How is global warming impacting where montane organisms live?
Climate change is changing where species live, how they time events in their annual cycle, and how they interact with other species. We know a fair bit about climate change's impact on temperate zone environments (e.g. North America, Europe), but almost nothing for tropical species. This is unfortunate, as the vast majority of biodiversity is found in the tropics, and theory predicts that tropical species may respond differently to climate change than temperate species. To address this data gap, I have conducted field surveys of bird communities along tropical mountain sides originally studied by Jared Diamond (New Guinea, in the 1960s) and John Fitzpatrick (Peru, in the 1980s). I am particularly interested in testing the widespread prediction that climate change will cause species that live only near the summit to disappear; although this idea of "mountaintop prediction" is commonly discussed, the empirical evidence that mountaintop extinctions are happening is actually very sparse. Thus far I have found that:
- New Guinean birds now live at much higher elevations than they did in the 1960s, when Jared Diamond completed his studies (Freeman and Class Freeman 2014 PNAS).
- The same is true for Peruvian birds (Freeman et al in review).
- Some species have shifted far upslope while others have not. We do not understand why (e.g. there are no correlations with traits such as body size, diet, etc.) (Freeman and Class Freeman 2014 PNAS, Freeman et al. in review).
- Tropical species in general are "strong responders" to global warming -- for a given temperature increase, tropical species shift their distributions upslope far more than do temperate zone species (Freeman and Class Freeman 2014 PNAS).
Why are there more species in the tropics and at low elevations?
Why do some places have higher biodiversity while others do not? And why do so many species live in the tropics (latitudinal diversity gradient) and, along mountain slopes, at low elevations (elevational diversity gradient)? The reason why these diversity gradients exist is due both to evolutionary factors-- geographic differences in speciation and the persistence of species -- and also the ecological factors that explain why species, once formed, do not simply expand their distributions and live everywhere (range limits). My speciation research focuses on birdsong, while my range limits research focuses on the elevational limits of montane birds.
Does birdsong drive speciation?
Mate choice is important to birds. Many species are perfectly capable of producing fertile offspring with several related species, yet in nature they only mate with their own species. This observation suggests that factors related to choosing mates (premating factors) are important barriers to reproduction. Birds typically use song and plumage to choose their mates. I conduct field experiments to study the evolution of birdsong, primarily by using playback experiments to measure whether populations recognize song of a related, geographically isolated population (upcoming experiments will investigate plumage as well...). Thus far I have found that:
- Birds with innate (genetically programmed) song evolve different songs faster than do birds that learn their songs (Freeman et al. 2017 Evolution). This is probably because there is a "downside to learning for speciation" -- learning increases variation, and higher variation within a population makes it more difficult to evolve the ability to discriminate against foreign song (because that foreign song has to be different enough from your population for you to discriminate against it, which is harder when song in your population is highly variable). These results refute the popular idea that song learning leads to faster song divergence in geographic isolation and hence accelerates speciation.
- Divergence in acoustic traits is only loosely correlated with behavioral discrimination (Freeman & Montgomery 2017 The Auk: Ornithological Advances ; Pegan et al. 2015). This means that playback experiments are often a better idea than analyses of recorded songs if you want to know whether song is likely to be an important premating barrier between two geographically isolated populations.
- There are myriad tropical populations that are classified as subspecies but deserve status as distinct biological species (we provide evidence for 21 such cases in Freeman & Montgomery 2017 The Auk: Ornithological Advances). The problem is that many tropical species complexes all look similar and so are categorized as one species, but sing very differently and thus likely have strong barriers to reproduction (they should be categorized as multiple species). Playback experiments are a good way to flatten this "latitudinal gradient in taxonomy."
Why do montane species specialize on narrow elevational zones?
One of the most obvious and interesting facets of species is that they do not live everywhere. Explaining why this is so -- the abiotic and biotic factors that limit species' distributions -- is a primary goal of ecologists and biogeographers. Interesting distributions occur everywhere, but I am particularly fascinated by the fact that tropical montane birds typically inhabit only a narrow elevational band. I find it astonishing that I can walk uphill through forest where a given species is absent, to forest where that species is abundant, and finally to forest that again lacks the species all in the course of 15 minutes of hard hiking. This despite the fact that the forest appears similar throughout (at least to my human eyes) and that it would be easy for birds to fly to upper or lower elevations. So why don't they? Is it a Goldilocks scenario where other places are too hot or too cold? Or perhaps birds live where their preferred food is most abundant? Thus far I have found that:
- Interspecific competition is important in determining species range limits. Interspecific competition between closely related bird species both drives divergent evolution of elevational distributions (Freeman 2015 Am Nat), and likely explains why related species that "replace" one another in different elevational zones along mountainsides often interact aggressively (Freeman et al. 2017 Ibis, Freeman & Montgomery 2015)
- New Guinean birds' thermal tolerances do not correlate with the temperatures they experience - evidence against the "Goldilocks" scenario (Freeman 2017 Diver & Dist).
- Tropical birds' distributions can be limited by their preferred food (Freeman & Mason 2015).
Natural history of poorly known birds
As a naturalist, I enjoy learning more about where species live, what they eat, how they interact with other species, how they attract mates and how they raise their babies. In short, what makes a species tick -- it's natural history. Ecology is built on understanding the natural history of organisms, and I have been fortunate to study the natural history (particularly breeding biology, see my publication list for many nest descriptions and documentation of breeding behavior) of many tropical birds. For example, I found an active nest of the lovely Gold-ringed Tanager in Colombia's Choco cloud forests, and was able to document that multiple adults bring food to provision Gold-ringed Tanager nestlings -- this species is a cooperative breeder. Modern ecology is conducted in a hypothesis-testing framework, but both generating testable hypotheses and conducting comparative analyses rely on accurate and thorough natural history information. For example, global analyses of cooperative breeding currently suggest cooperative breeding to be rare in tropical birds. New natural history data for poorly known tropical birds may challenge this assumption, potentially altering the conclusions of hypothesis-testing analyses of the phylogenetic and environmental drivers of cooperative breeding.