Effects Of Pollination

Pollination contributes to genetic recombination and survival of plant species in heterogeneous environments. Many plants can reproduce vegetatively or by self-fertilization, but these mechanisms are not conducive to long-distance colonization or genetic recombination. Species survival and adaptation to changing environmental conditions requires outcrossing and environmental selection among diverse genotypes. Some long-lived perennials may endure adverse conditions and persist by vegetative reproduction until conditions favor out-crossing and seedling recruitment. Such windows of opportunity are unpredictable, requiring annual investment in flower and seed production (Archer and Pyke 1991).

Pollinator-facilitated reproduction is a key factor maintaining populations of ephemeral or sparsely distributed plant species. Obligate outcrossing plant species that depend on insect or vertebrate pollinators for pollination are vulnerable to loss of these mutualists. Maintenance of rare plant species or restoration of declining species depends to a large extent on protection or enhancement of associated pollinators (Archer and Pyke 1991, S. Corbet 1997). Norman and Clayton (1986) and Norman et al. (1992) found that pawpaws, Asimina spp., in Florida, United States, depended on beetle and fly pollinators attracted to yeasty floral odors. Self-pollinated flowers occasionally produced fruits, but only seeds from cross-pollinated flowers germinated.

Differential pollination and reproductive success among plant species affect vegetation dynamics. Plant species that maximize pollination efficiency and increase outcrossing via animal pollinators are able to persist as scattered individuals. However, pollination efficiency by insects is strongly affected by plant spacing. Momose et al. (1998a) found that pollination by thrips and consequent fruit and seed development of a small (<8 m height) tree species, Popowia piso-carpa, in Sarawak declined dramatically when distances between trees exceeded 5 m. Changes in pollinator abundances and pollination efficiency affect plant population dynamics and persistence in communities. Environmental changes that increase the distance between conspecific plants may threaten their survival, as shown in the following examples.

Steffan-Dewenter and Tscharntke (1999) examined the effects of plant isolation on pollination and seed production in replicate grasslands surrounded by intensively managed farmland. They established small experimental patches of two grassland species, Sinapsis arvensis and Raphanus sativus, at increasing distances from the grassland boundaries and found that the number and diversity of bees visiting flowers, and seed production, declined with increasing isolation (Fig. 13.3). Number of seeds per plant was reduced by 50% at 260 m from the nearest grassland for R. sativus and at 1000 m for S. arvensis.

Changes in pollinator abundance, such as those resulting from ecosystem fragmentation, can affect plant reproduction and gene flow (Bawa 1990, Didham

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Relationship between the plant distance from the nearest chalk grassland and abundance of pollinating bees per 15 minutes (left) and number of seeds per plant (right).The regression lines are significant at P < 0.003. From Steffan-Dewenter and Tscharntke (1999) with permission from Elsevier. Please see extended permission list pg 572.

et al. 1996). Powell and Powell (1987) compared attraction of male euglossine bees to floral chemical baits in forest fragments in Brazil. Abundance and species composition did not differ among sites prior to fragmentation. However, after fragmentation, visitation rates for most species were correlated to fragment size, and the bee species trapped in clearings differed from the species trapped in forests (Fig. 13.4). Powell and Powell (1987) concluded that the reduced abundance and activity of particular pollinators in fragmented forests threatened the viability of their orchid hosts. Aizen and Feinsinger (1994) compared pollinator visitation among replicated blocks containing continuous forest and large (>2.2 ha) and small (<1 ha) fragments in subtropical dry forest in northwestern Argentina. The diversity and visitation frequency of native pollinators decreased significantly, and the visitation frequency of exotic honey bees, Apis mellifera, increased significantly with decreasing fragment size (Fig. 13.5). Fragments supported fewer bee species than did continuous forests. Although honey bees from the surrounding agricultural matrix replaced most of the lost visitation by native pollinators, some plant species could be threatened by loss or reduced specificity of pollinators.

Pollination also contributes to production of fruits and seeds that support associated food webs. Many animal species depend on fruit and seed production, at least seasonally (see later in this chapter). Hence, pollination of fruiting plants has consequences not only for plant reproduction but also for the survival of frugivores and seed predators (Bawa 1990).

Pollinators can affect ecosystem energy and nutrient fluxes. Roubik (1989) calculated the effects of social bees on energy and nitrogen budgets of tropical forests in Central America. He estimated that 600 colonies km-2 harvested 1.4 x 107 kJ year-1 and disposed of an equivalent energy value represented by dead bees scattered on the ground within a few dozen meters of each nest. This value exceeded estimates of energy fixed annually by primary producers, indicating that the energetics of flowering are greatly underestimated

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| Rates of visitation by male euglossine bees at chemical baits in intact forest, forest fragments of varying size (100 ha, 10 ha, and 1 ha), and recently deforested (500 ha). Modified from Powell and Powell (1987) with permission from the Association for Tropical Biology.

(Roubik 1989). The 600 colonies also distributed about 1800 kg trash (pupal exuviae and feces) ha-1 year-1. At 4% nitrogen content, this represents a flux of 72 kg ha-1 year-1 or about 1% of above-ground nitrogen in biomass. Pollinator effects on community structure also should affect ecosystem processes. These effects warrant further study.

| Rates of visitation by all pollinating insects, exotic honey bees (Apis mellifera) alone, and native pollinators alone on flowers of two plant species by treatment (continuous forest, and large [2.2 ha] and small [1 ha] fragments) and by time of day in Argentina. Vertical lines represent standard errors; bars under the same letter do not differ at P < 0.05. From Aizen and Feinsinger (1994) with permission from the Ecological Society of America.

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