Speciesism is species-selfishness.
It is arbitrarily to give priority to the interests of our species simply because it is OUR species, in a similar way as we could give more importance to the interests of our ethnic group, social group, continent, nation, tribe, clan, family and, ultimately, ourselves.
All these are examples of selfishness which start from its simplest form, selfishness relating to the individual, and extend to groups for no other reason than because the individual is part of them.
The 19th–century historian William Edward Hartpole Lecky wrote in his History of European Morals:
“At one time the benevolent affections embrace merely the family, soon the circle expanding includes first a class, then a nation, then a colation of nations, then all humanity, and finally, its influence is felt in the dealings of man with the animal world.”