Category: Educational

In terms of Ecology, the term ‘succession’ can be defined as ‘The change in species composition, community structure and function over time’ (MacKenzie et al 2001).

Succession of plant species follows a disturbance or initial colonisation of a new habitat. Take a back garden: the owners abandoned the house, stopped mowing the lawn, and the garden was left completely to its own devices. Eventually, after many stages of development, this garden would probably become a forest.

Grass is a competitive species, however with no regular mowing taking place; new opportunistic pioneering species (r-selected species) which grow easily quickly begin to take hold. Then, shrubs begin to jump in and smaller species die out. Eventually, tree saplings (K-selected species) grow large enough to dominate, and compete with smaller species by engulfing all the light, water and nutrients. The final set of ruling species is called the ‘climax community’.

Take a look around and begin to imagine what could be under your feet if nature was to take its course. The grass fields all around us are what is called a ‘plagioclimax’– this is, a climax community caused by human halting of natural succession. Should these fields be forest? Was the Oak woodland always like that or perhaps planted by humans? How about the dandelions that grow between the cracks of tarmac at a dis-used industrial site; are they, perhaps the first in the line of succession that would eventually become a natural jungle?

Primary succession; succession that occurs on newly formed surfaces (with no seed bank) such as dunes, lava flows and glacial forelands takes hundreds or thousands of years to facilitate. Secondary succession; succession that begins on previously vegetated areas such as fallow fields, clear-cut forests and embankments, vegetates rapidly.

Succession takes two main forms. ‘Autogenic’: natural, plant driven succession uninhibited by outside abiotic factors; and ‘Allogenic’: succession driven by abiotic, environmental factors such as human intervention, grazing animals, water flows, etc. Allogenic succession is far more common- true autogenic succession requires perfect growing conditions for all species, and virtually no human interference.

Henry Cowels (1899) was one of the first scientists to document this observable phenomenon. Using the example of his study site, Lake Michigan Sand Dunes, he noted the succession of species. Succession began with Marram Grass, which would facilitate within 20 years. Cotton Wood proceeded after 20-50 years, Jack Pine after 50-100 years, and Black Oak eventually dominates after 100-150 years. Each of these stages is commonly referred to as a ‘sere’.

In 1916 Fredrick Clements named this ‘facilitation’: complete replacement of species, where the preceding species inadvertently makes conditions habitable, or more favourable to other species. Clements held that ‘after a disturbance, an ecosystem would eventually return to its characteristic species assemblage’. He believed that succession was somewhat predicable and that eventually one would observe a monoclimax– the ‘King’ species of succession.

However, ‘many studies have shown successional pathways to be much more complicated and unpredictable than the classic facilitation model’ (Mac Donald 2003).

In the early 1920’s, Henry Gleason offered a more complex alternative to Clements’ model, suggesting that species responded individually to environmental factors and that the community structure was less predictable and more ‘coincidental’.

Years after Cowels’ benchmark study of Lake Michigan Sand Dunes, Jerry Olson (1958) conducted a long term study at the same site and concluded that the final, ‘climax’ community or assemblage differed depending on original conditions. He developed the idea of ‘multiple successional pathways’, depending on original conditions, extrinsic variables and colonisation patterns.

However, apart from environmental conditions and external factors, during the latter half of the 20^th century, the more complex relationships between plant species that would eventually begin to explain the differing colonisation patterns were being researched.

In 1950, ecologist Catherine Keever observed succession following field abandonment and detected allelopathic and competitive interactions taking place. Allelopathy is a biological phenomenon where one species releases a biochemical which hinders growth, survival and/or reproduction of another species. Keever writes that Broomsage dominates as the ‘King’ species for this reason.

It soon became apparent that there is no simple pattern of succession. Instead, an array of intertwining mutualistic and competitive relationships built upon a combination of existing conditions, external factors and species interrelationships. This means, similarly to a food web among the animal kingdom, a change in one factor or species can alter the eventual climax community.

In 1977 Joseph Connell and Ralph Slatyer published their work which summarised the three basic modes in which an ecological community interact. The Conell-Slatyer model of succession includes three broad mechanisms: Facilitation, Inhibition and Tolerance.

Facilitation is ‘competitive replacement of species’. Each community creates conditions favourable for more complex communities, and are eventually ‘overtaken’ by more competitive species. Tolerance is where all species involved are equally capable of establishing themselves, and tolerate conditions put upon them by other species (such as lack of light) in order to survive. Late successional species are incredibly tolerant and competitive. Lastly, Inhibition is where initial plants modify the environment so that it is less favourable to other species, such as through nutrient sapping and allelopathy.

Lawrence Walker and colleagues observed all three models taking place at once during a 7 year study in Puerto Rico. During the study, early successional forms and woody plants were removed, which eventually affected the long term successional species- it appeared that woody plants and ferns appeared to, in combination, facilitate long term forest development (Walker et al 2001).

In contrast, scrambling ferns inhibited succession and decreased woody plant richness. This phenomenon can be attributed to a host of different and interlinking factors such as alteration of soil pH and light availably.

Walker’s study highlights problems such as species arrival order, which make projections almost impossible when studying the contrasting effects of competition and facilitation. It becomes clear that, ‘although it is an observable site phenomenon, there is no unanimity of definition or casual explanation for the process’ (Joy Tivvy, 1993).

So if you go down to the woods today, you’re almost sure of a big surprise!

The Gaia Hypothesis, also known as the Gaia Theory or the Gaia Principle proposes that the Earth system operates as one (named Gaia). The theory states that organisms interact with their inorganic surroundings on Earth to form a complex, self-regulating system that in itself maintains conditions for life on the planet.
The hypothesis, which is named after the Greek Goddess Gaia, was formulated by scientist James Lovelock and co-developed by the microbiologist Lynn Margulis in the 1970s. Lovelock and Margulis are interested in how the biosphere and the evolution of life forms affect the stability of global temperature, ocean salinity, oxygen in the atmosphere and other environmental variables that affect the habitability of Earth.

The Gaia Hypothesis suggests that organisms co-evolve with their environment. That is they ‘influence their abiotic environment (for example, temperature and atmosphere) and that in turn the environment influences biota by Darwinian processes. James Lovelock (1995) gives evidence of this in his second book, showing evolution of the world from bacteria such as Stromatolites, towards the oxygen enriched atmosphere today that supports more complex life.

Less accepted versions of the theory state that changes in the biosphere are brought about by coordination of all living organisms and conditions are maintained through homeostasis. However, most scientists accept that each individual pursues its self-interest, and that their combined actions have a counterbalancing effect on environmental change (Wikipedia 2014).

However, humans are changing everything. Our actions have far exceeded the level of activity that is required by the Earth to keep its systems in homoeostasis. As a species we have not only brought about environmental change through our interaction with the abiotic systems, but by the elimination of biota that keep Earth operating as a self-regulating system.

Rockstrom and colleagues (2009) identified nine planetary boundaries that humans must not transgress in order to preserve the planet in its current state of homoeostasis that is imperative for our survival. This can be described as a ‘safe operating space for humanity’ (Steffen et al 2011). These include climate change, ocean acidification, stratospheric ozone depletion, biochemical flow cycles, global freshwater use, biodiversity loss, atmospheric aerosol loading and chemical pollution and change in land use.

If the Gaia hypothesis is to be taken into account, as a species, the single most important chance we now have to survive is retention of biodiversity. However, worryingly, biodiversity is the boundary that we have exceeded the furthest- due to our actions of creating pollution, removing forest and changing land use, burning fossil fuels and changing climate, and many other factors, which has sent many systems into dangerous positive feedback cycles rather than self-regulating negative ‘re-setting’ feedback loops. As a result, we are currently losing biodiversity at a rate of 150 – 200 species per day (Rockstrom et al 2009).

A study this decade shows that 13 – 37% of species will be ‘committed to extinction’ by 2050 if we are to continue in our harmful and wasteful vein (Midgley et al 2004).
The Gaia Hypothesis has been criticized for being teleological and contradicting the principles of natural selection- in that, a process or action in nature is designed to work together for a final cause, that is, to maintain the homoeostasis as we know it today.

This idea of ‘feedback-coupling’ assumes that evolution means survival of the individuals who do well in the environment that they and co-occurring species have created. Both feedbacks will evolve because any trait that improves conditions will also give reproductive advantage to its biota. In contrast, natural selection favours any trait that gives biota a reproductive advantage, whether it improves or degrades the environment, for example the human species.

However ‘degradation’ of the environment in one species’ eyes may not be degradation for another, adapted species. Life and the environment evolve together as a single system so that not only does the species that leaves the most progeny tend to inherit the environment but also the environment that favours the most progeny is itself sustained (Lovelock 1986).

At the same time, organisms that retain or alter conditions optimising their fitness leave more of the same- in this way conditions are retained or altered to their benefit (Lovelock and Margulis 1974).

 

A puddle would say ‘well, this depression in the ground here is really quite comfortable isn’t it? It’s just as wide as I am, just as deep as I am, it’s the same shape as I am… in fact, it conforms exactly to me in every detail. This depression in the ground must have been made just for me!’– Douglas Adams.

 

It would seem plausible that organisms must adapt to the constraints of the environment else they don’t survive. ‘It is inevitable that that sentient life should view its world as Eden, for any evolutionary linages to which this world were a Hell would not persist long enough to develop intelligent life forms’ (Kirchner 2002).

However, humans are changing Earth’s conditions so rapidly that most species that we co-inhabit this planet with, and reply so heavily upon for our survival, cannot evolve fast enough. The time has come to perhaps either attempt to re-set the systems that we need to keep within the threshold to support life as we know it, or accept that we will eventually meet the same demise as so many other species.

We may think we are one step ahead of nature, but in reality, the rate at which we are changing the Earth will probably deem it impossible for us to adapt fast enough to survive. Are we really clever enough to save ourselves from our own collective actions?

Perhaps, it is time to consider, rather than seeing our demise as the ‘destroying of the planet’ we are instead simply changing the planet ready to start over again from the simplest life-forms, which will eventually support the next generation of complex life. Our actions could all be indeed, part of Gaia’s great master plan.