Community Ecology

Species Interactions: Definitions and Consequences

Species interactions are fundamental to community ecology, influencing both the short-term distribution and abundance of organisms and their long-term evolutionary trajectories. The main types of species interactions are classified by their effects on the fitness of the interacting organisms.

• Competition: Occurs when individuals use the same limited resources, resulting in a decrease in fitness for both species. This is a - / - interaction.

• Consumption: Occurs when one organism eats or absorbs nutrients from another, increasing the consumer's fitness but decreasing the victim's fitness. This is a + / - interaction.

• Mutualism: Occurs when two species interact in a way that increases the fitness of both. This is a + / + interaction.

Example: Competition for water between plant species in a forest; predation of insects by birds; pollination mutualism between bees and flowering plants.

Competition and Niche Concepts

Mechanisms and Outcomes of Competition

Competition is a central ecological process that shapes species distributions and evolutionary adaptations. It is always a - / - interaction, but the degree of impact can vary.

• Asymmetric Competition: One species suffers a much greater fitness decrease than the other, indicating that one is a much better competitor.

• Symmetric Competition: Each species experiences a roughly equal decrease in fitness, indicating approximately equal competitive ability.

Niche Concepts

The niche of a species describes its ecological role and the conditions/resources it utilizes.

• Fundamental Niche: The full range of resources a species can use or conditions it can tolerate in the absence of competitors.

• Realized Niche: The range of resources a species actually uses or conditions it actually tolerates in the presence of competitors.

Example: A plant species may be able to grow in soil depths from 5-35 cm (fundamental niche), but due to competition, it is restricted to 15-35 cm (realized niche).

Competitive Exclusion Principle

When niches overlap completely, one species will outcompete the other, leading to the competitive exclusion principle:

• Competitive Exclusion Principle: It is not possible for species with identical niches to coexist indefinitely.

Example: If two species of plants compete for the same soil depth, the stronger competitor will exclude the weaker from that niche.

Niche Differentiation and Resource Partitioning

Natural selection can lead to evolutionary changes that reduce competition, a process known as niche differentiation or resource partitioning.

• Species may evolve to use different resources or occupy different habitats, reducing direct competition.

• This process is called character displacement when it results in changes in species traits.

Example: Two bird species may evolve different beak sizes to exploit different seed types.

Types of Consumption Interactions

Predation, Herbivory, and Parasitism

Consumption interactions are + / - relationships where one organism benefits at the expense of another.

• Herbivory: Consumption of plant tissues by animals.

• Parasitism: Consumption of small amounts of tissues or resources from a host by a parasite living in or on the host.

• Predation: Killing and consumption of another individual (prey) by a predator.

These interactions drive evolutionary arms races, with prey evolving defenses and predators evolving more efficient hunting strategies.

Example: Plants developing toxins to deter herbivores; predators developing sharper teeth or better camouflage.

Mutualism

Types and Examples of Mutualistic Interactions

Mutualism is a + / + interaction that benefits both species involved. These relationships are diverse and can involve various organisms.

• Plants and Pollinators: Plants provide nectar and pollen; pollinators provide reproductive assurance.

• Fungi and Plant Roots (Mycorrhizae): Fungi provide access to water and minerals; plants provide sugars.

• Ants and Acacia Trees: Ants provide protection from herbivores; trees provide shelter and food.

Example: Bees pollinating flowers while collecting nectar.

Short-Term and Long-Term Impacts of Species Interactions

Population and Evolutionary Effects

Species interactions have both immediate and evolutionary consequences.

• Short-Term Impacts: Affect the distribution and abundance of species within communities.

• Long-Term Impacts: Drive co-evolution, leading to adaptations and changes in species traits over generations.

Example: Predator-prey dynamics influencing population sizes; competition leading to niche differentiation.

Energy Flow in Ecosystems

Basic Concepts of Ecosystem Productivity

An ecosystem is defined by the flow of energy and nutrients between biotic (living) and abiotic (non-living) components. Energy enters ecosystems primarily through photosynthesis.

• Gross Primary Productivity (GPP): Total amount of energy fixed by autotrophs (primary producers) via photosynthesis.

• Net Primary Productivity (NPP): Energy remaining after cellular respiration, available to consumers.

Formula:

Example: Tropical wet forests have higher NPP than deserts due to greater energy fixation.

Photosynthesis Equation

Photosynthesis is the process by which autotrophs convert solar energy into chemical energy.

Food Chains and Trophic Levels

Energy flows through ecosystems via food chains and food webs, with organisms classified by their trophic level.

• Primary Producers (Autotrophs): Synthesize organic molecules from inorganic sources using energy from sunlight.

• Primary Consumers: Eat primary producers (herbivores).

• Secondary Consumers: Eat primary consumers (carnivores).

• Tertiary Consumers: Eat secondary consumers.

• Detritivores: Feed on detritus, the waste products or dead bodies of other organisms.

Energy dissipates as heat at each trophic level, while nutrients cycle within the ecosystem.

Table: Energy Flow in Ecosystems

Additional info: Table values inferred from provided notes; actual percentages may vary by ecosystem type.

Summary of Key Concepts

• Species interactions (competition, consumption, mutualism) shape community structure and drive evolutionary change.

• Niche concepts explain how species coexist and partition resources.

• Energy flow and nutrient cycling are fundamental to ecosystem function, with primary productivity setting the energy available to higher trophic levels.