Question

Draw up a food web of, say, six or seven species with which you are familiar and which spans at least three trophic levels. Take each species in turn and suggest the kind of community organization that would be necessary for this to be a keystone species.

Short answer

Construct a food web with grass, grasshopper, frog, snake, hawk, and decomposers, identifying the frog as a keystone species due to its role in controlling grasshopper populations.

Step-by-step solution

Define Trophic Levels

Trophic levels describe the position of species in a food chain. The primary levels are producers, primary consumers, secondary consumers, and tertiary consumers. In a typical ecosystem, producers like plants form the first trophic level, herbivores (primary consumers) form the second, and carnivores (secondary and tertiary consumers) occupy higher levels.

Select Species for the Food Web

Choose six or seven species that are familiar and span at least three trophic levels. For example, you might choose grass (producer), grasshopper (primary consumer), frog (secondary consumer), snake, and hawk (tertiary consumers), along with decomposers such as fungi or bacteria.

Construct the Food Web

Start with the producer at the base of the web. Connect it to the primary consumer that feeds on it. Then, link the primary consumer to secondary and tertiary consumers. For example, grass is eaten by grasshoppers, which are preyed on by frogs. Frogs are eaten by snakes, which in turn can be preyed on by hawks.

Identify Possible Keystone Species

A keystone species is one whose presence and role within an ecosystem have a disproportionate effect on other organisms. In this food web, consider each species' role. For example, the frog might be a keystone species if it regulates the population of grasshoppers, thus protecting plant life.

Community Organization for Keystone Species

For the frog to function as a keystone species, the ecosystem must be organized so that the frog maintains the balance of grasshopper populations. This requires a stable population of frogs, supported by suitable habitats and adequate food supply (grasshoppers), and minimal interference from other predators.

Analyze the Impact of Removing a Species

Consider the impact of removing the suggested keystone species (e.g., frog) from the ecosystem. This would likely lead to an increase in grasshopper populations, resulting in overconsumption of plants and a decline in biodiversity as plant life diminishes.

Key concepts

Trophic Levels

In any ecosystem, trophic levels are crucial because they provide the structure for understanding the flow of energy.
Each level signifies a different step in the food chain, starting with producers and moving up to various levels of consumers.
Let's break it down:

• Producers: These are organisms like plants and algae that manufacture their own food through photosynthesis. They form the base of the food web.
• Primary Consumers: Herbivores such as grasshoppers consume the producers. They are the first level of consumers in the food chain.
• Secondary Consumers: These are typically small carnivores like frogs, which eat primary consumers.
• Tertiary Consumers: Larger predators, such as hawks, make up this group. They prey on both primary and secondary consumers.

Understanding these levels helps in analyzing energy flow and ecological relationships within the food web.

Keystone Species

Keystone species play a pivotal role in maintaining the structure of an ecosystem.
Their presence ensures the balance of various populations and interactions. If a keystone species is removed, the ecosystem might undergo dramatic changes.

What makes a species keystone?

• Diverse Impact: Keystone species have a greater impact on their environment than their number suggests.
• Regulatory Role: They often control populations of other species, such as the frog regulating grasshopper populations to prevent overgrazing.
• Critical Habitats: These species often modify habitats, benefiting numerous other organisms.

Therefore, preserving keystone species is essential for ecosystem health and sustainability.

Ecosystem Balance

Ecosystem balance refers to the stability and healthy functioning of an ecosystem.
This involves the harmonious interaction of species and their physical environment.
Key factors include nutrient cycling, energy flow, and food web dynamics. These components work together to create a balanced ecosystem.
What contributes to ecosystem balance?

• Food Web Interactions: These ensure energy transfer between trophic levels, supporting diverse life forms.
• Role of Keystone Species: By maintaining predator-prey dynamics, keystone species keep certain populations from overpowering the ecosystem.
• Biodiversity: A high diversity provides resilience against disturbances, such as disease or climate change.

Maintaining ecosystem balance ensures long-term sustainability and productivity.