Synergistic impacts of aggressive species on small birds in a fragmented landscape

Abstract

Attempts to conserve threatened species in fragmented landscapes are often challenging because factors such as habitat loss, habitat degradation and dominant species interact to reduce threatened species’ capacity to survive and reproduce. Understanding how threatening and mitigating processes interact is critical if conservation measures are to be effective. We used data from long‐term monitoring of bird populations and multivariate latent variable models to quantify how Australian woodland birds respond to the presence of the Noisy Miner, a despotic species known to exclude other bird species. We then investigated the extent to which the presence of other aggressive species exacerbates the impacts of the Noisy Miner, and to what extent these impacts can be mitigated by dense midstorey plantings. We found strong synergies between the Noisy Miner and two other aggressive species (Grey Butcherbird and Pied Butcherbird), despite weak effects of butcherbirds in isolation. The impacts of aggressive birds are most pronounced for small‐bodied species, but these impacts are lessened in the presence of high midstorey cover. Synthesis and applications. Accounting for interactions reveals that revegetation may be capable of improving conservation outcomes even when the proximate cause of species declines (i.e. exclusion or predation by aggressive bird species) cannot be managed directly.

Long‐term monitoring in endangered woodlands shows effects of multi‐scale drivers on bird occupancy

“…The Noisy Miner has been shown to strongly defend individual habitat patches and is widely known to influence the range of bird species that can exist at a location (Maron et al, 2013). While not strictly an environmental attribute of a patch, we follow Westgate et al (2021) in treating Noisy Miner occupancy as an environmental attribute, rather than a response variable like other bird species. Our predictor variables also included the interaction term between Noisy Miner presence and midstorey, as the impacts of Noisy Miners can be mitigated by higher midstorey (Westgate et al, 2021).…”

“…Investigating large‐scale processes is particularly important where a vegetation type is sufficiently widely distributed that there is natural turnover of its dependent faunal communities across climate or latitudinal gradients. An additional challenge is that the assemblage of co‐occurring fauna present at a patch, and their ecological interactions, can also impact a species' ability to persist there (Maron et al, 2013; Westgate et al, 2021).…”

“…Our investigation spanned long‐term studies distributed across nine degrees of latitude (~900 km) and encompassed a strong climate gradient (Lindenmayer, Lane, Crane, et al, 2018), with factors like rainfall known to influence the extent of development of understorey vegetation (Specht & Morgan, 1983). Understorey, in turn, is an important factor influencing the occurrence of a range of bird species in Box‐Gum Grassy Woodlands, including an array of taxa of conservation concern (Montague‐Drake et al, 2009) and despotic species like the hyper‐aggressive native honeyeater, the Noisy Miner ( Manorina melanocephala ), which can drive away other (typically smaller‐bodied) species (Maron et al, 2013; Westgate et al, 2021). There are also differences in levels of woody vegetation cover in landscapes surrounding Box‐Gum Grassy Woodlands—from 3% to 27% in the southern half of our study region—and this too can influence the occurrence of bird species (Lindenmayer et al, 2020).…”

“…Furthermore, a large body of research has shown that patches occupied by the aggressive Noisy Miner support fewer birds from other species, and we expected to observe this same pattern in our study (Maron et al, 2013). However, the presence of understorey vegetation is known to be a prominent factor limiting the presence and abundance of despotic taxa like the hyper‐aggressive Noisy Miner that otherwise have marked negative impacts on smaller bodied bird taxa in temperate woodlands (Maron et al, 2013; Westgate et al, 2021).…”

Preserving wintering frugivorous birds in agro‐ecosystems under land use change: Lessons from intensive and super‐intensive olive orchards

“…Birds were identified and counted within a small band (30 m) to each side of transects (Bibby et al., 1992), to minimise variations in bird detection across habitat types and sampling occasions (e.g. weather, time of day, observer; Shave et al., 2018; Westgate et al., 2021). This detection distance was set considering other bird studies in olive (Rey, 1995) and other fruit orchards (Shave et al., 2018), and following a preliminary study consistently showing high bird detectability within this band width.…”