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Communities assemble through deterministic (environmental filtering, limiting similarity) and stochastic (dispersal, drift) processes. Environmental filtering removes species unable to tolerate local conditions; limiting similarity prevents too-similar niches from coexisting; stochastic dynamics maintain diversity despite deterministic forces. Assembly rules identify predictable principles, but communities often appear idiosyncratic due to historical contingency. Current composition results from local and regional biogeographic processes.
From community ecology, you know that species interact through competition, predation, and mutualism, and from niche theory, you understand that species partition resources to reduce competitive overlap. Community assembly asks the next question: given all the species in a regional pool, which ones actually end up coexisting in a particular local community, and why? The answer involves a series of filters — both deterministic and stochastic — that winnow the regional species pool down to the local community you observe.
The first filter is environmental filtering (also called habitat filtering). Not every species in the regional pool can survive the local abiotic conditions — temperature, soil pH, water availability, disturbance regime. A desert community excludes species that require constant moisture regardless of their competitive abilities. This filter tends to make local communities more similar to each other in terms of species traits than you would expect by chance, because only species with the right physiological tolerances pass through. If you measured the leaf traits of all plants in a dry grassland, you would find them clustered around drought-tolerant values — that clustering is the signature of environmental filtering.
The second filter works in the opposite direction. Limiting similarity (or competitive filtering) prevents species that are too ecologically similar from coexisting. If two species use exactly the same resources in exactly the same way, competitive exclusion predicts that one will drive the other extinct locally. This means that the species passing through the environmental filter must also be sufficiently different from each other in their niches — different feeding strategies, different microhabitats, different timing of activity — to coexist. While environmental filtering makes communities look more similar than expected, limiting similarity pushes them toward greater trait dispersion. The tension between these two forces shapes the functional structure of communities.
But deterministic filters alone do not fully explain community composition. Stochastic processes — dispersal limitation, ecological drift, and historical contingency — introduce unpredictability. A species perfectly suited to a habitat may never arrive if it cannot disperse there. Two communities with identical environments may contain different species simply because different colonizers happened to arrive first and established priority effects. Neutral theory, proposed by Stephen Hubbell, formalized the idea that some community patterns can be explained without invoking niche differences at all — just random birth, death, and dispersal among ecologically equivalent species. Most ecologists now view assembly as a continuum: strong environmental gradients favor deterministic filtering, while benign or homogeneous environments allow stochastic dynamics to play a larger role. Understanding where a community falls on this continuum is essential for predicting how it will respond to environmental change or species introductions.