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The graph below shows the population figures of different types of wild birds in the United Kingdom between 1970 and 2004.
Summarize the information by selecting and reporting the main features, and make comparisons where relevant.

The line chart shows the changing population numbers of three different types of wild birds, and then a cumulative number of all wild bird species in the United Kingdom from 1970 to 2004.

The index shows that 1970 is represented by the dotted line at 100, the changes in population figures are therefore given in relation to this. A similarity can be seen in the population figures for all the four categories during the first 7 years, growing slightly by around 10 index point.

However, figures shown in this chart observed dramatically different changes during the following 28 years. While the population of coastal species peaked at 140 index points, fluctuating around this figure slightly from 1991, the population of farmland species plunged to its lowest figure of approximately 53 index points in 1998 and stood at this lowest level until the end of the time period shown in this chart. After fluctuating between 90 and 100 index points before 1990, the population of woodland species remained steady at 90 index points from 1990 to 2004. As for the indication of the population for all species, we can see that this figure fluctuated between 100 and 110 index points throughout the period.

Overall, whilst there was a remarkable rise in the population of coastal species, the opposite trend was true for farmland species. At the same time, the number of woodland species experienced a slight downward trend; however, we can see that the average for all types of wild birds witnessed a marginal increase over this time period.

I am not an expert, but I have done all my best to assess your essay gaoyanjun. check my notes whether they are relevant or not if needed.
Introduction is not bad but you should avoid using the same verb (show) as provided in the description. You may use synonyms of it(illustrate, represent, depict)
Again using the same verb (show). A specific numbers are not willing to be used in the paragraph of the overview in my view.
I think it would be better to replace the word however to it can be seen from the graph that .
Too many words (258). Your essay should be reduced somehow. I would delete your second paragraph and replace it with the latest paragraph with changes of closing phrases into starting phrases.
Now check out my version to this graph.
The diagram depicts information about the population figures of various species of wild birds in the United Kingdom in 34 years time starting from 1970.
In the beginning all representatives had common index(100) however the trends illustrated quite fluctuated change during the period
It is clear from the graph that rates of all four species fluctuated slowly between 100 and 110 till 1977. Then coastal species went on rising steadily and reached the highest peak of 140, however this rate was followed by fluctuation in 10 indexes till end of the period. Farmland species decreased suddenly and the index dropped from 110 to about 65 in ten years time. Then there was a slow declining for the whole last period after a slight increase in 1989. Woodland species rate showed unsteadiness between the year of 1977 and 1991(in 85-90 indexes). Then it remained stable till the end of the period. The trend of all species represented the same fluctuation as woodland species with a little difference, however all species never fell under 100 during the period.
All in all 20 of coastal species owned the highest index among 111 other species in United Kingdom.