15:273 General Ecology - Final Exam

Chapter 18, 19, 20, 21, 22, 23, 24, Exam Review

Ecological Succession

• Process of change in the species composition of an ecological community over time

• What happens following a disturbance or the initial colonisation of a new habitat

Henry Chandler Cowles

• Studied vegetation development on sand dunes on the shores of Lake Michigan (Indiana Dunes)

• Recognised that vegetation on dunes of different ages might be interpreted as different stages of a general trend of vegetation development on dunes

Frederic Clements

• Was raised in the prairies

• Had a large-scale view of communities, leading to the idea of an organismal concept of communities

  • This concept dominated community ecology until the mid-20th century

Organismal Community Concept

• Clements regarded a community as a closely integrated holistic entity of mutually interdependent organisms and relatively discrete borders

• Succession has a predictable end-point, a climax community

  • Is a stable condition or equilibrium

Henry Gleason

• Was raised at the prairie-forest border, which cannot be fully described by a single climax community (non-equilibrium)

• Developed the individualistic (continuum) concept of communities

  • This concept dominated community ecology during the second half of the 20th century

Clement’s Predicted

• Discrete community types with sharp ecotones

Gleason’s Suggested

• There is continuous variation in communities

Robert Whittaker Findings

• Species show individualistic distribution patterns along gradients of environmental factors, resulting in a continuous change of community composition

  • This supports Gleason’s view

Modern Synthesis (of Theory)

• Gleason’s view is more widespread among scientists but Clements’ aspects of a community are frequently accepted

  • E.g. dominant tree species influence a lot of other organisms

• Discrete community units are pragmatic for applied disciplines such as forestry

Effects of Disturbance

• Leads to changes in community structure and composition over time

• Examples: volcanoes, earthquakes, storms, fires, and climate change

Intermediate Disturbance Hypothesis

• Species richness is the highest at intermediate levels of disturbance

At High Levels of Disturbance

• Many species fail to get established (non-equilibrium)

At Low Disturbance Levels

• Competitively superior species suppress the others species

Premises Base for The Intermediate Disturbance Hypothesis

1. Ecological disturbances have major effects on species richness within the area of disturbance

2. Interspecific competition results from one species driving a competitor to extinction and becoming dominant in the ecosystem

3. Moderate ecological scale disturbances prevent interspecific competition

Intermediate Disturbance Hypothesis Example

• Rabbit grazing →

  • Mean number of plants per plot in sand dunes is the highest at intermediate levels of rabbit grazing

Communities in Early Succession

• Are dominated by fast-growing, well-dispersed species (opportunist, fugitive, or r-selected life-histories)

  • As succession proceeds, these species will be replaced by more competitive (k-selected) species

Patterns of Diversity - Succession

• Formerly seen as having a stable end-stage called the climax, shaped primarily by the local climate

  • This idea has been largely abandoned by modern ecologists in favour of non-equilibrium ideas of ecosystems dynamics

Secondary Succession Example

1. Bare rock

2. Mosses/grasses

3. Grasses/perennials

4. Woody pioneers

5. Fast growing trees

6. Climax forest

Primary Succession Characteristics

• Begins on rock formations

• Lichen, algae, fungi develop soil

• These are replaced by plants, like grasses and shrubs

• Water and nutrient levels increase over time

• Nitrogen-producing species play a large role

• N is limited

Secondary Succession Characteristics

• Follows disturbance (fire, flood) or removal of an existing community (logging)

• Strongly influenced by pre-disturbance conditions (e.g. soil, and seed banks)

• Can be relatively rapid

• Species that dominate are usually present from the start

Community Composition

• Some species are naturally rare/abundant

• Some communities are dominated by one or a few species, some are species-rich

How Biodiversity is Measured

1. Alpha diversity (α-diversity)

2. Beta diversity (β-diversity)

3. Gamma diversity (γ-diversity)

Alpha Diversity (α-Diversity)

• Mean species diversity in a site at a local scale

• Number of species

Beta Diversity (β-diversity)

• Ratio between regional and local species diversity (gamma diversity divided by alpha diversity)

• Rate at which species composition changes across a region

• Overemphasises the role of rare species

  • Example: high diversity in matrices of recently disturbed land

Gamma Diversity (γ-Diversity)

• Total species diversity in a landscape

  • Scale is important to distinguish between alpha and gamma diversity

Autogenic Succession

• Brought about by changes in the soil caused by the organisms there

  • Includes accumulation of organic matter in litter or humic layer, alteration of soil nutrients, or change in the pH of soil due to the plants growing there

  • Structure of the plants can also alter the community

Autogenic Succession Example

• When larger species like trees mature, they produce shade on the forest floor that excludes light-requiring species → as a result, shade-tolerant species will invade the area

Allogenic Succession

• Caused by external environmental influences and not by the vegetation

  • Animals play an important role as they are pollinators, seed dispersers, and herbivores

Allogenic Succession Example

• Soil changes due to erosion, leaching, or the deposition of silt and clays. This can alter the nutrient content and water relationships in the ecosystem

(Main) Models of Successional Change

1. Facilitation

2. Tolerance

3. Inhibition

Facilitation Model

• Presence of an initial species aids and increases the probability of growth of a second species

Facilitation Model Example

• Presence of Alder plants aids the growth of Willow and Poplar seedlings in an Alaskan floodplain by increasing the availability of nitrogen

  • Alder roots contain nitrogen-fixing bacteria (which greatly increase inorganic nitrogen in soils)

Tolerance Model

• The climax community is composed of the most “tolerant” species that can co-exist with other species in a more densely populated area

  • Eventually, dominant species replace or reduce pioneer species abundance through competition

Tolerance Model Example

• Forest succession with survival in conditions of shade

  • Species better adapted to shady conditions will become dominant

Inhibition Model

• Earlier successional species inhibit growth of later successional species and reduce growth of colonising species already present

  • The environment is thus less hospitable to other potential colonising species

Inhibition Model Example

• Lantana (Lantana camara) sprawling shrubs exclude and inhibit the growth of certain tree species

Succession in Consumers

1. Initially, fauna is sparse (few mites, ants, and spiders living in cracks and crevices)

2. The animal population increases and diversifies with the development of the forest climax community (nematodes, insect larvae, ants, etc.)

3. Vertebrates enter the scene, such as squirrels, foxes, mice, etc.

Alternative Stable States (Questions)

1. Does succession under given environment always lead to the same climax community?

2. Does the final stage depend on the identity of colonising species?

3. Are there different stable states for a community?

Alternative Stable States - Normal Successional Trajectory

• There is a predictable way a “ball” will fall

Alternative Stable States - Disturbance

• The input of disturbance can push the “ball” back up the hill after it’s fallen

Alternative Stable States - Redirection

• The redirection of energy can lead to an alternative stable state

Succession in Mixed-Hardwood Forest

• When moose browsing is added to the equation, we would expect one of two possible outcomes:

  • If moose browse all species equally and they are sufficiently abundant, they may actually delay succession, keeping the forest in a more-open, shorter state

  • If moose brows preferentially, avoiding species they dislike (such as Spruce), they may redirect the trajectory of the forest. Mature forest which eventually takes hold will be dominated by these browse-resistant tree species

Energy

• Is required by most complex metabolic pathways (often in the form of adenosine triphosphate, ATP)

  • Especially those responsible for building large molecules from smaller compounds, and life itself is an energy-driven process

  • Living organisms would not be able to assemble macromolecules from their monomeric subunits without a constant energy input

Food Chains

• Linear representation of feeding interactions

• Only a part of the energy content of the food can be absorbed and utilised

• Do not accurately describe most ecosystems

Food Chain Example

1. Aphid eats plants

2. Ladybugs eats aphid

Food Web(s)

• Representation of all feeding interactions

• It expresses connections among the food chains in an ecosystem

• Three basic ways in which organisms get food are:

1. Producers (Autotrophs)

2. Consumers (Heterotrophs)

3. Decomposers (Detritivores)

Producers (Photoautorophs)

• Are responsible for almost all productivity of the biosphere

• Use sunlight as an energy source

  • Chlorophyll and other pigments capture photons

Photoautotrophs

• Plants, algae, and photosynthetic bacteria

• Serve as the energy source for a majority of the world’s ecosystems

Producers (Chemoautotrophs)

• Specialised bacteria that get their energy from inorganic chemicals (e.g., by oxidising sulphide minerals)

• Energy released by these exothermic reactions is used to drive biosynthesis of CO₂ and H₂O to form glucose

Hydrothermal Vent Ecosystem

• Supported by chemoautotrophic bacteria and organic material that sinks from the ocean’s surface

Deep-Sea Vents

• Species in deep-sea ecosystems have adapted to interact with each other in many ways - symbiosis between many species and chemosynthetic bacteria

Whale Falls

• Are an important influx of nutrients to the sun-starved deep ocean

Consumers (Heterotrophs)

• Consume rather than produce biomass energy as they metabolise, grow, and add to levels of secondary production

• There are different kinds of feeding relations: herbivory, carnivory, scavenging, and parasitism

The Ghost Plant (Monotropa uniflora)

• NOT an autotroph, is a parasitic mycoheterotroph and gets its energy from the surrounding trees, via a mycorrhizal fungus

• Lacks chlorophyll

• Occur in dark forests, where there isn’t enough light for photosynthesis

Decomposers - Fungi

• Heterotrophs that break down dead or decaying organisms

• Carry out decomposition, a process possible by only certain kingdoms, such as fungi

Fungi

• Are the primary decomposers in most environments

Decomposers - Detritivores

• Heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces)

• Ingest and digest dead matter internally

Decomposers - General

• Directly absorb nutrients through external chemical and biological processes

Decomposers - Scavengers

• Heterotrophs that consume dead organisms that have died from causes other than predation or have been killed by other predators

Scavenging

• Generally, refers to carnivores feeding on carrion, but is also a herbivorous feeding behaviour

Positions in a Food Web

1. Producers

2. Primary consumers

3. Secondary consumers

4. Tertiary consumers

5. Apex predators

Trophic Levels - Producers

• Plants and algae thats make their own food

• Level 1

Trophic Levels - Primary Consumers

• Herbivores that eat plants

• Level 2

Trophic Levels - Secondary Consumers

• Carnivores that eat herbivores

• Level 3

Trophic Levels - Tertiary Consumers

• Carnivores that eat other carnivores

• Level 4

Trophic Levels - Apex Predators

• Have no predators and are at the top of their food web

• Level 4

Second Trophic Level Example

• Rabbits eat plants at the first trophic level, so they are primary consumers

Third Trophic Level Example

• Foxes eat rabbits at the second trophic level, so they are secondary consumers

Fourth Trophic Level Example

• Golden eagles eat foxes at the third trophic level, so they are tertiary consumers

Decomposer Example

• The fungi on trees feed on dead matter, converting it back to nutrients that primary producers can use

Ecological Pyramids

• Are created due to inefficiency of energy transfers, the productivity declines with increasing trophic level

• If producers are productive but short-lived, their biomass may be less than that of the primary consumers

• Are not necessarily upright

Kinds of Ecological Trophic Pyramids

1. Pyramid of numbers

2. Pyramid of biomass

3. Pyramid of energy

Ecological Pyramid Example - Aquatic Ecosystem

• Sea lion (Top)

• Herring

• Zooplankton

• Phytoplankton (Bottom)

Ecological Pyramid Example - Terrestrial Ecosystem

• Snakes (Top)

• Mice

• Grasshoppers

• Grasses (Bottom)

Transfer Efficiency

• Proportion of energy transferred from one trophic level to the next

Lindeman’s 10% Law of Transfer of Energy

• Was not originally called a “law”, and was also cited ranging from 0.1% to 37.5%

• 10% of the transferred energy is stored as flesh

  • Remaining is lost during transfer (broken down in respiration, or lost to incomplete digestion by higher trophic level)

Food Chain Length

• Way of describing food webs as a measure of the number of species encountered as energy or nutrients move from the plants to top predators

Mean Chain Length

• Entire web is the arithmetic average of the lengths of all chains in a food web

Food Chain Length Example

• Simple predator-prey example

  • Deer is one step removed from the plants it eats (chain length = 1)

  • Wolf that eats the deer is 2 steps removed from the plants (chain length = 2)

Connectance

• Fraction of all possible links that are realised in a network

• C = L / [S(S - 1)/2]

  • L is the number of trophic links

  • S(S - 1)/2 is the maximum number of binary connections among S species

Mathematical Models on Networks

• Are used to determine structure, stability, and laws of food web behaviours relative to observable outcomes

Trophic Cascades

• Powerful indirect interactions that can control entire ecosystems

• Occur when a trophic level in a food web is suppressed

• Next lower trophic level is released from predation (or herbivory if the intermediate trophic level is a herbivore)

Trophic Cascade Example

• A top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behaviour of their prey

Top-Down Cascade Example

• Otters eat sea urchins but have declined due to overhunting and disease

  • Sea urchins eat kelp holdfasts, leading to a reduction in this ecosystem engineer

• Low sea otters → lead to sea urchins eating more kelp

Pacific Kelp Forest Trophic Environment

• Sea otters feed on sea urchins

• Sea otters have been hunted to extinction →

  • Sea urchins increase in abundance →

    • Kelp populations are then reduced

Yellowstone Park Trophic Environment

• Reintroduction of wolves (Canis lupus)

  • Reduced the number, and changed the behaviour of elk (Cervus canadensis)

    • Freed several plant species from grazing pressure

      • Led to the transformation of riparian ecosystems

Nutrients

• Substances necessary for the healthy physiology of organisms

• Must be sufficiently available

• The most limiting element that determines growth

Macronutrients

• Are required in larger quantities

• Carbon (C), Oxygen (O), and Hydrogen (H); form the backbone of most organic molecules

Photosynthesis

• How plants get Carbon, Oxygen, and Hydrogen

Mineral Nutrients

• Elements from the Earth and foods that organisms need to develop and function normally

• Examples: Nitrogen (N), Phosphorus (P), Calcium (Ca), Magnesium (Mg), Potassium (K), and Sulphur (S)

General Characteristics of Cycles

• Energy flows directionally through ecosystems. Entering as sunlight (or inorganic compounds) and leaving as heat during the many transfers between trophic levels

• Matter that makes up living organisms is conserved and recycled

Most Common Elements Associated with Organic Molecules

• Carbon (C), Nitrogen (N), Hydrogen (H), Oxygen (O), Phosphorus (P), and Sulphur (S)

• Can take various chemical forms, and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth’s surface

Biogeochemical Cycles

• The recycling of inorganic matter between living organisms and their environment

Anthropogenic

• Are human activities that alter all major ecosystems and the biogeochemical cycles they drive

Hydrosphere

• Where water movement and storage occurs

• Important for leaching certain components of organic matter into rivers, lakes, and oceans, and is a reservoir for carbon

Reservoir

• Place where a certain kind of material is stored, or resides, for some period of time

  • Place where material moves into and out of

Exchange Pool/Pool

• Holds an element or water for a short period of time

Exchange Pool Examples

• Atmosphere is an exchange pool for water - holds water (water vapour) for just a few days

• Glaciers; the soil layer; the aggregate of bodies of fresh water on the continents (rivers and lakes)

Flux

• Rate at which a given material moves between reservoirs

Steady State

• If the flux of material into and out of a given reservoir is the same for some period of time

  • Commonly, flux in and the flux out are not equal