title: Abundance distributions for tree species in Great Britain: A two-stage approach to modeling abundance using species distribution modeling and random forest.
abstract: High-quality abundance data are expensive and time-consuming to collect and often highly limited in availability. Nonetheless, accurate, high-resolution abundance distributions are essential for many ecological applications ranging from species conservation to epidemiology. Producing models that can predict abundance well, with good resolution over large areas, has therefore been an important aim in ecology, but poses considerable challenges. We present a two-stage approach to modeling abundance, combining two established techniques. First, we produce ensemble species distribution models (SDMs) of trees in Great Britain at a fine resolution, using much more common presence-absence data and key environmental variables. We then use random forest regression to predict abundance by linking the results of the SDMs to a much smaller amount of abundance data. We show that this method performs well in predicting the abundance of 20 of 25 tested British tree species, a group that is generally considered challenging for modeling distributions due to the strong influence of human activities. Maps of predicted tree abundance for the whole of Great Britain are provided at 1 km2resolution. Abundance maps have a far wider variety of applications than presence-only maps, and these maps should allow improvements to aspects of woodland management and conservation including analysis of habitats and ecosystem functioning, epidemiology, and disease management, providing a useful contribution to the protection of British trees. We also provide complete R scripts to facilitate application of the approach to other scenarios.
authors.affiliation: Department of Plant Sciences University of Oxford Oxford UK.
authors.forename: Louise
authors.identifier: 0000-0002-8061-4226
authors.initials: L
authors.lastname: Hill
authors.affiliation: Department of Plant Sciences University of Oxford Oxford UK.
authors.forename: Andy
authors.initials: A
authors.lastname: Hector
authors.affiliation: Sylva Foundation Sylva Wood Centre Long Wittenham Oxfordshire UK.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Hemery
authors.affiliation: Centre for Ecology & Hydrology Lancaster Environment Centre Bailrigg Lancaster UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Smart
authors.affiliation: Department of Plant Sciences University of Oxford Oxford UK.
authors.forename: Matteo
authors.initials: M
authors.lastname: Tanadini
authors.affiliation: Department of Plant Sciences University of Oxford Oxford UK.
authors.forename: Nick
authors.identifier: 0000-0002-9955-7858
authors.initials: N
authors.lastname: Brown
keywords: abundance distributions; abundance–occupancy relationships; biotic effects; mapping