How can a portal that connects two planets affect the environment?

In my world there are two planets within the same solar system, which are connected approximately every seven cycles through a portal that opens for a few days.

• The planets are very similar.
• The atmospheres are breathable.
• Both people and animals can move from one planet to another without problems.
• Gravity is the same (don't touch this).
• The fauna is different; different kinds of plants, flowers and animals exist on both sides.

Is it possible that these openings affect the flora and fauna, taking into account that they only remain open for a short period of time?

posted about 6 years ago

Hedufigo‭

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