Powerpoing: Feb 18 Ecological Succession

Learning Objectives 

• Describe the process of ecological succession
  Define pioneer species and climax community
  Define primary and secondary succession
• Describe the characteristic of species during early and late succession

Ecological Succession is the gradual process by which ecosystems change and develop over time.

Ecological succession can be described something like a story of an ecosystem. Lets start from the beginning.

Once upon a time, there was an ecosystem. Everything was well,  until catastrophe struck (DISTURBANCE)! A volcanic eruption had wiped away all life, and even the soil had been covered and burned (PRIMARY SUCCESSION). There was nothing left but bare rock. There was not a trace of soil to be found. It didn’t look like life was ever going to return.

Yet, there is hope.

Only a few months after the volcanic eruption, life was beginning to come back. Lichens and other hardy species, took root. They produced acids that broke down the rock, producing a thin layer of soil. Other organisms which need soil, such as weeds. Moss grasses then began to grow. As they died, they were broken down by microbes into soil and nutrients. The soil thickened. eventually shrubs and other longer lived, plants were able to grow. Animals that fed on these shrubs returned as well. Eventually, mature oaks, fir trees and other long lived trees came back. They outcompeted and shaded out the weeds and grasses that were there before. Eventually, the community stabilized (CLIMAX COMMUNITY).

Although the process of ecological succession itself may seem random, there are actually some predictable changes that occur.

1. Disturbance: the disturbance can be in many forms, from the fall of a log to an asteroid destroying 90% of all life on earth.
   • Primary succession: when the disturbance destroys or leaves no soil. Succession begins with bare rock. e.g. lava flow, a newly formed island and an asteroid
   • Secondary succession: when the disturbance destroys most of the life, but leaves the soil intact. Succession happens much more rapidly, since some seeds and life may still remain in the soil.
2. If it is primary succession, lichens are the first to colonize. We call these species that colonize first pioneer species. Pioneer species are the first to colonize an area that has suffered disturbance.
3. If it is secondary succession, because seeds and most underground life still remains, the speed of succession will be much higher.
4. The process of succession will continue, usually in the order of
   1. Weedy species (e.g. dandelions)
   2. Small shrubs
   3. Shade intolerant plants (e.g. Red Alder)
   4. Shade tolerant plants (e.g. Western Hemlock and Douglas Fir conifers)
5. Until a climax community is reached. A climax community is a community which remains relatively stable over a long period of time. Unlike earlier stages of succession, which may be as short as decades or centuries at the longest depending on local conditions, Climax communities may remain stable for thousands of years… until another disturbance resets the community.

Organisms in the early and late stages of succession have very different characteristics, since very different characteristics are needed for organisms to survive and thrive in early and late stages of succession.

1. Ability to Disperse: since organisms in early succession are in an environment with lots of spaces and little competition, a species that is able to disperse their offspring far and wide is most likely to proliferate and be successful.
2. Shade tolerance: species in the early stages of succession are in an area with few species. Tall trees that would shade the ground have not yet established. Therefore, species in the early stages of succession tend to be shade intolerant (need lots of sunlight). In the later stages of succession, because tall trees begin to shade the ground, species which can tolerate this shade are more likely to grow.
3. Lifespan: organisms in early stages of succession tend to be short lived. As the environment is constantly changing, having a shorter lifespan would mean that natural selection can happen at a faster rate (since each generation is short lived, and reproduction rates are high). This would mean the population as a whole will be able to adapt better to these changes.
4. Competitors: since not many species would have established in the early stages of succession species that establish in early stages would not benefit from being very good competitors, whereas species in later stages of succession need to compete with many other established species for space and resources.
5. Hardiness: the ability of an organism to be “hardy” (to live in an environment with extreme conditions such as low nutrients or blaring sunlight) is much more important in the early stages of succession than in the later stages.

Powerpoint: Feb 17 Symbiosis

Learning Objectives

• Define symbiosis
• Define and give examples of parasitism, commensalism and mutualism

Most organisms in an ecosystem share some sort of relationship with each other. In these relationships, at least one party will benefit.

Symbiosis close relationship between two species in which at least one species benefits from the other.

Challenge question: from an evolutionary standpoint, why don’t we see relationships where neither party benefits?

Powerpoint: Feb 16 Population Growth and Decline

Learning Objectives

• Explain an exponential growth curve and a logistic growth curve
• Describe how Density dependent and density independent factors impact population growth
• Interpret a population vs. time graph
• Identify and define carrying capacity and steady state
• Evaluate what happens when species are removed from their natural habitats

Imagine you had an infinite amount of food and comfort. Everything that you ever want, all the food you could eat, all the space you need, all the luxury you can think of. It’s no wonder that you might think, “What a great world, time to reproduce!”
And so you do. But you don’t just have one child, why stop there when there’s so much space and room for all? Why not have two? Three? Five?
Later on, your children have many children and their children have many children and it keeps going on and on.

Notice that for every generation, you’re getting more and more individuals being born than in the last generation. If we were to graph this growth in individuals, it would look something like this:

Exponential Growth Curve

If nothing stops the population from growing, the population would just keep expanding and expanding faster and faster. If we were to graph this, we would produce an exponential growth curve.

Its calculated that if eastern cottontail rabbits were allowed to reproduce to their fullest capacity (20 kits a year/pair), in seven years, we’d have 184, 597, 433, 860 rabbits!

Thankfully, our world is not yet flooded with rabbits and will likely not be. Realistically, a population cannot keep growing unchecked. Something will come along and beat the population down, as it grows too large. We call these factors density- dependent limiting factors.

Density-Dependent Limiting Factors

Density dependent limiting factors are factors that control population size more strongly on large populations than on smaller ones.

1. Competition: when populations become crowded both plants and animals compete, or struggle, with one another for food, water, space, sunlight, and other essentials of life. The more individuals the less space and resources per individual.
2. Predation: As predators become more numerous, they eat more prey than are born and the population of prey decreases. As the population of prey decreases, there is less food for the predators, and their numbers decrease. This predator-prey relationship keeps both species in check. We call this relationship cyclic growth.
3. Parasitism: Parasites are much like predators, but instead of killing them; they live off of them and weaken them. When the population is very large and crowded, parasites are able to travel from one individual to the other faster and the population would decrease. Parasites are detrimental to their prey, but often not deadly. Why? If the prey were to die the parasite would die too.
4. Crowding and Stress: Crowding creates stress and could lead to lowered health that would be detrimental to the population. Some fishes, birds and mammals are also extremely territorial. When population numbers increase, the amount of fighting for space will likewise increase and so will stress.

Thanks to the above four density-dependent limiting factors, organisms do not normally exhibit exponential growth. The growth curve looks more like this:

                                                                                                                Logistc Growth

Parts A and B still look like the exponential growth curve above. This is where there is still lots of space and resource for everyone and crowding has not become a problem. Birth rate >> death rate.

Part C the growth curve begins to level as less births and death of individuals due to predation, parasitism and competition increases. There is still growth though, so birth rate > death rate.

Part D at this population size, the population birth rate = death rate. For every one individual born, one dies. Which means, the population is not growing. Therefore, this part of the graph is called the steady state.

Since in the environment it is in, the population does not generally increase past this number, the number of individuals at the steady state is called the carrying capacity. It is, theoretically, the MAXIMUM number of individuals that can be held.

Density Independent Factors

Not all organisms have their numbers limited by density dependent factors though. Some are limited by factors that have nothing to do with their numbers.

1. Boom and bust populations: locusts and algae for example, grow in great numbers when conditions are right, but die in huge numbers suddenly (population crash).
2. Natural disasters: natural disasters such as floods, rainstorms etc. The population can essentially be wiped out. It doesn’t matter how large the population is at that point. 

Apply your knowledge to a new situation

In any one environment, organisms that have evolved in relation to each other have evolved to deal with each other’s strengths and weaknesses. For example, the lynx and hare each evolves over time to compete with each other, the hare evolving traits to run from the lynx and the lynx evolving traits that allow them to hunt hare down. Similarly in an environment where organisms have evolved together (co-evolved) for a long time, they help to keep each other in check.

However, when organisms are torn away from their environments, the checks and balances are also taken away and in some cases, the population has exploded past control.

We see this in invasive species, such as the scotch broom, European starling and House sparrow. These species, which were introduced from the British Isles, have since become pests that compete with and threaten native species.