Delayed biodiversity change: no time to waste Franz Essl 1, 2, 3 , Stefan Dullinger 3 , Wolfgang Rabitsch 2 , Philip E. Hulme 4 , Petr Pys ˇek 5, 6 , John R.U. Wilson 1, 7 , and David M. Richardson 1 1 Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa 2 Environment Agency Austria, Spittelauer La ¨ nde 5, 1090 Vienna, Austria 3 Division of Conservation Biology, Vegetation and Landscape Ecology, Faculty Centre of Biodiversity, University of Vienna, Rennweg 14, 1030 Vienna, Austria 4 The Bio-Protection Research Centre, Lincoln University, PO Box 84850, Canterbury, New Zealand 5 Institute of Botany, Department of Invasion Ecology, The Czech Academy of Sciences, 252 43 Pru ˚ honice, Czech Republic 6 Department of Ecology, Faculty of Science, Charles University in Prague, Vinic ˇna ´ 7, 128 44 Praha 2, Czech Republic 7 South African National Biodiversity Institute, Kirstenbosch Research Centre, Claremont, 7735, South Africa Delayed biodiversity responses to environmental forcing mean that rates of contemporary biodiversity changes are underestimated, yet these delays are rarely addressed in conservation policies. Here, we identify mechanisms that lead to such time lags, discuss shifting human percep- tions, and propose how these phenomena should be addressed in biodiversity management and science. Environmental change and delayed biodiversity responses Biodiversity often does not respond immediately to changes in the physical or biotic environment. Consider- able time lags (i.e., relaxation times) are involved in these responses [1,2]. However, the mechanisms at work, and the factors (e.g., feedbacks or abrupt changes at thresholds) that mediate delayed biodiversity responses to environ- mental forcing are manifold and poorly understood. Con- sequently, the full spectrum of consequences of rapid environmental changes on biodiversity is difficult to con- textualize and, thus, the implications are easily under- estimated. Here, we expand on recent assessments of these phenomena [2] by formulating 12 mechanisms that con- tribute to delayed biodiversity responses (Table 1), discuss how human perceptions regarding environmental change typically result in slow societal response, and then provide priorities for science and management on how to address this problem to mitigate future attrition of biodiversity. Progress and limitations in understanding the mechanisms of delayed biodiversity responses Delayed biodiversity responses to environmental forcing have been well studied over the past two decades, espe- cially in the context of habitat loss and fragmentation. Mechanisms and implications have been intensively ex- plored after the phenomenon of extinction debt was pro- posed by Tilman et al. [1], likely because habitat destruction is a dominant and clearly visible feature of environmental change and because it is relatively straight- forward to quantify (e.g., by using time-series of changes in habitat extent that can be backcast using historical maps). However, recent work has highlighted that biodiversity shows substantially lagged responses to other changes in biotic and abiotic pressures (e.g., [3,4]), and more needs to be done to elucidate the consequences of these processes (Table 1). Moreover, studies on delayed biodiversity responses have usually focused on particular kinds of environmental change in isolation (e.g., habitat loss or range filling of invasive species) and on one or a few taxonomic groups. In reality, such changes often act simul- taneously upon multiple components of biodiversity. For instance, habitat loss causes qualitative changes in the remaining habitat (e.g., due to edge effects), and alters connectivity between remaining habitat patches [4], simul- taneously affecting genes, species, and communities. In the worst case, such changes result in losses of ecosystem services that contribute to human well-being, but often with delays of several decades [2,5]. Delayed biodiversity responses can, of course, lead to increases as well as declines in population sizes and species diversity in communities. Available evidence suggests that relaxation times arising from the same pressure differ in length for biodiversity losses and gains. For instance, projections of the ranges of plant species in mountains under climate change have suggested that range losses at the trailing edge will take longer to unfold than range gains at the leading edge [5], thus creating a transient species surplus. Shifting baselines undermine the assessment of long- term environmental changes Humans tend to undervalue environmental changes that unfold slowly and incrementally over timescales of decades relative to those that play out over a single human gener- ation or less. In fact, assessments of environmental changes are often based on shifting baselines. As humans, we adjust our perception of the state of the environment unconsciously, based on recent impressions, giving scant attention to earlier changes [6]: the abnormal becomes the new normal. For instance, climate change during the 21st century will likely be drastic, and the global surface air temperature will probably exceed the limit of dangerous Forum 0169-5347/ ß 2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tree.2015.05.002 Corresponding author: Essl, F. (franz.essl@univie.ac.at). Keywords: extinction debt; global change; management; relaxation time; shifting baselines; time lags. TREE-1951; No. of Pages 4 Trends in Ecology & Evolution xx (2015) 1–4 1