Question

Population density (no. of individuals per \(\mathrm{m}^{2}\) ) of all organisms in all ecosystems falls with increasing body size, so that larger animals are less common. But for species of equal body size, aquatic organisms are \(10-20\) times more abundant in lakes than terrestrial organisms on land. Suggest two reasons why this might be. Cyr et al. (1997) discuss this issue.

Short answer

Stable environments and easier resource access in lakes support higher densities of organisms.

Step-by-step solution

Understanding Differences in Aquatic and Terrestrial Environments

Aquatic environments generally provide more stable conditions than terrestrial environments. Water bodies, such as lakes, have fewer fluctuations in temperature and moisture levels compared to land environments. This stability can lead to a more consistent habitat for aquatic organisms, supporting higher population densities.

Analyzing Resource Availability

In aquatic ecosystems, resources like oxygen and nutrients are more readily available throughout the environment. Water facilitates the diffusion of these essential substances, making them accessible to a larger population of organisms. In contrast, terrestrial organisms might face greater challenges in acquiring these resources due to factors like soil quality and climate conditions.

Evaluating Predation and Competition Pressures

Lakes might provide a more protected environment with fewer predators and lower competition for resources than terrestrial settings. The presence of natural boundaries in aquatic habitats can reduce the rate of predation and increase the survival rate of organisms, thus supporting greater abundance.

Key concepts

Aquatic and Terrestrial Environments

Aquatic and terrestrial environments represent two very different settings in terms of habitat stability and physical characteristics. Aquatic environments, such as lakes, oceans, and rivers, tend to have more stable conditions compared to terrestrial (land-based) environments. For instance, water bodies experience fewer extreme temperature fluctuations than land masses do. The temperature beneath the water surface can remain relatively constant for extended periods.
In water environments, moisture levels are consistently high, ensuring organisms do not face dehydration threats that are common on land. These stable and moist conditions support aquatic life forms, allowing them to thrive and be more abundant than their terrestrial counterparts.

• Temperature stability: Water helps in moderating temperature changes.
• Moisture availability: Constant moisture ensures survival without the risk of dehydration.

Resource Availability

Resources such as nutrients and oxygen are critical for organism survival, and their availability vastly differs between aquatic and terrestrial environments. In aquatic settings, resources are readily available. Water facilitates the diffusion of oxygen and nutrients, making it easier for organisms to absorb what they need for survival.
Oxygen, which is dissolved in water, spreads evenly, ensuring everyone gets their fair share. Similarly, nutrients are washed through water bodies, nourishing aquatic plants, which form the base of the food web. On land, organisms face challenges such as poor soil quality or alterations due to climate conditions, affecting resource access.

• Oxygen and nutrients spread evenly in water.
• Terrestrial challenges include varied soil and climate conditions.

Predation and Competition Pressures

Predation and competition are natural population controlling factors. However, the extent to which they impact organisms can vary between aquatic and terrestrial environments. Lakes and other water bodies often have natural boundaries that provide sheltered environments.
These boundaries can reduce predation risks, offering a somewhat safe haven for aquatic organisms. With fewer natural predators, aquatic populations can grow larger than those on land, where organisms might face continuous threats from predators.
Additionally, in aquatic settings, competition for resources like food might be lower due to abundant availability, allowing for higher organism survival rates. Terrestrially, competition might be fierce for limited resources, impacting population growth negatively.

• Aquatic environments can offer refuge with fewer predators.
• Abundance of resources in water can lead to lower competition.