Synopsis:
When you hear about climate change, what is the first thing you think about? Deforestation? Rising sea levels? Or global warming? Although the research behind these categories is already daunting, are we being told everything?
In this series, the Hidden Impacts of Climate Change, I will be digging deeper into the true effects that climate change has on our planet. Not only on humans, but also on the environment and animals.
This is part 1 of the Hidden Impacts of Climate Change.

What is Developmental Plasticity?

According to research studies carried out by the University of Michigan, in the last 40 years most species worldwide have declined in size. For example, salamanders in Appalachia, red deer in Norway and smaller fish in the Atlantic and Gulf of Mexico have all provided evidence for this trend. Also, the yellow-bellied marmot (Marmota flaviventris) has gradually emerged earlier from hibernation (a study led between 1975 and 1999) because of warmer air temperature earlier in the spring.
The bone, tarsus has decreased by an average of 2.4%, a bone in the lower leg of birds. Although a small difference, it is an alarming trend that correlates with the increase in temperature.

Developmental plasticity which is solely affected by the environment, is a hypothesis that intertwines with the hypothesis of selection pressure. Temperatures increase the metabolism meaning by the time animals reach adulthood, they are smaller as they age quicker relative to size. But temperatures can also cause animals to grow in size in high latitudes, as increasing temperatures and precipitation have given shrews, otters and martens more time and resources to grow before winter. Nonetheless, on average animals are shrinking due to them not being able to adapt to the unprecedented rate of change in temperature and other external factors, caused by human activity.

But why is this a problem? Well animals having smaller bodies has many knock-on-effects on the environment around it. This is exemplified by the fact that:

• Animals would be able to hold less eggs, leading to a lower population size in the long run.
• For amphibians who need to keep their skin wet to breathe, shrinking can mean a higher chance of them dying in a drought as their bodies absorb and hold smaller quantities of water.
• Predators may have to eat more shrinking prey, causing an unbalance in the ecosystem.

This image shows how plasticity can vary due to the environment. Image source: https://www.ncbi.nlm.nih.gov/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6709652_fgene-10-00720-g001.jpg

How does it work?

In acquiring and storing such information, the plastic nature of the central nervous system allows for the adaptation of existing neural connections, in order to accommodate new information and experiences, resulting in developmental plasticity. This form of plasticity that occurs during development is the result of three predominant mechanisms: synaptic and homeostatic plasticity, and learning (in humans and animals).

Synaptic Plasticity:

• Synapses are gaps between neurones where chemical neurotransmitters are diffused across to initiate a new electrical impulse.
• Minute-to-minute changes are continuously happening at the level of microscale connections between neurons. These changes in neuronal connections are the primary mechanism for learning and memory and are known as “synaptic plasticity”. 
• Synapses undergo an activity-dependent and selective strengthening or weakening so new information can be stored.
• Synaptic plasticity depends on numerous factors including the threshold of the presynaptic stimulus in addition to the relative concentrations of neurotransmitter molecules. 
• Synaptic plasticity has long been implicated for its role in memory storage and is thought to play a key role in learning. However during developmental periods, synaptic plasticity is of particular importance as changes in the network of synaptic connections can ultimately lead to changes in developmental milestones.

Homeostatic plasticity:

• Homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems. This mechanism ensures that the human body stays at optimal conditions.
• Homeostatic plasticity helps regulate prolonged excitatory responses, which lead to a reduction in all of a neuron’s synaptic responses. Excitatory responses are when neurones fire an action potential (An action potential occurs when a neuron sends information down an axon, away from the cell body).
• During development billions of neurons wire themselves up into complex networks and manage to reach a state where they can generate—and then maintain stable activity patterns throughout the life of an organism. They are constantly undergoing modifications to allow organisms to store information and adapt their behaviour to a changing environment.
• While the exact mechanisms by which homeostatic plasticity acts with remains unclear, recent studies raise the idea that homeostatic plasticity is modulated according to the period of development or challenges in existing neural circuits.

Learning:

• Synaptic plasticity is considered to be a by-product of learning, and learning requires interaction with the environment to acquire the new information or behaviour, whereas synaptic plasticity merely represents the change in strength or configuration of neural circuits.
•  By depending largely upon selective experiences, neural connections are altered and strengthened in a manner that is unique to those experiences. Therefore, animals or humans can experience a stimulus and adapt by learning the behaviour in response.

In summary, brain plasticity is an exciting, vital topic which explains how well a brain can function. It has the reasoning as to why and how the ways in which we and animals develop mentally and physically throughout our lives. 

Is this evolution?

As stated, the effects on developmental plasticity are already happening. You may ask, “But isn’t this just evolution?” The answer is no. Developmental plasticity determines the adults we and animals become, which is more serious than it sounds. Although there can be minor changes in climate and no effects are seen, some of these effects may not be seen until later adulthood.

Firstly, as species, we have evolved to become who we are today and are still evolving to adapt to constant external changes. The effects of evolution are only seen on a long-time scale. We can’t see animals just adapt genetically and physically over one generation due to the environment, it is impossible. The capacity of genetically similar individuals to produce substantially different phenotypes depending upon environmental conditions during early life (defined here as the period between conception and reproductive maturation) is known as ‘developmental plasticity’. In this case, it is our developmental plasticity determining growth and development from our early life. Therefore, it is important to realise that larger or smaller animals living in similar habitats over time is not evolution if there are no obvious stimuli causing changes in desirable qualities, or natural selection within the population.

Secondly, developmental plasticity does not correlate or is a causation of genetics. The impact of early conditions is so dramatic; it can affect survival and reproduction. For example, the ‘thrifty phenotype hypotheses’ posits that inadequate early nutrition triggers ‘nutritional thrift’, impairing the development of pancreatic function and pre-disposing the individual to metabolic disorders in adulthood. This hypothesis was later updated to posit that metabolic disorders in later life would be milder if adult nutritional conditions closely matched those in childhood. This shows that a poor early diet is a prerequisite of the future implications it has on the body, not genetics. Using this example, it infers that a family with no genetic diseases, that has a poor diet will mainly affect the children who may suffer with health in the future. However, evidence is limited for health gains following matched early life and adult nutritional environments, so this hypothesis can not be concluded.

Another example is the ‘allostatic load hypothesis’, which invokes stressors—during development and throughout life as challenges to which the organism must respond by modifying its physiology and behaviour. The accumulation of such challenges results in increased susceptibility to disease. This hypothesis is often overlooked in the developmental plasticity literature, but we include it here because, by explicitly invoking stressors during development as contributors to poor health in later life. In real life, this can be exemplified by racism, which can invoke worry, anxiousness and pressure, which all wear down the person throughout their life.

So why is climate change involved?

Temperature’s direct effects have only been specified so far in this article. But if temperature was the only factor disrupting developmental plasticity, it would only be the effects of global warming. So other factors of climate change that are negatively impacting upon developmental plasticity are going to be discussed in this section.Current plastic responses will only remain adaptive under future conditions if informative environmental cues are still available, when the time lag between these two environments increases, cues are expected to become less informative. So, the environment is a key part in developmental plasticity, factors that emphasise this are extreme weather events, melting ice caps, glaciers and competition.

1) Extreme weather events

More frequent flooding, stronger storms and heatwaves are all examples of extreme weather events. They affect everyone and everybody. Even plants have their developmental plasticity controlled by their meristem; a group of stem cells which produce the organs necessary during the plant’s development. Therefore, although we only focus on the short-term, devastating impacts of these events would not be noticed for the organisms affected, its impact will be felt for many years to come. 

For example, during a heatwave, humans can suffer from heatstroke and dehydration. We haven’t adapted to the extreme high temperatures so we suffer without necessary precautions. If we didn’t have artificial defence mechanisms against the heat many will die. The same applies for other animals and plants, but in their case they do not have artificial defence mechanisms. For plants and habitats, they will die off and competition for space and land would become fierce which will lead to ‘survival of the fittest’. However, for animals they will not be adapted to another habitat, so they are more likely to lose the battle for necessities, leading to endangerment. 

Therefore, the surviving young animal’s and plant’s developmental plasticity will be affected in the future, as their features will have to change as they grow to their altered environment. Not only this, but they may lack the natural resources for their phenotype to develop properly, e.g., deficiencies.

2) Melting ice caps and glaciers

This follows the same principles as extreme weather events in terms of habitats. When the ice melts, habitats are lost, sea levels rise and the weather becomes more unpredictable. The effects of habitats being lost has already been specified, and unpredictable weather is something we are seeing on a day-to-day basis worldwide, so we are going to focus on sea levels rising.

Sea levels rising have countless repercussions, ranging from the erosion of land to extreme weather. Flooding, loss of habitats and the low-lying land that is being lost are all consequences that aren’t hidden; we know these all stem from this problem. One of the less talked about effects is the thawing of permafrost. Permafrost is ground that is permanently frozen that holds large amounts of methane, a greenhouse gas. The phenomenon (climate change) that has melted more than 9.6 billion tonnes of glacial ice in the world since 1961, according to a 2019 satellite study by the University of Zurich, meaning the methane that is released increases the rate of global warming, causing an uncontrollable cycle of melting ice which would push us towards the worst predictions for climate change.

In the aforementioned study, the University of Zurich revealed that glacial melting has accelerated over the last three decades. This loss of ice has already reached 335 billion tonnes per year, which is 30% of the current rate of ocean growth. This also leaves less freshwater. These statistics highlight the effects that rising sea levels can have on developmental plasticity for organisms around the world.

3) Competition

Plants, animals and humans are all organisms that are competing for basic necessities and there are some examples below: 

• Plants- land, sunlight, nutrients from the soil (soil is being damaged; degradation)
• Animals- land, habitats, water, food 
• Humans- shelter, land, water, food

These examples are all being exacerbated by climate change. For humans, we are already seeing a housing crisis because of increasing population, war and conflict over land, and unrest against the actions being taken by world leaders. But how does this competition fare for animals and plants?

In the case of animals, they can also fight each other, but outright fighting is not a feature for every specie. For example, sea and land animals can lose their bearings of the land they are on due to drastic changes in weather, causing many deaths due to unexplainable changes in behaviour; like whales that have been more frequently washing up on shores, and this is exemplified through extreme weather events like hurricanes, which cause large decreases in air and water pressure. Many animals can quickly sense these changes and will often behave strangely, flee or hide for safety. Even tigers mass migrating is linked to climate change in terms of behavioural changes, but in the case of prey, this could be a deadly move. Consequently, diversity in the animal kingdom is decreasing due to the imbalances caused by climate change and animals are developing with a lower rate of survival.

However, this doesn’t apply to plants. Plants obviously can’t mass migrate or fight each other, so what do they do? They have little choice but to compete for the nutrients and to grow favourably towards the sunlight. But many will die of a lack of this as these as they are scarce, due to some areas becoming inhabitable and weather becoming less predictable. Their developmental plasticity is not fully understood, but their meristem can help them adapt over a short period of time. However, while living in the same habitat naturally, it is difficult for a plant’s organs to adapt to a sudden change in weather or environment. This will drastically affect biodiversity in the ecosystem, which will have a chain reaction that is not fully understood.

Image source: https://royalsocietypublishing.org/doi/10.1098/rspb.2017.0386#d3e396

Closing Words:

Global change encompasses modifications of the environment both at a global scale and at a local scale but in such large proportion that the whole planet is affected. The more research is done the more daunting the effects, it seems, but it is better to know then react, than to be naïve to non-mainstream consequences. Although we can’t change the way every animal and plant develop, the more awareness spread, the better. In this article, developmental plasticity is clearly shown to be affected by climate change, but as studies continue, we are yet to know the full extent of its reach and influence in the world we see tomorrow.

If you want to read more about developmental plasticity, the following are websites that go in depth:

• https://www.frontiersin.org/articles/10.3389/fgene.2019.00720/full#h3
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6709652/
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249629/
• https://www.ncbi.nlm.nih.gov/pmc/?term=plasticity
• https://academic.oup.com/emph/article/2017/1/162/4835137

Thank you for reading!

Written by Malick, a Year 12 Student at Dartford Grammar School