Functional and phylogenetic diversity of bird assemblages are filtered by different biotic factors on tropical mountains

Abstract

Aim It is not yet clear whether similar mechanisms influence the assembly of ecological communities across different continents. Here, we investigated the functional and phylogenetic diversity of bird assemblages along elevational gradients in two biogeographic regions in order to identify how these are driven by biotic factors, such as food resources and vegetation structure, and abiotic factors, such as ambient temperature and precipitation. Location Two 2,000‐m elevational gradients in the Ecuadorian Andes and on Mount Kilimanjaro, Tanzania. Taxon Forest‐dwelling bird species. Methods We recorded bird species abundances on 18 and 30 plots in the Ecuadorian Andes and on Mount Kilimanjaro respectively. We measured 10 functional morphological traits, related to bird feeding and movement, and utilized bird phylogenies to compare observed values and null‐model corrected effect sizes of functional and phylogenetic diversity along elevational gradients and between biogeographic regions. Furthermore, we assessed how observed values and effect sizes of functional and phylogenetic diversity were associated with the underlying gradients in available food resources, vegetation structure, temperature and precipitation. Results Functional and phylogenetic diversity were generally higher in species assemblages in the Ecuadorian Andes than on Mount Kilimanjaro. Both observed values and effect sizes of functional and phylogenetic diversity decreased significantly with increasing elevation in both biogeographic regions. Functional diversity consistently increased with increasing resource availability, whereas phylogenetic diversity increased with increasing vegetation heterogeneity and canopy closure in both biogeographic regions. Temperature and precipitation were not significantly associated with functional and phylogenetic diversity. Main conclusions Our results suggest that in both mountain systems the diversity of functional traits in bird species assemblages is the result of environmental filtering by available food resources, whereas phylogenetic diversity is primarily limited by vegetation structure. These findings suggest important differences in the main drivers of functional and phylogenetic diversity. We conclude that biotic factors might be more important for driving bird diversity patterns than abiotic factors and that a loss of resource availability and vegetation structure, e.g., through human impacts, is likely to trigger changes in community assembly on tropical mountains.

Biotic interactions help explain variation in elevational range limits of birds among Bornean mountains

“…Of course, change in abiotic factors across elevations may reflect indirect effects beyond physiological constraints, such as the availability of food (Ferger, Schleuning, Hemp, Howell, & Böhning-Gaese, 2014;Hanz et al, 2019;Janzen, 1973), given that adults of most species do not appear to be physiologically limited by temperature (Londoño et al, 2017). Indeed, the existence of elevational reversals in some pairs of Andean birds (Remsen & Cardiff, 1990;Remsen & Graves, 1995) also makes it unlikely that direct effects of abiotic factors are the primary limiting factor.…”

Forest structure determines spatial changes in avian communities along an elevational gradient in tropical Africa

“…Kilimanjaro. Hanz et al () reported that avian functional diversity increased with increasing resource availability, whereas phylogenetic diversity increased with increasing heterogeneity of vegetations and canopy closure along elevational gradients in Ecuador and Tanzania. However, to our knowledge, our study is the first to bring information about the role of detailed forest parameters.…”

The biotic interactions hypothesis partially explains bird species turnover along a lowland Neotropical precipitation gradient

“…Environmental gradients allow separation of stochastic from deterministic drivers of ecological communities Condit et al, 2002;Jankowski, Ciecka, Meyer, & Rabenold, 2009;Jankowski et al, 2012;Legendre, Borcard, & Peres-Neto, 2005;Swenson et al, 2011;Whittaker, 1960). In particular, latitudinal and elevational gradients, along which temperature, precipitation and possibly productivity vary, have been studied intensively to help identify the extent to which different biotic and abiotic variables determine community composition (Hanz et al, 2019;Jankowski et al, 2009Jankowski et al, , 2012Kraft et al, 2011;Qian & Ricklefs, 2007;Rodríguez & Arita, 2004;Swenson et al, 2011).…”

“…The harsh versus benign environment dichotomy that gave rise to the BI hypothesis is present in dry versus wet lowland tropical forests, yet this hypothesis remains largely untested in Neotropical bird communities (but see Hanz et al, 2019 for an analysis of bird community turnover along a temperature and precipitation gradient). Analogous to latitudinal and elevational temperature gradients, climatic harshness usually increases as precipitation decreases (when controlling for temperature), and the potential for biotic interactions (intra-and inter-trophic) increases as precipitation increases.…”