At the
University of Cambridge, Willis held a
postdoctoral research fellowship at
Selwyn College, a
Natural Environment Research Council (NERC) postdoctoral fellowship in the Department of Plant Sciences, and a
Royal Society University Research Fellowship (URF) in the Godwin Institute for Quaternary Research. In 1999, she moved to a lectureship in the School of Geography and the Environment at the
University of Oxford, where she established the Oxford Long-term Ecology Laboratory in 2002. Willis was made a professor of long-term ecology in 2008, and in October 2010 became the first Tasso Leventis Professor of Biodiversity and director of the James Martin Biodiversity Institute in Zoology. She was also an adjunct professor in the Department of Biology at the
University of Bergen, Norway. She is a trustee of
WWF-UK, a panel member on the advisory board for the
Commonwealth Scholarship Commission, a trustee of the
Percy Sladen Memorial Trust, an international member on the
Swedish Research Council's FORMAS evaluation panel, and a college member of the UK
Natural Environment Research Council (NERC). From 2012 to 2013 she held the elected position of director-at-large of the International
Biogeography Society. In 2013, she was appointed Director of Science at the
Royal Botanic Gardens, Kew, On 1 October 2018, Willis succeeded
Keith Gull as Principal of
St Edmund Hall, Oxford. Willis's research focuses on reconstructing long term responses of
ecosystems to environmental change, including
climate change,
human impact and
sea level rise. She argues that understanding long-term records of ecosystem change is essential for a proper understanding of future ecosystem responses. Many scientific studies are limited to short-term datasets that rarely span more than 40 to 50 years, although many larger organisms, including trees and large mammals, have an average generation time which exceeds this timescale. Short-term records therefore are unable to reconstruct natural variability over time, or the rates of migration as a result of environmental change. She also argues that a short-term approach gives a static view of ecosystems, and leads to the conceptual formation of an unrealistic "norm" which must be maintained or restored and protected. Her research group in the Oxford Long-term Ecology laboratory therefore attempts to reconstruct ecosystem responses to environmental change on timescales ranging from tens to millions of years, and the applications of long-term records in
biodiversity conservation. She has argued that the impacts of contemporary climate change on plant biota is uncertain and potentially not as severe as researchers envision, and challenged assumptions made in the interpretation of spatially constrained temperature records. Kew's ''State of the World's Plants'' report (2016) pinpoints land cover change as the major threat to global biodiversity, not climate change. Willis's research has been published in
Nature,
Science,
Philosophical Transactions of the Royal Society B,
Biological Conservation.{{Cite journal | doi = 10.1016/j.biocon.2011.11.001| title = Determining the ecological value of landscapes beyond protected areas| journal = Biological Conservation| volume = 147| pages = 3–12| year = 2012| last1 = Willis | first1 = K. J. | last2 = Jeffers | first2 = E. S. | last3 = Tovar | first3 = C.| last4 = Long | first4 = P. R. | last5 = Caithness | first5 = N.| last6 = Smit | first6 = M. G. D. | last7 = Hagemann | first7 = R.| last8 = Collin-Hansen | first8 = C.| last9 = Weissenberger | first9 = J. ==House of Lords==