The population is the driving force of your empire. Without it, you won't build any fleets nor settle down on a new planet. Each civilization has their own default representatives, but they can also come from other races - if only you engage in conquering or participating in different events. Fortunately, the race of a hero doesn't influence their loyalty - only the happiness in the system does. Each new colony will initially be alluding to your own population. Only if in the given system a different population already exists is there a chance of that race growing in numbers on your planet. You can also spend luxury resources to incite other races to inhabit your worlds.
Bigger populations of races allow to unlock more traits and bonuses. In the senate tab, you can find a shortcut to the population screen, also available from system view (if you click on a symbol of a race). You can see here the numbers of other races within your empire (1), see what luxury resources will incite them to migrate (2), see planet bonuses (3) the Collection Status (4) which offers considerable bonuses if the number of citizens is bigger than 50. Then, there's info concerning the influence of the aliens on the given party per turn (5) as well as their reactions to different actions and the situation in the empire or systems (6).
Ideologies. Every race reacts differently to different ideologies (6,7). Every action influences the support for the given ideology, hence the given party. By hovering over the ideologies, you can see which actions influence voters of this party. For militarists support, you will need things such as building military ships, recruiting militaristic heroes or assigning them to fleets and systems, blocking or conquering systems and fighting in battles. It's analogical for any other ideology.
However, every race reacts differently to different ideological issues within the empire. Here things become more tricky. On the illustration (7), you can see a wheel for humans. Events of religion, pacifism or militarism have a direct influence on the popularity of that party. BUT - events for ecologists have only 50% of the influence, and events of the industrialists have a 200% influence within a system, and events for scientists have an identical impact on scientists and industrialists. Hence, it will be easier for the industrialists to seize power within the empire of the Imperials are the dominant race. The reason is twofold: the industrial events themselves generate more influence, plus any even in science also boosts the industrial influence.
If the supported ideology is not ruling, it negatively impacts happiness in the system and vice versa. If you're not sure what sort of actions can improve the support of the population, you can check that. In the side menu, between the governor and population bars is the ideology support bar (1). There you can see how many representatives in the senate are there for the given party, and what is the current support for a party. You can click on the stats to get more info. Here, you can see what influences the support within the coming turns (2). The numbers next to the events tell you how many more turns will the given modifier be in effect. If there's no number it means that the modifier is in effect until further notice.
The advantages of having one or more different races in the empire:
- A single race gives more control over ideologies and facilitates management. You're not forced to micromanage the population and juggle citizens between systems. You can obtain bonuses provided by the support of the given population much faster.
- A few races provides a welcomed balance between management and bonuses. Your dominating race will still be the most important in the empire, but you will also be able to use the bonuses provided by other races (for example better production on the given type of planet). You will often be able to get bonuses for many representatives. On the other hand - ideological unease can be problematic during elections and can negatively impact happiness, although it's possible to choose races that have similar ideologies.
- Many races ensure diverse population. It's characterized by a myriad of bonuses. Additionally, if you manage to assimilate them, their race traits are included in the characteristics of your empire. The problem is that with such diverse society, it will be very hard to maintain the chosen party as the ruling one. It will also be difficult to keep track of which heroes should go to which systems.
It's not an easy choice because there are always pros and cons here. It all depends on the faction you choose as the first one and the playstyle you'd prefer. As Horatio, for example, you will look forward to assimilating more races to be able to use their genes. These races may become useless then. As Cravers you need other races for slaves.