Ecology Notes

Species Diversity

• Focuses on the variety of species within a community.

Trophic Structure: Producers

• The trophic structure of a community is a hierarchy of trophic levels.

• These levels are defined by the feeding relationships among species.

• First trophic level (primary producers or autotrophs):

  • Plants and photosynthetic organisms capture sunlight and convert it to chemical energy.

• Animals are consumers (heterotrophs):

  • They acquire energy and nutrients by consuming other organisms or their remains.

• Second trophic level (primary consumers):

  • Herbivores that eat plants.

• Third trophic level (secondary consumers):

  • Carnivores that eat herbivores.

• Fourth trophic level (tertiary consumers):

  • Carnivores that eat other carnivores.

• Omnivores:

  • Organisms like humans and some bears feed at multiple trophic levels simultaneously.

Detritivores and Decomposers

• Scavengers (detritivores):

  • Animals that ingest dead organisms, digestive wastes, and cast-off body parts (e.g., earthworms and vultures).

• Decomposers:

  • Small organisms like bacteria and fungi that feed on dead or dying organic material.

• Ecological function:

  • Detritivores and decomposers reduce organic material to inorganic molecules, which producers can assimilate.

Food Chains and Webs

• Food chain:

  • The trophic structure where one organism eats another.

  • Each link points from the food to the consumer.

• Straight-line food chains:

  • Rare in nature.

• Food web:

  • A set of interconnected food chains with multiple links, portraying complex relationships.

Food Web Analysis

• Links between trophic levels contribute to community stability when environmental disturbances eliminate some species.

• In species-rich communities, losing one or two species has minor effects on overall community stability.

• The proportions of species at high, middle, and low trophic levels are reasonably constant across communities.

• Regardless of species richness, a community includes two to three prey species for every predator species.

Interspecific Competition

• Interspecific Competition:

  • Can cause local extinction or prevent new species from establishing, decreasing species richness.

  • More studies focus on competition in K-selected species than in r-selected species, which may overestimate the importance of competition.

  • In communities where resource partitioning and character displacement occur due to past competition, its current importance may be underestimated.

• Ecologists' Views:

  • Some ecologists are undecided about the influence of interspecific competition on species composition and community structure.

  • Plant and vertebrate ecologists believe competition profoundly affects species distributions and resource use.

  • Insect and marine ecologists suggest predation, parasitism, and physical disturbance govern community structure.

Predators Can Boost Species Richness

• Predators can increase species richness by stabilizing competitive interactions among prey.

• Example: Sea Stars and Mussels:

  • Predatory sea stars preferentially eat mussels, reducing their numbers.

  • This allows other species to grow.

  • When sea stars (keystone species) were removed, mussels outcompeted other species, reducing diversity from 18 to 2 or 3.

Effects of Herbivores on Food Plants

• Periwinkle Snails (Keystone Species):

  • Periwinkle snails in Massachusetts graze on the green alga Enteromorpha.

• In tidepools:

  • Enteromorpha outcompetes other algae.

  • Moderate herbivory allows less competitive species to grow, increasing species richness.

• On high rocks:

  • The red alga Chondrus is dominant.

  • Periwinkles feeding on Enteromorpha reduce algal species richness.

Effects of Disturbance on Community Characteristics

• Great Barrier Reef Study:

  • From 1963 to 1992, researchers tracked the effects of five major cyclones.

  • Changes resulted from external disturbances removing coral colonies and internal processes (growth and recruitment).

  • The community never reaches equilibrium due to slow growth and recruitment, and frequent disturbances.

Intermediate Disturbance Hypothesis

• Species richness is greatest in communities with fairly frequent disturbances of moderate intensity.

• This allows K-selected species to survive while creating openings for r-selected species to colonize.

• Severe and frequent disturbances:

  • Communities include only r-selected species with fast life cycles.

• Mild and rare disturbances:

  • Communities are dominated by long-lived K-selected species.

Ecological Succession: Responses to Disturbance

• Ecological succession:

  • A series of changes in species composition in response to disturbance.

• Primary succession:

  • Begins when organisms colonize terrestrial habitats without soil (e.g., after volcanic eruptions or glacier retreat).

  • Lichens are usually the first visible colonizers.

  • They erode rock, initiate soil development, and produce organic material.

Ecological Succession

• Soil Accumulation:

  • As soil accumulates, r-selected plants colonize the site.

  • Their decaying remains enrich the soil (facilitated by detritivores and decomposers).

• Development:

  • As soil gets deeper and richer, bushes and eventually trees are supported.

• Climax Community:

  • A relatively stable, late successional stage (climax community) is often dominated by long-lived K-selected species.

  • It persists until an environmental disturbance eliminates it, allowing other species to invade.

Primary Succession

• Glaciers retreat, leaching minerals (especially nitrogen) from the newly exposed substrate.

• Lichens and mosses establish.

• Mountain avens (Dryas) grow on the nutrient-poor soil, benefiting from nitrogen-fixing bacteria.

• Shrubby willows (Salix), cottonwoods (Populus), and alders (Alnus) take hold in drainage channels, also symbiotic with nitrogen-fixing microorganisms.

• Young conifers (hemlocks - Tsuga, and spruce - Picea) join the community.

• After 80 to 100 years, dense forests of Sitka spruce (Picea sichensis) and western hemlock (Tsuga heterophylla) crowd out other species.

Secondary Succession

• Occurs after existing vegetation is destroyed or disrupted (e.g., by fire, storm, or human activity).

• Early stages proceed rapidly because the soil is ready for colonization and may contain numerous seeds.

• Later stages parallel those of primary succession.

Alternative Successional Sequences

• Similar climax communities sometimes arise from alternative successional sequences.

• Example: Pond to Hardwood Forest (Aquatic Succession):

  • Debris from rivers and runoff accumulates, transforming a pond into a swamp.

  • Transpiration by larger plants dries the soil, allowing other species to colonize.

  • Eventually, the swamp may become a meadow or forest with moist, low-lying ground.

Processes Underlying Succession

• Facilitation Hypothesis:

  • Species modify the local environment to make it less suitable for themselves but more suitable for the next successional stage.

  • Succession is orderly and predictable.

• Inhibition Hypothesis:

  • New species are prevented from occupying a community by existing species.

  • Succession is neither orderly nor predictable.

• Tolerance Hypothesis:

  • Succession proceeds because competitively superior species replace competitively inferior ones.

  • Early-stage species neither facilitate nor inhibit later-stage species.

  • Competition eliminates species that cannot harvest scarce resources.

Processes Underlying Succession

• Succession results from a combination of facilitation, inhibition, and tolerance with differences in dispersal, growth, and maturation rates.

• Disturbance and density-independent factors play important roles, sometimes speeding up successional change.

• In other cases, disturbance inhibits successional change, establishing a disturbance climax (disclimax community).

Variations in Species Richness Among Communities

• Species richness follows a latitudinal gradient for many plant and animal groups, with the most species in the tropics and a decline toward the poles.

• Hypotheses for high tropical species richness:

  • High reproductive rates.

  • Low migration.

  • Few environmental disturbances.

• Hypotheses for maintaining high tropical species richness:

  • High availability of energy and other resources.

Theory of Island Biogeography

• The equilibrium theory of island biogeography addresses variations in species richness on islands of different sizes and levels of isolation.

• The number of species on an island is determined by the equilibrium between immigration and extinction.

Theory of Island Biogeography

• The mainland forms a species pool from which species immigrate to offshore islands as new colonizers.

• As the number of species on an island increases, the extinction rate rises due to competition and predator-prey interactions.

Effect of Island Size

• At equilibrium, large islands have more species than small islands.

• Reasons:

  • Large islands have higher immigration rates (larger target).

  • Large islands have lower extinction rates (support larger populations, greater range of habitats and resources).

Effect of Distance from Mainland

• At equilibrium, islands closer to a mainland source have more species than more distant islands.

• Reasons:

  • Islands near the mainland have higher immigration rates (easier to locate).

• Distance does not affect extinction rates.