Populations

Ecology studies the interactions between organisms and their environment, focusing on four levels: organisms, populations, communities, and biomes. Population ecology examines groups of individuals of the same species living in a defined geographical area, sharing resources, and potentially interbreeding.

Population Attributes

Populations, unlike individuals, possess attributes like birth rates and death rates (expressed per capita), sex ratio, and age structure. The age distribution, plotted as an age pyramid, indicates population growth status (growing, stable, or declining). Population size (density, N) can be measured by total numbers, per cent cover, or biomass, depending on the species and context. Indirect estimation methods, like pug mark counts for tigers, are often used.

Population Growth

Population size fluctuates due to four basic processes: natality (births), mortality (deaths), immigration (individuals entering), and emigration (individuals leaving). Population density increases when births and immigration exceed deaths and emigration (Nt+1 = Nt + [(B + I) – (D + E)]).

Two primary growth models describe population dynamics:

• Exponential Growth: Occurs when resources are unlimited, allowing a population to grow geometrically. The rate is described by $dN/dt = rN$, where $r$ is the intrinsic rate of natural increase. This results in a J-shaped curve.
• Logistic Growth: Realistic growth occurs in habitats with limited resources, leading to competition. The population grows initially, then slows down as it approaches the habitat's carrying capacity (K), resulting in a sigmoid (S-shaped) curve. The equation is $dN/dt = rN(K-N)/K$.

Life History Variation

Organisms evolve life history traits that maximize their reproductive fitness (r-value) in their habitat. Some species reproduce only once (semelparity, e.g., Pacific salmon, bamboo), while others reproduce multiple times (iteroparity, e.g., most birds, mammals). Reproductive strategies also vary in offspring number and size: producing many small offspring (e.g., oysters) versus few large offspring (e.g., birds, mammals). These variations are shaped by environmental constraints.

Population Interactions

In natural habitats, different species interact in various ways, forming complex communities. These interspecific interactions can be beneficial (+), detrimental (-), or neutral (0) to the species involved:

• Predation (+/-): One species (predator) kills and consumes another (prey). Predators control prey populations, maintain species diversity, and act as conduits for energy transfer. Prey evolve defenses like camouflage (cryptic coloration) and warning signals (toxicity).
• Competition (-/-): Occurs when species compete for limited resources (food, space, mates). It can be for resources or interference competition. Gause's Competitive Exclusion Principle suggests that two species competing for the same limited resources cannot coexist indefinitely, but resource partitioning and behavioral adaptations allow for co-existence.
• Parasitism (+/-): One species (parasite) benefits by living on or inside another (host), deriving nutrients at the host's expense. Parasites often evolve host specificity and complex life cycles, while hosts develop resistance mechanisms. Ectoparasites live externally (e.g., lice), while endoparasites live internally (e.g., Ascaris).
• Commensalism (+/0): One species benefits, and the other is neither harmed nor benefited. Examples include orchids growing on mango trees (epiphytes) and barnacles on whales. Cattle egrets foraging near grazing cattle is another instance.
• Mutualism (+/+): Both interacting species benefit. Examples include lichens (fungi + algae/cyanobacteria), mycorrhizae (fungi + plant roots), and plant-pollinator interactions (e.g., figs and wasps, orchids and bees), often involving co-evolution.
• Amensalism (- / 0): One species is harmed, while the other remains unaffected.

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