How Chemical Signals Connect Physics, Chemistry, and Biodiversity. Within a forest, there are many different chemical molecules being released by plants. Some are responsible for the smell of leaves and flowers whilst others play a role in interactions between plants, insects and predators. Could the biodiversity present in each season, also be dictated by the chemical interactions between these organisms?
Plants release volatile organic compounds (VOCs) which are carbon containing molecules that can easily enter the atmosphere. These VOCs have a role in ecological interactions or atmospheric chemistry. The rate of release will depend on the plant physiology and environment. These molecules will move due to physical processes and their impact will depend on their chemistry and the biology of the organisms encountering them.
This forms an interesting link between the fields of physics, chemistry and biology.
Physics will dictate how much of the compound is released, chemistry will dictate how it reacts and biology will dictate how it is produced, detected and responded to.
Seasonal biodiversity can be studied by looking at how the physical, chemical and biological factors change.
1. Physical Factors
The first aspect to the invisible chemical language of plants begins with the physical factors.
Due to the angle of the earth’s axis there will be seasonal changes in the amount of daylight and solar energy reaching much of the earth.
This will influence the temperature, plant growth and the environment in which the volatile compounds are produced.Temperature will be a key factor. As temperature increases the vapour pressure of many volatile substances will increase meaning there is a greater chance of them entering the atmosphere. However, the amount actually released will depend on how much is produced, the plant physiology and stresses in the environment.
Light can also have an impact on the amount released. Carbon compounds will be produced by photosynthesis to aid the growth of the plant and other biochemical processes. Some of these will aid in the production of VOCs. Some emissions will be greatly influenced by both temperature and light whilst others will be triggered by different physiological or environmental factors.
A landmark study undertaken by Guenther et al (1995) titled ‘A global model of natural volatile organic compound emissions’ created a model to estimate the amount of emissions coming from vegetation based on environmental factors including temperature and light. This helped to show how biological emissions can be incorporated into larger scale atmospheric models.
This is significant as it shows how the physical factors of the environment can link to atmospheric chemistry via measurable biological factors.
Wind can be an issue. The compounds will be transported and diluted by the wind. There will be turbulence in the atmosphere so the concentration of the chemical will vary around the plant.
Two plants releasing the same compound may have very different concentrations around them on a warm calm day compared to a cool windy day. Whilst the chemical itself may not have changed, the concentration and chance of reaching another organism may have changed significantly.
This is key as the chemical signal will be influenced by physical factors.
2. Chemical Factors
Plants produce a variety of VOCs including terpenes and green leaf volatiles. The structure of these molecules will determine how volatile they are, how reactive they are, how long they last and how they will interact with biological receptors.
Some terpenes will contribute to the smell of the plant whilst others will contribute to defence processes and atmospheric chemistry. Green leaf volatiles can be released quickly when there is damage to the plant tissue causing the recognisable smell of freshly cut grass.
However, there is an ecological importance of these compounds.
Turlings, Tumlinson and Lewis undertook a landmark study in 1990 titled ‘Exploitation of herbivore-induced plant volatiles by parasitic wasps’ where they looked at how parasitoid wasps use the odours released by plants damaged by herbivores. Through experimentation they showed that parasitoid wasps can use the odours released by herbivore damaged plants to find their hosts.
This provided evidence for the incredible ecological process of one organism damaging the plant causing it to change its chemical emissions which can influence another organism.
There is no conscious message being sent by the plant, but the biochemical process has changed the information in the environment.
Some of these biogenic Volatile Organic Compounds will react with oxidants such as hydroxyl radicals and ozone. Some of the oxidised products will contribute to forming secondary organic aerosols.
This means the molecule released from the plant may influence an organism nearby, or it may react with other atmospheric chemicals. Or it may be transported and diluted.
The atmosphere can be seen to have a large chemical influence.
This shows how studying seasonal biodiversity can benefit from knowledge of chemistry as the molecule and its reactions matter.
3. When Plants Ask for Help
Research undertaken by Turlings et al raises the question of whether the chemical response of one organism can impact the survival of another.
If a plant is being eaten by a caterpillar, the plant may activate defence pathways due to the herbivore damage. This can cause a change in the amount of VOCs being released. In certain plant systems this can attract natural enemies of the herbivore.
The wasps may use the chemical information along with other factors to find suitable hosts. This can decrease the chances of the herbivore surviving.
This is often referred to as indirect defence.
The ecological importance of this is the relationships it creates. By changing the plant biochemistry it can influence the behaviour of an insect which can influence the activity of a predator/parasitoid. This may change the structure of the local food web.
However, different plants produce different chemical mixtures and different insects will respond to different cues. The weather, availability of alternative hosts and natural enemies can all have an impact.
This shows that whilst plants may not be consciously controlling other organisms using chemical messages, it does show that the chemical emissions can influence interactions between species. The chemistry of individual organisms can be important when looking at the dynamics of a larger ecological community.
4. Linking to Seasonality
How does this relate to seasonal biodiversity?
Environmental factors, plant physiology and the activity of other organisms in the season can all influence the amount of VOCs released.
Temperature and sunlight can influence the amount of a specific VOC released. Water availability and environmental stress can cause changes in plant metabolism and chemistry. Depending on the compound and species, this can have a varying effect.
A good example of this is isoprene. Many species of plants will release isoprene and this can be greatly influenced by temperature and light. This means that depending on the changes in these factors the amount released from vegetation will change.
However, not all plants release isoprene.
Other VOCs may be affected by temperature, herbivore damage, drought or development.
The organisms that may respond to these will also be influenced by the season. Insects will emerge, reproduce, migrate or go into dormancy depending on the species and environmental cues. Only if the correct organisms are present and able to respond to the chemical will there be an effect on the ecology.
If a plant released compounds that attracted natural enemies of an insect herbivore, but the natural enemy was only active during one period, there may be a difference in the ecological impacts.
Or the wind and temperature may change meaning there is a difference in the concentration and movement of the compounds. This can affect whether the natural enemy encounters them.
This shows that the ecological importance of the chemical will depend on when it is released, how it moves and if the correct organisms are present.
However, it is important to measure the trends. An increase in emissions during the summer may be due to temperature, plant development, insect damage or a combination of these factors.
It is important to understand why there are changes.
5. Atmospheric Chemistry
The importance of the VOCs released by plants is not just in the interactions between organisms.
Once in the atmosphere they will react and can influence the composition and particle formation.
Some of the oxidised VOCs will contribute to secondary organic aerosols. These can have an impact on visibility and cloud formation depending on the atmospheric conditions and properties of the particles.
Biogenic VOCs will also react with nitrogen oxides and other atmospheric components. Depending on the chemical environment they can contribute to the formation or destruction of ground level ozone.
This is particularly important for vegetation as ground level ozone can cause damage to plant tissues and can inhibit photosynthesis.
This can lead to a feedback loop. Plants release compounds that take part in atmospheric chemistry and this can influence the health of the plants.
This can be complex as the VOCs may not have the same effect on the atmosphere. The effect will depend on factors such as sunlight, temperature, nitrogen oxide concentrations and the type of compounds.
However, this shows an important point that organisms are not just victims to their physical and chemical environments. Their biological activity can help them to manipulate their environment.
Seasonal biodiversity will be linked to processes from a single leaf to the atmosphere above the forest.
6. Climate Change
As the environment changes these relationships become increasingly important.
Climate change can cause changes in temperature, rainfall, drought frequency and the timing of events in the season. This can influence plant physiology and VOCs.
Certain compounds may be released in higher amounts due to increased temperature. However, if there is severe drought, this may limit plant activity and cause changes in emissions.
The composition of the atmosphere can also influence how the VOCs react and how long they last.
Insects and other organisms may respond differently to the changes in the environment. If there are changes in the plant emissions but the organisms do not change their seasonal activity this can cause changes in ecological interactions.
For example, if a plant released chemicals that attracted a specific natural enemy but this natural enemy became less active during the time the plant needed protection.
Whilst this may be a likely mechanism it will need to be investigated for specific species.
This is important as biodiversity will not just depend on the presence of individual species. It will depend on interactions between species such as pollination, herbivory, predation, competition and decomposition.
Many species may still be present within an ecosystem but the timing and effectiveness of the interactions may change.
Research into seasonal chemical communication could help scientists to understand not just which species survive due to changes in the environment but how the relationships between them are impacted.
7. How Could Scientists Investigate?
Although the chemical signalling is invisible it can be investigated.
Scientists could measure the VOCs being released by the plants using analytical techniques such as gas chromatography and mass spectrometry. This would allow them to separate and identify the compounds in the sample.
They could repeat this throughout the seasons noting the temperature, sunlight, humidity, wind, soil moisture and plant condition.
Experiments could be carried out to provide causation. Scientists could expose plants to different temperatures/light whilst keeping other variables the same. They could measure the emissions and see how insects reacted.
However fieldwork would also be required as there are many variables in natural ecosystems.
Scientists could look at how much VOCs are released, herbivore abundance and parasitoid activity throughout the seasons. They could see if changes in chemical emissions are helping to explain differences in the biological interactions.
The best studies would look at:
1. What physical factors are influencing the production and movement of specific compounds?
2. What compounds are changing and how?
3. Are there measurable differences in behaviour, survival or interactions of organisms?
This would link physics, chemistry and biology with measurable evidence.
Conclusion: The Invisible Links to Support Life
Things are seasonal, and hence biodiversity can be quantified in the obvious manifestations. Blossoms in spring, leaves in autumn, bugs in spring, and a wide assortment of different birds between environments. In any case, there’s significantly more going on beneath the surface.
Temperature and sunlight will affect plant physiology and VOCs. The chemistry will affect how the compounds behave in the atmosphere. Some of these molecules will be recognised by biological systems as cues. This can influence the interactions between plants, herbivores, predators and other organisms.
These interactions are complicated and will vary by species, environment and timing of events. But that makes them interesting scientifically.
The key point to keep in mind is that seasonal biodiversity is not simply about what’s going up and what’s going down. We need to look at the physical conditions and the chemical and biological interactions.
Even if the forest seems still, its denizens are experiencing a shifting chemical milieu. Molecules are being secreted, transported, modified and, on occasion, detected by other organisms.
Perhaps the most amazing thing about a season is that the invisible links between life change.
By understanding these links we can better understand biodiversity by seeing how it is a dynamic network influenced by the continuous interaction between physics, chemistry and biology.
By: Meghna Gupta
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