This article is a review about species distribution modeling, or, the ability to predict how species ranges could change under global warming. Authors highlight the fact that models published in the past have overlooked microclimates (small places that provide species with climate different than in the overall area). For example, in a place where heat waves are felt, some species could be able to use the surrounding vegetation to remain cool, and essentially be unaffected by the heat wave. Willis et al. (2009) show how important it is to consider such areas when estimating species’ future ranges under climate change since they could halt or at least slow many species’ extinctions.

Thermal limits of ectotherms (cold-blooded species) are often much higher than the temperature of the surrounding air. When limits are exceeded however, it was previously thought that organisms internally regulated their body in order to survive. However, this study shows that most ectotherms must use their behavior by retreating to suitable habitat when the air temperature becomes unsuitable (too cold or too hot). These behaviors are costly, obligating ectotherms, like bees, to direct their energy into preserving their heat/cool, instead of directing it into reproduction, feeding, and other essential activities. Extreme temperatures are predicted to happen more often as climate change progresses, and is likely to be problematic to bees.

Authors discuss the effect of heat waves on bumblebee decline. Heat waves and drought occurrences in France, the United Kingdom, Scandinavia, and Turkey led to bumblebee declines. It highlights that bumblebees are sensitive to extreme temperature events.

Different aspects of heat waves could be affecting bees, as discussed in this paper. It could be that temperatures are rising above a tolerable threshold killing the bees, the length of the heat wave, the drought that is often associated with heat waves, starvation if bees are overheating and cannot feed themselves properly, or that heat waves are occurring during winter which could interrupt the queens’ hibernation while flowers providing their food are not yet available. Climate change is estimated to increase the probabilities for the occurrence of extreme temperature events in the future, a factor that may further threaten bee decline.

Authors described an approach to model species’ distributions under climate change which takes into consideration ecological characteristics of species like reproduction, dispersal, and adaptation. We call these “mechanistic” models because they include mechanisms into their predictions.

Using this approach, they modeled the distribution of 12 butterfly species and found that the ability to colonize unoccupied areas that are newly suitable and maintain populations in these areas is essential for species to track their suitable conditions. Otherwise, species can fall behind the pace of climate change and loose range.

Testing for the effect of pesticides was important as field experiments have their negative impacts on bumblebees in the past. Whitehorn et al. (2015) showed that Bombus terrestris colonies exposed to pesticides in a lab had a lower growth rate and produced 85% fewer queens (bees that would eventually leave the colony to start a new colony) than those that were not exposed to the chemical. While this was observed on a small scale, the effect of pesticides was not measurable when Kerr et al. (2015) asked if it affected bumblebees’ wide scale shrinking range.

This review explains the multiple threats that bees are currently facing. Pesticides and other agrochemicals, parasites spread by humans, climate change, and land use change, are threats that either act on their own or interact, to cause negative effects on bees.

For the study design, Kerr et al. (2015) took these threats into consideration to measure what effect was responsible for the results that they obtained, and chose to look at pesticides and land-use change; effects which, surprisingly, were not responsible for bumblebee range loss in the south and the inability for them to expand their range in the north. This is not to say pesticides and land-use change cannot create population declines at a smaller scale.

This study showed that Drosophila (a type of fruit fly species) adapted to hot and dry regions were more resistant to heat. The upper thermal limits tolerated by Drosophila could influence range limits. Authors conducted trait analyses and found that heat tolerance was an ancestral trait, kept over time. Trait-based analyses for bumblebees conducted by Kerr et al. (2015) also correspond to these results.

This paper answers the research question “Can species physiologically adapt to climate warming?” They analyzed data from numerous other studies that described the thermal tolerances of species worldwide. Main findings of this study are that tolerance to heat was mostly conserved within closely related species, whereas tolerance to the cold was a trait that could vary.

Trait-based analyses conducted by Kerr et al. (2015) correspond to these results. The upper thermal limits of bumblebees were found to be an ancestral trait, since it was conserved across closely related species.

Authors conducted a meta-analysis, a type of analysis that looks at a group of studies on the subject. They demonstrate that many terrestrial species shifted their range higher in altitude as well as further north. Chen et al. (2011) found that the rate at which species can shift varies strongly between species. Thus, it is likely for individual species traits to play an important role in their ability to shift.

Findings by Chen et al. (2011) were important to frame the research question in Kerr et al. (2015). Since species from many different taxa have shifted, it was reasonable to ask if bumblebees are shifting as well.

This paper is a review of the different ways to measure how much species are threatened by climate change, and how they might respond. For instance, you can do this by estimating how a species range could change in the future. Understanding how climate change is affecting specific species is the first step to develop conservation strategies. Kerr et al. (2015) accomplished a first step toward this goal by demonstrating that climate change is linked to widespread losses and an overall shrink in distribution.

It is now clear that climate change has affected species, as shown by endless amounts of papers in the primary literature. This paper estimates sizeable species extinctions by 2050 linked to climate change. They do this with species distribution modeling. This technique uses known species distributions and climatic conditions, and estimates how the distribution could change in the future according to models of future climate. Authors used optimistic and pessimistic estimates of future climate data (since, we do not know how much humans will cut down on activities that aggravate climate change), and even took into account the capacity for some species to move to and colonize other places. They estimated extinctions of 15% to 37% of the species included in their study. Studies that look at future impacts of climate change yield uncertain results since these cannot be verified.

The IPCC (Intergovernmental Panel on Climate Change) put together a report summarizing climate literature. Refer to the PDF of the synthesis of this report here if you are interested in finding out more information on climate change: https://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf

Like PGLS, this method examines the relationship between two or more variables. However, this method does not account for phylogeny. OLS regression was used to compare with PGLS results to see if it would produce similar results.

The Akaike information criterion (AIC) is a statistic that measures whether some models are more or less informative than others. This means that a model with a lower score for AIC is more informative than a competing statistical model and may tell us something that is meaningful in a biological sense.

Like many other aspects of statistics, we have to be careful about blindly believing test results, so we apply our experience as scientists to make sure that numbers like the AIC score actually make biological sense!

Click here for a more in-depth view of what AIC is, how it is calculated, and what it can (and can't) do.

This paper demonstrates how bird and butterfly communities have changed over time, and while they shifted through time, they failed to keep up with climate change. They show that northward shifts for bird communities shifted by 37 km, and butterfly communities shifted by 114 km. The climate shifted much faster than this, leaving lags of 212 km for birds, and 135 km for butterflies. These are the distances that birds and butterflies would need to travel to reach temperatures similar to that of their historical range shifts.

Phylogenetic Generalized Least Squares (PGLS) is a method that accounts for phylogenetic relationships within a group of species (phylogeny reveals how closely related species are to each other). This method will determine if variables of interest are closely related.

In this table, models of trait evolution indicated whether traits are ancestral and kept over evolutionary time. Species’ upper thermal limit (one of the traits studies here) was found to be shared by close relatives. The authors pointed out in this paper that niche conservatism (i.e. the tendency for species to keep their ancestral traits) could explain such findings.