In the last article of the “Ecology concepts” series, we went through the definition of ecosystem and oversaw what trophic webs are. Briefly recap, an ecosystem could be defined as the physicochemical environment, the living organisms in it, and the relationship between them and the medium.

In today’s “Ecology concepts”, we will delve more into these relationships. When two nodes of our trophic webs connect, it means that two species are interacting. And the question to answer today is: what are those interactions and how can we study them?

Interactions in nature can be reduced to transfers of energy, nutrients, or biomass [1]. If you remember the trophic webs, you’ll know that there are primary producers that can transform energy from light into biomass (i.e.: plants growing through photosynthesis). The next interaction in the web may be a primary consumer, which consumes that energy. The energy, in the form of biomass, moves through the nodes of the trophic web as the organisms interact with each other. In these interactions, the organisms can either win, lose, or remain with the same energy, which we will refer to as either positive (+), negative (-), or neutral (0) outcomes of the interactions. If we plot the possibilities of these with two different species, we get a table with six different types of interactions that may happen in nature: 

Fig 1: The six different types of interactions that happen in nature. Author 2024

There is an ongoing discussion in the Ecology world about neutrality because it is very difficult to happen. Someone will always either get something or lose it, When both the organisms involved in the interaction do not get anything, it would mean that there was no interaction at all.

As for the rest, when both the parties involved get something (energy, nutrients, or biomass), we call it mutualism, or symbiosis and “everyone is happy” [2]. One of the most known examples is the clownfish symbiotic relationship with anemones, in which Nemo can hide from predators inside the anemone and can eat the parasites there. At the same time, it provides the anemone with nutrients – mainly through feces – but fortunately, anemones are not picky eaters.

When someone gains and the other one remains unaffected – no negative repercussion, but neither a positive one – it is known as commensalism. A classical commensalism case could be the whole group of plants called Epiphytes. These plants grow over other plants, using them as substrate. They can take their nutrients and water from the sap of the tree they are growing on, without damaging any part of it [3].

Fig 2: Photograph by Dirk van der Made. Epiphytes near Santa Elena, Costa Rica, January 2004. {{CC-BY 1.0}} on Wikimedia Commons.

Similar to commensalism, but with a negative outcome for one species and a neutral one for the other one, we get ammensalism. It is a little bit trickier as it is rarer, but sometimes one organism will produce some kind of toxin even if the other is not present, which is kind of counterproductive, but nature is not perfect.

The classical interaction that we think of when talking about trophic webs is the prey-predator interaction. Here, someone gets energy, and someone loses it, in other words, someone eats, and someone gets eaten. If there is a loss for both species involved, we will get what is known as interspecific competition, when two species have to lose energy competing for the same resource.

These two interactions have been deeply analyzed in the Ecology field, but there were two non-ecologists that simultaneously and independently from each other put some math to it. Alfred Lotka and Vito Volterra were two mathematicians who proposed in 1925 and 1926 respectively, similar theories to analyze interactions between biological systems. The equations are now known as the Lotka-Volterra model [4]. The beauty of it is that it can be applied to both cases, the competition situation in which two species fight for a resource or the prey-predator stage, in which one species is the resource the other is interested in.

The key parameter is – no pun intended – the K, which refers to the carrying capacity of an ecosystem, or the maximum number of individuals that may inhabit it in an “equilibrium situation”, without causing the collapse of the ecosystem.

Two species will balance their numbers, and there will be a maximum number of individuals of one species (K1) and a maximum number for the other one (K2) that sum up to the total K. The Lotka-Volterra equations offer 4 different outcomes for the competition between the species:

1. Species 1 wins and species 2 is displaced from the ecosystem
2. Species 2 wins and species 1 is displaced from the ecosystem
3. An unstable equilibrium point, in which one of the species will win but we are not sure which one 
4. Coexistence, or stable equilibrium point, in which both species can share the ecosystem and if one species grows, the other decreases and the other way around

As we were saying, not only these equations can be used for competition, but they are also present in the prey-predator relationship. And here, we also have equilibrium points, but in the prey-predator model, they are slightly different. The populations cyclically influence each other: an increase in prey numbers will cause an increase also in predators, as they have more food. However, more predators means that they will compete among themselves for prey and consume prey faster, so the prey numbers will decrease. With a decrease in prey, predators will be forced to decrease numbers as well, as they cannot be fed. The reduction of predators allows prey to grow back again, and so we will start again. 

Fig 3: Ian Alexander (parameters, PNG version) Krishnavedala (vectorisation), CC BY-SA 4.0, via Wikimedia Commons.

If you are wondering about applications, let me give you one of the most known examples of the importance this has. Yellowstone is one of the main Natural Parks in the USA, and it was in a terrible state at the beginning of the last century. Why? Because the equilibrium of the ecosystem had been altered, by removing the predators from the model: the wolves had been eradicated. Through reintroducing them, equilibrium was reached again, and the park is now getting back to its proper shape. 

Thanks to Ecology, we can get an understanding of the rhythms of Nature, and help it heal and recover from the perturbations we may have brought upon it. That is why it’s so important to understand it.

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Cover- and preview image: Wolf chasing a deer (2015, 3 August). Photo by Supercarwaar, CC BY-SA 4.0, on Wikimedia Commons.