Ecology: Community Structure

Flashcards covering Lecture 20 on Community Structure, including holistic vs. individualistic views, ecotones, community assembly rules, food web dynamics, keystone species, and trophic cascades.

What are the primary objectives of community ecology?

To understand the functioning and stability of ecological systems, as well as the diversity and composition of co-occurring species.

How did F. E. Clements ($1916$) conceptualize an ecological community in the holistic view?

As a "super-organism" composed of integrated parts (analogous to cells or organs), forming a closed community with distinct boundaries and deterministic, predictable species composition and succession.

How did H. A. Gleason ($1926$) conceptualize an ecological community in the individualistic view?

As an open, unique assemblage of species brought together stochastically (by chance) with no inherent interspecific organization, where species distributions change gradually and independently along environmental gradients.

According to the provided diagram, how do closed communities differ from open communities along an environmental gradient?

Closed communities (a) have coordinated species distributions with sharp boundaries separated by distinct ecotones, whereas open communities (b) consist of species distributed independently with overlapping ranges and no sharp boundaries.

How did R. H. Whittaker's gradient analysis of trees in the Siskiyou Mountains and Great Smoky Mountains challenge the holistic community concept?

Whittaker showed that species abundances (e.g., Red oak, Beech, White oak) peak independently along moisture and elevation gradients rather than forming discrete, tightly bounded community clusters.

What is an ecotone?

A region of rapid turnover of species along a transect that acts as a zone of transition between adjacent communities.

Along a soil transect between non-serpentine and serpentine soils, which distinct elements characterize each soil type?

Serpentine soils are characterized by elevated concentrations of nickel ($\text{Ni}$), chromium ($\text{Cr}$), and magnesium ($\text{Mg}$), while non-serpentine soils have higher concentrations of iron ($\text{Fe}$) and copper ($\text{Cu}$).

From an evolutionary perspective, what is the fundamental flaw of the Clementsian "super-organism" view of communities?

Natural selection acts on the fitness of individuals, meaning populations evolve to maximize the reproductive success of individual members rather than benefiting the community as a whole (relying on the discredited concept of group selection).

What are the three main rules of community assembly that determine species composition?

1. Species must be able to disperse to the location.\n2. Environmental conditions must allow species to colonize, survive, reproduce, and maintain a population.\n3. Interspecific interactions (e.g., competition, predation) refine the community composition.

Based on the community assembly experiment depicted below, what did soil transplant experiments reveal about the physiological tolerances of Pygmy cypress and Sargent cypress?

Although restricted to acidic ($\text{A}$) or serpentine ($\text{S}$) soils in nature, Pygmy cypress grew much better and Sargent cypress grew almost as well in normal ($\text{N}$) soils, demonstrating that biotic competition rather than abiotic physiological limits excludes them from normal soils in nature.

How does community structure differ from population structure?

Population structure describes single-species attributes such as spatial density, spacing, and age-sex structure, whereas community structure describes multi-species attributes such as species richness, trophic level composition, and feeding guilds.

What is omnivory in the context of food webs?

Omnivory occurs when an organism feeds on species from more than one trophic level.

Based on the pitcher plant communities shown below, how do species richness and food web complexity relate across Seychelles, Madagascar, and West Malaysia?

The number of trophic levels increases with species richness (from $5$ to $14$ species), while the number of links per species remains relatively constant and independent of species richness ($2.9$ to $3.4$).

What are the two distinct components of community stability?

1. Constancy: The ability of an ecological system to resist change.\n2. Resilience: The ability of an ecological system to return to "normal" after a disturbance.

In Robert Paine's intertidal experiment depicted below, what occurred when the keystone predator Pisaster was excluded?

Species richness collapsed from $15$ species down to $8$ species because the bivalve mussel Mytilus competitively excluded other species by monopolizing available rock space.

How does the chrysomelid beetle act as a keystone species in old-field plant communities?

By feeding on dominant goldenrod plants, the chrysomelid beetle prevents goldenrod from monopolizing light and space; applying insecticide eliminates the beetle, leading to goldenrod dominance and a decline in plant diversity.

What distinguishes connectedness webs, energy flow webs, and functional webs as depicted in the diagram?

1. Connectedness web: Highlights all feeding relationships among organisms.\n2. Energy flow web: Quantifies interactions according to the flux of energy.\n3. Functional web: Emphasizes the strong influence of specific populations on the growth rates of other populations.

What is a trophic cascade?

An ecological phenomenon where the indirect effects of consumer-resource interactions propagate across multiple consecutive trophic levels rather than being limited to adjacent levels.