Full census results are available from 1841 until 1911 at present, but censuses were also taken in the years 1801, 1811, 1821 and 1831. Only summaries of findings from these censuses are available; apparently, the census forms themselves were destroyed. Nonetheless, it does permit us to get a glimpse of the population of this country as a whole, and focused on the four parishes of Lewis in particular. These findings have been published on the Histpop.org website.
Two conclusions can be drawn: the population of the island doubled in the 40 years between 1801 and 1841, and the clearance of the Park district had no impact on the population of the parish of Lochs. And neither has the migration of 1851 had a major impact, as shown on the 1861 census.
I will pose a personal opinion at this point. Migration (forced or otherwise) appears to have had little impact on Lewis in terms of numbers. This does not mean that the 'Clearances', which did happen during the 19th century in Lewis, had a negligible impact.
The Park area, cleared in the years up to 1821, remains virtually depopulated today; by Park I mean in this context the area south of Loch Shell and east of Loch Seaforth.
The execution of the Clearance of 1851 is graphically described in the diaries of the Chamberlain of the Lews - graphic for its lack of compassion.
If explanation is sought for my interest in derelict villages, you can find it in these facts. Whether through compulsion, or through economic necessity, each former township represents lives lived, and lives ripped from their roots.
With thanks to fellow researcher Direcleit for highlighting the source website.