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They may affect invasion either by limiting migration or by preventing ecesis.

In the first place, an association acts as a barrier to the ecesis of species invading it from associations of another type, on account of the physical differences of the habitats.

The absence of pollinating insects is sometimes a curious barrier to the complete ecesis of species far out of their usual habitat or region.

They restrict or prevent ecesis either by the destruction of invaders or by placing them at a disadvantage with respect to the occupants.

Man and animals operate as marked barriers to ecesis wherever they alter conditions unfavorably to invaders or where they turn the scale in competition by cultivating, grazing, camping, parasitism, etc.