Study Notes on the Paradox of the Plankton

Paradox of the Plankton

• Phytoplankton's Nutritional Needs:

  • Phototrophic organisms can thrive in inorganic media with CO2, nitrogen, sulfur, and phosphorus, along with trace elements including Na, K, Mg, Ca, Si, Fe, Mn, B, Cl, Cu, Zn, Mo, Co, and V.

  • Certain species have additional vitamin requirements (e.g., thiamin, cobalamines).

• Coexistence of Species:

  • The paradox arises from many species competing for limited nutrients in a homogeneous environment.

  • Enrichment experiments show nutrient deficiencies in summer, intensifying competition.

  • Competitive exclusion theory suggests only one species would dominate under stable conditions.

• Equilibrium Dynamics:

  • Hutchinson posits that diversity arises from failure to achieve equilibrium due to changing external conditions.

  • Three scenarios of species competition based on reproductive rates and environmental changes:

  1. $t ext{ (reproductive time)} ext{ } << ext{ } t_e ext{ (environmental change time)}$ - Competitive exclusion occurs before significant changes.

  2. $t ext{ } ext{ (approx. } = t_e)$ - No equilibrium is attained.

  3. $tc ext{ (competitive time)} ext{ } >> te$ - Exclusion occurs during changing environmental conditions.

• Species Distribution:

  • MacArthur's distribution model indicates that species abundance can follow specific mathematical distributions (Type I, Type II, Type III).

  • Type I fits specific species populations but fails for diatom and arthropod communities.

  • Type IV indicates uneven distributions caused by heterogeneous environmental conditions.

• Role of Ecology:

  • Light gradients and chemical variations in the water can marginally influence niche diversity.

  • Symbiotic and commensal relationships may allow coexistence, particularly among planktonic species with different nutrient requirements.

• Predation Impact:

  • Predators can differentially limit competing species, thus supporting diverse prey populations.

• Non-equilibrium Conditions:

  • Theoretical models suggest that non-equilibrium allows diverse planktonic communities to persist.

  • Historical evidence of dominant species persisted through geological time supports this idea.

• Validity of Phytoplankton Concept:

  • Phytoplankton may derive from benthic species rather than being evolutionary independent.

  • Dynamics in lakes show no correlation between lake area and phytoplankton species diversity.

• Conclusion:

  • Competitive exclusion theory provides a framework to explore ecological paradoxes in phytoplankton diversity, suggesting empirical anomalies may suggest alternative ecological dynamics.