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Plant-pollinator interactions in fragmented tropical dry evergreen forests, southern India

Plant-pollinator interactions in fragmented tropical dry evergreen forests, southern India
K. Geetha Nayak

2008

Salim Ali School of Ecology and Environmental Sciences, Pondicherry University, Pondicherry 605 014, INDIA.

ABSTRACT

The Tropical Dry Evergreen Forest (TDEF) is unique to the Coromandel Coast of southern India, as much of these forests have been destroyed over past century leaving only fragments. It is likely that, the deep ravines in some parts of Pondicherry are probably formed dues to massive soil erosion during rampant destruction of trees for timber wood early part of twentieth century (Raja and Keshari, 2005).

TDEF fragments harbour species that are unique and adapted to a long dry season prevalent in this region. The species have wide distribution and are found in other forests in Peninsular India as well. However, there are variations in some morphological characters specifically adapted to suit the drier climate in the region. TDEFs are the only natural vegetation found in and around Pondicherry known to serve as refuge for migratory birds such as Forest Wagtail and Blue throated flycatcher.

Many forested areas in India are now becoming fragments due to deforestation and many developmental activities such as road or dam construction and the expansion of agricultural lands, in a recent study Jha et al. (2000) reported loss of 22.6% of forest cover in 22 years. In a study area of 40,000-Km2 alone, and degraded forest increased by 26.64%. Managing these fragmented forests frim further degradation is a challenging task and management actions will determine the nature of wildland habitats for centuries to come (Janzen, 1988). The ‘management or restoration’ of deforested areas in many regions in India, is done by planting exotic species such as Eucalyptus and Acacia species. For example, heavily mined areas in rainforests if Kundremukh National Park in Karnataka state has been planted with fast growing, commercial exotic species. Even TDEF forests area cleared for expansion of agricultural fields and abandoned lands are now being planted with Casuarina species.

The restoration of TDEF in Pondicherry is perhaps one of the most successful restoration programs in India. Restoration of barren or degraded land is extremely hard dues to the lack of a soil seed bank and seed rain input. On the other hand, planting and seedling transplanting can be an expensive affair. Therefore, restoration activities should have specific target species and emphasis should be given to the species which have specific requirements like growing under shade or reliance on pollinators etc. In the present study, the species like Lepisanthes tetraphylla and Walsura trifolia were found to be very few and had self-incompatible breeding systems, whereas, species like Memecylon umbellatum and Glycosmis pentaphylla were abundant. Some species like Eugenia bracteata, Memecylon umbellatum, Canthuim species were bird dispersed and could easily colonize new areas. Self-incompatible and species that have low recruitment should be given priority in planting programs. Adequate population sizes should also be maintained.

Plant-pollinator interactions in TDEF
Flowering TDEF plant species receive a wide variety of insect visitors, but only a few of the actually pollinate the plants. The plant species studies were recorded as mostly pollinator generalists receiving visitors from more than one taxa. Social bees such as Apis cerana, Apis dorsata and Trigona iridipensis were found to be predominant visitors. Of the 13 species, breeding systems were determined for ten species and six of them (namely: Capparis brevispina, Derris ovalifolia, Eugenia bracteata, Glycosmis pentaphylla, Memecylon umbellatum and Tarenna asiatica) were self-compatible (SC) and four species (namely: Capparis zeylanica, Ixora pavetta, Lepisanthes tetraphylla and Walsura trifolia) were self-incompatible (SI).

Fruit set was noted to be significantly lower in SI species compared to SC species and pollinator limitation was higher in SI species. Hand-cross pollination increased fruit set in all species, indicating that plant species were pollinator limited and the population sizes of cross-compatible individuals (i.e. number of flowering individuals) were more less adequate to facilitate cross-pollination. Pollinator specialization did not affect fruit set, although generalist SI species suffered higher levels of pollinator limitation than generalist SC species. For example, Walsura trifolia, as SI species with generalist pollination mode showed higher levels if pollinator limitation than Capparis zeylancia, a SI species pollinated by sunbirds. Although, Walsura trifolia had highest number of floral visitors, it had lowest fruit set from natural pollination as well as hand-cross pollination. This is probably a result of lack of effective pollinators and low population size as well.

The population size is not related to the fruit set, but when species were analysed individually, the population size as negatively related to fruit set in three species: Derris ovalifolia, Glycosmis pentaphylla and Ixora pavetta, which is probably an indication of paucity of effective pollinators in the fragments or lack of sufficient pollinator visit to large flowering aggregations. Nearest neighbour distance of the conspecifics appear to be in an important factor for the plants. The natural fruit set significantly decreased with increasing distances, suggesting the Allee effect. The hand cross pollen augmentation not only increased fruit set, but also the fruit and seed mass. Fruit set from CP1 and CP3 had higher weight than OP and SP treatments. Self-pollen significantly reduced the fruit set and fruit weight in all the plants; therefore, lack of out crossing opportunities in small isolated populations will probably result in eventful inbreeding depression.

The adult density if SC and SI species significantly differed: SC species had higher adult density but sapling density did not differ. This perhaps is due to ongoing disturbance and low fruit set per se in the fragments.

Although, several aspects to understand factors determining the reproduction in plants in these fragments have been analyzed in the present study, it seems to be confounded by combination of several caused and processes like availability of pollen and pollinators, population sizes of reproductive individuals, interplant distances etc.

Owing to undocumented declines in pollinators and lack of undisturbed control, this study completely had to rely on controlled pollination experiments. Documenting population declines of pollinators, especially insects is so challenging, that there are hardly any case studies. On the other hand, measuring pollination deficits of plants is easy, and there are dozens of studies (Thomson, 2001). Although cross-pollination experiments are not equivalent to ‘ideal pollination’ by pollinators, as they may also carry heterospecific pollen.

However, the resulting increase in fruit set as well as increased in fruit and seed size in this study confirms the pollinator deficit, rather than resource limitation. Ehrlen and Eriksson (1995) found that supplementally pollinated plants of Lathyrus verna increased their seed production by 3:1 times, but then seeds shrank in size.

Fragmentation could affect plant population directly through reduction in habitat area or indirectly through reducing the pollinator pool. The decrease in pollinators would result in lower fruit set especially in case if self-compatible species. In other words, lower pollinator activity can increase the probability of self-pollination, which can lead to reduce fruit set. Davidar et.al. (2007) found many species had very low or no recruitment and Dasmohapatra et.al. (in prep) found many species were not represented in the soil seed bank. Therefore, it is possible that eventually self-incompatible species in these fragments will be replaced by self-compatible species. A hypothetical framework based in Davidar et.al (2007) and Dasmohapatra et.al (in prep) and the present study given in the figure 6.1

The presence if restored sites also helped to rest to what extent functional aspects (Plant-pollinator interactions) have been reinstated in these sites apart from structural aspects. The study by Davidar et.al. (2007) examined the relationship between adult and sapling densities in natural and restored fragments done in the same study sites. The pollinator limitation and visitation diversity in these two types of forests was examined in this study. With all the species pooled, it is showed the fruit set is natural forest fragments were higher than in restored fragments, but when each species was analyzed individually there was no difference in the fruit set, expect for three species. The visitation diversity did not differ, however, the dominance index differed. The pollinator importance index of five principal pollinators for plant species was similar. This indicates that restoration activities have helped to establish plant-pollinator communities: however, planting densities have to be increased to improve pollinator effectiveness in restored sites. This study along with Davidar et.al (2007) perhaps are the first empirical studies that examine both structural and functional aspects of restoration. Studies such as these, can provide insights into the factors associated with the successful establishment and regeneration of species.