Abundance across geographical species ranges and the rare edge hypothesis

Species geographical ranges have long fascinated biogeographers.Do species ranges have universal properties that provide predictive relationships or is each species' range idiosyncratic and unique in its essence?Perhaps something in between.One question that has been straightforward to ask, but complicated to answer, is whether species abundances (i.e., population occurences, sizes, density of individuals, etc.) peak in central areas of species ranges, also known as the abundant centre hypothesis (ACH) [1].If true, the ACH would have myriad ramifications for ecological and evolutionary theory, phenomena and practice, including predicting regions of species ranges with the highest diversity, conservation value, evolutionary rates and dispersal patterns, etc.In this issue, Martin et al. [2] ask this question and frame a related but new question and hypothesis: are peripheral areas of species ranges rare in species abundances?Examining species range patterns of abundance using a highly robust dataset of North American birds and a powerful modelling approach, Martin et al. offer compelling evidence for this 'rare edge hypothesis' (REH).They found consistently lower bird relative abundance and occupancy near species range edges in nearly all cases examined.Additionally, abundance most often peaked not at the strict centres of species ranges, but somewhere in the interior or inner half of ranges.Although it has been widely invoked, the ACH has also been questioned owing to doubts about its universality as a rule, as many studies have examined it and found inconsistent results over the past half century (e.g.[3][4][5]).It is challenging to evaluate hypotheses like the ACH and the REH.First, it is challenging to quantify abundance across a species' entire geographical range, especially for wide-ranging species, which can cover a continent or more.Different abundance patterns can appear similar depending on sampling density and coverage (figure 1), and so robust and even sampling is necessary to verify different patterns.Second, there are many ways to quantify abundance, including the frequency of occurrence, the number of 'populations,' the density of individuals, etc. [6].Which type of abundance, if not all, should be expected to conform to such patterns if they exist?Third, estimates of abundance are further complicated by the great diversity of species, with such varied life histories, modes and types of dispersal, mating systems, life spans, etc. Fourth, there are many methods to define, analyse and test for spatial patterns [5], including whether the 'centre' refers to the exact (strict) centre, or to interior, 'core' regions of species ranges [7].Finally, defining central and non-central areas of species ranges and their edges can be challenging, and sampling between central and edge regions is rarely even [2].Martin et al. proposed the REH and provided multiple tests of it and the ACH using some of the best available data on bird species abundance patterns to date.The results were strongly in agreement with the REH, even when various measures and considerations were taken.Using the publicly available eBird dataset of breeding birds of North America-collected mainly through community scientists-gave a very large dataset of occurrences across many bird species ranges from which relative abundance could be modelled.Birders themselves seem to be abundant in North America and provide relatively reliable data, regardless of where in the given species range a report comes from.Birders often report on multiple species of the bird