Human impacts on pollination, reproduction and breeding systems in tropical dry forest plants

Abstract

Over the last two decades several studies have shown that plant species of contrasting life-forms ranging from small herbs to large trees may experi-ence a decline in reproductive success following habitat fragmentation and population disruption (Bawa 1990; aizen and Feinsinger 1994; aguilar et al. 2006). Such outcome has been shown for many plants throughout the tropics, particularly trees, where human activities have resulted in elevated rates of habitat fragmentation and degradation (Ghazoul and Shaanker 2004; Quesada and Stoner 2004; Quesada et al. 2004). Because almost 90 percent of angiosperms (i.e., flowering plants) depend on animals for effec-tive pollination and sexual reproduction (Buchmann and Nabhan 1996), it is of central concern to understand the capacity of pollinators for transfer-ring pollen among individuals and its consequences on plant reproduction in newly created anthropogenic landscapes.While evolutionary dependence of plants on animal mutualists for sexual reproduction has improved pollen transfer to stigmas, it has also prompted increased plant susceptibility to fragmentation and other forms of anthropogenic disturbance that characterize today?s landscapes (e.g., ai-zen et al. 2002; ashworth et al. 2004). Changes in abundance, composi-tion, and/or foraging behavior of pollinators as a consequence of habitat disturbance will have an effect on the amount and/or quality (autogamous 174 seasonally dry tropical forestvs. xenogamous) of pollen deposited on stigmas, thus affecting reproduc-tion and the genetic structure of plants (Wilcock and Neiland 2002). Much research has been conducted with regard to the effects of habitat loss and fragmentation on pollination, plant reproduction, and genetic di-versity of plant populations over the past 20 years. Nevertheless, there has been certain research bias in the selected natural systems evaluated, where species from tropical forests represent only 16 percent of the entire studied species around the world (aguilar et al. 2006, 2008). Moreover, no specific analysis of this subset of species has yet been conducted. this comparative underrepresentation of tropical plant species in fragmentation studies high-lights the need to focus more thoroughly on population studies from these threatened and fragile habitats.Some expected outcomes of habitat fragmentation include local extinc-tion of plant and animal populations, the alteration of species richness and abundance, and changes in the trophic structure of communities. these negative effects of habitat fragmentation can be expressed at the landscape and population levels. at the landscape scale, fragmentation involves the transformation of a large area of habitat into several patches of smaller size, isolated from each other by surrounding anthropogenic habitats different from the original. Such loss and breaking apart of the habitat alters nega-tively the connectivity, functioning, and biodiversity within the matrix of the fragmented habitat (Fahrig 2003). at the population level, habitat frag-mentation may reduce the effective population size and the magnitude and direction of gene flow, which in turn would produce negative changes in the population and genetic structure of plant species (Young et al. 1996; aguilar et al. 2008). the reduction of both gene flow and effective popula-tion size by habitat fragmentation may cause inbreeding, genetic drift, and a consequent decline of genetic variation. therefore, the loss of genetic diversity may limit the ability of local populations to respond selectively to varying local conditions, compromising their persistence and increasing their risk of extinction due to inbreeding depression. Habitat fragmentation may not only lead to a reduction in population size and genetic variation but also disrupt key interactions of the plants with their pollinators and seed dispersers. the interaction between plants and pollinators can be disrupted by habitat loss, reduction of pollinator abundance, changes in floral resource availability and distribution, or com-petitive exclusion from floral resources by inefficient or exotic pollinators. Most of the plant-pollinator interactions may depend on the relative abun-dance of floral resources, thus changes in plant abundances caused by forest disturbance may lead to modification in the composition, functioning, and Human Impacts on Pollination, reproduction, and Breeding Systems 175 maintenance of plant-pollinator webs (aizen and Feinsinger 2003; Lope-zaraiza et al. 2007). We should expect small isolated or fragmented plant populations to be less attractive to pollinators than large populations. as a result of this, rates of pollinator visitation and seed production may often be lower in small than in large populations of plants pollinated by animals. the negative consequences of habitat fragmentation for plant popula-tions could be exacerbated by the complex interactions of reproductive (sex expression) and mating systems (selfing vs. outcrossing or mixed strategies) in combination with population size and pollination and seed dispersal sys-tems. Previous studies of seasonally dry tropical forests (SDtFs) indicate that the reproduction of plants is dependent on the presence of natural pollinators (Frankie et al. 1974; Bullock 1985). therefore, changes in the abundance and activity patterns of pollinators induced by habitat fragmen-tation are expected to reduce gene flow between isolated plant populations. the negative effects of forest fragmentation on the viability of populations could be particularly noticeable in tropical tree species that posses self-in-compatibility systems and depend on pollinators for sexual reproduction (Bawa 1974, 1990; aguilar et al. 2006). Disturbances that impact animal vectors of pollen transfer may therefore affect the reproductive output of tropical trees. Pollination of tropical plants is mainly conducted by animal vectors such as bees, butterflies, flies, birds, and bats, and the natural popu-lations of these animals inhabit and depend on the existence of forests.the objectives of this chapter are to (1) evaluate the effects of forest fragmentation on plant-pollinator interactions, plant phenology, reproduc-tive dynamics, and genetic parameters of tropical plants; (2) describe and compare plant life-history traits, pollination systems, and plant reproduc-tive traits between tropical forests; and (3) predict vulnerability patterns to forest fragmentation based on ecological and reproductive traits of plants.