Question

Trophic cascades are weaker in terrestrial systems compared with aquatic ones (Hall et al. 2007 ). One suggested explanation for this difference is that differences in body size between plants and their herbivores could be responsible (Shurin et al. 2006 ). Discuss the relative size of plants and their herbivores in aquatic ecosystems and in terrestrial ecosystems, and speculate how these differences might affect trophic cascades.

Short answer

Aquatic ecosystems have stronger trophic cascades due to smaller, faster-growing plants compared to terrestrial ecosystems where large, slow-growing plants weaken cascades.

Step-by-step solution

Understanding Trophic Cascades

Trophic cascades refer to the top-down impacts that predators exert on lower trophic levels. In simple terms, a change in predator populations can affect the whole food web, influencing the abundance and distribution of prey species and thus impacting the entire ecosystem.

Examining Aquatic Ecosystems

In aquatic ecosystems, plants, usually phytoplankton or algae, are much smaller compared to their herbivorous consumers such as zooplankton or small fish. The small size and rapid reproduction rates of aquatic plants allow them to quickly respond to changes in predation pressure, contributing to stronger trophic cascades.

Exploring Terrestrial Ecosystems

In contrast, terrestrial plants are generally larger, like trees or shrubs, compared to their herbivores, ranging from insects to larger mammals like deer. This size difference means that plants in terrestrial ecosystems grow slower and have more complex structures that are less rapidly influenced by predation pressure, making trophic cascades typically weaker.

Analyzing the Impact of Size Differences

The relative size of plants and their herbivores influences the strength of trophic cascades because smaller plants, like those in aquatic systems, can quickly increase or decrease in biomass in response to predator controls on herbivores. In terrestrial systems, larger plants with longer growth cycles provide more stable structures, making herbivore impacts less immediate and thus weakening cascade effects.

Speculating on Ecosystem Differences

Given the size dynamics, aquatic systems, with their rapid plant turnover and direct link between predators and primary production, exhibit stronger trophic cascades. In contrast, terrestrial systems, where large plants dominate and herbivore effects are more diffuse, show weaker cascades due to lesser sensitivity to changes at higher trophic levels.

Key concepts

Terrestrial Ecosystems

Terrestrial ecosystems are characterized by the prominent presence of land-based plants such as trees, shrubs, and grasses. These plants form the primary producers and play a critical role in forming the basis of the food web in these ecosystems. In terrestrial environments, plants and their herbivores vary significantly in size, with plants often being much larger and more complex than their herbivorous counterparts. Common herbivores in these ecosystems include insects, birds, and mammals, ranging from tiny aphids to larger animals like deer and elephants.

Because terrestrial plants often have extended life cycles and robust structures, they are less immediately responsive to changes in herbivore populations. For example:

• A tree may take decades to reach maturity and cannot quickly adjust its growth in response to herbivory.
• Herbivores like insects may consume plant leaves, but the plant's larger structure often remains largely unaffected in the short term.

This slow response rate means that predators influencing herbivores have a less pronounced impact on the plant population, leading to generally weaker trophic cascades in terrestrial systems.

Aquatic Ecosystems

In aquatic ecosystems, the dynamics are quite different due to the nature of the plants involved. Aquatic plants are typically smaller organisms like algae and phytoplankton that serve as primary producers. These producers are consumed by herbivorous species such as zooplankton or small fish.

One key characteristic of aquatic plants is their high reproductive rate, allowing them to quickly adjust their population levels in response to changes in their environment. For instance:

• Algae can reproduce rapidly within days, enabling them to respond swiftly to the changes in consumer populations.
• This quick biomass turnover means any predatory effects on aquatic herbivores can lead to rapid shifts in algae abundance.

The direct and robust link between plant biomass and predator control over herbivores results in stronger trophic cascades compared to terrestrial ecosystems, as changes can ripple through the system more efficiently.

Plant-Herbivore Interactions

The interaction between plants and herbivores is a crucial factor in understanding the strength of trophic cascades. In any ecosystem, plants provide the essential nourishment herbivores need for survival, influencing their population dynamics.

In terrestrial systems, the interaction is often influenced by the size and structural integrity of plants. For example:

• Larger plants can absorb more damage and continue growing, moderating immediate impacts from herbivores.
• The complex and varied structures of terrestrial plants offer refuge and less accessibility to herbivores.

Conversely, in aquatic ecosystems, plant-herbivore interactions are more dynamic due to the small, less defensive nature of underwater flora. Rapid reproduction cycles allow:

• Quick restitution of biomass following herbivore consumption.
• Immediate population adjustments following any predation impacts.

Therefore, the relative sizes and reproductive features of plants and their herbivores play a pivotal role in guiding the strength and pattern of trophic cascades within these different environments.