The response of endemic biodiversity to climate change in Earth’s temperate climate zones is complex. A new study suggests that species that have evolved in regions with relatively high natural climate variability may at the same time be more resilient and more vulnerable to the effects of steadily rising temperatures:

Genetic diversity of species from temperate climate zones, stemming from the large seasonal and multi-annual climate fluctuations of these regions, may place them at an adaptive advantage in face of 21^st-century climate change. This same natural climate variation may however also blind these temperate-zone species to the underlying climate trend, misguiding them to disperse or even evolve in false directions, decreasing their chance of survival.

Species that have evolved and live in regions with high natural temporal climate variability may genetically be better adapted to cope with the consequences of warming. They may also have more difficulty to respond to underlying climate trends and migrate to a climatically suited new home. How such advantages and disadvantages may balance out? You tell us…

Temperate zone biodiversity is under various pressures

First of all let’s emphasize that biodiversity in Earth’s temperate climate zones – so in between tropical & subtropical warm climates, and polar cold climates – is in general not doing well. This is not (yet) the effect of 21^st-century climate change, but of a human-dominated landscape, with widespread habitat destruction that already started many centuries ago, and even millennia if you include extinctions from overhunting.

This relatively slow-paced historical prelude of the current extinction crisis is rapidly accelerating in modern times, as illustrated by the recent bombshell insect decline study: a nation-wide population collapse of flying insects in German nature of 76 percent in the last 27 years, representative of general insect declines in other low-lying nature reserves throughout Northwest Europe.

So where Earth as a whole may still be lingering on the brink of the Holocene-Anthropocene Mass Extinction, Earth’s temperate climate zones – that’s most of North America and most of Eurasia – are ahead of that development.

Effects of climate change on ‘temperate zone biodiversity’

But what about the consequences of climate change? Well there the story gets more complex. Earth’s temperate climate zones are not an extreme of the spectrum, but rather a connecting part between the hot tropics and the cold Polar Regions.

Therefore, as species disperse under climatic warming – some may (on the northern hemisphere that is) survive by migrating to the North, while ‘new’ (non-endemic) species may enter from the South. Therefore locally biodiversity might first increase before it decreases – with subsequent ecosystem disturbances and an increasingly distorted balance of competition between species as a possible extinction amplifier.

Now to the new study: it’s titled ‘Climates Past, Present, and Yet-to-Come Shape Climate Change Vulnerabilities’, was performed by ecologists Christopher Nadeau and Mark Urban of the University of Connecticut and Jon Bridle of the University of Bristol, and published last month in Trends in Ecology & Evolution.

These researchers try to improve prediction of the vulnerability of populations to the effects of climate change, by linking the future development to the specific climatic conditions that populations currently live in, and the climatic characteristics of the historical environment that species evolved in.

Conditions of spatial and temporal climate variability that species evolved in and live in today, may determine their chances to survive future climate change – through various mechanisms.

Spatial and temporal climate variability – on a species level

They distinguish Earth’s biodiversity in populations that evolved (past climate) and live (current climate) in either high or low temporal and either high or low spatial natural variability, so if the natural climatic conditions show relatively high or low variations over time (seasonal, annual, decadal – often relatively low in tropical regions, high in temperate climates) and across the landscape (for instance mountain ranges can have relatively high spatial climate variability, continental plains may have low spatial variability).

Now both spatial and temporal climatic variation can be assessed per species. Then spatial variation can increase with a species’ ability to disperse – for instance a bumblebee is more likely to encounter changing climatic conditions when it migrates for a couple of days than a tree. Temporal climate variation can increase with a species’ average generation time. The bumblebee is unlikely to experience unusually hot or cold years for the simple fact that it only lives one season, perhaps two – trees however… Etcetera.

7 testable predictions for species’ adaptive capacity to climate change:

From there they define 7 ways in which this background might influence the future response of species to anthropogenic climate change – as 7 testable predictions:

1. Populations from Climates with High Temporal or Spatial Variation Will Maintain Higher Genetic Diversity Which Increases Their Intrinsic Response Capacity
2. Populations from Climates with High Temporal Variation Will Have Higher Phenotypic Variation, Thereby Increasing Their Intrinsic Response Capacity
3. Populations from Climates with Low Spatial or High Temporal Variation Will Evolve Higher Dispersal Propensity Which Increases Their Intrinsic Response Capacity
4. Populations from Climates with High Temporal Variation among Generations Will Evolve Broad Thermal Tolerances That Decrease Their Sensitivity to Climate Change
5. Climate-Tracking Will Be More Effective in Climates with High Spatial Variation, Which Increases the Extrinsic Response Capacity of Populations
6. Populations Will Track Suitable Climates More Slowly in Climates with High Temporal Variation, and This Decreases Their Extrinsic Response Capacity
7. Evolutionary Adaptation of Populations Will Lag Further behind Long-Term Climate Change in Regions with High Temporal Variation, Thereby Decreasing the Extrinsic Response Capacity of Populations

Very interesting. So for an individual species coming from high natural climate variability can both be an advantage and a disadvantage.

Intraspecific biodiversity helps protect biodiversity

One way having evolved in a variable climate can be an advantage the authors suggest, increasing a species’ adaptive capacity, is through higher genetic diversity within the total population – see prediction 1, and also 2, as phenotype is a product of how genes express (different morphs within one species).

We think this is something to take proper note of, as genetic diversity is actually an intrinsic part of biodiversity – one that is easily overlooked when focussing merely on the species level.

In Ecology genetic diversity is therefore also referred to as intraspecific biodiversity – and under 21^st-century climate change this intraspecific biodiversity is forecast to decline even more rapidly than species diversity. That’s because Earth’s current extinction crisis is not caused by a sudden asteroid strike, but by a gradual increase of many ecological stressors (gradual warming is a clear example, progressive deforestation is another) – and therefore accompanied by general species population decline that can be directly linked to loss of smaller subpopulations.

Therefore common life forms may survive as a species, but many of their subpopulations, containing a big chunk of the total amount of unique genes (possibly defining them as subspecies, possibly as morphs), may still be lost – leading to intraspecific biodiversity decline.

Now biodiversity loss is a cascading process, with the total amount of biodiversity acting as a weight on the break: biodiversity protects biodiversity – if you want to protect it, you better make sure you have a lot of it. A degraded forest becomes susceptible to plagues. And, apparently, a gene-diverse species is more resilient to climate change.

Two morphs, one species. In Biology a distinction is made between ‘genotype’ and ‘phenotype’. To keep this very simple, the genotype is all of the genes, the phenotype how these genes express (not just in the morphology, but also in for instance behaviour). Now some genetic switches can be influenced externally through environmental factors that can also be climatic. The extent to which a species’ phenotype is not a direct representation of a species’ genotype is called ‘phenotypic plasticity’. High phenotypic plasticity can be a product of high natural climate variation, and can in turn also increase the adaptive capacity of a species to cope with future climate change. In the end though, whether direct or indirect, it’s al genetic. And as genetic diversity is an intrinsic part of biodiversity, the overlying statement is (again) that you need high biodiversity in order to protect biodiversity. Shown in this picture is a famous evolutionary example of phenotypic expression in the peppered moth (Biston betularia). As you can deduce from its Latin name, this moth likes birch trees and has adopted very suiting camoflage. During the British Industrial Revolution increasing numbers of this moth expressed as the black morph, as it adapted to a soot-polluted environment. The trick lies in ‘jumping genes’ or ‘transposable elements’: mobile segments of DNA that can change their position within a genome and alter the expression of other genes. Again, you have to have developed this genetic flexibility to have this adaptive capacity. Credit: University of Liverpool.

From the micro to the macro level – from Genes to Geography

We think it’s also interesting to pile all of biodiversity together and not differentiate on taxonomy, but on geography – in order to assess overall climate adaptive capacity.

Then high or low spatial climate variation can be defined not as an individual species’ trait, but as a landscape factor, one that can in turn be linked to larger climatic zones, and categorise biodiversity as endemic biodiversity from for instance mountain ranges (high spatial), from plains (low spatial), ocean-dominated biodiversity (low temporal) or continental biodiversity (high temporal) – or, a very broad differentiation, tropics and subtropics, polar regions, and temperate climate zones – with the latter on average being a zone with relatively high natural temporal climate variability.

But can we take that liberty? And could the above 7 predictions for species’ adaptive capacity still hold true – in this much blunter categorisation?

We decided to ask one of the authors, Mark Urban, a specialised climate biologist, some of whose research we’ve tried to cover in previous episodes of this series.

Well, disclaimer, ‘perhaps’ – to some extent:

Q (Rolf): “I was wondering if the 7 predictions could also be applied for all of biodiversity, and then per climate zone. So temperate zone biodiversity would in general be high temporal, whereas tropical biodiversity in general would be low – mountain biodiversity high spatial, etc. This would be in general, so non-specific. I’m asking because I want to understand temperate climate zone biodiversity risks from climate change better… Do the predictions also apply on that level?”

A (Mark): “Yes, I think you can make some broad generalizations, and more so for temporal versus spatial variation. But you should still consider the generation length of species in mediating that variation. I could imagine some short-generation taxa experiencing lower climate variation even in temporal zones.”

Urban also explained how to properly interpret climate variability on a species level:

“We’re saying that geographic variation is only relevant at the scale of a particular organism. So the geography doesn’t change, but it is cut up in different ways depending on if you’re dealing with a mouse or a wolf. The same is true for time.”

“So I think you can make some broader recommendations based on the geography of an area, but those predictions will always be modified by the spatial scale that the organism encounters. So a flat mountaintop might function like a plain for a mouse, but a mountain for a wolf, if that makes sense?”

[Yes, practical examples work very well for us. We have a mouse living on the very kitchen table that this article is written on that also thinks it lives on a plain – and seems to be fine with more distant spatial limitations on the horizon of its ecosystem. It doesn’t seem to like the clicking noise of this laptop computer, nor my face. Therefore I will leave once this piece is ready and uploaded. Ecology really is about knowing your place.]

So, which predictions might apply to the biodiversity of Earth’s temperate climate zones?

As in this article we focus on the specific vulnerability of ‘temperate climate zone biodiversity’ to anthropogenic climate change – we take a closer look at all connections with either high or low temporal variability, noting again that for temperate climate zones this variability tends to be relatively high.

In 6 of the 7 predictions a connection is suggested between populations’ climate adaptive capacity and natural temporal climate variability. Four times this connection is positive (predictions 1-4) – implying a relatively low climate vulnerability – and two times negative (predictions 6-7), implying decreased adaptive capacity for populations coming from high temporal.

High genetic diversity and (resulting) high phenotypic variation both increase adaptive capacity to climate change

Predictions 1 and 2 may show how temperate zone species may have developed genetic diversity from ‘climatic diversity’ – and that especially for species that can live very long, therefore experience climatic changes [one word: trees] this may have led to heterogeneous communities with larger overall adaptive capacity. The authors explain as follows:

“Temporal environmental variation that occurs among generations can preserve genetic variation by favoring different traits at different times and preventing one genotype from dominating a population. This process can be enhanced for long-lived species or species with propagule banks because old individuals or propagules can be less affected by episodic natural selection and therefore persist in the population despite many generations experiencing different selective optima.”

“For example, interannual temperature variation maintains genetic variation in silver birch (Betula pendula) stands by favoring the recruitment of different genotypes in different years. This genetic variation could facilitate evolutionary adaptation to climate change over the next 33–55 years.”

Boreal forests may look much less diverse than tropical rianforests, that have far larger tree species diversity. Within tree species genetic diversity may however be relatively large. This not only goes for birch, but also for spruce trees, because some populations actually survived during ice ages – and individual trees may be thousands of years old, so lived through most of the Holocene climate fluctuations.

“In another example, seasonal temperature variation maintained genetic variation in Drosophila subobscura that facilitated a rapid evolutionary response to a recent heat wave.”

If you can maintain genetic diversity with genetic diversity then how do you maintain genetic diversity? The geographical model is tricky…

However, if there is one thing we’ve learned pondering Earth’s various sustainability crises it is that you can’t mine a resource forever if you don’t grow it back equally quick…

Therefore noting general genetic diversity as a strength against biodiversity loss, may turn out to bite you in the tail as a tiny petitio principii – in the sense that it can’t be a given that you can maintain the high genetic diversity that maintaining genetic diversity depends on – not in our geographical model at least, in which biodiversity is simultaneously threatened by various stressors.

This is sadly more specifically the case for Earth’s temperate climate zones that are more densely populated by humans and where as a consequence general endemic biodiversity decline has progressed further than elsewhere on Earth. And as we’ve discussed in this article, such biodiversity loss is accompanied by decreasing genetic diversity within species, and therefore perhaps –we fear– gradually making predictions 1 and 2 less applicable in our geographical model.

Do you like to travel? Then maybe that was advantageous to the survival of your ancestors. Are you a stay-at-home? Then maybe travelling was risky where your great-grandparents grew up. Now some of these genes may determine your species’ extinction risk under future climate change…

Now another interesting statement that may apply to temperate climate zone biodiversity is number 3, that looks at whether it would have made sense for species to develop the propensity to disperse.

Dispersal has risks and advantages. In an environment with high spatial climate variability risks may be large, therefore species from mountainous landscapes may have been discouraged to develop this trait. However in an environment with high temporal climate variability, like (lowland) temperate climate zones, the opposite may be true.

An example of seasonally adapted species are of course migratory birds, some of which cross the entire width of Earth’s temperate zone between summer and winter (and some of which are actually trying to reroute under current climate change). But here it’s also good to note that the temperate zones saw very large paleoclimatic fluctuations during the past 2.5 million years of the Pleistocene (when Earth’s tropics experienced some precipitation changes, but had a much more stable temperature) – and that temperate species that developed the trait to disperse easily, did very well moving South during glaciations, and recolonising previous habitat in subsequent interglacials.

The authors combine the possible conflicting effects of spatial and temporal climate variability in a dragonfly illustration:

“For example, European dragonflies from standing freshwater systems have higher dispersal propensity than those from running freshwater systems because standing systems are more ephemeral on long-time scales, although other explanations exist. The higher dispersal propensity of dragonflies from standing systems allowed them to recolonize central Europe after the last glaciation, occupy a greater portion of suitable habitat, and track contemporary climate change better than species from running systems.”

Shown in this video by WeLoveEarth.org are four European endemic species: one dragonfly and three damselflies. One of the damselfly species (called banded demoiselle (Calopteryx splendens)) prefers running water, the other three species do very well in standing water. Especially in the flat landscape of the Netherlands, where this is filmed, running water environments are less common than standing water (therefore Calopteryx splendens is actually rare). Now according to above logic Calopteryx splendens would have been wise not to develop the propensity to disperse, whereas the other species of dragonfly and damselfly may be more inclined to cover long distances – possibly increasing their adaptive capacity to climate change.

This sounds really useful to have: thermal tolerance genes…

Now another genetically inherited trait that temperate biodiversity may benefit from in the face of future climate change, is literally a temperature change tolerance – something that temperate (and polar) biodiversity may have relatively more of, as explained under prediction number 4:

“[It has been] observed that endotherms have a broader thermal neutral zone in the arctic than the tropics. [It has been] suggested that temperate ectotherms evolved broader thermal tolerances than tropical ectotherms in response to greater temperature seasonality in temperate regions. Recent studies confirm these patterns and demonstrate a clear link between thermal tolerance breadth and seasonal temperature variation.”

Big question of course is does thermal tolerance to temperature fluctuations from the past, also equate to tolerance for the future supra climate – with temperatures higher than they have been for several million years?

The answer is no and yes, the authors explain: although “upper thermal tolerances vary little within and among species across broad temperature gradients” – there are still many benefits to the ‘buffering capacity from large thermal safety margins’, for instance because species tend to live not on, but under their thermal optimum – and also because “species with broader thermal tolerances often have larger geographical ranges, and this can reduce their vulnerability to climate change because their range is more likely to incorporate low vulnerability regions. Therefore, temperate organisms are often predicted to be less vulnerable to climate change than tropical organisms, despite higher predicted increases in temperature in temperate versus tropical regions.”

(Very interesting, we think. This also seems to emphasize though that there are ecological threshold values to future climate change. If temperature rises beyond species’ upper thermal limits then also having a broad thermal tolerance is no longer an answer – something that may contribute to the pattern that biodiversity loss tends to accelerate under progressive warming.)

Thermal tolerance of species to climate change: ‘temperate biodiversity’ may have a broader range. Although upper thermal limits may be fixed, having a broader range can probably still help species survive climate warming.

Most species don’t carry a compass – or rather don’t know their best chances lie North

Now if you want to survive future warming there are also downsides to living in a high temporal climate – one major one has to do with a blurred sense of directions.

We, humans, all know that for northern hemisphere species your best chance to survive a climate that becomes warmer than you can tolerate, is migrating North – because then you are likely to encounter climatic conditions that are (relatively) similar to what you were used to – at least when average temperature is your concern.

For many other species the story might be a bit different – and there’s jargon to emphasize this: ecologists prefer not to speak of species’ climate migration, but rather of their dispersal [or “biodiversity redistribution“] – as it’s non-intentional, or at least more of a trial-and-error process: dispersal is moving around until it feels comfortable, and then settling down, an unguided process that can send you down the wrong path – and for Earth’s temperate climate zones possibly even the opposite path.

Again, the authors explain, under testable prediction number 6:

“In climates with high temporal variation, weather during a relatively short period (e.g., days, weeks, decades) can differ substantially from the long-term trend. For example, February 2015 in the northeastern USA was the second coldest on record despite a 3.9 °C increase in average February temperature since 1900.”

“Periods that deviate from the long-term trend can slow climate-tracking if climates along range-shift pathways become temporarily unsuitable or by eliminating populations that colonized regions that recently became suitable. For example, amphibians in the western USA might not track suitable climates because decadal climate fluctuations cause gaps between areas where climate is currently suitable and areas predicted to be suitable in the future.”

“In addition, a short cold snap in winter 2010 led to range retractions of exotic species that had previously expanded their range from the Caribbean into the USA. Decreased climate-tracking rates can increase extinction risk under climate change, especially for populations and life-stages that are sensitive to short-term climate fluctuations.”

If climatic changes usually don’t really hurt you, you may forget to evolve when really you have to

And lastly, having a relatively high adaptive capacity can also make you lazy – or at least an evolutionary slow-responder to what’s happening underneath. Also the last testable prediction, number 7 (about lagging evolutionary adaptation due to the confusing signal of natural variation), can be compared to a human trait: “we saw a snowflake [or someone waving a snowball] last winter – so probably the climate isn’t warming.”

Brilliant cartoon by Adam Zyglis about obstruction to the evolution of Science. The inability to see beyond horizons (or to see trends through natural variation) is not an exclussively human trait – other temperate zone species can also find this difficult.

Non-human species in temperate climate zones may show similar behaviour, the authors fear:

“Recent predictions of the evolution of wing melanin in alpine and subalpine butterflies demonstrate how temporal variation in weather can slow evolutionary adaptation to climate change. In this example, temperature variation caused variation in the direction (for or against wing melanin) and the magnitude of selection, resulting in very little directional evolution under recent climate change despite directional changes in temperature.”

They end with a nice positive note though:

“Under some circumstances, however, high climatic variation can aid evolutionary adaptation. For instance, extreme weather events can remove maladapted adults of long-lived organisms, and this can facilitate the recruitment of better-adapted individuals.”

Let’s hope that last mechanism will apply to Earth’s temperate zone biodiversity. And just perhaps – life depends on strokes of luck – that may also apply to those climate-lagging individuals within our own species. You can’t wave a snowball forever.

We thank the authors for their very elaborate study that got us all thinking. Biodiversity effects of climate change are very complex. That’s why we need scientists that actually dare to go this deep. Because, let’s all agree, these uncertainties matter – we are talking about the future outlook of life on Earth, all of it.

So next time you bump into your local climate-specialised ecologist, give him or her a tap on the back – and don’t forget to ask them if they’ve found something new. If so, please let us know: our series continues.

© Rolf Schuttenhelm | www.bitsofscience.org