Functional Traits Predict Sensitivity of Forest Birds to Habitat Disturbance in Tropical Countryside

This blog post is provided by Ian Ausprey and tells the #StoryBehindthePaper for the paper ‘Functional response traits and altered ecological niches drive the disassembly of cloud forest bird communities in tropical montane countrysides‘, which was recently published in Journal of Animal Ecology. In their study they examine how functional response traits related to morphology, diet, foraging behaviour and environmental niche breadth of birds predict species-specific abundance changes in countryside habitats compared to forests.

It’s a well-known fact that species composition changes across gradients of human disturbance, such as agricultural landscapes or “countrysides”. But why do some species appear while others disappear? To get at mechanistic reasons for the loss and turnover of species in Anthropogenic landscapes we hypothesized that a species’ sensitivity to habitat disturbance can be predicted by specific characteristics known as functional response traits. In particular, we turned to the idea of ecomorphology originally developed in the 1980s. Simply put, a species’ morphological traits tend to be tightly correlated with key facets of its ecology, such as diet or dispersal ability. A classic example is bill size and shape for birds: species that eat arthropods on the wing tend to have flat bills for flycatching, whereas those that persist on plant-based resources tend to have either long bills for probing nectar or thick bills for ripping fruit and cracking seeds.  For our study we conducted intensive surveys of bird communities in the species-rich Andes of northern Peru across a gradient of agricultural land use ranging from large tracts of contiguous primary forest to isolated forest fragments to early successional shrubby vegetation to silvopasture. We also safely captured birds using mist nets and measured their morphological traits before releasing them. We then combined these measurements with published databases on diet, foraging behavior, elevational range size and habitat breadth to construct a suite of 16 functional traits hypothesized to predict sensitivity to agricultural disturbance.

Peruvian field assistant Elio Nuñez Cortez measuring the bill of an Emerald Toucanet in the Andean cloud forests of northern Peru. Bill length was one of 9 morphological traits collected in the field and used to predict avian abundance changes across a gradient of agricultural land use change.

For the first half of our analysis we took a simple yet rarely used approach. Instead of using our traits to simply compare functional diversity metrics across the land use gradient (as done in most studies taking a functional approach) we used collections of functional response traits to actually predict changes in species-specific abundances between forest and each disturbance level. We did this by integrating hierarchical distance sampling abundance models that account for imperfect detection with functional trait groups while controlling for phylogenetic relationships. Key traits predicting increased abundance in early successional and mixed-intensity agricultural areas included (1) morphological adaptations to dense understory habitats (short-rounded wings, long tarsi), (2) small eyes that reduce glare in brightly lit environments, (3) plant-based diets (flowers, fruit, and seeds), and (4) broad elevational range limits and habitat breadth. Particularly interesting were results for elevational range breadth – species occupying mixed and high-intensity agricultural land use regimes had mean elevational range limits 20-60% wider than species found within forests. This is one of the first studies to demonstrate such relationships with elevational range breadth for birds and supports the general idea that generalist species adapted to a wider range of environmental conditions are less sensitive to Anthropogenic disturbance.

Forest specialists with narrow elevational range limits, such as the endemic Rusty-tinged Antpitta and Masked Fruiteater, tended to disappear in agricultural environments. Photos used by permission: Jon Irvine & Ian Ausprey.

We then used hypervolumes to calculate the total ecological niche space available to communities across the land use gradient for different functional trait groups. By overlapping community hypervolumes for communities within forests with those in different countryside habitats we found striking differences in how distinct functional trait groups explained changes to the ecological niche. Specifically, ecological niche space expanded within agricultural habitats for traits related to diet and environmental niche breadth, while contracting for foraging and dispersal traits. Even more interesting was the fact that such changes were driven by species with unique functional trait combinations. These included terrestrial species with unusually rounded wings and long legs, like the Undulated Antpitta, seedeaters with extremely thick bills like the Golden Grosbeak, and, perhaps the species with the most extreme example of morphological adaption – the Sword-billed Hummingbird.

Species with unusual functional trait combinations, such as the Undulated Antpitta, Golden Grosbeak and Sword-billed Hummingbird, tended to contribute the most to unique components of ecological niche space found within forests and countryside habitats. Copyright: Tom Cantwell, Brian Ralphs, & Ian Ausprey.

Collectively, our research reveals the dynamic changes to ecological niche space that underlie variation in community composition across agricultural landscapes and highlights how functional traits can be used to identify mechanistic changes in community structure across disturbance gradients. We particularly note the importance of forest fragments and large tracts of early successional vegetation that maintained high levels of functional diversity in our system, while cautioning that substantial amounts of functional diversity related to foraging and dispersal traits found within contiguous forest sites were lost in countryside habitats. By identifying key functional trait groups that aligned with different countryside habitats, we demonstrated how conservation practitioners can contribute to the retention of avian functional diversity in agricultural landscapes while emphasizing the central role that intact forests play in maintaining unique elements of ecological function throughout the world.

Read the paper

Read the full paper here: Ausprey, I.J., Newell, F.L. & Robinson, S.K. (2022). Functional response traits and altered ecological niches drive the disassembly of cloud forest bird communities in tropical montane countrysides.  Journal of Animal Ecology

We were lucky to spend over two years living with rural campesino communities while conducting our research. This provided ample outreach opportunities at local schools during which children learned about two of their famous avian neighbors with particularly striking functional traits: the Emerald Toucanet and the Marvelous Spatuletail Hummingbird!

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