Teaching and the Division of Entomology
Nicholson arrived in Australia in 1921 and taught entomology at the University of Sydney. In 1930 he joined the Council for Scientific and Industrial Research (CSIR) in Canberra.
He became acting head of its entomology division in 1933; the appointment was confirmed in 1936. He remained chief until retiring in 1960. The division, subsequently part of CSIRO, combined agricultural insect problems with basic biological research.
Density feedback as an explanation to test
Nicholson sought to explain why populations neither increased indefinitely nor generally disappeared. In his account, changes in numbers altered competition for resources, which then affected survival and reproduction.
With V. A. Bailey he developed mathematical treatments of interactions such as those between hosts and parasites. Density feedback was central to his interpretation of regulation, while external conditions could also modify population changes.
References: [1]
Controlling resources in insect cultures
Laboratory cultures allowed Nicholson to vary resource supply and follow successive generations. Sheep blowfly experiments examined how competition for larval food affected later adult numbers and reproduction.
Effects were separated in time: larval competition could influence adults and the next generation only after development. Controlled external conditions therefore did not necessarily produce constant numbers; the cultures could show continuing oscillations.
References: [1]
Forms of competition and research organization
Nicholson distinguished competition in which many individuals shared and exhausted resources from arrangements in which some secured resources while others were excluded. He examined their different consequences for population dynamics.
As division chief he supported taxonomy, physiology and other specialisms alongside practical entomology. His population research developed within this broader program rather than apart from agricultural insect science.