germination, consumption by animals, seed burial, and pathogen attack (Marone et al. 2000). When seeds germinate they are lost to the seedbank. In desert environments the timing of rainfall is important for seed germination. So germination may be more important in some biotic zones, e.g., Great Basin (152.1), Mohave desert (153.1), Sonoran desert (153.2), Chihuahuan desert (154.1), than in other biotic zones, e.g., Rocky Mountain Subalpine Conifer Forest and Woodland (121.3) (biotic zones after Brown 1994).

Other factors that have an impact on germination are scarcity of seeds, lack of suitable sites, and competition from other plants (Marone et al. 2000). Trees offer suitable sites (microhabitats) in desert environments. Many desert annuals and perennials germinate under the cover of trees. Whisperingbells (Emmenanthe penduliflora) and Arizona fiestaflowers (Pholistoma auritum) have been observed growing under Parish�s desert-thorn (Lycium parishii), but not growing out in the open (Walker, personal observation). There is higher germination under established trees than in the open. This is a facilitative effect. Germination has also been found to be higher for bare ground in exposed sites than in those that had cover (Marone et al. 2000).

Seed burial is a mechanism that can have a profound effect on seed flux. One of the processes that is responsible for seed burial is solifluction. This is where an organic soil is buried "by the slow, downslope erosional deposition of mineral soil" (McGraw, et al. 1991). Small-seeded species, like Sporobolus cryptandrus and Descurainia sp., are most prone to deep burial. However, medium to large-seeded species like Sphaeralcea miniata, Phacelia artemisioides, and Glandularia mendocina are also prone (Marone et al. 2000).

Pathogen attack, i.e., the impact of bacteria and fungi, should be more carefully assessed to determine its influence on seed flux within seedbanks (Marone et al. 2000).

The mechanisms outlined above, germination, deep burial and pathogen attack, should be researched so that the effects of granivory can be better understood. For example rodents in North American deserts can consume up to 86% of seed production each year. This is in contrast to South African deserts where less than 1% of seed production is consumed by rodents (Kerley & Whitford 1994).

The Monte desert in South America has been shown to have a seedbank size that is similar to that of the deserts of North America, yet granivory is lower in South America than in North America (Marone & Horno 1997). Marone found that birds consumed 7 kg ha^-1 during the fall-winter months, virtually all the seed available. The family Heteromyidae, which includes kangaroo rats and pocket mice, is responsible for the high levels of seed removal in North American deserts (Kerley & Whitford 1994).

It is only after the study of all of the possible mechanisms, that an accurate determination of the effects of granivores on plant dynamics can be made.

References

Brown, D.E. (1994) Biotic communities southwesterrn United States and northwestern Mexico. University of Utah Press, Salt Lake City, Utah.

Kerley, G.I.H. & Whitford, W.G. (1994) Desert-dwelling small mammals as granivores: intercontinental variations. Australian Journal of Zoology, 42, 543-555.

Marone, L. & Horno, M.E. (1997) Seed reserves in the central Monte desert, Argentina: implications for granivory. Journal of Arid Environments, 36, 661-670.

Marone, L., Horno, M.E. & del Solar, R.G. (2000) Post-dispersal fate of seeds in the Monte desert of Argentina: patterns of germination in successive wet and dry years. Journal of Ecology 88, 940-949.

McGraw, J.B., Vavrek, M.C. & Bennington, C.C. (1991) Ecological genetic variation in seed banks, establishment of a time transect. Journal of Ecology, 79, 617-625.

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