Chapter 20: Competition, Predation, and Population Biology

Competition, Predation, and Population Biology overview of population biology and ecological interactions explores how groups of interbreeding organisms, known as populations, evolve through changes within the effective unit, the deme, and their occupation of an ecological niche. A foundational concept in population studies is population growth, which is quantified by the rate of increase (r). While reproduction and fecundity determine the potential for exponential growth, environmental limitations invariably lead populations to stabilize near the environment's maximum sustainable size, known as the carrying capacity (K). Population structure is complex and influenced by both density-dependent agents (like crowding) and density-independent agents (like climate), often requiring populations to continuously adapt to modified environments, a concept sometimes referred to as the Red Queen hypothesis. The chapter details three crucial biological interactions that influence evolutionary trait selection and can lead to speciation: competition, predation, and symbiosis. Competition, which arises when species rely on limited, shared resources, leads to key evolutionary responses: resource partitioning, where groups utilize different aspects of a shared resource (exemplified by warblers and Anolis lizards); character displacement, where morphological traits, such as bill depth in Darwin’s finches, diverge in coexisting species to reduce competition; and the competitive exclusion principle, which states that two species cannot permanently coexist if they utilize the exact same niche. Extensive studies on Darwin’s finches highlight that rapid evolutionary change is possible, often linked to simple developmental-genetic mechanisms, such as the expression levels of the Bmp-4 gene correlating with beak size. Predation acts as a significant ecological factor, frequently leading to oscillating population cycles between predator and prey, but it also reduces competitive exclusion by preventing a single dominant species from reaching carrying capacity. Finally, symbiosis covers close biological associations categorized primarily as mutualism, where both species benefit (such as gut bacteria or the relationship between protozoans and algae), or commensalism, where one species benefits without affecting the other (like barnacles on whales). Understanding these complex interactions is essential for assessing the dynamics of demes, populations, and species.