Temperate grassland songbird species accumulate incrementally along a gradient of primary productivity

Abstract

Global analyses of bird communities along elevation gradients suggest that bird diversity on arid mountains is primarily limited by water availability, not temperature or altitude. However, the mechanism by which water availability, and subsequently primary productivity, increases bird diversity is still unclear. Here we evaluate two possible mechanisms from species-energy theory. The more individuals hypothesis proposes that a higher availability of resources increases the total number of individuals that can be supported, and therefore the greater number of species that will be sampled. By contrast, the more specialization hypothesis proposes that increasing resource availability will permit specialists to exploit otherwise rare resources, thus increasing total diversity. We used 5 years of surveys of grassland songbird communities along an elevational gradient in British Columbia, Canada, to distinguish between these hypotheses. Vegetation changed markedly in composition along the gradient and contrary to the expectations of the more specialization hypothesis, bird community composition was remarkably constant. However, both total abundance and species richness of birds increased with increasing water availability to plants. When we used rarefaction to correct species richness for differences in total abundance, much of the increase in bird diversity was lost, consistent with the expectations of the more individuals hypothesis. Furthermore, high species richness was associated with reductions in territory size of common bird species, rather than the fine-scale spatial partitioning of the landscape. This suggests that bird diversity increases when greater resource availability allows higher densities rather than greater habitat specialization. These results help explain a pervasive global pattern in bird diversity on arid mountains, and suggest that in such landscapes conservation of grassland birds is strongly linked to climate and hydrology.

Urbanization and species-energy relationships

“…the Great Kiskadee (Pitangus sulphuratus) or the Rufous Hornero (Furnarius rufus). On the other hand, the NP hypothesis is consistent with a gradual change of species composition along the disturbance gradient (Harrower et al 2017). Indeed, our data showed that bird communities along the urban-rural gradient were dominated by different species (Table 2).…”

“…The three processes tested have different management decisions to increase species richness along urbanization gradients (Harrower et al 2017). If the HH hypothesis was supported, only efforts to create more diverse habitats would be adequate.…”

“…Moreover, as the number and quantity of resources increases, they are divided more equitably among a greater variety of foraging habitats; then, a lower dominance is expected as energy availability and habitat diversity increase (Hurlbert 2004). Due to this increase of new resource types changes along the gradient, a shift of species composition is expected because generalist species are replaced by specialist species (Harrower et al 2017).…”

Metrics for conservation success: Using the “Bird‐Friendliness Index” to evaluate grassland and aridland bird community resilience across the Northern Great Plains ecosystem

“…These regions tend to be drier with more sparse vegetation, as well as lower soil moisture and productivity. While these regions provide critical habitat for aridland bird species such as Brewer's Sparrow, they are less suitable for grassland birds that prefer lusher and more productive habitats (Fedy et al., 2018; Fisher & Davis, 2010; Harrower et al, 2017; Renfrew et al., 2013). As a result, they have fewer bird species, many of which share similar functional traits (e.g., ground‐dwelling insect‐ and granivores; Table 1).…”

Avian species richness and abundance show stronger responses to bison grazing intensity than to ecosystem productivity

“…Higher productivity supports a greater number of individuals, thereby reducing local species extinction rates and supporting a greater number of species (Srivastava & Lawton, 1998). Harrower et al (2017) suggest that for grassland songbirds, higher productivity may translate to greater availability of resources such as plant biomass and prey abundance, thus indirectly supporting avian abundance and species richness. Across North America, the most productive areas have undergone extensive land‐use change and are overwhelmingly under heavy human use (Scott et al, 2001).…”