Assembly Rules within a Contingent Ecology

Ecologists have searched for general principles, or assembly rules, which determine how species combine to form communities. Two major strands of research highlight the interaction of assembly rules operating at different levels of community organisation. and within uncertain local and historical contingencies. Here, we review the aterature on community assembly within the context of three principal determinants: dispersal constraints, environmental constraints and internal dynamics. This classification separates external factors (dispersal and environmental constraints) from mternal processes (internal dynamics). We assert that assembly rules are general, deterministic and mechanistic, and operate on internal dynamies within the constraints imposed by the local environment and the history of species invasions. Several assembly rules have been proposed to operate within and across trophic levels, as well as on whole systems. The rules act through resource dynamics and spatial dynamies to select species or subsets of species that can coexist, and may lead to patterns of increasing biomass and non-random spatial distributions. Environmental constraints restriet species establishment and mediate interactions among successful colonists; a change in environmental constraints, either exogenous or endogenous. may drive community change. Even in systems at equilibrium, environmental constraints influence the outcome of assembly rules. Dispersal constraints determine the pool of potential colonists available at a particular time and place, and are a complex amalgam of species-specific traits, storage effects, landscape ecology, and history. The order and timing of species invasions (dispersal constraints) interact with assembly rules to produce priority effects. The directionality of community assembly may be lost if either the rate of species invasions or the rate of non-selective species extinctions exceeds the rate at which species are selected by assembly rules. The falling apart, or disassembly, of a community through breakdown of species interactions may follow different rules or a different assembly trajectory.