Green areas are dwindling in an unprecedented rate in the cities all over the world. World wide urbanization in the fifties and sixties was marked by extensive use of construction materials to build contiguous anthrosphere costing destruction of natural environment. Technological advancement and economic solvency the urban people achieved made them forget the pastoral and rural lifestyle their parents and ancestors lived. They became quite oblivious to understand the importance of nature and natural goods and services in their lives.
The common trend for urbanizing a locality is destruction of nature and natural feature. The problem is more intense in developing countries. Bangladesh as a developing country located in South Asia is facing a great transition time in terms of urbanisation and industrialisation. More people are migrating towards the cities. For accommodating the incoming people, pressures on the land is escalating day by day. In such a situation the remnant green spaces of Dhaka city are at stake. The complex of problems encountered by the green spaces hinders the ecosystem, economic and social services that should come from them. There are various ecological roles of urban green spaces that are yet to be recognised.
Base of ecosystem in the urban complex
The remnant green spaces in the city are the presence of nature there. The diversity and integrity of nature is seriously being damaged due to destruction of wild vegetation and conversion of agroecosystem into built environment. The birds are directly dependent on the trees for their existence. The amphibians, reptiles and the mammals depend for their food on trees and other plant species indirectly. These animals feed on the insects and birds those directly depend on the plants of varying types. All plants, animals and microorganisms comprise the biotic community. There exists intricate relationship among these organisms. The biotic communities along with physical environment comprise the holistic environment.
Maintaining water cycle
The urban water cycle is tremendously influenced by the vegetation cover present in the city. The evapotranspiration, precipitation, infiltration and run-off all are influenced by presence, quality and quantity of green space in a city. Cycling of water in the environment is crucial for maintaining the life sustaining processes. Moreover, availability of water in useable form is most important in the urban environment because of huge water requirement by the densely located population. The green spaces considerably increase the infiltration of rainwater in urban areas where most of the land is covered by impervious materials.
The dependence of city dwellers on the groundwater is on rise. At present Dhaka WASA depends on the ground water source for 80% of its water supply. This is ultimately staking the city's existence.
Providing fuel, fruits and medicine
The green spaces in the urban environment can provide various goods to the citizens. These goods include various types of fruits, fuel for cooking and medicinal materials. The urban poor get the maximum benefits from these green spaces. Since the poor has limited capacity to meet their needs by purchasing or growing materials, the goods provided by the green spaces are very crucial for their livelihood. Moreover, the shrinkage of open spaces in the cities made these land parcels very crucial for the poor. The youngsters of poor sect get considerable nutrients from the fruits and nuts grown on roadside, parks and graveyards. The women collect foliage from the roadside and parks to cook with. The value of medicinal matters collected from the various plant species of the green spaces is great to these people.
Wildlife habitat and species conservation
Urban green spaces can play a significant role for conservation of native plant ,animal and microbial diversity .With progress of urbanisation the land of a city is mostly used up for more economically benefit raising purposes. The native plant and animal species face hard time for existence. The remnant green spaces become last abode for wild plants and animals fighting to exist before being locally extinct.
Pollution abatement
The green spaces act as purifier of the total environment. First of all, the plants of the green spaces remove suspended particulate matters and volatile organic compounds from air. The root systems of these trees help microbes convert the organic waste materials into plant nutrients. Secondly, the infiltration of rainwater through soil ultimately aids to purify the down moving water. This down moving along with infiltration is highly associated with green spaces i.e. places covered with vegetation where water can be captured and allowed to go into soil. Most of the land area in a city is covered with asphalt or concrete that reduces the infiltration rate. Finally, the plant stands purifies the air by removing the particulate matters, absorbing hydrophobic compounds and capturing other toxic materials on their leaves and barks.
Conditioning the total environment
The ambient temperature, humidity and solar heat are three basic components of natural environment in the city. The comfort of living in a city mostly depends on these factors. The citizens are usually not conscious about miraculous effects of the green spaces in controlling these factors. It was found in several studies that temperature of the green spaces in a city in hot summer is much cooler than the surrounding locality. Again, humidity of air is raised by green spaces in winter.
Conservation of soil and waste assimilation
Soil conservation and value extraction from fertility of soil can be ensured from the green spaces that comprise the botanical gardens, public parks and places where cultivation is done in a city. The soil of the city can be better utilized and conserved by covering the open spaces with green plantation. Green spaces can assimilate organic waste materials by the process of biodegradation.
Conclusion
The ecological role of urban green spaces is multifaceted. The green spaces condition the urban environment by controlling the temperature and humidity. The green spaces play significant role in maintaining water cycle, abating pollution and harbouring a variety of plants, animals and microbes. From ecological perspective, the green spaces are only productive areas in the city, the biotic community of city somehow or other depends on them. The human being is also benefactor of the green spaces; the green spaces provide various ecosystem goods and services. But the paradox is this that the developing countries are facing great crises of loosing greenery from their cities while the western world is now trying to bring back the nature to their cities.

Mahfujur Rahman, works as a Research Officer at Bangladesh Centre for Advanced Studies (BCAS) and can be reached at [email protected]