Worldbuilding opinion has it that if you create a situation where aliens come to Earth, they must be ‘aggressive’. I’ve thought about this a long time in terms of population ecology (which is what I was trained to do) and have come to the conclusion that yes, this is correct, and no, it is not.
The problem lies, I think, with the fact that population ecology attaches a different meaning to the word aggressive than the general public does.
An aggressive species is one that actively colonises available niches, one that has the resilience and comfort range to thrive in a wide range of conditions, and one that produces vast numbers of seeds/eggs/young. Such species will often tolerate high population densities. They will be found at the disturbed edges of established communities (roadsides, building lots). They will have short lifespans, and will often provide food for a lot of species higher in the community’s development.
Rabbits are aggressive species. When given new open land (think golf courses and paddocks), they breed like crazy. Until the trees move in, and the foxes, but by this time, rabbits have already moved onto the next open ground.
Human society, by comparison is an ecological community by itself. Within each human culture, there are those who will colonise new niches and those who build on that colonisation effort. Whether you talk about the first early Africans who moved into colder climates, the first Chinese who moved to Australia, or the first English people who moved to Dubai, it doesn’t matter. Within their community of origin, these ‘first’ groups paved the way for others, in a purely ecological sense. In a plant community, these two functions would be performed by two different species. We can conclude that humans are good at colonising within a fairly narrow temperature range, and are good at holding our territory once we have it.
Are we aggressive? Moderately so, probably. Many writers have dreamed up situations in which ‘truly aggressive’ aliens come to visit us. And this is the part where people confuse aggression with violence.
Ecological aggression–the type that will lead to colonisation of new territory–is a characteristic that needs both back-end pressure (it’s too busy in the home country) and an aim (we need new land). Ecological aggression does not mean destruction. It means invasion of an available ecological niche through settlement and breeding.
If a storm blows over a tree in a forest, and subsequently the pigs dig up the surrounding area for roots, invasive plant species (ones we call weeds) will come up in the newly-bared soil.
Taking this analogy to aliens, or a situation where humans are the invaders, what the newcomers will want is a space to live. They are not numerous enough to take on the population of an entire planet in a battle, and why would they, anyway. The locals are their best bet to learn about what’s available on the planet. They may want to trade, for technology, for unknown riches, or just for a pair of hands to do some work. But they’ll want a place to live for their expanding community.
They will most likely be friendly and cooperative, but oh-so-aggressive.