Individuals, populations, communities and ecosystems
Definition of Species
A species is defined as a group of organisms that share common characteristics and can interbreed to produce fertile offspring. This concept is fundamental to understanding biodiversity and ecosystem dynamics.
Example: The grey wolf (Canis lupus) is a species of canine that can interbreed with other grey wolves to produce fertile offspring. However, while wolves can mate with domestic dogs (Canis lupus familiaris), producing fertile offspring, they are considered subspecies due to their distinct evolutionary histories and ecological roles.
The biological species concept, while widely used, has limitations. For instance, it doesn't apply well to organisms that reproduce asexually or to extinct species known only from fossils.
Identification of organisms
Taxonomists use a variety of tools to identify an organism. One such tool is the dichotomous key. It is a tool consisting of questions with two choices in each step and answering the questions leads the user to the right identification.
An example of a dichotomous key is below.
Population
A population is a group of organisms of the same species living in the same area at the same time, and which are capable of interbreeding.
The concept of a population is scale-dependent. For example, we could discuss the population of rabbits in a specific field, or the population of rabbits in an entire country.
Abiotic and Biotic Factors
Factors that determine the distribution of a population can be biotic or abiotic.
Abiotic Factors
Abiotic factors are the non-living, physical components of an ecosystem that influence organisms. These include:
- Temperature
- Sunlight
- pH
- Salinity
- Precipitation
- Wind
- Soil composition
- Altitude
In a desert ecosystem, key abiotic factors include high daytime temperatures, low precipitation, and intense sunlight. These factors shape the adaptations of desert organisms, such as the water-storing capabilities of cacti or the nocturnal behaviour of many desert animals.
Biotic Factors
Biotic factors involve the interactions between living organisms within an ecosystem. These include:
- Predation: Where one organism (predator) hunts and consumes another (prey).
- Herbivory: Consumption of plants by animals.
- Parasitism: A relationship where one organism (parasite) benefits at the expense of another (host).
- Mutualism: A relationship where both organisms benefit.
- Disease: Interactions between pathogens and their hosts.
- Competition: When organisms compete for limited resources.
In a coral reef ecosystem, mutualism is exemplified by the relationship between coral polyps and zooxanthellae algae. The algae provide the coral with nutrients through photosynthesis, while the coral provides shelter and compounds needed for photosynthesis to the algae.
Population interaction
Populations interact with the ecosystems and each other through various methods as mentioned below:
- Predator-Prey Relationships: Often shown as oscillating populations, where an increase in predators leads to a decrease in prey, followed by a decrease in predators due to lack of food.
- Mutualism: Relationship between two species in which both of them benefit.
- Parasitism: Relationship between a parasite and a host.
- Herbivory: The consumption of plant material by animals
Population size
Population size is regulated by density-dependent factors and negative feedback mechanisms. Density-dependent population limiting factors like disease tend to regulate the population more significantly.
Population size would significantly decline as a result of disease outbreaks if the population size is large rather than small.
Carrying capacity
Carrying capacity refers to the maximum population size of a species that an environment can sustain indefinitely. Density-dependent factors significantly determine is a population reaches carrying capacity or not. Limiting factors of human population growth have been increasingly eliminated, resulting in unsustainable ecosystems. These factors have been eliminated through technological and health advancements, education and infrastructure development.
In a pond ecosystem, the carrying capacity for a fish species might be determined by factors such as available food, oxygen levels, and suitable spawning areas. As the fish population approaches this capacity, limiting factors like increased competition for food and space would slow population growth.
Population Growth Curves
Population growth is often represented graphically using two main types of curves:
- S-curve (Logistic Growth):
- Characterized by slow initial growth, followed by rapid growth, and then levelling off as the population approaches carrying capacity.
- Typical of populations in stable environments with limited resources.
- J-curve (Exponential Growth):
- Shows rapid, unrestricted growth.
- Occurs when resources are abundant and there are no significant limiting factors.
The reintroduction of wolves to Yellowstone National Park in 1995 provides an interesting case study in population dynamics. Initially, the wolf population grew rapidly (J-curve), but as they approached the carrying capacity of the ecosystem, their growth slowed and began to level off (S-curve).
Community
A community is a collection of interacting populations within the ecosystems. It comprises of several populations interacting in the ecosystem.
Habitat and Niche
Habitat
Habitat refers to the environment in which a species normally lives. It encompasses both biotic and abiotic factors that influence the organism's survival and reproduction.
The habitat of a polar bear (Ursus maritimus) includes Arctic sea ice, coastal areas, and tundra regions. These areas provide the necessary conditions for the polar bear's survival, including access to prey (mainly seals) and suitable denning sites.
Niche
The niche of an organism is the particular set of abiotic and biotic conditions and resources to which it responds. An ecological niche is the role of a species in an ecosystem.
Fundamental vs. Realized Niche
- Fundamental Niche: This represents the full range of conditions and resources in which a species could potentially survive and reproduce if there were no competition or other limiting factors.
- Realized Niche: This is the actual set of conditions and resources that a species utilizes in the presence of other species and environmental constraints.
Consider the Galápagos finches studied by Charles Darwin. The fundamental niche of a particular finch species might include a wide variety of seed types and sizes. However, due to competition with other finch species, its realized niche might be limited to specific seed sizes that it is most efficient at handling.
Conclusion
By understanding these concepts, students can understand the complex interactions within ecosystems, calculate population abundance and appreciate the challenges of managing and conserving populations in changing environments.
