A Global-Scale Mid-Domain Effect Cannot Explain the Latitudinal Gradient in Species Richness.

Area of Science:

• Ecology
• Biodiversity Science
• Biogeography

Background:

• The latitudinal gradient in species richness is a fundamental biodiversity pattern.
• The geometric mid-domain effect is a proposed explanation for this gradient.
• Existing explanations rely on potentially flawed assumptions.

Purpose of the Study:

• To critically evaluate the geometric mid-domain effect hypothesis.
• To demonstrate why a global-scale mid-domain effect cannot arise.
• To clarify the mechanistic basis for the latitudinal gradient in species richness.

Main Methods:

• Literature review of the mid-domain effect hypothesis.
• Geometrical analysis of Earth's surface and domain definitions.
• Critique of assumptions regarding geophysical and bioclimatic boundaries.

Main Results:

• The Equator is not a valid geometric mid-domain for Earth's surface.
• Geophysical or bioclimatic boundaries lack geometric relevance at a global scale for this effect.
• A global-scale mid-domain effect cannot mechanistically explain the latitudinal gradient in species richness.

Conclusions:

• The geometric mid-domain effect hypothesis is not a valid explanation for the latitudinal gradient in species richness.
• Dispensing with this hypothesis allows for focus on more plausible explanations for biodiversity patterns.
• Accurate understanding of biodiversity patterns requires rigorous evaluation of proposed mechanisms.