Biological Communities: Structure, Interactions, and Competition Models

Definition and Hierarchical Structure of Biological Communities

In the study of biological organization, life is categorized into several hierarchical levels. Starting from the individual organism, these levels progress to the population, the community, the ecosystem, the biome, and finally the biosphere or ecosphere. A biological community is formally defined as an assemblage of individuals belonging to different species that occupy a specific geographical area and interact with each other either directly or indirectly.

The study of communities is divided into two primary aspects: structure and function. Structure refers to the species composition, the spatial distribution of these species, and how they vary over time. Function involves the complex interactions between species and their subsequent effects on energy flux (the flow of energy) and the cycles of matter within the environment. According to Miller and Travis (1996), the structure and function of a community are manifestations of a complex series of interactions that bind all members together in an intricate web.

Types of Biological Interactions

Interactions within a biological community are generally divided into two categories: intraspecific and interspecific. Intraspecific interactions occur between individuals of the same population, while interspecific interactions occur between populations of different species that constitute the community. These interactions are fundamentally characterized by their impact on the individuals involved, often represented by symbols indicating positive ($+$), negative ($-$), or neutral ($0$) effects.

Neutralism ($0/0$) describes a situation where two species live together but do not influence each other. Amensalism ($-/0$) occurs when one species is inhibited or damaged while the other remains unaffected. Commensalism ($+/0$) involves one species benefiting while the other is unaffected. Proto-cooperation and Mutualism ($+/+$) are interactions where both parties benefit. Predation and Parasitism ($+/-$) involve one species benefiting at the direct expense of another. Competition ($-/-$) is an interaction where both species are negatively affected because they seek the same limited resource.

The Dynamics of Interspecific Competition

Competition is defined as the interaction between two organisms seeking the same resource. This interaction leads to a measurable reduction in the fecundity, survival, or growth of the individuals involved. This pressure significantly influences population dynamics, species distribution, and evolutionary trajectories. Thomas Park (1954) distinguished between two main types of competition: direct and indirect. Direct competition, also known as interference competition, involves a real contest for resources where individuals cause direct physical or chemical damage to others (mutual inhibition). Indirect competition, or exploitation competition, occurs when the use of resources by some individuals deprives others of those same resources, without direct physical interaction.

Schoener (1983) further refined these categories into six specific types defined by the capabilities of the organisms and their habitats. Consumption competition involves the use of a shared resource. Preemption competition is characteristic of sessile organisms that occupy space first, preventing others from settling. Overgrowth competition occurs when one organism grows over another, depriving it of resources like light or food. Chemical competition involves the release of toxins. Territorial competition is the active defense of a territory by mobile animals. Finally, encounter competition involves non-territorial physical interactions. In terrestrial plants, competition for preemption and overgrowth is the most common.

Mathematical Modeling: The Lotka-Volterra Competition Equations

To understand how species can coexist despite the negative effects of competition, Alfred James Lotka (1880–1949) and Vito Volterra (1860–1940) developed mathematical equations. These models build upon the basic principles of population growth. In an environment with unlimited resources, a population grows exponentially, described by the formula:

$dN/dt = r N$

In this equation, $r$ represents the exponential growth rate. However, real-world growth is limited by the carrying capacity ($K$) of the environment, leading to the logistic growth model:

$dN/dt = r N (1 - \frac{N}{K})$

The Lotka-Volterra model expands this to competition between two species ($N_1$ and $N_2$). The growth rate of each species becomes a function of not only its own density but also the density of the competing species:

For Species 1: $dN_1/dt = r_1 N_1 [1 - \frac{N_1 + Α_{21} N_2}{K_1}]$

For Species 2: $dN_2/dt = r_2 N_2 [1 - \frac{N_2 + Α_{12} N_1}{K_2}]$

The terms $Α_{21}$ and $Α_{12}$ are the competition coefficients. Specifically, $Α_{21}$ represents the per capita competitive effect of species 2 on species 1, essentially acting as a conversion factor to turn individuals of species 2 into an equivalent number of individuals of species 1. If $Α_{21} > 1$, the impact of species 2 on species 1 is greater than the impact of species 1 on itself. If $Α_{21} < 1$, the impact is smaller than intraspecific competition. If $Α_{21} = 1$, the impacts are equal.

Conditions for Equilibrium and Coexistence

To find the conditions for coexistence, we look for points of equilibrium where growth for both species is zero ($dN/dt = 0$). Under the assumption that $r > 0$, equilibrium is reached when:

$1 - \frac{N_1 + Α_{21} N_2}{K_1} = 0$

$1 - \frac{N_2 + Α_{12} N_1}{K_2} = 0$

This identifies the densities where population growth stops, known as the Zero Growth Isoclines (ZGI). On these isoclines, the carrying capacity of a species effectively decreases as the density of the other species increases. Plotting these isoclines on a Cartesian plane reveals four possible outcomes based on the values of $K_1$, $K_2$, $Α_{12}$, and $Α_{21}$. If one species is a much stronger competitor, it will lead to competitive exclusion, where one species dominates and the other goes extinct. If interspecific competition is very strong for both ($Α_{12} > 1$ and $Α_{21} > 1$), the winner depends on the initial population densities. Stable coexistence occurs only when interspecific competition is weak relative to intraspecific competition (Α coefficients for both species are less than $1.0$). In this case, each species inhibits its own growth more than it inhibits the growth of the competitor.

The Principle of Competitive Exclusion and the Ecological Niche

Gause (1934) formulated the Principle of Competitive Exclusion, stating that two species cannot occupy the same niche and compete for the same resources in the same habitat indefinitely; eventually, one will exclude the other. A classic experimental test of this was performed by Tilman et al. (1981) using diatoms. When competing for the resource silicate, Synedra ulna was shown to be a more efficient exploiter of the resource, driving Asterionella formosa to extinction.

This principle is the foundation of the ecological niche concept. While a habitat is simply the place where an organism lives, an ecological niche is the functional role of the organism and its position along environmental gradients. Hutchinson (1957) proposed the multidimensional or hypervolume niche, which describes the niche by considering all environmental factors (dimensions) important to the species, such as temperature and food size. The "Fundamental Niche" (or potential niche) represents the total set of conditions under which a species can persist without competition. The "Realized Niche" is the actual subset of conditions the organism occupies in the presence of biotic stressors like competition. The realized niche is always a smaller section of the fundamental niche.

Niche Parameters and Evolutionary Effects

Niche breadth (amplitude) refers to the variety of resources used or the range of environmental tolerances of a species. According to optimization theory, in resource-rich environments, species tend to specialize (narrower niche), while in resource-poor environments, they generalize (broader niche). When a competitor is introduced, if it reduces resource availability homogeneously, a species may expand its realized niche. If the reduction is dishomogeneous, the species may restrict its niche.

Niche overlap defines the amount of resources shared by two organisms. Species can coexist through niche differentiation, which can occur in space (microhabitat differentiation), in time (differences in seasonal or daily activity), or through differing resource use. If coexistence is observed with differentiated niches, it is sometimes attributed to the "Ghost of Competition Past" (Connell, 1980). This theory suggests that current differences may be the result of past natural selection that favored divergence to reduce competition.

An example of this is character displacement, where two species living together diverge in morphological or physiological traits that overlap. This is seen in Darwin’s finches (Geospiza fortis and Geospiza fuliginosa) on the Gal
pagos Islands. On islands where they co-occur, their beak sizes diverge significantly to allow for the consumption of different seed types, a process requiring genetic variation and long evolutionary timescales. In modern communities, competition is most extreme among sessile organisms in crowded conditions, whereas studies suggest phytophagous insects and herbivores compete less frequently because they are rarely limited by resource availability.