Organisms and Populations

Learning Outcomes

1. Understand the concepts of population, ecology, and interspecific interactions.
2. Grasp the mechanisms of population growth and density dynamics.
3. Learn about competitive exclusion, mutualism, predation, and other interspecies interactions.
4. Differentiate between logistic and exponential growth models.
5. Analyze the evolutionary adaptations of populations for reproductive success.

Populations and Population Ecology
The living world is diverse and highly complex. To understand its intricate processes, we must study various levels of biological organization: macromolecules, cells, tissues, organs, organisms, populations, communities, ecosystems, and biomes. Populations, an important aspect of ecology, consist of organisms of the same species inhabiting a defined geographical area, sharing similar resources, potentially interbreeding, and interacting with the abiotic environment. Population ecology is crucial as it connects ecology with genetics and evolution.

Population Attributes

Populations possess attributes that individual organisms lack. These key population attributes include:

1. Birth Rates and Death Rates: Instead of focusing on individual births or deaths, populations are characterized by their per capita birth and death rates. For instance, if a population of 20 lotus plants in a pond increases to 28 after adding 8 new plants, the birth rate is 0.4 lotus per plant annually.
2. Sex Ratio: While individuals are either male or female, a population has a sex ratio. For example, in a population where 60% are females and 40% are males, the sex ratio influences population growth dynamics.
3. Age Distribution: Populations consist of individuals of varying ages, forming an age pyramid when graphed. This reflects whether a population is growing, stable, or declining. Age pyramids are key indicators of the population’s growth status.
4. Population Size and Density: Population size, technically referred to as population density (N), reveals the population’s status in its habitat. Density can be measured in various ways. For instance, measuring biomass or percent cover may be more meaningful than counting individuals when dealing with very large or difficult-to-count populations.

Important Note:
Population size, though generally expressed in numbers, may also be described in terms of biomass or percent cover when direct counting is impractical or less meaningful.

Population Growth

Population growth fluctuates over time, impacted by several factors such as food availability, predation, and weather. The four primary processes that influence population size are:

1. Natality: Refers to the number of births that add to the population during a specific time.
2. Mortality: The number of deaths during a given period reduces the population size.
3. Immigration: Refers to the influx of individuals from other populations, increasing population size.
4. Emigration: The movement of individuals out of the population decreases its size.

The population’s density at a given time is influenced by these factors, as represented in the equation:
Nt+1 = Nt + [(B + I) – (D + E)]
Where Nt+1 is the population density at time t+1, B is births, I is immigration, D is deaths, and E is emigration.

Growth Models

Populations generally follow two growth models:

1. Exponential Growth: When resources are unlimited, populations grow exponentially, as represented by the equation dN/dt = rN. In this model, r represents the intrinsic rate of natural increase. This results in a J-shaped curve, signifying rapid population growth.

• For example, even a slow-growing species like elephants can exhibit massive population growth if left unchecked.

1. Logistic Growth: As resources become limited, populations exhibit logistic growth, which is represented by the equation:
   dN/dt = rN[(K-N)/K]
   Here, K is the carrying capacity, the maximum population size that the environment can sustain. This model results in an S-shaped curve, showing population growth that initially accelerates, then decelerates, and finally stabilizes as it approaches the carrying capacity.

Important Note:
While exponential growth occurs under ideal, resource-rich conditions, logistic growth is more common in nature due to the finite nature of resources.

Life History Variation

Populations evolve in response to environmental conditions, maximizing their reproductive fitness (Darwinian fitness). Life history strategies vary widely between species, depending on the abiotic and biotic constraints in their habitat:

1. Semelparity: Some organisms, like Pacific salmon and bamboo, reproduce only once in their lifetime, producing a large number of offspring.
2. Iteroparity: Other species, such as birds and mammals, reproduce multiple times over their lifetime, often producing fewer, but larger, offspring.

Population Interactions

Species do not exist in isolation; instead, they interact with other species within their habitat. These interactions are classified into six categories based on their effects:

1. Mutualism: Both species benefit.

• Example: The symbiotic relationship between fungi and plants in mycorrhizae or between algae and fungi in lichens.

1. Competition: Both species are harmed as they compete for limited resources.

• Example: In Gause’s competitive exclusion principle, when two species compete for the same resource, the superior competitor eventually eliminates the inferior one.

1. Predation: One species benefits, and the other is harmed.

• Example: Predators like tigers regulate prey populations, preventing ecological imbalance. Even herbivores like cattle are predators when they consume plants.

1. Parasitism: One species benefits, while the host is harmed.

• Example: The malaria parasite requires a mosquito as a vector, while parasites like lice and ticks live on the external surfaces of their hosts.

1. Commensalism: One species benefits, and the other is unaffected.

• Example: Cattle egrets benefit from grazing cattle by feeding on insects stirred up by the cattle.

1. Amensalism: One species is harmed, and the other is unaffected.

• Example: The secretion of certain chemicals by walnut trees inhibits the growth of surrounding plants.

Predation and Plant Defenses

Predators play a vital role in maintaining ecosystem balance. They regulate prey populations and transfer energy through trophic levels. However, prey species have evolved various defensive mechanisms:

1. Camouflage: Many species, like insects and frogs, use cryptic coloration to avoid predators.
2. Toxins: Some species, like the Monarch butterfly, produce chemicals that make them distasteful to predators.
3. Morphological Defenses: Plants have evolved defenses such as thorns and chemical deterrents (e.g., the Calotropis plant, which produces toxic glycosides).

Competition and Competitive Exclusion

Competition can occur even when resources are not limiting. Interference competition reduces the feeding efficiency of one species due to the presence of another. Competitive exclusion states that two species competing for the same resource cannot coexist indefinitely. However, species often evolve mechanisms to avoid direct competition, such as resource partitioning, where different species exploit different aspects of the same resource.

Parasitism and Host Evolution

Parasites have evolved numerous adaptations to exploit their hosts. For example:

1. Ectoparasites: Live on the host’s surface, like lice on humans.
2. Endoparasites: Live inside the host, like tapeworms in the intestines of mammals.
3. Brood Parasitism: Birds like the cuckoo lay their eggs in the nests of other birds, relying on the host to incubate their eggs.

Mutualism and Co-evolution

Some of the most remarkable examples of mutualism involve plant-animal interactions. Plants rely on animals for pollination and seed dispersal, offering rewards like nectar and fruits. In turn, animals depend on plants for food and shelter

. One well-known mutualism is between fig trees and wasps, where each species has evolved to depend entirely on the other for reproduction.

MCQ
What is the primary outcome of Gause’s competitive exclusion principle?
a) Two species can coexist indefinitely in the same habitat
b) One species eventually eliminates the other due to superior competition
c) Both species increase in population size simultaneously
d) One species evolves mechanisms to avoid competition
Answer: b) One species eventually eliminates the other due to superior competition