Question

Draw a vertical food chain (part of a food web) showing that sea urchins eat kelp and that sea otters eat sea urchins. Use this model to: (1) show or explain how a trophic cascade works; and (2) offer an explanation for why otters are considered a keystone species.

Short answer

A vertical food chain can be illustrated with kelp as primary producers, sea urchins as primary consumers, and sea otters as secondary consumers. A trophic cascade occurs when changes in one species population, such as sea otters, lead to significant changes in other species populations, such as sea urchins and kelp. Sea otters are considered a keystone species because their predation on sea urchins helps regulate urchin populations, preventing overgrazing of kelp and maintaining ecosystem balance.

Step-by-step solution

Draw the food chain

First, let's draw a vertical food chain that shows the feeding relationships between kelp, sea urchins, and sea otters. Place kelp at the bottom representing the primary producers, sea urchins in the middle as primary consumers, and sea otters at the top as secondary consumers.

Explain trophic cascade

A trophic cascade is an ecological phenomenon in which changes in the abundance of one species within a food chain lead to significant changes in other species populations throughout the chain. In our example, if the sea otter population were to decrease significantly, the sea urchin population would increase due to reduced predation. The increased sea urchin population could then overgraze the kelp, leading to a decrease in available kelp for other species in the ecosystem. This chain reaction of effects is called a trophic cascade.

Offer an explanation of otters as a keystone species

A keystone species is a species that plays a disproportionately important role in an ecosystem, contributing significantly to the balance and diversity of species within the ecosystem. In our food chain, sea otters are considered keystone species because their predation on sea urchins helps regulate urchin populations and prevent overgrazing of kelp. By doing so, sea otters maintain the balance of the ecosystem and provide habitat for various other species that depend on kelp. If otters were removed from this ecosystem, the balance would be disrupted, leading to significant and negative consequences for other species within the ecosystem.

Key concepts

Food Chain

A food chain is a fundamental concept in ecology, forming a sequence of organisms each dependent on the next as a source of food. At its simplest, a food chain starts with the primary producers, typically plants or algae, that create their own food through photosynthesis. Primary consumers, or herbivores, feed on these producers and are in turn eaten by secondary consumers, or carnivores. Secondary consumers may then be preyed upon by tertiary consumers, and so on, creating a linear progression of energy transfer through different trophic levels.

For instance, in a marine ecosystem, kelp acts as the primary producer, sea urchins serve as primary consumers, and sea otters are secondary consumers. The stability of each level is crucial, as disruptions can lead to a ripple effect impacting the entire food chain. Visualizing this vertical chain can clarify the direct connections between different members of an ecosystem and accentuate the consequences each level has on the next.

Keystone Species

The concept of a keystone species, like the sea otter in our example, is akin to the central stone at the apex of an arch in architecture. Just as the keystone holds the arch in place, a keystone species maintains the structure and integrity of an ecosystem. Such species have an outsized influence on their environment, far exceeding what might be expected based on their abundance.

Oftentimes, keystone species are predators that regulate populations of other species, thereby preventing any single species from dominating and reducing biodiversity. Their activities, such as predation, can also shape the physical environment, indirectly supporting a wide range of other organisms. The absence of a keystone species can lead to dramatic changes in ecosystem composition and even cause collapses in certain cases.

Ecosystem Balance

Ecosystem balance refers to the state of equilibrium within an ecological community, where species populations remain within a range that is sustainable over time. This balance is crucial for the health and functionality of ecosystems, as it influences biodiversity, resource availability, and the overall structure of the environment.

Maintaining the balance often involves the regulation of various interactions, including competition, predation, and symbiosis. A balanced ecosystem can recover from disturbances more quickly and is more resilient to changes, while imbalances can lead to extinctions, habitat degradation, and reduction of ecosystem services. Factors like keystone species, trophic interactions, and external impacts all play critical roles in determining the balance of an ecosystem.

Predation

Predation is a biological interaction where one organism, the predator, hunts and consumes another organism, the prey. It is a driving force in the maintenance of ecological balance, influencing both the structure and diversity of communities.

Predators can control the population size of prey species, thereby preventing overgrazing or depletion of resources, as seen with sea otters controlling sea urchin populations. This dynamic process can shape the evolution of species, promoting traits that enhance survival and reproduction. The presence of predators can create a landscape of fear for prey, influencing their behavior, habitat selection, and ultimately, the distribution of species within an ecosystem. Understanding the role of predation within the food chain is essential to grasp the broader implications of ecosystem dynamics.