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18.3: Patterns of Diversity Following Disturbance

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    In general, communities in early succession will be dominated by fast-growing, well-dispersed species (opportunist, fugitive, or r-selected life-histories). As succession proceeds, these species will tend to be replaced by more competitive (K-selected) species (See Life History Strategies). Trends in ecosystem and community properties in succession have been suggested, but few appear to be general. For example, species diversity almost necessarily increases during early succession as new species arrive, but may decline in later succession as competition eliminates opportunistic species and leads to dominance by locally superior competitors.  

    Ecological succession was formerly seen as having a stable end-stage called the climax, sometimes referred to as the 'potential vegetation' of a site, and shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of non-equilibrium ideas of ecosystems dynamics (See Intermediate Disturbance Hypothesis). Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state. Additions to available species pools through range expansions and introductions can also continually reshape communities.

    Several graphs line up along a timeline labeled forest succession over time in six stages. A line graph shows time on the x-axis with a series of peaks showing the shift over time from bare rock to mosses and grasses to grasses and perennials to woody plants and pioneers to fast growing trees and finally a climax forest made up of some of all previous stages. A diagram below shows disturbance labeling symbols for fire, humans, water, and biohazards. Arrows point from the 6 stages to simple illustrations of them. An additional line graph on the bottom shows a gentle logarithmic increase in biodiversity, biomass, and soil layer over time as the land becomes a climax forest.

    Figure \(\PageIndex{1}\): Trajectory of forest secondary succession following an intense disturbance, resulting in nearly no original biomass, to a mature forest state. Biodiversity, biomass, and soil depth all increase with time, and community composition changes. Image by Lucas Martin Frey is licensed under CC BY 3.0.

    Patterns of Diversity in Primary vs. Secondary Succession

    Primary succession begins on rock formations, such as volcanoes or mountains, or in a place with no organisms or soil (eg, an abandoned parking lot or railroad). In primary succession pioneer species like lichen, algae and fungi as well as abiotic factors like wind and water start to develop soil and initiate other important mechanisms for greater diversity to flourish. These pioneer species are then replaced by plants better adapted to less harsh conditions, these plants include vascular plants like grasses and some shrubs that are able to live in thin soils that are often mineral-based. Water and nutrient levels increase with the amount of succession exhibited.1

    The early stages of primary succession are dominated by species with small propagules (seed and spores) which can be dispersed long distances. The early colonizers—often algae, fungi, and lichens—stabilize the substrate. Nitrogen supplies are limited in new soils, and nitrogen-fixing species tend to play an important role early in primary succession.2

    Successional dynamics following severe disturbance or removal of a pre-existing community are called secondary succession. Dynamics in secondary succession are strongly influenced by pre-disturbance conditions, including soil development, seed banks, remaining organic matter, and residual living organisms. Because of residual fertility and pre-existing organisms, community change in early stages of secondary succession can be relatively rapid. Secondary succession is much more commonly observed and studied than primary succession. Particularly common types of secondary succession include responses to natural disturbances such as fire, flood, and severe winds, and to human-caused disturbances such as logging and agriculture. 

    Unlike in primary succession, the species that dominate secondary succession, are usually present from the start of the process, often in the soil seed bank. In some systems the successional pathways are fairly consistent, and thus, are easy to predict. In others, there are many possible pathways, potentially leading to alternative stable states. For example, nitrogen-fixing legumes alter successional trajectories.3

    Patterns of Diversity at Different Scales

    While we will discuss ways to measure biodiversity more in a later chapter (see section 22.2 on Diversity Indices), it's helpful to have some vocabulary to describe patterns of biodiversity when talking about succession and disturbance. 

    How is biodiversity measured? Part 1, Alpha, Beta, and Gamma Diversity \(\PageIndex{1}\)

    Alpha diversity (α-diversity) is the mean species diversity in a site at a local scale. At it's simplest, it is the number of species in a given location. More complex metrics of diversity take into account not only how many species there are, but how even their abundances are. Examples of places with extremely high alpha diversity include tropical rainforests. 

    Beta diversity (β-diversity) is the ratio between regional and local species diversity and connects alpha and gamma diversity. It describes the rate at which species composition changes across a region. For example, if every wetland in a region was inhabited by a similar suite of plant species, then the region would have low beta diversity; in contrast, if several wetlands in a region had plants communities that were distinct and had little overlap with one another, the region would have high beta diversity. Beta diversity is calculated as gamma diversity divided by alpha diversity. Beta diversity as a measure of species turnover overemphasizes the role of rare species. The difference in species composition between two sites or communities likely reflects the presence and absence of some rare species in the assemblages. Examples of places with high beta-diversity include matrices of recently disturbed and undisturbed land.

    Gamma diversity (γ-diversity) is the total species diversity in a landscape. Here, questions of scale become very important to appropriately distinguish between alpha and gamma diversity. 

    These terms were introduced by R. H. Whittaker4,5. Whittaker's idea was that the total species diversity in a landscape (gamma diversity) is determined by two different things, the mean species diversity in sites at a more local scale (alpha diversity) and the differentiation among those sites (beta diversity). This idea can be mathematically notated in this way: β=γ/α. The simplest calculations of alpha and beta diversity involve reworking this equation algebraically, with the result being that alpha, beta, and gamma diversity scale together. 

    Disturbance and following successional change can increase biodiversity at the alpha, beta, and gamma scales, although the patterns of diversity increase are scale dependent. For instance, a recently disturbed area might have higher alpha diversity than an undisturbed area but this is a result of different patches within the area where existing biomass either was or was not removed, allowing for colonization by novel species. Thus, it is not always clear whether the increase in diversity should be attributed to alpha or gamma diversity. However, the presence of patches in different successional stages also increases beta diversity (and gamma diversity) by supporting cohorts of species with different life history traits and characteristics in the same landscape, whether or not the straightforward number of species present in any given patch is higher than any other patch. Simply comparing alpha diversity in each patch might not demonstrate the impact disturbance has on patterns of biodiversity, whereas calculating beta and gamma diversity provide a more accurate picture. 

    Exercise \(\PageIndex{1}\): Calculating Alpha, Beta, and Gamma diversity

    Three panels show three ecoregions with three rocky peaks in each region. In Ecoregion 1 the first peak shows species A, B, C, D, E, F, and G. The second peak shows species A, B, D, E, and F. The third peak shows species B, C, D, E, F, and G. In Ecoregion 2 the first peak shows species B and C. The second peak shows species C, E, F, and G. The third peak shows species C, D, and E. In Ecoregion 3 the first peak shows species A, C, and E. The second peak shows species B, C, D, F, and H. The third peak shows species D, G, I, and J.

    Figure \(\PageIndex{2}\): This figure shows biodiversity for nine mountain peaks across three Ecoregions. Each symbol represents a different species; some species have populations on only one peak, while others are found on two or more peaks.

    1. Calculate alpha, beta, and gamma diversity for each mountain/Ecoregion.

    2. Does the "most diverse" mountain or Ecoregion change depending on which metrics we use?

    Answer

    The same ecoregion peaks shown above are recreated here. Ecoregion 1 has an alpha of 6, gamma of 7, and a beta of 1.2. Ecoregion 2 has an alpha of 3, gamma of 6, and a beta of 2. Ecoregion 3 has an alpha of 4, gamma of 10, and a beta of 2.5.

    Ecoregion 1 has the highest highest average species richness (alpha diversity) across its three peaks. If we were only to look at average alpha diversity, we might consider it the most diverse region. However, Ecoregion 3 has much higher turnover (beta diversity) and higher species diversity at the landscape scale. Furthermore, all of the species represented in Ecoregions 1 and 2 are also represented in Ecoregion 3. 

    References

    1. Fujiyoshi, M., et al. (2005). Effects of arbuscular mycorrhizal fungi and soil developmental stages on herbaceous plants growing in the early stage of primary succession on Mount Fuji. Ecological Research, 21(2), pp. 278-284.
    2. Korablev, A.P., & Neshataeva, V.Y. (2016). Primary plant successions of forest belt vegetation on the Tolbachinskii Dol Volcanic Plateau (Kamchatka). Izv Akad Nauk Ser. Biol., 4, pp. 366-376.
    3. Chapin, F.S., Matson, P.A., & Mooney, H.A. (2002). Principles of terrestrial ecosystem ecology. New York: Springer, pp. 281–304. ISBN 0-387-95443-0.
    4. Whittaker, R.H. (1960). Vegetation of the Siskiyou Mountains, Oregon and California. Ecological Monographs, 30, pp. 279–338. doi:10.2307/1943563
    5. Whittaker, R.H. (1972). Evolution and measurement of species diversity. Taxon, 21, pp. 213-251. doi:10.2307/1218190

    Contributors and Attributions

    Modified by Castilleja Olmsted (University of Pittsburgh) and Kyle Whittinghill (University of Vermont) from the following sources:

    18.3: Patterns of Diversity Following Disturbance is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.