Ecosystems and Population Dynamics: Key Concepts

Posted on Dec 11, 2024 in Biology

Ecosystems: Definition and Types

An ecosystem is a system composed of a set of interrelated elements. Specifically, it consists of a biotope and a biocenosis, which produce many complex interactions between its components.

• Biotope: A substrate comprising physical and chemical characteristics.
• Biocenosis: All living organisms within the ecosystem.

Types of Ecosystems

1. Terrestrial Ecosystems: These are conditioned by climate. Ecological factors are dependent on weather conditions. There is a great diversity of terrestrial ecosystems.
2. Aquatic Ecosystems: These are classified according to water salinity. We distinguish between saltwater (marine) and freshwater ecosystems. They can be further classified based on water dynamics:
   • Lentic (still waters), such as lakes.
   • Lotic (flowing waters), such as rivers.

Biotope-Biocenosis Interactions

The characteristics of the biotope influence living beings. There are significant differences between organisms of different environments. For example, plants in arid regions and aquatic “pinxo” plants have very large leaves. In terrestrial environments, organisms are more influenced by climatic and edaphic (soil) factors. The aquatic environment depends on the physical and chemical properties of water.

The characteristics of living beings can change the biotope. For instance, the primitive atmosphere lacked oxygen until bacteria evolved to carry out photosynthesis.

Interaction Between Abiotic Factors

Any factor can influence the other components of the biotope. For example, wind favors the dynamics of water, both marine and desiccation. Elevated temperatures may evaporate large quantities of water and cause increased salinity.

Interaction Between Biotic Components

In an ecosystem, relationships occur between organisms of the same species, but they may also occur between different species.

The Population

• Habitat: The place where a species normally lives.
• Ecological Niche: The role of a species in an ecosystem. For example, bobcats prey on rabbits; their ecological niche includes the conditions necessary for their survival.

Concept of a Population

A population is a group of organisms of the same species, living in the same time and space. They have a high probability of mating and share genes. Unlike individuals, a population is immortal; reproduction ensures its perpetuation.

Relations Within a Population

The most representative relationships within a population are competition and intraspecific associations.

1. Intraspecific Competition: This is the main interaction within populations. For example, individual oak trees compete for light and soil nutrients.
2. Intraspecific Associations:
   1. Family Groupings: These are formed for reproduction and protection of offspring. In sexual reproduction, gene mixing influences the choice of genetic heritage. Pairings can be:
      • Monogamous (one male and one female).
      • Polygamous (one of one sex and several of the opposite sex), which can be:
        • Polygynous (one dominant male with all the females).
        • Polyandrous (one female with two or more males).
   2. Gregarious Clusters: These are temporary associations for obtaining benefits such as finding food, defense, or caring for offspring.
   3. Colonies and Societies: These represent a complex organization of populations. In colonies, individuals are physically united and originate from the same parent. In societies, individuals are organized into castes.

Changes in Population

Population Growth

The number of individuals composing a population is often represented by the letter “N”.

The variation of individuals over a period of time (ΔN/t) is obtained by adding the number of births (B) and immigrants (I), and subtracting the number of deaths (M) and emigrants (E):

ΔN/t = B + I – M – E

The rate of population growth (r) is calculated by dividing the population increase in a time period (ΔN/t) by the initial population size (N). If environmental conditions are favorable, the population grows over time, and the growth rate is positive (r > 0). The greater the number of reproductive individuals, the higher the population growth.

However, a population cannot grow indefinitely due to environmental resistance, such as lack of food or competition for space. Consequently, the growth rate stops (r = 0) or decays (r < 0).

Carrying Capacity (K) is the maximum number of individuals in a given population that the ecosystem can sustain; it is the limit imposed by environmental resistance.

Population Strategies

Population growth varies greatly between species. Some tend towards equilibrium, like the oaks of the Mediterranean, which barely change their numbers over time. Other populations fluctuate sharply depending on factors such as seasonal changes and nutrient availability; examples include insects and grasses.

There are two main types of population strategies:

1. Stable or Equilibrium Populations: These are in equilibrium with the environment, and their population size (N) approaches the carrying capacity (K). They are called “K-strategists” and correspond to large organisms with a small growth rate (r).
2. Opportunistic Populations: These are characterized by significant fluctuations in population size (N). They are generally small animals with a high growth rate (r), and are called “r-strategists”.

Interacting Populations in the Ecosystem

Interaction between individuals of different species is unavoidable. These interspecific relationships have different implications and can be beneficial, harmful, or indifferent.

Interspecific Competition

When two or more species use the same resources or habitats, and resources are scarce, one species may displace another.

Consumer-Resource Interactions

The most representative examples are predation and parasitism. In both, the consumer benefits while the resource is harmed.

1. Predators: Organisms that kill other creatures (prey) for food.
2. Parasites: Beings that exploit others (hosts) without killing them. Parasites that live on the surface of the host are called ectoparasites (e.g., ticks), while those that live inside are called endoparasites.

Commensalism and Inquilinism

These are interactions that are beneficial to one species but indifferent to the other.

1. Commensalism: One species, called the commensal, benefits from feeding on the leftovers of a predator.
2. Inquilinism: One species benefits from being housed in another, while the host species is indifferent. For example, a crab may live in the cavities of a sponge.

Mutualism and Symbiosis

In these relationships, two species cooperate to achieve a mutual benefit.

1. Mutualism: The relationship between the two species is not mandatory. For example, plants develop fruits that animals ingest; the undigested seeds are expelled, aiding in their dispersal.
2. Symbiosis: Neither of the two species can live separately. An example is mycorrhizae (fungi associated with roots), where a fungus feeds on substances provided by the roots and, in turn, assists in nutrient absorption.