Species #0 is not a species. It is a classification bucket. The federation's standard interspecies language is built on mathematics and game theory, not linguistics, and Species #0 is a game-theoretic designation: the all-defector. Any entity whose behavior pattern constitutes unconditional defection in iterated cooperative frameworks - species that are fundamentally incompatible with the mutual-benefit equilibria the federation exists to maintain - receives the #0 designation and is excluded from the registry of participating sapients.
This is not a moral judgment. It is a behavioral classification. The federation does not use the word "evil." It uses the phrase "does not converge to cooperative equilibrium under any known payoff structure," which means approximately the same thing but is harder to argue with.
The Species #0 designation is applied when a sapient entity meets one or more of the following behavioral criteria, as assessed over a sufficient interaction history to establish pattern stability:
Unconditional defection. The entity's dominant strategy is defection regardless of the other party's strategy. No combination of incentives, concessions, threats, or cooperative overtures produces sustained cooperative behavior. The entity defects not because cooperation has failed but because defection is the preference, irrespective of outcome for the other party.
Utility functions predicated on harm. The entity derives positive utility from the negative utility of others. This is distinct from indifference to harm (which is common and manageable) - it describes entities for whom the suffering of other sophonts is an input to their own satisfaction. In the formal notation: their payoff matrix includes terms where U(self) increases as U(other) decreases, and this relationship is not instrumental but terminal.
Non-recognition of external experience. The entity does not model other sophonts as having experiences. This is not a failure of empathy - many federation species have limited or no empathy in the human sense and cooperate perfectly well through rational self-interest. This is a failure of modeling: the entity's cognitive architecture does not represent other minds as having internal states at all. Other sophonts are treated as environmental features, not as agents. Negotiation is not possible because the concept of "the other party's interests" is not representable in their cognition.
Structural incompatibility with bounded interaction. The entity's expansion or resource-acquisition behavior does not respond to satiation, territorial agreement, or resource sufficiency. Growth is the terminal value, not a strategy in service of other values. Coexistence requires boundaries; these entities do not recognize boundaries as a concept their behavior can incorporate.
Species #0 classification is not exile, imprisonment, or war. It is non-engagement. The federation does not attempt to reform, contain, or destroy Species #0 entities. It withdraws from interaction with them. Trade ceases. Diplomatic channels close. Transit rights are revoked. The federation's cooperative infrastructure - which is substantial and which most spacefaring species rely on - becomes unavailable.
The practical consequence is isolation, not because isolation is the goal but because the federation's architecture is built on cooperative game-theoretic assumptions and entities that do not cooperate cannot interface with it. It would be like trying to plug a device into a power grid when the device only outputs current and never draws it. The socket doesn't reject the device out of malice. The physics simply doesn't work.
The designation is reviewable. If a Species #0 entity's behavior pattern changes - which can happen through internal cultural evolution, external pressure, or the extinction of the specific cognitive architecture that produced the all-defection pattern - the classification can be revisited. This has happened. It is rare.
The entities classified as Species #0 share a behavioral outcome (non-cooperation) but not a cause. Some are predatory. Some are expansionist. Some are solipsistic. Some are simply structured in ways that make cooperation impossible for reasons that have nothing to do with hostility. A Species #0 entity that does not recognize your experience as real is not choosing to disregard you any more than a human chooses not to negotiate with bacteria. The cognitive architecture simply does not support the operation.
This means that Species #0 entities are wildly diverse in biology, cognition, technology, and threat level. The designation tells you one thing: the federation has determined that cooperative equilibrium with this entity is not achievable. It does not tell you why, and the why matters enormously for anyone who has to deal with the consequences.
The federation's Species #0 registry is not fully public. The following are entities referenced in other HXFG entries or otherwise known to human researchers through published federation records.
Status: No longer apparently extant. Designated Species #0 retroactively based on behavioral reconstruction.
The unnamed entity that colonized Regez, the homeworld of the Ansale'wit, and conducted mineral extraction operations that destroyed sections of the planetary fungal network. When the Ansale'wit resisted, the colonizers escalated to active hostilities, including the deployment of novel bioweapons against civilian network infrastructure.
The colonizers did not respond to diplomatic overtures. They did not negotiate. Post-hoc analysis of their behavior pattern is consistent with non-recognition of external experience: the Ansale'wit network was treated as a geological feature to be exploited, and Ansale'wit resistance was treated as an environmental hazard to be managed, not as communication from agents with interests.
The entity is no longer extant in the Regez system. The circumstances of their departure or destruction are unclear (see Ansale'wit: History With Us for available details). The Species #0 designation was applied retroactively by the federation based on the Ansale'wit's historical records.
The federation's interspecies framework is built on game theory because game theory is the mathematics of strategic interaction between agents, and strategic interaction between agents is what the federation is. The standard language of galactic diplomacy is not a spoken language. It is a formal notation system for expressing payoff structures, strategy spaces, equilibrium conditions, and preference orderings. Spoken language is used for nuance, culture, and the things that don't reduce to math. The structural agreements - treaties, trade frameworks, coexistence protocols - are written in math because math doesn't mistranslate.
The Species #0 designation exists because the framework needed a way to express: "this agent's strategy space does not contain any strategy that produces a stable cooperative equilibrium with any other agent in the federation, under any payoff structure we can construct." Registry number zero. The null partner. The entity with whom no game has a cooperative solution.
The term "all-defector" is the human-readable gloss. The formal notation is more precise and less evocative. But humans prefer words that carry weight, and the weight here is appropriate.