The element iodine sublimes whereas lithium does not, even if the molar mass of iodine is a lot greater than that of lithium.

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged

The population of unionid mussels decreased after zebra mussels were introduced. zebra mussels competed with the unionid mussels for food. the zebra mussels were able to displace the unionid mussels, allowing the zebra mussel population to increase  over the years. the unionid mussel population decreased because  of lack of food.