Marginal distribution (biology)

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Marginal distribution (biology)

The margins of a species’ range are the edge areas where populations are found outside the core, central part of the range. Core populations occur near the center; marginal populations live at the boundary. Edges are often limited by factors the species cannot fully adapt to, preventing expansion beyond them.

Terminology and concepts
- Chorology: the science of where organisms are distributed.
- Core vs. marginal (peripheral): core is the center of a species’ range; marginal is at the edge.
- Realized ecological niche: the approximate ideal habitat inferred from the range core.
- Leading edge: the expanding part of the range.
- Rear edge: the retreating part of the range.
- Central-marginal hypothesis: margins often have less genetic diversity and more genetic differentiation than the core, due to smaller, more isolated populations and different ecological conditions. Gene flow from center to edge can slow differentiation, with some exceptions.

Refugium
- When climates restrict distribution, small, favorable areas called refugia (for example, southern Europe during ice ages) can maintain populations at the edge.

Abiotic factors
- Climate and other non-living factors create physiological limits to dispersal. Too much or too little of a factor lowers survival and reproduction.
- Warming can push ranges northward; precipitation and soil moisture can cap how far species can live; other factors include dissolved oxygen, conductivity, alkalinity, and pH.

Biotic factors
- Interactions with other organisms can limit distributions: predation, competition, and parasitism.
- Some species depend on hosts (parasitism or mutualism) and may spread differently at the edge.
- Marginal populations can have higher parasite loads in less favorable conditions.

Anthropogenic factors
- Humans can shift distributions through deforestation, climate change, introducing species, and creating new habitats.
- These changes can enable some species to expand their ranges or threaten native species elsewhere.

Combined influences
- Most range limits result from a mix of abiotic and biotic factors working together. For example, some plants depend on fire to germinate and on ants to disperse seeds.

See also
- Edge effects
- Species distribution
- Niche construction
- Mutualism–Parasitism continuum

This concise overview captures how marginal distributions arise, why edges matter, and the key factors shaping where species can live.