Petr Keil

Introduction Differences in species composition between communities in space and time have been quantified by various indices, with a heated debate about which are better for different purposes (Brualdi and Sanderson 1999, Gotelli 2000, Vellend 2001, Koleff and Gaston 2002, Gaston et al. 2007, Jost 2007, Jurasinski et al. 2009, Ulrich et al. 2009, 2017, Tuomisto 2010b, Baselga 2010, 2012, Podani and Schmera 2011, Almeida-Neto et al. 2012, Chao et al. 2012, Carvalho et al. 2012, Newbold et al.

1 Introduction Excessive loss of biodiversity via species extinctions and extirpations is a serious threat to human wellbeing and ecosystem functioning (IPBES 2019). Assessments of how fast and where species disappear are thus necessary to identify causes of the loss and for effective conservation decisions. The problem with such assessments is that both species diversity and its loss strongly depend on the area over which they are assessed.

Abstract Assessment of the rate of species loss, which we also label extinction, is an urgent task. However, the rate depends on spatial grain (average area A) over which it is assessed; local species loss can be on average faster, or slower, than regional or global loss. Ecological mechanisms behind this discrepancy are unclear. We propose that the relationship between extinction rate and A is driven by two classical ecological phenomena: the Allee effect and the Janzen-Connell effect.