We study the origin and loss of biodiversity. But as the sixth mass extinction unfolds around us, we are increasingly focused on how climate change is leading to species loss. The speed of global change has transformed the fundamental community ecology question of what limits species’ geographic ranges – why a species lives here but not there – to one of the few most pressing applied questions of our time. We use observations, experiments and big datasets to test how climate and species interactions shape where species live—and how species are on the move as climate continues to change.

Motivating questions include:

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 have found that:

• In Peru, climate change has set in motion an “escalator to extinction” for birds. Species are shifting upslope, but high elevation species have nowhere higher to go. Several previously common high elevation species are now locally extinct (Freeman et al 2018 PNAS).

• Across the globe, high elevation species are generally shrinking in range size. This pattern is much stronger in the tropics than the temperate zone (Freeman et al. 2018 GEB).

• 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 2018 PNAS).

• Tropical species are tracking temperatures much more strongly than temperate species: for a given temperature increase, tropical species have shifted their distributions upslope far more than temperate species (Freeman et al. 2021 Ecology Letters).

Why do tropical 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 we are particularly fascinated by the fact that tropical montane birds typically inhabit only a narrow elevational band. It is astonishing that one 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 an ornithologist) 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? We have found that:

• Interspecific competition is important in determining species range limits (Freeman et al. 2022 Science). 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 Condor). Behavioral interactions are key drivers of these patterns — closely related species that are strongly territorial tend to replace one another while close relatives that do not defend territories tend to overlap in range (Freeman et al. 2019 Ecography; Freeman et al. 2022 Science).

• New Guinean birds' thermal tolerances do not correlate with the temperatures they experience - evidence against the "Goldilocks" scenario (Freeman 2017 Diversity & Distributions).

Why are there more species in the tropics and at low elevations?

Why do some places have higher biodiversity than others? 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). We have found that:

• Rates of evolution tend to be faster in the temperate zone than the tropics: I found this pattern in a global analysis of bird beaks, and also in a synthesis of the published literature (Freeman et al. Ecology Letters 2022). This means that we can provisionally reject hypotheses for the latitudinal diversity gradient that invoke faster rates of evolution in the tropics.

• Studies of the latitudinal diversity gradient may be hampered by the “latitudinal taxonomy gradient”, in which populations that would be classified as distinct species in the temperate zone are instead classified as belonging to the same species if they live in the tropics (Freeman & Pennell 2021 TREE). I have argued that, for songbirds, playback experiments can help flatten this latitudinal gradient in taxonomy (Freeman & Montgomery 2017 The Auk: Ornithological Advances).

• Rates of species interactions are not necessarily higher near the equator or at low elevations (Freeman et al. 2020 Am Nat, Lau et al. 2023 J Biogeography, Roesti et al. 2020 Nature Communications). Interestingly, adaptation in response to strong species interactions in the tropics may flatten the latitudinal gradient in interaction rates (Freeman et al. 2020 Am Nat). These findings are relevant to hypotheses for the latitudinal diversity gradient that suggest species interactions drive the origin and maintenance of diversity.

Natural history of poorly known birds

We 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 studied the natural history (particularly breeding biology, see my publication list for many nest descriptions and documentation of breeding behavior) of many tropical birds. Ben still remembers the exciting moment when he found an active nest of the lovely Gold-ringed Tanager in Colombia's Choco cloud forests, and provided the first evidence that this species is a cooperative breeder. Modern ecology is conducted in a hypothesis-testing framework, but conducting comparative analyses relies on accurate and thorough natural history information. For example, global analyses of cooperative breeding currently suggest cooperative breeding to be rare in Neotropical birds. New natural history data for poorly known Neotropical birds may challenge this assumption, potentially altering the conclusions of hypothesis-testing analyses of the phylogenetic and environmental drivers of cooperative breeding.