# Ecological levels of organization

Ecological systems are defined by the interactions of organisms and the physical processes affecting those organisms through space and time. As a result, ecological systems are complex, and to simplify the study of specific processes or interactions, ecologists have organized them in hierarchical levels (Lidicker, 2007). The lowest level of ecological organization is the population, defined as individuals of the same species that inhabit a given area simultaneously (Hannan & Freeman, 1977; Berryman, 2002). A collection of local populations that interact with each other and the larger area or region with a balance between extinction and recolonization rates is called a metapopulation (Hastings & Harrison, 1994). Individual populations are further organized into ecological communities. A community is an assemblage of populations of different species that live in a shared environment and interact with one another, forming together a distinctive system (Fauth et al., 1996).

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When several communities are grouped in their abiotic surroundings, they are considered an ecosystem (Tansley, 1935). Furthermore, all communities exist in the broader spatial context of the landscape. Each landscape is composed of unique communities and ecosystems, and the broad-scale of geological and climatic patterns occurring around the globe give rise to regional patterns in the geographic distribution of ecosystems. Geographic regions that have similar geographical and climatic conditions support similar types of communities and ecosystems. These broad-scale regions dominated by similar types of ecosystems are referred to as biomes (Clements, 1916), and biomes are further grouped in what we call the biosphere, the sum of all the organisms on the planet and their environment, considered as a system of interacting components.

Two components, structure and dynamics, characterize every ecological level. The system structure results from demographic parameters such as the density of individuals that belong to specific populations or communities, the relative abundance of those individuals, or the number of species found in any given community. This species richness is the simplest measure of community structure, but not all species are equally abundant or affect the community in the same way. Foundation species are the base of a community and play a significant role in defining its structure (Ellison et al., 2005). When a single or a few species predominate within a community, those species are referred to as dominants (Grime, 1987), while a less abundant species with a disproportionate impact on community structure and dynamics are called keystone species (Holling, 1992). Population and community dynamics result from the interactions between the individual organisms and their environment and the individuals' interactions. Dominant or keystone species are essential within a community because small changes affecting this species can affect the whole system. This chapter will focus on how epigenetic diversity can influence ecological processes and interactions at many different organizational levels and how changes in lower levels can affect higher levels.
