KARACHI: Monoculture (the practice of growing a single crop or plant) increases the risk of damages from environmental changes, disease and infestation and has caused huge losses in many urban landscapes of the world.

Karachi, too, is exposed to similar threats as the city seriously lacks diversity in plantation, says a study that analyses changes inplantation along a major road of the city over the past two decades.

T itled Changes in diversity and composition of flora along a corridor of different land uses in Karachi over 20 years: Causes and implications, the research has been published in an international journal and conducted by Prof Zafar Iqbal Shams of Karachi University`s Institute of Environmental Studies.

The study investigates changes in the taxonomic diversity and composition of plant species growing along a 15.2-km long corridorcomprising M. A. Jinnah Road, Jahangir Road, S. M. Taufiq Road and Shahrah-iPakistan.

It compares data with a 1993 study conducted on the same corridor, which starts from the Merewether Tower and ends at Sohrab Goth, passing through the most densely populated 19 union councils of the city.

The study says that the Karachi Metropolitan Corporation has been extensively planting certain species along thestreets over the last few decades without properly studying their suitability to the environment.

`This is evident from the fact that many species were extensively planted in Karachi over the decades but were later removed for various reasons. For instance, eucalyptus was extensively planted during 1980s and 1990s along the streets, which was later removed due to their fast growing roots and shoots that damaged many civic services, it says.

According to the study, monoculture is commonly practiced in Karachi and currently conocarpus erectus is the most common species, which alone has 83.55 per cent individuals on the street under study.

The practice of growing one particular plant species that has gained momentum in recent years in Karachi may increase the risk of potential damages from environmental changes, diseases and insect infest ation, it says.

Another area of concern, the study points out, is the large scale plantation of exotic species along the streets. It has also been observed that native species were replaced with exotic species during different plantation campaigns.

The study highlights importance of native species and says that most global plantation campaigns are now focusing on indigenous species to help restore nativeecosystems in urban landscape. For instance, the Million Trees Programme of South Australia is dedicated to planting three million native plants across Adelaide metropolitan areas.

`Native trees require reduced external inputs and offer foraging resources to birds that may be reduced or absent in exotic streetscape. They also have cultural importance. For instance, Ficus religiosa (locally known as peepal) has a religious value in South Asia, Azadirachta indica (neem) and Ficus benghalensis (banyan) have been sufficiently celebrated in the literature ofIndian subcontinent,` it says.

Findings A total of 9,548 individuals of 28 exotic and 25 native species were found growing along the entire the corridor. Out of which, only 866 plants (9.07pc) were native while 8,682 plants (90.93pc) were exotic. Concarpus erectus, an exotic species, alone had 83.55pc individuals on theecosystems in urban landscape. For instance, the Million Trees Programme of South Australia is dedicated to planting three million native plants across Adelaide metropolitan areas.

`Native trees require reduced external inputs and offer foraging resources to birds that may be reduced or absent in exotic streetscape. They also have cultural importance. For instance, Ficus religiosa (locally known as peepal) has a religious value in South Asia, Azadirachta indica (neem) and Ficus benghalensis (banyan) have been sufficiently celebrated in the literature ofIndian subcontinent,` it says.

Findings A total of 9,548 individuals of 28 exotic and 25 native species were found growing along the entire the corridor. Out of which, only 866 plants (9.07pc) were native while 8,682 plants (90.93pc) were exotic. Concarpus erectus, an exotic species, alone had 83.55pc individuals on thestreets under study and was found growing in all transects of the corridor.

In 1993, the corridor did not have a single individual of this shrub and eucalyptus was the most common plant of the corridor. The streets were represented by 32 native and 35 introduced species that had 4,613 individuals, out of which 3,135 individuals (67.96pc) were exotic and 1,478 individuals (32.04pc) were native.

Seventeen exotic and 13 native species, which were reported growing in 1993, were not found in 2013 anywhere along the corridor.

In 2013, the corridor did not have a single plant of Jatropha integerrima (a native of tropical America), which had sizeable individuals in 1993.

The study shows that plant diversity of the corridor has declined substantially from 8.57 in 1993 to 1.41in 2013. However, the `commercial-cum-residential area` has demonstrated greater diversity of native species in 2013 compared to that in 1993. This category has also shown thegreatest richness of native species in 2013.

According to the study, biologically diverse species are preferred for plantation in urban landscapes to reduce potential damage from pests and diseases. A diverse composition of species and their population is better adapted to environmental changes, diseases and insect infestation.

It also points to the colossal replacements of large-stature trees with mediumstature plants, which may have substantially reduced the air pollutant sequestering capacity and other ecosystem services.

In 1993, the corridor had 42.19pc largestature trees while in 2013 the corridor was represented by 90.63pc plants of medium-stature trees.

`Small-stature trees provide reduced ecosystem services. Studies show that large trees are more scenic and provide more shade to buildings than small trees.

And, above all, remove more air pollutants, about 60-70 times greater, than those removed by small ones,` it says.