Interdependencies Between Species (VCE SSCE Biology): Revision Notes
Interdependencies Between Species
What's in an ecosystem?
Ecology is the study of how organisms interact with one another and their environment. No species exists in complete isolation. Every organism is part of a larger system that includes other living things and the non-living environment around them.
To better understand these complex systems, scientists organize biological systems into five main levels, from smallest to largest: cell, organism, population, community, and ecosystem. Each level builds upon the previous one, and changes at one level can ripple through the entire system.
1. Cell – The smallest functional unit of a living organism. Individual cells or systems of cells can be examined to understand how organisms maintain survival.
2. Organism – A living thing made up of one or more cells, such as an animal, plant, or single-celled life form.
3. Population – A group of individuals of the same species living in the same geographic location. For example, all the red kangaroos in central Australia form a population.
4. Community – A group of interacting populations of different species in the same geographical region. Think of all the different species living together in a coral reef.
5. Ecosystem – Multiple communities interacting with one another and their physical environment. Ecosystems include both biotic factors (living things like predators, prey, and competitors) and abiotic factors (non-living things like temperature, water, and nutrients).
Understanding these levels helps scientists study how changes at one level can affect the entire system. For example, changes to a keystone species (organism level) can dramatically affect the entire ecosystem.
Population size, distribution, and density
When studying populations within an ecosystem, three key characteristics help us understand their structure: size, distribution, and density.
Population size and carrying capacity
Population size is simply the number of individuals in a population. This number changes over time based on four main factors:
- Births – new individuals born into the population
- Deaths – individuals that die
- Immigration – individuals moving into the population from elsewhere
- Emigration – individuals leaving the population
The population change can be predicted using this equation:
Where:
- = future population size
- = initial population size
Understanding Carrying Capacity
Populations cannot grow indefinitely. Every environment has a carrying capacity – the maximum population size that an environment can sustain indefinitely. Think of carrying capacity like seats on a bus – there are only a set number of seats, and while you could squeeze more people on, it would be uncomfortable. Different species have different carrying capacities in the same environment, just like a bus can fit more toddlers than adults.
When population density is low, growth can be exponential (rapid and unrestricted). However, as the population approaches the carrying capacity, growth slows and eventually plateaus. This creates an S-shaped curve known as logistic growth.
r and K reproductive strategies
Different species have evolved different strategies for reproduction, forming a spectrum from r-selection to K-selection.
r-selection (the "quick and many" strategy):
- Produces large numbers of offspring regularly
- Provides little or no parental care
- Offspring reach sexual maturity quickly
- Higher mortality rate
- Population growth is often "boom and bust"
- Examples: oysters, insects, many aquatic animals, rodents
K-selection (the "slow and few" strategy):
- Produces smaller numbers of offspring
- Provides significant parental care
- Offspring reach sexual maturity slowly
- Lower mortality rate
- Population remains stable near carrying capacity
- Examples: humans, elephants, whales, large mammals
Most species fall somewhere along this spectrum rather than at the extremes. You can say a species is "more r-selected" or "more K-selected" when comparing different organisms. This reflects the reality that reproductive strategies exist on a continuum rather than as two distinct categories.
Population distribution
Population distribution refers to the range of geographical areas where members of a population can be found. This is limited by the ideal habitat of a species and its ability to tolerate different environments.
For large geographical areas, distribution is often shown on maps using colored regions. For example, red kangaroos are found across most of central Australia's hot and arid regions, whilst the critically endangered Leadbeater's possum survives in only two small populations northeast of Melbourne.
Within a local area, individuals can be distributed in three main patterns:
Uniform distribution – individuals are evenly spaced throughout the area. This might occur when individuals require a large amount of personal space or territory.
Random distribution – individuals are scattered without any predictable pattern. This is rare in nature.
Clumped distribution – individuals are grouped together in clusters. This often occurs when resources are patchily distributed, or when individuals benefit from living in groups.
Population distribution is not static and changes over time due to environmental factors such as temperature, rainfall, invasive species, and habitat loss. Monitoring these changes helps scientists understand how species respond to environmental pressures and predict future distribution patterns.
Population density
Population density refers to the number of individuals in a population per unit area (for example, individuals per square kilometer). Dense populations use up available resources more quickly than less dense populations.
Many factors that affect population size can be classified as either density-independent or density-dependent:
Density-independent factors – These affect populations the same way regardless of how dense the population is. A drought will impact a high-density population the same way it impacts a low-density population.
Density-dependent factors – These factors have stronger effects as population density increases. For example, disease spreads more easily through dense populations than sparse ones.
As population density increases, density-dependent factors become more severe, slowing population growth until the population size stabilizes at the carrying capacity. This is why populations don't grow exponentially forever – the environment's limitations eventually constrain growth through these density-dependent mechanisms.
Interactions between species
Within ecosystems, many species share habitats and interact with each other in various ways. These interactions can have beneficial, neutral, or harmful effects on the species involved. Currently, there is not a single species on Earth that exists completely independently.
Understanding the +/0/– Notation System
We can classify species interactions using a simple system: + (benefit), 0 (neutral), and – (harm). This notation shows the effect on each of the two species involved. For example, predation is marked as (+/–) because the predator benefits (+) while the prey is harmed (–).
Mutualism (+/+)
A mutualistic relationship exists when both organisms benefit from interacting with each other. Both species end up better off than if they didn't interact at all.
Example: Coral and Algae Partnership
Coral reefs get their dazzling colors from photosynthetic algae living inside coral tissue. The algae produce glucose and oxygen for the coral, whilst the coral provides the algae with a safe place to live. Both organisms benefit from this arrangement – the coral receives food and oxygen, while the algae receives protection and access to sunlight.
Commensalism (+/0)
Commensalism describes interactions where one organism gains some benefit whilst the other experiences no significant benefit or harm. True commensalism is quite rare in nature because closer investigation often reveals subtle effects on both species.
Example: Cattle Egrets and Buffalo
Cattle egrets follow herds of buffalo around the African savanna, feeding on insects stirred up from the grass. The birds benefit from easier access to food. Whilst this initially seems to have no effect on the buffalo, egrets actually feed on ticks attached to buffalo and may warn them when predators approach, making this relationship more mutualistic than commensal.
Predation (+/–)
Predation describes interactions where one organism (the predator) hunts and kills another organism (the prey) for food. The predator benefits by obtaining food, whilst the prey is harmed by being killed.
In response to predation pressure, many prey species have evolved adaptations to evade predators, such as camouflage that helps them blend into their environment. Conversely, predators have evolved adaptations like sharp claws, teeth, and improved eyesight to hunt more effectively. This ongoing evolutionary pressure between predators and prey is sometimes called an evolutionary arms race.
Example: Predator-Prey Cycles in Canadian Forests
In northern Canadian forests, populations of snowshoe hares and their main predator, the lynx, show cyclical patterns that mirror each other:
- When hare populations increase, lynx populations increase shortly after due to abundant food
- As lynx numbers rise, they reduce hare populations through predation
- With fewer hares available, lynx populations then decline
- Lower lynx numbers allow hare populations to recover
- The cycle repeats approximately every ten years
Research by Charles Krebs and colleagues proved that these population cycles are heavily influenced by both predation and resource availability. Experiments showed that excluding lynx or supplementing food both increased hare populations, with the combination of both having the greatest effect.
Parasitism (+/–)
Parasitism describes interactions where one organism (the parasite) obtains nutrients at the expense of another organism (the host). Unlike predation, parasites don't immediately kill their host, though they may weaken it enough to eventually cause death.
Parasites can be classified as:
- Ectoparasites – live external to the host (e.g., mosquitoes, ticks, lice)
- Endoparasites – live inside the host (e.g., parasitic worms, fungi, certain amoeba)
Example: Parasitic Wasps
Some parasitic wasps lay their eggs inside live caterpillars. The wasp larvae hatch and feed on the caterpillar's tissue from the inside, eventually killing the host. This strategy allows the wasp to provide a guaranteed food source for its offspring without having to care for them after laying eggs.
Amensalism (0/–)
Amensalism describes interactions where one organism experiences a negative effect whilst the other experiences neither benefit nor harm.
Example: Trampling by Ungulates
Ungulates (hooved animals like cattle, sheep, and deer) often trample grasses and small shrubs whilst walking, killing or damaging them. However, the animals themselves receive no benefit or harm from this interaction.
Competition (–/–)
Competition describes interactions where two or more organisms rival for the same pool of resources. When organisms compete, the availability of resources decreases for all competitors. This has a negative effect on all organisms involved.
The Reality of Competition
There is never a true "winner" in competition. Rather, organisms try to "outcompete" each other by obtaining resources more efficiently or effectively. Even the most successful competitor still experiences negative effects from having to compete for resources.
Competition can occur between:
- Different species (interspecific competition) – weeds and vegetables in a garden competing for water and nutrients
- The same species (intraspecific competition) – two male red kangaroos fighting to establish dominance and win over a female mate
The influence of keystone species
Some species play a much larger role in maintaining ecosystem structure and composition than others. These are called keystone species – species whose effects on an ecosystem are greater than expected relative to their population size.
Identifying Keystone Species
Keystone species are difficult to identify because removing any species has some effect on the ecosystem. However, removing a keystone species typically has a much larger and more dramatic impact than removing other species. The term "keystone" comes from architecture – just as a keystone holds an arch together, these species hold ecosystems together.
Keystone species commonly fulfill two main roles: apex predators and ecosystem engineers.
Apex predators
Apex predators are predators that sit at the top of the food chain with no natural predators of their own. These species control the numbers of their prey and, subsequently, the numbers of many other organisms throughout the ecosystem.
Example: Great White Sharks as Apex Predators
Great white sharks prey upon sea lions, seals, and large fish species, keeping their populations at healthy numbers. By controlling sea lion populations, sharks indirectly control populations of medium-sized fish (which sea lions eat), and small fish (which medium fish eat), and so on down the food chain.
Sharks also help keep prey populations physically and genetically healthy by removing weaker, older individuals that are easier to catch. This process of natural selection strengthens the overall prey population.
If great white sharks were removed from their ecosystem:
- Sea lion populations would increase dramatically
- Overpredation of medium fish would occur
- The cascade would disrupt the entire food web
- The ecosystem's balance would collapse
Ecosystem engineers
Ecosystem engineers interact with and significantly alter the physical environment of their ecosystem, creating habitats and opportunities for other species.
Example: Greater Bilby
Whilst foraging for food, bilbies dig many small pits in Australian soil. These pits fill with leaves, trap seeds, increase water permeability, and provide microhabitats. Each hole becomes a fertile pocket of nutrients, allowing native grasses, shrubs, and trees to grow in nutrient-poor soil.
Example: North American Beavers
Beavers construct dams that create wetlands where many fish species (such as salmon and trout) live. These wetlands also provide habitat for numerous other species. A single beaver dam can create an entirely new ecosystem where none existed before, supporting hundreds of different species.
The Yellowstone wolves – a trophic cascade
One of the most dramatic examples of keystone species impact comes from Yellowstone National Park, USA.
Case Study: Wolves in Yellowstone National Park
Background: By the 1920s, grey wolves had been hunted to extinction in the park. Over the next 70 years, elk populations exploded, leading to intense grazing and trampling of vegetation. The ecosystem suffered considerably.
The Reintroduction (1995): A small number of wolves were reintroduced, and their impact was profound. The presence of wolves changed elk behavior, causing them to avoid dangerous areas. In these areas, vegetation rapidly recovered, which catalyzed many more changes:
Direct Effects:
- Birds and beavers moved into newly forested areas
- Beaver dams provided habitats for otters, ducks, fish, reptiles, and amphibians
- Wolves hunted coyotes, leading to more rabbits and mice
- More small mammals supported more hawks, weasels, foxes, and badgers
Indirect Effects:
- Ravens and bald eagles scavenged wolf kills
- Bear numbers increased due to more carrion and berry trees
- River banks stabilized due to vegetation
- Rivers became fixed in their paths with more pools and habitat
This scenario demonstrates a trophic cascade – an ecological process that starts at the apex predator and cascades all the way down the food chain, affecting the entire ecosystem structure. The reintroduction of just a few dozen wolves transformed the entire landscape of Yellowstone.
Remember!
Key Points to Remember:
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Ecosystems are interconnected systems – Organisms can be studied at five levels (cell, organism, population, community, ecosystem), with each level influencing the others. No species exists in complete isolation.
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Populations have three main characteristics – Size (number of individuals), distribution (where they're found), and density (how crowded they are). These are affected by both density-dependent factors (like disease and competition) and density-independent factors (like climate).
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Species interactions follow six main patterns – Mutualism (+/+), commensalism (+/0), predation (+/–), parasitism (+/–), amensalism (0/–), and competition (–/–). Each notation shows whether organisms are benefited (+), harmed (–), or unaffected (0).
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Keystone species have disproportionate impacts – These species affect ecosystems far more than their population size would suggest. They commonly serve as apex predators (controlling prey populations) or ecosystem engineers (altering physical environments).
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Removing keystone species causes cascading effects – The removal of a keystone species can trigger a trophic cascade that disrupts the entire ecosystem, as demonstrated by the removal and reintroduction of wolves in Yellowstone National Park.