Kennedy W. *Nyongesa1 Matonyei Lelei and Francis O. Obiria
1,2 Kitale National Polytechnic, Department of Mathematics and Applied Sciences.
Kisii University- Francis O. Obiria
Corresponding email: firstname.lastname@example.org
The Monoclimax theory proposes that the generation of a climax community is solely the control of the climate of the region. Therefore, the climax community becomes the “index” of that region. In most temperate forests in United States of America.; climate change led to substantial increase in some species and decline in some. In tropical Africa, prediction of biotic responses to future climate change trends is based on two modelling approaches: bioclimatic species envelope models and dynamic vegetation models. Another complementary but underused approach is to examine biotic responses to similar climatic changes in the past as evidenced in fossil and historical records. This paper discusses climate change as one of the drivers of ecological succession and how it determines the type of climax community obtained. A Climax community is the final community in the process of ecological succession. Ecological succession is a fundamental concept in ecology. It refers to changes in the composition or structure of an ecological community; a series of progressive changes over time. Succession may be initiated either by formation of new, unoccupied habitat such as a lava flow, a severe landslide or by some form of disturbance such as fire or logging. The former case is referred to as primary succession, the latter as secondary succession. In primarysuccession, newly exposed or newly formed rock is colonized for the first time. In secondarysuccession, an area previously occupied by organisms is disturbed then re-colonized following the disturbance. The trajectory of ecological change can be influenced by site conditions, by the interactions of the species present, and by stochastic factors such as availability of colonists, seeds or climatic and weather conditions. Some of these factors contribute to predictability of succession dynamics; others add probabilistic elements. Communities in early succession will be dominated by fast-growing, well-dispersed or r-select species. As succession proceeds, these will be replaced by more competitive k-selected species. This review paper brings in view a concept map that relates the different concepts and sub-concepts in ecological succession.
Key Words: Climate Change, Ecological Succession, Climax Communities, Concept Map, Garden Plot
Ecologists have a strong interest in understanding how communities form and change over time; this stage of climax community has been the object of much scrutiny and attention from ecologists, because it is rare and so many different possibilities can theoretically arise, depending on the path of succession. However, studies have shown that every climax community has a certain specific set of characteristics: 1) They have high tolerance to ecological disturbances. 2) They have a “middle path”, i.e. they have moderate conditions, also called mesic conditions. 3) They have high species diversity, and transfer of energy is in the form of complex food webs, not simple food chains.4) The size of organisms in the community is large, and they all have their specific niches. 5) Net community production is low, while biomass and organic matter is high. 6) Mineral cycles and nutrient exchange in the community is slow. But this is where the unanimous agreement of ecologists ends. How does the climax community originate and what controls their composition? This has been a matter of great debate for much of the 20th and 21st centuries. Three theories have taken the forefront, and the ecological community is split between them.
a) Monoclimax theory
Clements (1916) advanced this theory when he studied communities in the early part of the 20th century. This theory states that the generation of a climax community is solely the control of the climate of the region. Therefore, the climax community becomes the “index” of that region. He argued that within the same type of climate, the adaptations and requirements of organisms remain the same. It is also possible that two communities will have formed with the presence of the same pioneer species. Hence, the way they modify their surroundings (which eventually leads to change in communities) will also be largely the same. Consequently, each climactic region is characterized by climax communities where a few specific species are dominant in the climax community.
The Monoclimax theory, as developed by Clements (1916, 1936), is often described as the origin of dynamic ecology. According to this theory there is only one climax community in a climatic or geographical region. But topographic differences and different soil types form other communities in the same zone. These communities are known as subordinate communities. The subordinate communities may be proclimax, post climax, disclimax, preclimax, and subclimax communities: (a) Proclimax: The community which is more or less stable resembling the climax community is called proclimax (b)Disclimax: The community disturbed by man or other animals is called disclimax community. Its example is grassland in forest area (c) Subclimax or preclimax: The community in which development is stopped in the subtinal matte of succession due to burning, cutting or grazing and flooding is called subclimax or preclimax community (d) Postclimax: The community in which a strip of vegetation of higher life form is found within a climax is called post climax community. Its example is forest strip along a stream in grassland.
Daubenmire (1968) criticized this theory, proposing that climate is the sole control of climax communities. According to Daubenmire, believing in the Monoclimax theory meant admitting that all factors other than climate are secondary in nature. What is the alternative? It is the possibility that all environmental factors are equally important, and accordingly, many climax communities can exist.
b) Polyclimax theory
Tansley proposed this in 1939. This theory suggests just that according to a controlling influence by each environmental factor, which becomes dominant purely as a matter of chance, there can be several climax communities. According to this, there is a particular climax community for a region called climactic-climax community (Tansley, 1920., 1929., 1939). However, other factors can prevent the community from reaching this stage. For example, consider a community that has been affected by a road construction project. This area has an overriding influence enforced by humans; therefore, it cannot reach its climactic-climax. One of the earlier seral stages may be stabilized in such a situation and forms a “pre-climax” or a “sere-climax”.
It is also possible that an entirely new community can be established, that is vastly different from the expected climactic-climax. Since the overriding influence was of humans, the climax is known as anthropogenic climax. Different climaxes, based on different factors having an overriding influence have been proposed. During succession the floristic convergence is only partial. There may be more than one type of stable endpoint. These alternative stable endpoints are controlled by local factors such as topographic position or soil type, consequently, called topographic climax, edaphic climax, fire climax among others as opposed to “climatic climax”. In contrast, Clements regarded these as “sub climaxes.” Tansley’s Polyclimax theory is: (a) an equilibrium model (b) less deterministic than Monoclimax theory. According to this theory a number of different climax communities are present in climatic or geographical region. Thus proclimax, disclimax, preclimax, and postclimax communities are all climax communities. They are all self-reproducing communities. They can maintain themselves or an indefinite period of time. This theory does not rule out climax communities. But it gives other stages of communities as full status of climax communities. Thus these are not regarded as in minor communities
c) Climax-Pattern Hypothesis
Whittaker (1951) proposed this concept that was entirely new. According to him, the climax community was controlled by all environmental factors equally. Therefore, no one climax is established in isolation. A series of climax communities are established, parallel to each other depending on the changing environmental gradients. According to this theory the structure, composition and other characters of the climax communities are determined by the total environment of the ecosystem. It is not determined by a single factor. A Climax community is composed of a number communities.
To that end, the authors in this review paper agree with the Polyclimax theory as far as saying that many different factors can influence the occurrence of the climax community and its composition. There will also be some factors more dominant than others in a given area. However, it is also important to understand that communities never occur in isolation. There is always a gradual change from one community to another. Therefore, in a given region with changing communities, the environmental factors will also change accordingly. This will alsohave an impact on the resulting community composition. Different community structures based on different environmental parameters can occur parallel to each other as these conditions gradually change from one to another.
For example, a community of trees at a particular elevation will gradually change into another community composition as the elevation changes. Along with this change in elevation, temperature and soil conditions will also change. Different permutations and combinations arise with just these three factors (3×3=9), resulting in the possibility that any number of climax communities out of the possible 9 can occur in that area simultaneously.
Foundational Studies on Ecological Succession and Climax Communities
Early sources of the concept of climax include and the American work of Cowles (1899, 1901) and Clements (1916). Ecologists seek to capture with concepts the meaning of vegetation change Whittaker (1974). The idea that this change could be described and understood was a major development in the history of ecology, that may be traced back to Kerner (1863 in Whittaker,1974), Hult (1885, 1887 in Whittaker,1974), Warming (1891, 1896 in Whittaker,1974), Lüdi (1921, 1930 in Whittaker,1974), Cain (1939) and Whittaker (1951). In counterbalance to the idea of vegetation change it was natural to recognize the condition of relative vegetational stability that came to be known as “climax”.
Ecological Succession in Context
When the planet first formed, there was no soil. Hot magma and cold water make hard rocks, as seen by newly formed islands. Primary ecological succession is the process of small organisms and erosion breaking down these rocks into soil. Soil is then the foundation for higher forms of plant life (He, 2008); these higher forms can produce food for animals, which can then populate the area as well. Eventually, a barren landscape of rocks will progress through primary ecological succession to become a climax community.
Morin and Thuiller (2009) assert that, after years and years, the soil layer increases in thickness and harbors many nutrients and beneficial bacteria that are required to support advanced plant life. Further, if this primary ecosystem is disturbed and wiped out, secondary succession can take place, and small plants will come back first. After they create a solid layer of vegetation, larger plants will be able to take root and become established. At first, small shrubs and trees will dominate. As the trees grow, they will begin to block the light from most of the ground, which will change the structure of the species below the canopy. Eventually, the ecosystem will arrive at a climax community, which may or may not be the similar to the original community. It all depends on which species colonize the area, and which seeds are able to germinate and thrive.
Cyclic ecological succession happens within established communities and is merely a changing of the structure of the ecosystem on a cyclical basis (Deluca et al., 2009); some plants thrive at certain times of the year, and lay dormant the rest. Other organism, like cicadas, lay dormant for many years and emerge all at once, drastically changing the ecosystem.
Bryant et al. (1998) posit that, while ecological succession is a term coined by botanists, it also applies heavily to animal a population that goes through a disruption. For instance, a coral reef; the coral reef as an ecosystem did not just happen into existence, but like many plant communities had to be formed over time through ecological succession. The primary ecological succession in a coral reef is the colonizing of rocks by small coral polyps. These polyps will grow and divide many times to create coral colonies. The shapes and shelter of the coral colonies eventually attract small fish and crustaceans that live in an around the coral. Smaller fish are food for larger fish, and eventually a fully functioning coral reef exists. The principles of ecological succession, while developed in context to plants, exist in all established ecosystems.
Temperate deciduous forests are among the major biomes on earth and provide important ecological, economic, and social services to society (FAO 2012); most temperate deciduous forests in the eastern U.S. experienced heavy exploitation in the 19th and early 20th centuries and some of these were completely deforested while others are now recovering. These recovering forests are in intermediate stages of development and are undergoing rapid succession changes as a result of growth, competition, dispersal, and establishment (Shifley et al., 2012). Most temperate forests in U.S. are recovering from heavy exploitation and are in intermediate succession stages where partial tree harvest is the primary disturbance. Changes in regional forest composition in response to climate change are often predicted for plant functional types using biophysical process models. These models usually simplify the simulation of succession and harvest and may not consider important species-specific demographic processes driving forests changes.
According to Schumacher and Bugmann (2006), by use of the forest landscape modeling approach to project changes in density and basal area of 23 tree species due to succession, harvest, and four climate scenarios from 2000 to 2300; on average, succession, harvest, and climate change explained 78%, 17%, and 1% of the variation in species importance values at 2050, respectively, but their contribution changed to 46%, 26%, and 20% by 2300.
Climate change led to substantial increases in the importance of red maple and southern species e.g. yellow-poplar and decreases in northern species e.g. sugar maple and most of widely distributed species e.g. the white oak (Schuler, 2004); harvest interacted with climate change and accelerated changes in some species like the increase in southern red oak and decrease in American beech, while ameliorated the changes for others such as increasing red maple and decreasing white ash. Succession was the primary driver of forest composition change over the next 300 years by this model. The effects of harvest on composition were more important than climate change in the short term but climate change became more important than harvest in the long term (Brandt et al., 2014).
According to Iverson et al. (2008), the results show that it is important to model species-specific responses when predicting changes in forest composition and structure in response to succession, harvest, and climate change. As large-scale industry timber harvest moved from temperate deciduous forests to other less explored forest biomes, fine spatial scale (<10 ha) non-industrial timber harvest, mostly in the form of partial harvest of large trees of preferred species, becomes the primary anthropogenic disturbance (Shifley et al., 2012). These disturbances can change forest composition and structure and alter succession trajectories (Deluca et al., 2009). Furthermore, climate change may affect species establishment and mortality and alter forest composition in the region (Morin and Thuiller, 2009).
Predictions of forest change at regional scales (>100 million ha) often rely on niche and biophysical process models (Morin and Thuiller, 2009). Niche models (also called bioclimatic envelope models) relate observed species’ distributions to environmental predictors using a variety of statistical methods and have been extensively used to predict the potential impacts of climate change on tree species distributions (Guisan and Thuiller, 2005, Boulangeat et al., 2012). Recent advances in niche models include dispersal functions to predict species range shifts accounting for the effects of seed source, habitat fragmentation, and dispersal distance (Iverson et al., 2008, Meier et al., 2012). Biophysical process models, on the other hand, predict stock (e.g., biomass) and fluxes (e.g., aboveground net primary production) of plant functional types by incorporating leaf photosynthesis, carbohydrate allocation, and nutrient and water cycling (Morin et al., 2008, Medvigy et al., 2009). Despite the recognized importance of succession and harvest, both niche and process models usually use coarse spatial resolutions (e.g., 10–20 km) in regional scale predictions and consequently succession and disturbances (e.g., harvest and fire) are either simplified or ignored (Neilson et al., 2005, Purves and Pacala, 2008, Iverson et al., 2011, McMahon et al., 2011). Therefore, we still lack an understanding of the relative importance of succession, disturbance, and climate change in determining future forest composition changes.
Forest landscape models are explicitly designed to incorporate site-scale succession and landscape-scale disturbance to simulate forest change at landscape scales (He and Mladenoff, 1999). They have been recently used to examine the relative importance of succession, disturbance, and climate change in affecting forest change (e.g., biomass) at landscape scales (e.g., 104–107 ha). For example, Scheller and Mladenoff (2005) used the LANDIS II forest landscape model in northern Wisconsin and found that tree harvest and wind were as important as the effects of climate change alone in influencing the magnitude of forest composition change and the direction of tree species richness change.
Schumacher and Bugmann (2006) used the LANDCLIM forest landscape model in the Swiss Alps and projected that climate change would cause extensive forest cover changes beginning in the coming decades, fire was likely as important as climate, and harvest was less important compared with the direct effects of climate change. Gustafson et al. (2010) used the LANDIS II model in south-central Siberia and found that biomass was more strongly affected by timber harvest and insect outbreaks than by the direct effects of climate.
Wang et al. (2014) posit that studies investigating the relative importance of these endogenous and exogenous processes at regional scales and spanning short- to long-time frames have rarely been done; this is mainly because forest landscape models were unable to operate at sufficiently large geographic extents with a resolution fine enough to represent site- and landscape-scale processes. Maximum simulation capacity (number of pixels) of forest landscape models was in the range of 106 ~ 107 cells (He, 2008).
Recent advances in the LANDIS PRO forest landscape model have expanded the simulation capacity to 108 cells at 90–270 m resolution (Wang et al., 2014); the simulation capacity of LANDIS PRO makes it possible to predict forest change for a large temperate deciduous forest region under alternative climate scenarios while mechanistically simulating site- and landscape-scale processes. The future forest composition and structure can be assessed based on projected density, basal area, importance values, and biomass of tree species down to the raster cell level.
Climate change is degrading current habitats and will likely create novel habitats as species with good dispersal mechanisms redistribute themselves to track a shifting climate envelope (IPCC (2007); climate change is impacting and changing the species composition of natural habitats, but the refugia where these natural habitats occur nevertheless represent our best option for long-term biodiversity conservation. Existing natural communities are defined by unique arrays of environmental characteristics and the suite of species that interact within them. Rare species are often indicators of specialized or unique habitats. Even if some species are lost from special habitats, and some migrate in or out, the unique set of environmental characteristics will remain to provide the basis for a rich palette of opportunities for species in the future. Rare species and special habitats should be a priority for conservation action. Successful adaptation strategies for fish and wildlife will require understanding and reducing the combined effects of both climate-related and non-climate stressors. The cumulative effects of habitat loss and alteration, pollution, invasive species, and pathogens in addition to climate change may prove to be a deadly combination for many species. Ecological succession is one of the major observations on any Nature Trail. It is possible to observe both the on-going process of succession and the consequences of past succession events at almost any point along the trail. The rise and the decline of numerous species within our various communities illustrate both of the types of motive forces of succession: the impact of an established species to change a site’s environmental conditions, and the impact of large external forces to suddenly alter the environmental nature of a site. Both of these forces necessarily select for new species to become ascendant and possibly dominant within the ecosystem.
Some specific examples of ecological succession observable in tropical and temperate regions include:
- The growth of hardwood trees (including ash, poplar and oak) within the red pine planting area in tropical and temperate forests. The consequence of this hardwood tree growth is the increased shading and subsequent mortality of the sun loving red pines by the shade tolerant hardwood seedlings. The shaded forest floor condition generated by the pines prohibits the growth of sun-loving pine seedlings and allows the growth of the hardwoods. The consequence of the growth of the hardwoods is the decline and senescence of the pine forest.
- The raspberry thickets growing in the sun lit forest sections beneath the gaps in the canopy generated by wind-thrown trees in temperate forests. Raspberry plants require sunlight to grow and thrive. Beneath the dense shade canopy particularly of the red pines but also beneath the dense stands of oaks, there is not sufficient sunlight for the raspberry’s survival. Whenever there is a tree fall the raspberry canes have proliferated into dense thickets. Within these raspberry thickets, are dense growths of hardwood seedlings. The raspberry plants are generating a protected “nursery” for these seedlings and are preventing any major browser of tree seedlings from eating and destroying the young trees. By providing these trees a shaded haven in which to grow the raspberry plants are setting up the future tree canopy which will extensively shade the future forest floor and consequently prevent the future growth of more raspberry plants.
- The Succession “Garden” plot. This plot was established in April, 2000. The initial plant community that was established within the boundaries of this plot was made up of those species that could tolerate the periodic mowing that “controlled” this “grass” ecosystem. Soon, though, other plant species became established as a consequence of the removal of the stress of mowing. Over time, the increased shading of the soil surface and the increased moisture retention of the undisturbed soil-litter interface allowed an even greater diversity of plants to grow and thrive in the Succession Garden. Eventually, taller, woody plants became established which shaded out the sun-loving weed community. In the coming years we expect tree seedlings to grow up within the Succession Garden and slowly establish a new section of the forest.
Concept maps are graphical tools for organizing and representing knowledge. They include concepts, usually enclosed in circles or boxes of some type or without circles or boxes; and relationships between concepts indicated by a connecting line or arrow line linking two concepts. A concept is perceived as regularity in events or objects or records of events or objects, designated by a label. The label for most concepts is a word, although sometimes symbols such as + or %, and sometimes phrases more than one word are used. Propositions are statements about some object or event in the universe, either naturally occurring or constructed. Propositions contain two or more concepts connected using linking words or phrases to form a meaningful statement. Sometimes these are called semantic units, or units of meaning.Figure1 shows the Concept Map as contemplated in this review paper; the Concept Map shows the multidimensional inter-connectedness of the different related concepts and sub-concepts in ecological succession, climax communities and climate change.
Figure 1: Multidimensional Inter-connectedness of the different related concepts and sub-concepts in ecological succession.
Sources: Nyongesa and Matonyei, 2019
Ecological succession is a force of nature. Ecosystems, because of the internal species dynamics and external forces, environmental drivers including climate change are in a constant process of change and re-structuring. To appreciate how ecological succession affects humans and also to begin to appreciate the incredible time and monetary cost of ecological succession, one only has to visualize a freshly tilled garden plot. Clearing the land for the garden and preparing the soil for planting represents a major external event that radically re-structures and disrupts a previously stabilized ecosystem. The disturbed ecosystem will immediately begin a process of ecological succession. Plant species adapted to the sunny conditions and the broken soil will rapidly invade the site and will become quickly and densely established. These invading plants are called “weeds”. “Weeds” have very important ecological roles and functions, but weeds also compete with the garden plants for nutrients, water and physical space. If left unattended, a garden will quickly become a weed patch in which the weakly competitive garden plants are choked out and destroyed by the robustly productive weeds, most of which could be invasive species, resistant to herbicides or domestically poisonous, and with unknown economic value. A gardener’s only course of action is to spend a great deal of time and energy weeding the garden. This energy input is directly proportional to the “energy” inherent in the force of ecological succession. If you extrapolate this very small scale scenario to all of the agricultural fields and systems on Earth and visualize all of the activities of all of the farmers and gardeners who are growing our foods, and the natural forest ecosystem managers, you begin to get an idea of the immense cost in terms of time, fuel, herbicides and pesticides that humans pay every growing season because of the force of ecological succession. The natural forest ecosystem managers have to put in more time, knowledge and skills of Adaptive Resource Management in wake of global climate change.
I sincerely thank Professor Shem O. Wandiga, Professor Dan Olago and Professor Nicholas Oguge for having been my mentors and supervised my Doctoral research at the Institute of Climate Change and Adaptation (ICCA), University of Nairobi. They demonstrated profound knowledge about the complexities and dynamics encapsulate in the study of Global Climate Change. I wish to thank my co-author for the input.
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