Interspecies conflict in sci-fi campaigns

[fuindordm]

Thanks for the input everyone.

[MENTION=46713]Jhaelen[/MENTION], Reasons for conflict abound... once multiple species are sharing the same world or habitat. What I was looking for was reasons for conflict that could justify the risk of destroying expensive starships, under the assumption that starships cost a significant fraction of a civilization's economic output to rebuild.

Naturally, if building starships is cheap, even massive ones like the Enterprise or the Galactica, then species will fight each other for more petty reasons.

 

[TBeholder]

TL;DR: This thread is asking you to contribute new reasons for mutiple alien species to come into conflict with each other in a hard science fiction setting.

I'm juggling ideas in my head for a new hard science fiction campaign. I want the PCs to be protagonists of a story involving multiple alien species/empires, and I want the setting to include a compelling and believable reason for the aliens to come into conflict with each other. [...] Here's what I've though of so far. Any and all other ideas are welcome.

In other words, any generic.
Because none of these is fitting for conflicts between political entities (or cultures for #5) of different species any more or any less than that of the same species or mixed.

If you want it to have roots in basic biology, consider terraforming.
The initial situation: a planet fits some species or another, or maybe no one. Since it's a natural lottery, there are no serious conflicts - besides, if you escalate it too much, no one will be able to use the resulting wasteland, which is a loss compared to having neighbours with whom you can trade, even if you have reasons to dislike them.
Why everything is upset: now that the environment can be reliably changed on planetary scale without making the place unfitting for anyone, this stops being a lottery and becomes a resource that cannot be shared arbitrarily. On top of this, many consider terraforming "unfair" in general - mostly those who are quite fine with how the "lottery" goes, of course. Conversely, among the species who could use only very rare sorts of environment and have noticeable population pressure there was much rejoicing at the news. And another implication of terraforming is that the same methods greatly increase level of damage to a planet that can be recovered and thus can be deemed acceptable...
Will such a situation proceed very far without someone shooting at someone? Unlikely.

For possible clashes on the level of psychophysiology, read ConSentiency series by Frank Herbert.