This dissertation introduces R-selected spacecraft as a field of study that draws from concepts in ecology, and introduces the Monarch spacecraft as a case study for a system designed in accordance with the principles of this field. The Monarch is a 2.5-gram spacecraft that is the first to trade quantity, rather than cost, for low mission risk. By taking advantage of recent technological advancements in unrelated disciplines and taking a statistical approach to mission assurance, R-selected spacecraft open the door to an entirely new paradigm in space access and exploration. This dissertation describes the challenges and advantages unique to gram-scale, R-selected spacecraft. It also presents a number of use cases --- involving distributed in-situ sensing and planetary science --- that are unique to spacecraft of the Monarch's diminutive size and large quantity. This dissertation presents a routing policy for moving information through large collections of Monarchs in low-Earth orbit, and results from simulated lunar impact survival tests. Demonstrations of distributed sensing, leaderless cooperation, routing, and actuation are presented and discussed to illustrate the viability of some entirely new mission concepts. The final chapters anticipate future capabilities for Monarchs and present a method for extracting insights from the sorts of datasets which swarms of Monarchs will produce. The appendices discuss applications for distributed in-situ sensing in digital agriculture, and present datasets gathered by the Monarchs from vineyards and dairy calves.