Competition can also take place between male members of the same species seeking females, in this case a limited resource, in order to gain access to reproduction and to be able to pass their genes onto another generation. As we saw earlier, the competition between members of the same species plays a primordial role in natural selection. The males’ competition for the females and the selection that the females make of the males with most desirable characteristics is called sexual selection, yet another chapter in the complex and fascinating encyclopaedia of natural selection.
“Males of the same species gather and perform a variety show of displays, like songs and dances, hoping to attract female attention.”
Turning the focus onto plants now, once seeds germinate, competition begins for nutrients that they can extract from the ground or the substratum where they grow, as well as the water and light they need to flourish. This fierce competition for resources can be seen when a tree is pulled down by the wind, the rain or simply because the weight of epiphytes or other plants ends up bringing down the old trees weakened in their structures by illness or termites, creating a clearing in the forest.
“A clearing in the forest creates a great opportunity for life to be reborn.”
On falling to the ground its crown squashes all the vegetation with it. Furthermore, trees are connected to each other through vines and when a tree falls, they act as cables that pull and bring down the nearby trees.
A clearing in the forest creates a great opportunity for life to be reborn, as seeds that have lain dormant in the ground start to germinate, thanks to the great quantity of light. The small plants start to grow in an accelerated fashion, triggering stiff competition for nutrients and available light.
The tree that gets to the highest part of the clearing first has the greatest opportunities to develop, widen its stem and cover the clearing with its branches. The plants that lose the race stay below in the shade and their growth is suddenly halted, or in some cases, they die if they cannot tolerate the shade.
The plants that are able to colonise and develop first in a clearing are known as pioneers and are the first step in the transformation of this clearing in dense forest.
“Death is inextricably linked to life, ensuring the continuation of the forest over the course of time.”
Generally, in a clearing, herbaceous plants grow first, which are then replaced by shrubs and finally fast-growing trees appear. Pioneer species need a lot of light to live, so when the shade of another tree ends up covering them, their growth slows down and they may even die.
After the establishment of pioneer plants and once the ablest species have grown and covered the clearing, another tough battle commences, when the slow-growing plants start to reclaim their space in the clearing. These plants can tolerate shade, which gives them an edge over the pioneers. The succession process ends when the slow-growing species replace the pioneer plants. Many of the fine woods, like copal or the Mashpi Magnolia, are slow-growing trees and are typical of primary forest.
Clearings are very important as they support the biodiversity of the ecosystem, creating microclimates and unique conditions for certain organisms. This cycle of clearing formation and forest reconstruction is known as ecological succession. In this way, death is inextricably linked to life, ensuring the continuation of the forest over the course of time.
Figure 2.- The tree of seven leathers (Miconia), is a pioneer species that is mainly found in forests in regeneration and is an indicator of the processes of ecological succession.
This is an ecological interaction in which a species, the predator, benefits from another species, the prey, which is damaged. Many people think that depredation only occurs when a large predator like a puma hunts a deer, which is correct, but predation also includes simple things like a wasp eating a larva or an insect eating seeds.
Figure 3.- Collared trogon devouring a moth.
A curious example of predation occurs on the cloud forest floors which are very thin on nutrients as the soil is very acidic due to their clay origin (infertile oxisols) and throughout time it has lost nutrients, washed away by rains.
“Plants only borrow nutrients and then on dying return them to the soil.”
Dead leaves fall to the floor, forming a layer. These leaves are then predated by hundreds of decomposing organisms like fungi, some insects, earthworms and other microscopic organisms like bacteria, and the organic material starts its process of decomposition, which in essence is the liberation of inorganic components trapped in dead tissue of the ground, so that in turn these feed the plants of the forest.
In this way, plants only borrow nutrients and then on dying return them to the soil so that new plants can take advantage of them, closing the cycle. Death, in this case, feeds life.
Figure 4.- Millipede feeding on a decomposing tree trunk.
Just like predation, parasitism is a relationship between two species where one (the parasite) benefits and the other (the host) is damaged. But the difference is that in predation, an animal kills and eats the other, but in parasitism, the parasite slowly feeds on the host and usually doesn’t kill it.
There are internal parasites like protozoa and different types of intestinal worms. And there are external parasites like leeches, ticks and mites. Even a deer munching on the leaves of a tree could be considered a type of parasite.
“Plants obtain toxic chemicals as a by-product of metabolism and over the course of hundreds of years of evolution, they have proved to be an effective defence against their predators.”
If we say that herbivores are a kind of parasite, we can see how the different plants and their herbivores relate to one another. In an experiment conducted in the Amazon, the total biomass in one hectare of forest was measured, coming to 900 metric tonnes. Of this total, only 0.2 metric tonnes corresponded to animals and insects living within this hectare. That is to say, in comparison with the vegetal biomass, the animal biomass corresponded to just 0.02%.
The most incredible finding was that only 7% of organisms in this hectare fed on living leaves, while around 69% fed on dead vegetal material or on decomposition. So the rest of the organisms that lived in this hectare hunted and fed on others. But if plants are an apparently abundant food source, why do only a small percentage of organisms feed on them?
The answer can be found on the inside of leaves, full of chemicals that are toxic for herbivores. Plants obtain these chemicals as a by-product of metabolism and over the course of hundreds of years of evolution, they have proved to be an effective defence against their predators.
Among the main chemical defences are: alkaloids, saponins, cyanogenic and cardiac glycosides, tannins, phenols, terpenes, toxic amino acids and calcium oxalate.
Figure 5.- Soft leaves covered by a gelatinous substance, one form of protection against voracious insects.
The arms race: plants versus herbivores
Herbivores and plants are embroiled in a perpetual arms race: plants are not the only ones that have developed an arsenal of defences. In both physical and chemical variations, evolution functions in both directions, so herbivores have also developed strategies to be able to trick plants and take advantage of their nutrients.
“If you have read Lewis Carroll’s Alice in Wonderland, you might recall a passage in which Alice, trying to flee from the Queen of Hearts, runs as fast as she can but does not make any progress because the ground is moving at the same speed in the opposite direction, almost like a running machine…”
Bear in mind that the evolutionary development of many defence strategies in plants has occurred due to the same pressure of selection that herbivores exercise on plants, but once these strategies have evolved, they can act in reverse, as a force of selection of herbivores, which then have to evolve new strategies to be able to feed themselves on the plants without being damaged, and vice-versa.
We call this effect the Queen of Hearts Hypothesis. If you have read Lewis Carroll’s Alice in Wonderland, you might recall a passage in which Alice, trying to flee from the Queen of Hearts, runs as fast as she can but does not make any progress because the ground is moving at the same speed in the opposite direction, almost like a running machine. In the same way, herbivores have to compensate for the best defences of plants to be able to feed on them, and plants have to continually invent defence mechanisms against herbivores. This theory also explains how the constant perfecting of defence and feeding strategies helps to maintain the balance between the host and its parasite or between the predator and its prey. Therefore, the hyper specialisation of one does not lead to the extinction of the other.
Plants versus Insects
“There are plants that use latex that makes it impossible for beetles to penetrate their seeds.”
Let’s take passion flowers as another example. With their leaves rich in cyanide, they are very toxic for almost all herbivores, yet the caterpillars of heliconius butterflies are specialised in feeding only on these plants because evolution has allowed these caterpillars to develop enzymes that are able to absorb cyanide. In this case, specialisation defeated their attempts, because by limiting its food source, this species also limits the number of defences it has at its disposal.
There are also plants that use latex that makes it impossible for beetles to penetrate their seeds, or mucus that protects the shoots.
Figure 6.- Heliconius butterfly depositing its eggs on the leaves of the passion flower.
Perhaps one of the most extreme events of parasitism that occurs in the forest is when the spores of the cordyceps fungus invade the bodies of some insect species. The mycelium invades the body of the insect, replacing its tissues, even affecting its nervous system and changing its behaviour.
Figure 7.- Cordyceps fungus attacking a parasitoide wasp.
This is a positive interaction between individuals of different species, in which both participants benefit. To get a better understanding of mutualism let’s look at a couple of examples.
A great example of mutualism is pollination, in which plants and pollinators commit to a close interaction, where the pollinator offers a service in exchange for a resource. So, plants offer nectar as a food resource, while the pollinator acts as a means of transport for the pollen, which helps the plant to reproduce.
“Over a long period of time, hummingbirds have been able to adapt their beaks to the shapes of the different flowers available in the forest.”
We can witness this in the hummingbird garden of Mashpi, where these odd little birds visit the flowers from where they drink nectar. But as they insert their beaks into the flower, their heads are covered in pollen, and when they fly to the next flower they help with the reproduction of that plant.
Over a long period of time, hummingbirds have been able to adapt their beaks to the shapes of the different flowers available in the forest, just as flowers have changed their structures to adapt to the shape and size of the beaks of the hummingbirds. It’s clear here that the pressure of selection has worked on both parties, affecting the evolution of the two species equally, consolidating their mutual cooperation. So by specialising in one type of flower, hummingbirds reduce competition for the resource, while plants have gained a reliable pollinator that boosts its chances of reproduction. This relationship, in which one species directly influences the evolution of another is called coevolution.
Figure 8.- Coronita aterciopelada (Boissonneaua jardini) feeding on the flower of a rubiaceae in our hummingbird garden.
One mutualist relationship that plays a fundamental role in the ecology of the tropical forest floor is the association of plant roots with mycorrhizas. Here, these organisms exchange resources for other resources. Plants provide water to mycorrhizas, which in turn help the roots to quickly absorb minerals from the soil, especially phosphorous and nitrogen. The nitrogen-fixing rhizobia and leguminous plants exchange nitrogen for carbohydrates.
Figure 9.- Forest floor covered by dead leaves, providing nutrients to plants.
By themselves, termites are not able to digest wood. Therefore, a unicellular organism, the protozoa, that lives in the intestine of the termites, produces digestive enzymes to digest wood. Neither the termites or their internal assistants can live without each other.
Figure 10.- Soldier termites defending their termitarium.
The enemy of my enemy is my friend: other plant defence strategies
There are also relationships in which organisms give services mutually, as is the case of the cecropia trees and “quita calzón” ants. The ants live inside the tree where not only do they find a home but also food, so when the leaves of the plant are threatened by caterpillars or other predators, hordes of ants storm out to protect their home. Hence the time-worn adage: the enemy of my enemy is my friend.
“Caterpillars store cyanide within their tissues, to become toxic themselves.”
Figure 11.- “Quita calzón” ants defending a cecropia tree where they live.
Predators can also use the defence weapons of the plants in their favour. This is the case with heliconius butterflies who feed on passion flowers whose leaves, as we mentioned earlier, are rich in a compound derived from cyanide. On using enzymes to absorb the cyanide and taking advantage of the nutrients of the leaves, caterpillars at the same time store cyanide within their tissues, to become toxic themselves.
Heliconius butterflies never pass by unnoticed due to their flashy colours, indicating to predators that they are toxic.
Figure 12.- Heliconius sara in the butterfly garden.
This is a relationship in which a species benefits and the other is not affected, or not in any obvious way. Examples of these are: epiphytes, orchids and bromeliads. They grow in the trunks or branches of trees, where they find a space to live and from what we know, neither damage nor help the trees, only take advantage of the height to capture more light and water.
Herons are commensal: these birds follow cattle and feed on insects and other small animals that move out of the way as the herd grazes. As far as we know, the cattle don’t mind (not even about a certain loss of dignity when the herons perch on top of them.)
An example of commensalism in the forest is displayed in certain species of anteater birds. A group of birds follow the driver ants while the ants feed. As the ants hunt insects that live among the dead leaves of the forest, these insects often try to escape, and it’s at this time that the birds hunt the insects. Apparently these birds do not damage the ants, nor do the ants benefit directly from the birds, but this vehicle is very strong and in many instances it is difficult to see the anteater birds without the presence of driver ants. We recomend you to read Housekeeping army ants
Figure 13.- Zeledon anteater bird (myrmeciza zeledoni)
The incredible diversity that inhabits the tropical forests around the world is down to the combination of all the interactions that occur within them. Species form complex networks in which they closely interact, each one depending on the other.
“Due their fragility, the specificity of many species means that each change in their environment directly affects their survival.”
So if competition, predation, parasitism, mutualism and even commensalism have helped to strengthen ecological niches, making species becoming more specialised, deforestation, the introduction of foreign species that come to compete with native one, the appearance of illnesses and forest fragmentation have upset the balance between these relationships and have endangered diversity in tropical forests.
Conservation of all tropical forests is the responsibility of everybody. Due their fragility, the specificity of many species means that each change in their environment directly affects their survival, and the disappearance of one or more species of the forest puts the entire ecosystem at risk.