WK7 - INTERACTIONS: Competition: Part 2: Intraspecific Competition

Intraspecific Competition and Logistic Growth

Intraspecific Competition

• Competition between individuals of the same species (conspecifics).
• Underlies Darwin's theory of natural selection: survival and reproduction of the fittest.
• Individuals better at competing for limited resources pass on their genes.

Intraspecific Competition and Logistic Population Growth

• Logistic growth curve: population growth slows as it approaches carrying capacity ($k$).
• Initial rapid growth transitions to slower growth as intraspecific competition increases.
• The closer a population is to $k$, the greater the intraspecific competition.
• Growth stops when the population reaches $k$.
• Density-dependent process: higher population densities increase intraspecific competition.
• Overshooting carrying capacity: rapid population growth due to favorable conditions can lead to a temporary overshoot, followed by a decline to $k$ due to resource limitations.

Example: Common Tern (Sterna hirundo)

• Sigmoidal growth in nature.
• Global distribution; example data from the East Coast of the USA (north of New York City).
• Population decline in the 1900s due to feather use in the hat industry.
• Protection legislation introduced, leading to population recovery.
• Current global population trend is still negative.

Population Dynamics

• X-axis on the graph: Year.
• Y-axis: Number of tern nests (proxy for the number of birds, where one nest ≈ two birds).
• Post-legislation: exponential growth followed by leveling off around the 1990s.
• Fluctuations due to seasonality and environmental changes.
• Carrying capacity (k): approximately 2,000 birds or roughly one nest per square meter.
• Each nesting pair needs about one square meter to breed successfully.

Fecundity

• Initially high fecundity (≈ two chicks per pair).
• Decline in fecundity over time, leveling off at ≈ one chick per year.
• High initial fecundity: less intraspecific competition, more time for reproduction and chick care.
• Increased intraspecific competition: more competition for nest space and food, fewer fledglings per pair.
• The leveling off of number of nests coincides with the decline in fecundity.

Interspecific Competition in Plants: Self-Thinning

• Plants compete for soil, space, light, and nutrients.
• Self-thinning: trade-off between density and size of individuals.

Density and Time

• X-axis: Time of sowing.
• Y-axis: Density of surviving plants.
• Initially high density decreases over time as seedlings grow and compete for resources.

Plant Size and Density

• X-axis: Plant density.
• Y-axis: Biomass (dry mass).
• Plant size decreases as density increases.
• High density: plants remain small.
• Increased competition leads to self-thinning: weaker individuals die, density decreases, and remaining individuals grow larger and stronger.
• Slope of the relationship is often around $-\frac{3}{2}$, but there are exceptions to this rule.

Deviation from the $-\frac{3}{2}$ Rule: Brown Trout

• Initial slope close to $-\frac{3}{2}$ at smaller body sizes (juvenile stage).
• Slope becomes steeper as brown trout grow larger.

Possible Reasons for Change in Slope

• Habitat change: juveniles in rivers, adults in larger bodies of water (lakes, oceans).
• Different limiting factors for juveniles and adults.
• Greater decrease in mass with increasing density in adults.

Conclusion

• Intraspecific competition examined in both animal and plant species.
• Next lecture: interspecific competition (competition between species).