Introduction

Welcome to AP Biology topic 8.5: Community Ecology! In this article, we’ll build on our previous knowledge of natural selection and ecosystems and review the Simpson’s Index, communities, types of symbiotic relationships between organisms, trophic relationships, energy flow between trophic levels, and ecological succession.

Simpson's Index

Circle back to 8.3 and think of a forest, including trees and animals that all interact with each other. We defined a population as a single species in this forest. Meanwhile, a community would be the entire forest and its interactions that are subject to change over time.

Biodiversity is measured by using the Simpson's Diversity Index, based on the number of different species and the proportion each species represents of all individuals in the community.

In this, N represents the total number of organisms, and n represents the number of organisms in each species. There’s a positive correlation between the diversity index and biodiversity; a high diversity index indicates high biodiversity and vice versa.

The equation for Simpson’s Diversity Index is: $\text{Diversity Index }=1-\sum {\left(\frac{n}{N}\right)}^2$

Dynamic Communities

Communities are groups of different species that live in the same environment at the same time. These community structures are measured in terms of species composition and species diversity as seen in Simpson’s Index. For example, a small forest community may consist of vegetation (shrubs and grass), trees, bugs, deer, and wolves. The way these communities interact amongst themselves and with other communities determine how energy and matter move through the environment; influence population sizes; and shape the physical structure of the habitat. These relationships may seem random and arbitrary, but they are the result of evolutionary history, resource availability, and environmental pressures.

Types of Community Interactions

Competition (-/-)

When two communities require the same limited resource, they must compete and fight for it, reducing the well-being of both communities. Competition can occur for food, space, mates, or other resources. Over time, species may undergo resource partitioning, or using resources in different ways or at different times to reduce direct competition. A classic example of resource partitioning is different bird species using different parts of the same tree. This concept is illustrated in the image below:

Image Source: Spatial Partitioning | SaveMyExams

Predation ( + / – )

Predation occurs when one organism obtains energy and nutrients by consuming all or part of another organism. By this definition, predation extends beyond traditional predator-prey relationships, such as that of wolves and elk. It also includes interactions such as herbivores grazing on plants and unicellular organisms, like amoebas, engulfing bacteria. In both cases, one organism (the “predator”) obtains energy by consuming another (the “prey”).

Predation can affect prey abundance, behavior, and distribution, leading to indirect effects on plants, other animals, and overall community structure in the form of trophic cascades. Oftentimes, predators are examples of keystone species (see article 8.6: Biodiversity) in their environments.

Symbioses

Some species have close, long-term interactions known as symbioses or symbiotic relationships:

Trophic Relationships and Energy Flow

Communities are structured into trophic levels — producers, consumers, and decomposers —linked in food chains and more complex food webs. Disturbances to any one population can affect multiple other species through these networks, usually by affecting the availability of food. The stability of a community often depends on the number and strength of these connections.

Image Source: Ecological Pyramid with energy and biomass | Wikimedia Commons

Trophic levels in a typical ecosystem. Notice that only about 10% of the available energy actually gets transferred to the next trophic level upon predation. This is because the remaining 90% of energy is lost in various forms, such as heat loss and the energy it takes for the predator to eat the prey.

Ecological Succession

Community composition changes over time through succession:

Succession is driven by changes in resource availability, species interactions, and environmental conditions, eventually leading to a more stable community—unless disrupted again.

Summary

Practice MCQ