Analyses of topography show that mountains do not monotonically decrease in area with elevation as is commonly believed and that in reality land area often increases at higher elevations. This finding bodes well for the future of biodiversity since it means that in many parts of the world there are sufficient upslope areas for low- and mid-elevation species to migrate into as temperatures increase. However, more attention needs to be given to determining if migrating species can actually reach these expansive high-elevation areas. Many factors can prevent species from migrating upslope including stable ecotones. Often ecotonal boundaries are not set by mean temperatures alone and thus are not shifting upslope with warming. An example of this are tropical alpine treelines, which are not shifting upslope despite rapid warming potentially due to the stabilizing influences of climatic factors other than mean temperatures (e.g., extreme cold events) or non-climatic factors (e.g., soil or human disturbances). Stable ecotones can potentially prevent species from expanding their ranges into upland areas in which case the amount of land at higher elevations is irrelevant and species may face “mountain top extinctions” long before they reach the actual tops of the mountains.

The tropics have long been a focal point of interest in ecology and evolutionary biology - but where actually are the tropics? Classically, the tropics have been defined as all areas lying between 23.4° North and South as these zones receive direct overhead solar radiation at some point during the year. However, a suite of different environmental and climatic characteristics have also been employed to classify regions as tropical or not. The aims of this paper are to 1) briefly review some of the different criteria commonly used to define the tropics, 2) map the extent and distribution of tropical land areas according to these different criteria, and 3) asses the concordance between these criteria and a sample of recent “tropical” studies. More specifically, we review eight criteria that are frequently used (implicitly or explicitly) to define the tropics. We then map the location and extent of land areas that are “definitely tropical” (i.e., the core tropics) and areas that are “tropical by most definitions”. Finally, we examine how the different classifications apply to tropical research through an analysis of the study locations of >200 recent tropical biology articles. Depending on the definition, the extent of the terrestrial tropics ranges from 23 million to 66 million km² – a nearly threefold difference. Likewise, the classification of many areas as being tropical vs. non-tropical depends on the specific criterion employed. Of the tropical studies reviewed here, only 44% were based on data collected from the core tropics, and 12% of tropical studies were based on data collected from sites outside of the geographic tropics but with tropical climates. Many different criteria are used to classify areas as tropical vs. non-tropical, leading to inconsistencies when estimating the extent of tropical areas and variation in the classification of ecosystems and species as being tropical vs. non-tropical.

Tropical plant species are systematically underrepresented in large-scale analyses or synthesis looking at the potential effects of global climate change.  The reason being that we simply don’t know enough about the distributions and ecologies of most tropical plant species to predict their fate under climate change. This gaping hole in our knowledge is extremely worrisome given the high diversity of tropical plants, the crucial roles that they play in supporting global diversity and ecosystem function, and the elevated threats that climate change may pose to tropical species in general.

Temperature-change trajectories are being used to identify the geographic barriers and thermal ‘cul-de-sacs’ that will limit the ability of many species to track climate change by migrating. We argue that there are many other potential barriers to species’ migrations. These include stable ecotones, discordant shifts in climatic variables, human land use, and species’ limited dispersal abilities. To illustrate our argument, for each 0.5° latitude/longitude grid cell of the Earth’s land surface, we mapped and tallied the number of cells for which future (2060–2080) climate represents an analog of the focal cell’s current climate. We compared results when only considering temperature with those for which both temperature and total annual precipitation were considered in concert. We also compared results when accounting for only geographic barriers (no cross-continental migration) with those involving both geographic and potential ecological barriers (no cross-biome migration). As expected, the number of future climate analogs available to each pixel decreased markedly with each added layer of complexity (e.g. the proportion of the Earth’s land surface without any available future climate analogs increased from 3% to more than 36% with the inclusion of precipitation and ecological boundaries). While including additional variables can increase model complexity and uncertainty, we must strive to incorporate the factors that we know will limit species’ ranges and migrations if we hope to predict the effects of climate change at a high-enough degree of accuracy to guide management decisions.