Agroecosystem is defined as a spatially and functional coherent unit of agricultural activity and includes the living and non-living components in that unit as well as their interactions. An agroecosystem can be viewed as a subset of conventional ecosystem it is man-engineered ecological system which includes all biotic and abiotic components interacting with each other. An area with similar types of agro ecosystem can then be termed as an agricultural region. This is the system where in other to obtain more food, timber, cloth, medicine and other useful products, men become responsible for the replacement of natural systems. To secure maximum production, man makes such planned manipulations in the environment. Different biological modification includes crop intensification, mixed cropping, intercropping, multilayered cropping, crop rotation, using improved and high productive & high yielding crops, etc. This ecosystem aimed to produce species of their own choices. We have ecosystem of dominant crop likes maize, wheat, jowar, paddy, sugarcane, vegetables, etc. which grow and gives suitable yield under favorable condition. There are many factors that play a vital role in changing ecosystems. Farmers need for subsistence for their livelihood and government policy and researcher's interest may bring changes from one type to another.

Generally in the context of Nepal, Rice, maize and wheat are the main common agricultural products produced. During rainy season mostly there is the rice ecosystem prevailed which required wet and warm climate. As the season go on, during winter the rice ecosystem changes to wheat ecosystem with cool and dry climate. And after wheat before rice cultivation maize is cultivated. Thus there is one type of balance biological change in agro ecosystem. But due to more demand of crops, the double and triple farming system is increasing. Rice is cultivated twice and many more crops between two successive cultivation of rice. Also rice intensification is increasing. The crops are growing in mixed pattern cropping patterns have undergone drastic changes over the years. Provision of irrigation, development of short duration, photo-sensitive & high yielding varieties crops have resulted in growing number of crops in a year. And according to the producers and farmers interest and scope the one crop has been replaced by other crop.

The population is increasing gradually and the demand of foods and vegetables are growing very high. Due to the high demand and better scope of selling and increasing market, farmers are more attracted to the vegetable production, both seasonal and off- season. As a result, the rice ecosystem is converted to vegetable ecosystem. Similarly, due to the lack of the farm labors for wheat threshing and easiness of threshing rapeseed and mustard, the farmers nowadays prefer oil to wheat and wheat is replaced by oil seed crops. Also the government policy has played a vital role for the biological changing in agro ecosystem as priority is given to the different crops such as tobacco, sugarcane, jutes, etc. when one type of agro ecosystem changes to another, changes in other biotic components also occurs. When rice ecosystem is replaced by vegetables, terrestrial weeds will replace aquatic and semi-aquatic weeds. The rice field contains weeds like Monochoris vaginalis, Lemna spp, Spirodella spp, etc. but as soon as the rice is replaced weeds are also replaced as each crop has its own characteristics weeds and they thrive well when the same crop is grown successively.

Likewise, the changing of crops in agro system also has effects or dynamic change on insect and pests species. The insects and pests are also specific according to the crops grown. During the growing of rice, insects like rice grasshoppers, leaf hoppers, etc are infested on rice and they are absent during growth of other crops. Similarly, sorghum earhead bugs, sorghum shoot bugs, etc are infested in sorghum, wheat aphid, wheat army- worm, wheat weevil, etc. in wheat and maize borer, maize European corn borer and maize Asian maize borer is incurred in maize. Also there is effect of changing crops on diseases and their pathogens. Respective family and crops have their own specific disease and causal organisms, it occurs only in the specific host plants.

Thus the agro ecosystem is the dynamic process and human engineered system which are always changing with respect to farmers need for livelihood and with government policy. Nowadays due to different factors there are more aggressive biological changes occurring in our agro ecosystem such as mixed cropping, intercropping, crop rotation, change in cropping pattern, etc. the shifting cultivation has been changed into the permanent system of the agriculture. These all changes are the demand of the present situation of the population growth and food insecurity, also the agricultural productive land are vanishing due to more housing, settlement and constructive work. The biological changes occurring resulted in the more production and yield and also controlled the insects, weeds and pests biologically itself due to crop rotation, intercropping, etc. Thus in order to meet the demand of growing population for basic needs of food, different biological changes on the cropping system is effective within agricultural ecosystem which seems not only to be necessary but also mandatory in the present context.