Changing densities of generalist species underlie apparent homogenization of UK bird communities

Abstract

13Generalist species are becoming increasingly dominant in European bird communities. This has been 14 taken as evidence of biotic homogenization, where generalist 'winners' systematically replace 15 specialist 'losers'. We test this by relating changes in the average specialisation of UK bird 16 communities to changes in the density of species with different degrees of habitat-specialisation. 17Although we find the expected continued decline in community specialisation, this was driven by a 18 combination of a strong increase in the density of the most generalist quartile of species and declines 19 in the density of moderately generalist species. Contrary to expectation, specialist species increased 20 slightly over the 18 year study period, but had little effect on the overall trend in community 2Changes to the environment, such as climate change and land-use intensification, do not affect all 28 species equally (Rader et al. 2014). Habitat specialists may be more vulnerable to environmental 29 change than habitat generalists, due to their more restricted habitat requirements and potentially lower 30 ability to exploit new opportunities (Shultz et al. 2005). Environmental change could therefore lead to 31 a loss of differentiation in species composition between habitats, as a few generalist 'winners' replace 32 specialist 'losers ' (McGill et al. 2015). This is supported by negative relationships between 33 population growth rate and specialisation in a wide range of taxa (Munday 2004, Matthews et al. 34 2014, Timmermann et al., 2015, including birds (Julliard et al. 2004, Jiguet et al. 2007, Salido et al. 35 2012, and by observations of communities becoming increasingly composed of individuals of 36 generalist species (Davey et al. 2012, Timmermann et al. 2015. 37However, while individuals of generalist species make up an increasing proportion of 38European bird communities (Davey et al. 2012, Le Viol et al. 2012, it is unclear whether these 45We follow changes in UK bird communities over 18 years (2598 ± 597 SD 1 km squares 46 monitored each year). Our aims are to (1) quantify changes in community specialisation and (2) assess 47 the extent to which these changes reflect changes in the populations of specialist and generalist 66In this study, we used records in the first two bounded distance bands, and excluded records 67 of flying birds, with the exception of swifts, hirundines and raptors, as these species either are aerial 68 feeders or hunt from the air, so flying birds of these species are likely to be using resources within the Table S1 for a list of species). 79In order to turn raw abundances into estimates of density, we estimated detection probabilities Table S2 for other broader habitat classes). These covariates allow variation 88 in detectability over the breeding season and between habitats to be modelled. These models were 89 used to predict the probability of individuals of a species being detected in each transect section, and 90 these were averaged per species to obtain the pre...

The sensitivity of breeding songbirds to changes in seasonal timing is linked to population change but cannot be directly attributed to the effects of trophic asynchrony on productivity

“…Species which have advanced their lay dates most substantially exhibit more positive population trends over the last three decades. Advances in spring migration arrival dates have similarly been found to correlate with population trends Newson et al, 2016).…”

“…For example, the observed relationship may be a consequence of habitat and/or diet specialization being simultaneously correlated with both population change and sensitivity (Miller-Rushing, Høye, Inouye, & Post, 2010). More specialized species may show greater sensitivity to asynchrony than generalists, and are also likely to be more sensitive to changes in habitat quality, land cover and other pressures; their populations have tended to decline relative to the most widespread generalists in the UK (Sullivan, Newson, & Pearce-Higgins, 2016).…”

Roads as a contributor to landscape-scale variation in bird communities

“…Shifts in species assemblages in areas of high human disturbance have been identified in both urban 4,5 and agricultural 57 environments, and in response to climate change 5,6 . Rather than declines of so-called 'loser' species happening in isolation, simultaneous replacement of those species by expanding 'winner' species occurs 3,7,8,58 . These processes, it is suggested, are leading to homogenisation, or simplification, of biodiversity in large areas.…”

“…Extinctions and population declines are widespread 1,2 , but not evenly spread across taxa. It has been argued that differences in species' abilities to tolerate anthropogenic disturbance are leading to the simplification of species assemblages in human-disturbed environments [3][4][5][6][7][8] .…”

Do conservation designations provide positive benefits for bird species and communities?

“…Thus, positive effects of PA extent were most apparent for species associated with woodland and wetland habitats, both relatively rare and fragmented natural or semi-natural habitats in the British countryside (Martay et al 2018) that have been the target of much conservation effort. The pattern of increased abundance of urban species is indicative of wider increases in generalist species (Sullivan et al 2016) and outside pressures on PA generally. While the lack of a general relationship with abundance trend may indicate that PA are not being appropriately managed, the interpretation of such patterns is complex and requires detailed consideration (Wauchope et al 2021).…”