The Incomplete Filling of the N‐dimensional Hypervolume

Abstract

Starting as an undergraduate in the early 1980's, and going on to graduate school later in the decade, I immersed myself in the classic papers of evolutionary ecology, especially Hutchinson's 1957 niche paper and its embellishments by MacArthur, Horn, Levins, and Whittaker through the 1960's and 1970's. Working in Fakhri Bazzaz's lab, we saw the world as a kaleidoscope of environmental gradients, with responses to biotic and abiotic factors intimately interwoven and always dependent on environmental context. Hutchinson's N-dimensional hypervolume was realized in multi-factorial niche experiments, capturing spatial heterogeneity in forests and fields, or the interplay of spatial and temporal variation that emerges in disturbance regimes. Lab meetings were consumed by long discussions of how to interpret 2nd, 3rd, and even 4th order interactions in analysis of variance. Starting in the mid-1980's, greenhouse experiments increasingly focused on direct effects of elevated CO2, and interactions with temperature, water, and nutrient availability. The conceptual foundations provided by niche theory provided the critical tools to address the emerging questions related to global warming and directional environmental change. Over the next 10 years, my own work expanded in temporal scope, thinking about trait evolution across phylogenetic lineages, and the consequences for the assembly of communities in a biogeographic and evolutionary context. Somewhere in this process, I read Jackson and Overpeck's 2000 paper in Paleobiology: “Responses of plant populations and communities to environmental changes of the Late Quaternary” (JO2000). I wish now I could remember who suggested I read it, or how I may have stumbled across it, as I don't routinely scan the pages of Paleobiology. For me, though, this was the right paper at the right time; it had an immediate impact on the direction of my work, and I can trace the thread of its continuing importance through to my current research in climate change and conservation biology. JO2000 covers a lot of ground, and as with all papers, each reader will find a different idea, theme, or thread of information that intersects with their own work. For me, the key idea that I took away from this paper is that not all environments exist at a given point in time, whether considered regionally or at a global scale. JO2000 highlighted the importance of looking at large-scale climate patterns in climate space, the analog to Hutchinson's niche space from more than 40 years before. In climate space, where each factor is plotted on a separate axis, it becomes clear that not all combinations of conditions actually exist. Here in the Bay Area where I work, there are coastal locations where winter minimum temperatures average 6°C, and interior locations where summer maxima average 30°C and above. But there is nowhere that experiences this combination of winter and summer. Under future climate scenarios, however, this combination is projected to occur as warming winters and summers create new combinations. Over the past decade these new combinations have come to be called “novel climates,” and conversely conditions that appear today and may vanish may be called “disappearing climates” (Williams et al. 2007). Linking back to niche theory, the fact that not all conditions exist has profound implications for our understanding of biological distributions. Hutchinson's key formalization was the idea of the fundamental vs. realized niche, i.e., the roles of abiotic and biotic factors as limits to niche breadth and hence spatial distributions. By definition, the observations of where species live in field environments only informs us of the dimensions and breadth of the realized niche. But JO2000's observation that not all conditions exist means that we actually can't know the climatic limits of the realized or fundamental niche in certain portions of climate space, creating a fundamental limitation in our ability to extrapolate from modern distributions into potential distributions under past or future climates. JO2000 is where I first encountered these basic ideas, and it led me down a path of thinking about the geographic distribution of climate and climate space, how that will change in the future, and the implications for biological impacts of climate change. Published in the first year of the 21st century, it is a foundational paper that is shaping the new century of biology and the synthesis of climate science, ecology, and evolution with profound implications for our ability to project our future path, and perhaps intervene to stave off some of the potential impacts of climate change on biodiversity and our own well-being.