The phrase "are declining in number" is correct and usable in written English.
It can be used when discussing a decrease in the quantity or population of something over time.
Example: "The species of certain birds are declining in number due to habitat loss and climate change."
Alternatives: "are decreasing in quantity" or "are diminishing in population."
The authors estimate that gorillas, already a critically endangered species, are declining in number by 2.7percentnt each year.
While most respondents still felt exploiting the gas would bring economic benefits to the UK, those people are declining in number.
Three-quarters of Britain's 59 native species of butterfly are declining in number and a countryside completely bereft of butterflies is already a reality: 38 summer visits to the WCBS 1km squares recorded no butterflies at all in 2012.
The analysis estimates that at least one quarter of the 5487 species are threatened with extinction and that one half are declining in number.
Entire populations of birds have dwindled because of radiation from the nuclear power plant in Chernobyl, but some species are declining in number at a rate far greater than others are, a new study finds.
The nomadic pastoralists are declining in number and resort to all means of occupation, exacerbated due to efforts of the governments to sedentarize them as well as due to severe environmental conditions such as droughts which force the nomads to search for wages in urban areas.
The khaps are declining in numbers, Mr. Chautala added.
"Family farms are declining in numbers, commercial farms are becoming larger and increasingly corporate, entry into the industry is increasingly difficult and the average age of farmers is worryingly high," said land campaigner, Simon Fairlie.
As my colleague Andy Revkin reported in his Dot Earth blog two years ago, 7 of the 19 subpopulations of polar bears in the Arctic are declining in numbers and only one is increasing.
About half the species are declining in numbers, including one in five of those not at risk for extinction right now.
Older, white voters are declining in numbers, recent immigrants' figures are rising.
