(Less) ① First, select large-scale quadrats, identify and count the names and quantities of each species. ② According to the introduction of April 1st, in mowing grassland, with the increase of mowing intensity, the aboveground biomass of grassland showed a downward trend as a whole, but the plant species in No.4 Middle School showed an increasing trend.

(3) The increase of grazing and mowing will lead to the increase of soil salinization, but laying hay layer can effectively control saline-alkali land, because under the action of decomposers, inorganic nutrients of hay continue to return to the soil, which is beneficial to the growth of plants and can effectively control saline-alkali land.

(2) From the changes of the number of five plants in the past five years, it can be seen that the population density of ragweed is gradually increasing, while the population density of native species is gradually decreasing, indicating that improper introduction of organisms will lead to ecological invasion, so the basic principle of ecological engineering that urban greening projects should follow is mainly the principle of coordination and balance.

(3) In Section A ~ of No.4 Middle School, with the increase of population density, the competition for living conditions such as food and space intensified, and the number of natural enemies also increased, which led to the increase of locust survival resistance and the decrease of growth rate.

② Point A in No.4 Middle School is the point with the fastest population increase. In order to effectively control the locust plague, the population should be controlled in time before point A. Using sex attractants to lure and kill male insects will change the sex ratio of the population and lead to a decline in the birth rate, thus preventing the emergence of point C. 。

The purpose of locust control is to make energy flow to beneficial parts of human beings continuously and efficiently.

So the answer is:

(Less) ① Name (species, richness) and quantity (biomass) of each species.

② aboveground biomass plant species (biodiversity, species richness)

③ Under the action of decomposers (microbial decomposition).

(2) the principle of coordination and balance

(3)① With the increase of population density, the competition for living conditions such as food and space intensifies, and the number of natural enemies also increases. H's sp; & amph's sp; & amph's sp; & amph's sp;

②a & amp； H's sp; & amph's sp; & amph's sp; & amph's sp;

③ Sex ratio &; H's sp; & amph's sp; & amph's sp; & amph's sp;

(4) Make energy flow to the part beneficial to human beings continuously and efficiently.