Biodiversity in Communities

Species Diversity and Its Measurement

Biodiversity refers to the variety of life forms within a given ecosystem, biome, or the entire Earth. In community ecology, biodiversity is often quantified using two main metrics: species richness and species diversity.

• Species richness: The simple count of species present in a defined region.

• Species diversity: A weighted measure that incorporates both species richness and evenness (the relative abundance of each species present).

Relative abundance data are rarely available, so the terms are sometimes used interchangeably, but they are distinct concepts.

Phylogenetic and Functional Diversity

Beyond species counts, ecologists also consider:

• Phylogenetic diversity: Measures evolutionary distinctiveness using branch lengths among species in a phylogeny.

• Functional diversity: Measures ecological distinctiveness by categorizing and counting the functional traits of species.

Biodiversity Changes Through Time

Biodiversity is dynamic, changing through processes such as speciation (which increases diversity) and extinction (which decreases it). New ecosystems may form due to abiotic changes, while disturbances can destroy existing ecosystems.

Mapping and Hotspots of Biodiversity

Biologists use surveys and mapping to estimate species richness globally. Areas of highest biodiversity differ among taxonomic groups, but a prominent pattern is higher diversity near the equator (latitude effect) and in regions with greater geographical variation.

Biodiversity hotspots are regions with exceptionally high numbers of endemic species and significant habitat loss. These areas are a high priority for conservation efforts.

Community Structure and Species Interactions

Defining Communities

A biological community consists of all the populations of interacting species living within a defined area. Community structure is characterized by:

• Total number of species

• Sum of interactions among all species

• Relative abundance of those species

• Physical attributes (abiotic and biotic factors)

Types of Species Interactions

Species interactions are classified based on their effects on the fitness of the interacting species:

• Commensalism (+/0): One species benefits, the other is unaffected.

• Competition (-/-): Both species are harmed by the interaction.

• Consumption (+/-): One species benefits (consumer), the other is harmed (prey/host).

• Mutualism (+/+): Both species benefit.

Commensalism

Commensalism is difficult to study because it is challenging to demonstrate the absence of an effect on the host's fitness. Examples include birds perching on large mammals or barnacles on whales.

Competition

Competition occurs when individuals use the same resources, resulting in lower fitness for both. It can be:

• Intraspecific: Within the same species

• Interspecific: Between different species

The concept of the niche is central to understanding competition:

• Fundamental niche: The total range of environmental conditions a species can tolerate.

• Realized niche: The portion of the fundamental niche actually occupied, given competition and other limiting factors.

Gause's competitive exclusion principle states that two species with identical niches cannot coexist indefinitely.

Niche Differentiation and Character Displacement

To reduce competition, species may undergo niche differentiation (resource partitioning) and character displacement (evolutionary changes in traits that reduce niche overlap).

Consumption

Consumption is a +/- interaction where one organism eats or absorbs nutrients from another. Major types include:

• Predation: Predator kills and eats prey.

• Herbivory: Herbivore consumes plant tissues.

• Parasitism: Parasite consumes part of a host, usually without killing it.

Top-Down and Bottom-Up Control

Population dynamics can be regulated by:

• Top-down control: Consumers (predators) regulate prey populations.

• Bottom-up control: Resource availability (nutrients, sunlight) regulates primary producers and thus the entire community.

Coevolutionary Arms Race

Predator-prey and parasite-host interactions can lead to a coevolutionary arms race, where reciprocal adaptations occur over time.

Defensive Adaptations

Prey species have evolved a variety of defenses:

• Constitutive defenses: Always present (e.g., cryptic coloration, toxins, armor).

• Inducible defenses: Produced in response to consumer presence (e.g., chemical toxins in plants).

Mutualism

Mutualism is a +/+ interaction where both species benefit. Examples include:

• Mycorrhizal fungi and plant roots

• Pollinators and flowering plants

• Cleaner fish and host fish

Mutualisms can be exploited by "cheaters," turning the interaction into a +/– relationship.

Community Structure and Dynamics

Food Chains and Food Webs

Species interactions form complex networks. A food chain links species through consumption, while a food web summarizes all consumption interactions in a community.

Keystone Species and Ecosystem Engineers

Some species, called keystone species, have a disproportionate influence on community structure. Their removal can cause dramatic changes (trophic cascades). Ecosystem engineers modify the physical environment, creating habitat for other species (e.g., corals, beavers).

Disturbance and Succession

Disturbance is any strong, short-lived disruption that changes resource distribution. The disturbance regime is defined by the type, frequency, and severity of disturbances. Recovery after disturbance is called succession:

• Primary succession: Occurs when disturbance removes soil and all organisms (e.g., volcanic eruption).

• Secondary succession: Occurs when disturbance removes some organisms but leaves soil intact (e.g., wildfire).

Successional Pathways and Species Traits

Early successional communities are dominated by pioneer species (short-lived, good dispersers). Late successional communities are dominated by long-lived, competitive species. The sequence of species appearance is the successional pathway.

Species Interactions During Succession

As succession proceeds, species interactions become more important than abiotic factors. Existing species can:

• Facilitate the establishment of later species

• Be neutral (tolerance)

• Inhibit the establishment of others

Additional info: Community ecology integrates concepts from population biology, evolution, and ecosystem science to understand how species interactions shape the structure and dynamics of biological communities.