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The Theory of Ecological Communities Vellend 2016 (High-level processes…

- - - - The focal set of species (Sec. 2.1)
      - The spacial scale of analysis (Sec. 2.2)
      - The community properties of interest (Sec. 2.3)
    - - we can use the maximally inclusive definition of a community as “a group of organisms representing multiple species living in a specified place and time” (Vellend 2010; see also Levins and Lewontin 1980)
      - 2.1.1.
        A Focus on Horizontal Ecological Communities
        
        Individuals of different species share similar abiotic and biotic constraints on fitness.
        
        Community dynamics are closely analogous to the evolutionary dynamics of genotypes in a single species’ population (Nowak 2006; see also Chap. 5).
        
        Fitness can be quantified in a comparable way across individuals of the same or different species,
        
        Many theoretical models from population genetics apply just as easily to species in communities as they do to alleles or genotypes in populations.
        
        Such models are based on just four high-level processes:
        
        selection, drift, mutation, and gene flow in population genetics.
        
        selection, drift, speciation, and dispersal in community ecology.
        
        Multiple trophic levels
        
        The analogy is weaker.
        
        As indicated previously, it simply treats consumers or pathogens or mutualists as being outside the direct study object of interest— a component of the biotic environment that potentially exerts strong selection on the focal community and that might itself respond to changes in this focal community.
      - Objects of study in community ecology for a hypothetical terrestrial ecosystem
        
        Ecological community sensu lato
        
        As a theoretical ideal, it is useful to consider the complete set of organisms belonging to all species (viruses, microbes, plants, animals) living in a particular place and time
        
        A food web
        
        focus on feeding relationships, often ignoring differences among species within trophic groups, and externalizing nonfeeding interactions and even some feeding interactions (e.g., nectar consumption by pollinating insects)
        
        A consumer-resource module
        
        focus on a small number of strongly interacting species— “community modules” sensu Holt (1997)— such as particular consumer- resource pairs (e.g., the lynx and the hare)
        
        A mutualistic network
        
        focus on two sets of interacting species, such as plants and their pollinators or mycorrhizae, externalizing everything else
        
        Horizontal communities
        
        focus on species at a particular trophic level (e.g.,
        plants) or in a particular taxon (e.g., birds or insects), again externalizing everything else (Fig.
        
        Ecologists have referred to such a unit of study (or something like it) as an “assemblage” (Fauth et al. 1996), a “guild” (Root 1967), a set of “species having similar ecology” (Chesson 2000b), or a “horizontal community” (Loreau 2010).
    - - The key point here is that it is rarely if ever possible to define one “correct” scale for studying eco-logical phenomena of interest (Levin 1992), especially if we are interested in multiple interacting processes that might determine community structure and dynamics.
      - Although the focal scale of observation can clearly vary continuously from tiny study plots to entire continents, for sake of convenience community ecol- ogists often recognize discrete scales, such as “local” (the smallest), “global” (the largest), and “regional” (somewhere in between) (Ricklefs and Schluter 1993b, Leibold et al. 2004).
    - - The basic quantitative description of a community is a vector of species abundances, which we can call A.
      - First-Order Properties of Single Communities
        
        Species richness
        
        The number of species in the site (i.e., the number of nonzero elements in A).
        
        When comparing sites in which different numbers of individuals were observed, researchers often standardize species richness across sites by calculating the number of species observed in repeated random samples of a given number of individuals in a given site, a procedure called “rarefaction.”
        
        Unstandardized species richness is sometimes called “species density” (Gotelli and Colwell 2001).
        
        Species evenness/diversity
        
        Any index calculated from the vector of species abundances A in which, all else being equal, a more even distribution of abundances across species leads to higher values.
        
        For example, a forest in which total tree abundance is split 50:50 between two species has higher evenness than a forest in which one of the two species abundances greatly exceeds the other. Common
        
        Common indices are the Shannon- Weiner Index, Simpson’s Index, and various kinds of entropy calculations (Magurran and McGill 2010). These indices are typically based on the frequencies, freqi = ai /∑(ai), of each species i.
        freqi is the frequency of species i (its abundance divided by the sum of species’ abundances for that site)
        
        Species composition
        
        The vector of abundances (sometimes just recorded as presence/absence) itself, unmodified (A), can be considered a multivariate property of the community that one might wish to explain or predict.
        
        Species abundance distribution
        
        Regardless of which species has which abundance, the distribution of these abundances (e.g., is it log- normal or some other shape?) has been of great interest as a community property (McGill et al. 2007).
      - First- Order Properties of Multiple Communities (i.e., a Metacommunity)
        
        Beta diversity
      - Second- Order Community Properties Incorporating Species Characteristics
        
        Trait diversity
        
        Trait composition
      - Second- Order Community Properties Incorporating Site Characteristics
        
        Diversity- environment relationships
        
        Composition- environment relationships: