Bird diverzity along altitudinal gradients

Results from the project

This project was started in 2011, as the main PhD topic of Katerina Sam. The field work was conducted between 2011 and 2013, and one more survey was added in 2015. So far, we published 4 papers about avifauna of Mt. Wilhelm, while we have the last one (2015) submitted as a part of the project about El Nino.

Sam, K., Koane, B., Bardos, D. C., Jeppy, S., & Novotny, V. Species richness of birds along a complete rain forest elevational gradient in the tropics: Habitat complexity and food resources matter. Journal of Biogeography.
Marki, P. Z., Sam, K., Koane, B., Kristensen, J. B., Kennedy, J. D., & Jønsson, K. A. (2016). New and noteworthy bird records from the Mt. Wilhelm elevational gradient, Papua New Guinea. Bull. Brit. Ornitholog. Club, 137(4), 263-271.
Colwell, R. K., Gotelli, N. J., Ashton, L. A., Beck, J., Brehm, G., Fayle, T. M., … Sam, K., … & Klimes, P. (2016). Midpoint attractors and species richness: modelling the interaction between environmental drivers and geometric constraints. Ecology letters, 19(9), 1009-1022.
Sam, K., & Koane, B. (2014). New avian records along the elevational gradient of Mt. Wilhelm, Papua New Guinea. Bull Br Ornithol Club, 116-133.

Video about our field work

Bacground and hypotheses

Elevational gradients continue to present a specific challenge for the understanding of basic ecological patterns. After a century of research, the variations in patterns of elevational species richness and the causes thereof, are still not well understood (Rahbek 2005, McCain 2009). However, there has been a recent revival of interest in the analysis of elevational zonation (Hofer et al. 1999, 2000, Kessler 2000, Hemp 2002, Tuomisto et al. 2003, Mena & Vázquez-Domínguez, 2005).
Studies on tropical elevational gradients have traditionally been descriptive (e.g. Fjeldså & Lovett 1997, Patterson et al. 1998), and there has been a lack of rigorous analyses of zonation on local transects (Romdal & Rahbek 2009, McCain personal communication). Most of the works (87%, McCain 2009) on altitudinal gradients are compiled from regional bird field guides and faunal surveys that included elevational ranges for each species. The non-compiled works are affected by sampling method, length of altitudinal gradient and disturbance. Thus, this meta-analysis support need for actual and rigorous bird surveys along complete altitudinal gradient in undisturbed areas.

Thousands of elevational gradients are distributed across the globe on all continents and on most islands in various latitudes, climates and habitats. Moreover, birds are the best known faunal group, and have been common taxa to investigate many different patterns. Consequently, no surprise that there are about 190 elevational gradients of bird species richness in more than 150 published studies. In these works, several diversity patterns are exhibited on montane gradients (Rahbek 1995, 2005; McCain 2005, 2007a): decreasing diversity with increasing elevation, high diversity across a plateau of lower elevations then decreasing monotonically, a unimodal pattern with maximum diversity at intermediate elevations, or in rare instances increasing monotonically.

Proposed drivers of biodiversity can be grouped into four main categories: current climate, space (and mid-domain effect), evolutionary history and biotic processes (Pianka 1966, Gaston 2000, McCain 2007a), possibly explaining large-scale patterns in species richness. Sampling method could be another factor influencing apparent diversity patterns. All these drivers predict diversity patterns in following manners:

1. Sampling – Differences in sampling effort across the gradient may result in an experimental bias in diversity estimation (e.g. Colwell and Coddington 1994). For example, in cases where elevational bands were sampled with unequal effort, a relationship between diversity and elevation could simply be a result of differential sampling effort.

2. Space – On mountains, the „species-area” predicts that elevational bands covering more area (e.g. mountain base) should harbour more species than elevational bands covering a small area (e.g. mountain tops) (Rahbek 1997; McCain 2007b).

3. Mid-domain effect – The MDE assumes that spatial boundaries (e.g. the base and top of a mountain) cause more overlap of species ranges toward the centre of an area where many large- to medium-sized ranges must overlap but are less likely to abut an edge of the area (Colwell et al. 2004, 2005 and references therein).

4. Temperature – Climatic tolerances put restrictions on how many species can survive at different locations and elevations (e.g. Brown 2001). On mountains, temperature decreases monotonically by an average of 0.6 °C per 100-m elevational gain (Barry 1992). If temperature is a main determinant of bird diversity, the predominant elevational diversity pattern predicted is decreasing diversity with decreasing temperature and increasing elevation.

5. Temperature and water – Temperature decreases with elevation on all mountains, while rainfall and water availability follow more complex relationships with elevation depending on the local climate. On arid mountains, water availability is highest at intermediate elevations where rainfall and soil water retention are highest and evaporation lowest. On humid mountains, water availability is high across a broad base of lower elevations and only decreases toward the tops of the mountains, again due to higher runoff and decreases in rainfall. Bird species richness is predicted to be positively related to the warmest and wettest conditions elevationally, predicting mid-elevation peaks in bird species richness on arid mountains and decreasing diversity on warm, wet mountains.

6. Evolutionary history – Wiens et al. (2007) think that, although some studies have addressed correlations between elevational species richness patterns and climatic variables (e.g. McCain 2005, 2007; Oomen and Shanker 2005), patterns of species richness must ultimately be explained in terms of the processes that directly change the number of species in a region, namely speciation, extinction and biogeographic dispersal (e.g. Ricklefs 2004, Wiens and Donoghue 2004). Climate and other factors may still play a critically important role (and may be tightly correlated with species numbers), but they must act on these three processes to directly change the species numbers within a region.

Several authors have suggested that montane regions could have increased speciation rates at mid-elevations or increased extinction rates at the lowest and highest elevations (see species pump model; Moritz et al. 2000). An alternative hypothesis is that rates of diversification are similar at different elevations, and that intermediate elevations might have higher species richness because (for a given group) they have been colonized for longer periods of time than lowland or extreme high elevations.

McCain tested all these main drivers of diversity in her review (2009). Bird elevational diversity strongly supports current climate as the main driver of diversity, particularly combined trends in temperature and water availability. Bird diversity on humid mountains is either decreasing or shows a low-elevation plateau in diversity, while on dry mountains it is unimodal or a broad, low-elevation plateau usually with a mid-elevation maximum.These results emphasize that water in a necessary factor modifying the temperature effect on elevational diversity. May be the evapotranspiration could be even better predictor but there are no data available.