Distribution Patterns

Distribution Patterns 1. Effects of Dispersal on Population Density Dispersal affects population density by redistributing individuals within and between populations: • Reduction in Source Area: Dispersal reduces population density in areas of high density (source populations). • Increase in Target Area: Dispersal raises population density in areas where individuals settle (sink populations). • Balance Across Populations: Dispersal maintains stable densities by equalizing numbers between connected populations. • Colonization of New Habitats: Dispersal allows populations to expand into unoccupied or newly available habitats. 2. Emergence of Distribution Patterns Distribution patterns describe how individuals are spaced within an area. These patterns include: 1. Clumped Distribution: • Caused by uneven resource distribution, social behavior, or environmental heterogeneity. • Example: Trees clustering around water sources. 2. Uniform Distribution: • Results from territoriality, competition, or human management. • Example: Penguins nesting at fixed distances to reduce conflict. 3. Random Distribution: • Occurs when resources are evenly distributed and there are no strong interactions between individuals. • Example: Dandelions dispersed by wind in a uniform field. 3. Evidence for Female Choice Female choice is a mechanism of sexual selection where females actively select mates based on specific traits: • Direct Observations: Females often select males with distinct traits (e.g., peahens prefer peacocks with larger tails). 1 • Controlled Experiments: Manipulated traits in laboratory settings confirm preferences. • Fitness Correlations: Traits preferred by females are often linked to higher reproductive success (e.g., complex bird songs). • Nuptial Gifts: Mating preferences based on direct benefits (e.g., larger prey gifts in hangingflies). 4. Sexual Dimorphism and Male Traits Sexual dimorphism, where males and females look different, arises from sexual selection: • Male-Male Competition: Leads to traits like larger size or antlers to outcompete rivals. • Female Choice: Results in showy traits such as bright plumage or elaborate courtship displays. • Trade-offs: Male traits may increase reproductive success but reduce survival (e.g., increased predation risk for showy traits). 5. Phenotypic Plasticity and Morphs Phenotypic plasticity refers to the ability of an organism to change its phenotype in response to environmental conditions: • Increased Plasticity in Small Organisms: Smaller size in early life stages often allows greater adaptability and more morphs. • Morphs and Speciation: When morphs experience restricted gene flow, they may diverge genetically and reproductively, leading to speciation. 6. r-Selected Species and Parental Investment r-selected species exhibit a reproductive strategy focused on maximizing offspring quantity: • Low Parental Investment: Minimal resources are invested per offspring; survival depends on sheer numbers. • Traits: High reproductive rates, small offspring size, and short lifespans. • Adaptation to Unpredictable Environments: Strategy is advantageous in fluctuating or ephemeral habitats. 2 Characteristic r-Selected Species Reproductive Rate High Parental Investment Low Offspring Number Many Growth Rate Rapid Lifespan Short Environment Unpredictable Population Dynamics Boom and bust K-Selected Species Low High Few Slow Long Stable Near carrying capacity Table 1: Comparison of r-selected and K-selected species. 7. Comparison of r-Selected and K-Selected Species r-Selected Species • Thrive in unpredictable and disturbed environments. • Produce large numbers of offspring with minimal parental care. • Experience high mortality rates among offspring. • Rapid reproduction and short lifespans ensure population recovery after declines. • Examples: Insects, weeds, small fish. K-Selected Species • Adapted to stable environments near carrying capacity. • Invest heavily in fewer offspring, with high parental care. • Low mortality rates among offspring due to significant care. • Long lifespans and slow reproduction ensure population stability. • Examples: Elephants, whales, large mammals. 3