(4/17) Lecture + Posted Notes: Ecosystem Engineers and Community Stability

Speech Recognition Diversity and Key Species in Ecosystems

• Community Impact

  • Some individual species can significantly affect their community.

  • These species are called keystone species and play an essential role within their ecosystem.

  • Paradoxically, these species often have a low biomass yet exert a substantial impact on their community's stability and diversity.

• Understanding Biomass

  • Biomass is a term describing the total mass of living matter within a given area, often expressed in terms of weight per unit area.

  • Example: If we survey a specific area for red-backed salamanders, the biomass would be the total weight of all sampled salamanders combined.

  • Important to note that biomass is proportionate; for example, the biomass of plants in an ecosystem is usually greater than that of herbivores, which is greater than that of carnivores.

• Keystone Species vs. Ecosystem Engineers

  • Keystone Species

  • Have relatively low biomass but a disproportionate effect on species richness.

  • Their presence increases the number of species in a community, whereas their absence significantly reduces species richness.

  • Ecosystem Engineers

  • Change their environment (for good or bad) significantly but do not necessarily have a low biomass.

  • Influence habitat structures, which can alter species diversity but not directly related to biomass alone.

  • Examples include corals (as autogenic engineers) versus beavers (as allogenic engineers).

• Understanding Species Diversity

  • Species Richness vs. Species Diversity

  • Species Richness: The count of different species present within a given area.

  • Species Diversity: Considers both species richness and the relative abundance of each species.

  • Change in species diversity reflects changes in abundance rather than mere increases or decreases—"Diversity only ever changes."

• Types of Ecosystem Engineers

  • Autogenic Engineers

  • Create structural habitats just by growing larger; for example, coral reefs and kelp forests.

  • Their growth leads to habitat complexity, thereby attracting more species.

  • Allogenic Engineers

  • Change the environment through their activities, like beavers that create ponds and wetlands.

  • They carve out habitats that various organisms may use, influencing community structure.

• Beavers as Ecosystem Engineers

  • Beavers cut down trees and build dams, creating ponds that serve as secure habitats for various species.

  • As they alter the landscape, they promote new growth and increase the abundance of nitrogen-rich plants, benefiting the community.

  • Their activities can vastly change stream systems into pond systems, thereby raising the biomass and changing the community dynamics.

  • Example of changes due to beaver activities: Slow water flow leads to increased sediment retention and new species introduction.

• Consequences of Beaver Activity

  • Decreased water flow speed upstream, leading to flooded areas that were previously dry, promoting new growth and nutrient retention.

  • Growth of water plants results in changes to food webs, influencing herbivore dynamics.

  • When beaver populations decline or they leave, they set the stage for a fertile environment conducive to secondary succession, where new species can thrive and establish.

• Summary of Ecosystem Changes

  • The structural changes brought about by beavers create various new niches, thus changing the biodiversity in those environments.

  • They can either enhance or decrease the overall equilibrium of a community but ultimately promote a dynamic environment rich in diverse life forms.

Lecture Week 13 (4/17)

Community Ecology: Succession

I. Introduction

A. As we know from our classes on Evolution, Mendelian Genetics, Population

Genetics, Population Ecology, and Exploitative and Mutualistic Interactions the biotic

elements of our environment are continually changing. Across these lectures, we

have changed scale from what happens to the individual (Selection, Mendelian

Inheritance), to the population (Evolution, Population Genetics, Population Ecology),

and to between species (Competition, Exploitation and Mutualism). We are now

going to change scale again and consider events at the Community level.

B. Community Ecology may be defined as how species interact within a community.

And there is some truth that the interspecific interactions we have covered thus far

represent some understanding of community ecology. However, in application,

community ecologists are as concerned with the community as a whole as they are

with the individual interacting parts. That is, community ecologists are interested in

the species diversity within a community and how that diversity changes over time,

and what causes those changes (or how might we predict those changes). Community

ecology is the study of how a community, as a whole, functions and changes over

time.

C. Before we start talking in earnest about community ecology, we must first determine

what is meant by “species diversity”. Species Diversity refers to the number and

relative abundance of species in an area. Whereas Species Richness refers to the

number of different species in an area. Typically, people are really referring to

species richness when they say “species diversity”. As an example, consider the two

forests of equal size, and numbers of trees within those forests, below:

Forest #1 Forest #2

Species Number Species Number

Eastern Hemlock 15 Eastern Hemlock 33

Sugar Maple 59 Sugar Maple 20

American Beech 37 American Beech 58

Total 111 111

Both forests have 3 species in them. Therefore, they have the same species richness.

Additionally, they have the same total number of individual trees (111). However,

the relative numbers of each species is different between them (e.g., over 50% of

Forest #1 is made of sugar maples, where sugar maples represent fewer than 20% of

the trees in forest #2). Therefore, the two forests have a different species diversity.

When people say that we need to “maintain species diversity” in a particular

ecosystem, or a “lack of diversity” is the downfall of an ecosystem, they are really

talking about species richness.

Is this too picky? Maybe, but consider this. In order to take care of the planet, we all

need to know what others are talking about. We cannot afford to get lost in

conceptual confusion. The potential costs are too great.

II. Community Change

A. Succession is the gradual change in an area over time. That is, it is the changing of

communities, in species diversity and richness. When change occurs, different

species may be present in an area, where they were not before, and relative abundance

of existing species may be different as well.

1. Successional study may come in the form of Vegetative Succession, Animal

Succession, or Geologic Succession. Within these categories, you may also

consider things such as Stream Succession, Old Field Succession, and Aquatic

Plant Succession, to name a few. Literally, any change over time in an area

can be a study in succession.

2. When people talk of succession, they generally mean vegetative succession

for a couple of reasons. First, it’s one of the more well-studied forms of

succession, and we are at the point where we can make generalizations about

succession based on data from vegetative succession studies. Second,

vegetative succession tends to be more predictable than other forms of

succession. Third, with changes in vegetation over time, you typically get

predictable changes in other areas as well. For example, with the change from

old field to shrubs to pioneer trees, you tend to get animals (vertebrates and

invertebrates) that favor each vegetation stage only, and the diversity and

richness of animal species changes with the vegetation over time.

B. Types of Succession:

1. Primary Succession: results from the creation of new substrates, such as by

volcanoes (deposition of volcanic ash), shifting sand dunes, and glaciers. No

original soil remains and new soil must be created again by pioneer plants and

their decomposition.

2. Secondary Succession: arises when the community is diminished but the soil

remains intact, such as following wind, fire, overgrazing, logging and

hurricanes, to name a few.

a. Primary succession starts with bare rock (or rock pieces) continues

through lichens and mosses then to herbaceous species.

b. Secondary succession starts with exposed soil and moves into

herbaceous species.

c. Example: Glacier Bay, Alaska

3. Traditionally, vegetative succession follows a specific pattern of change. For

example: bare soil to herbs to shrubs to pioneer trees to intermediate trees to

climax forest.

4. A local community is often defined by what is or will be the species number

and composition of the late successional stage (climax) at that location. For

example, the area around the Binghamton University campus is considered a

sugar maple, hemlock forest even though there are many areas on and around

campus that do not include maples or hemlocks. The assumption is that if the

natural areas here were left to change on their own, they would end up being

dominated by maples and hemlocks.

5. Be careful! It is incorrect to think of succession as necessarily leading to a

local climax forest, and it is incorrect to assume that succession must follow

the progression described in #3 above. It is entirely likely that a local habitat

will never reach the climax stage typified in the area, especially if repeated

disturbances interrupt the flow of succession. Additionally, successional

stages may be skipped or repeated, depending on local circumstances. The

pattern presented in #3 above is considered an idealized model of change

only.

6. Therefore, you can say that the pathway, or progression, of succession in any

area is influenced by two general factors: Randomness and the History of the

area. Over time, events may randomly occur that destroy the current suite of

community members (e.g., say a fire burns a forest down). As a result,

succession doesn’t happen in the predicted way. Additionally, certain stages

may be skipped because there may not be source populations of the species of

that stage to help colonize the area (a reflection of the history of the area).

C. 3 mechanisms describing succession: How does succession take place?

1. Facilitation: One species makes it easier for subsequent species to live in the

same habitat. For example herbaceous species like legumes help to fix

nitrogen and make it available to other species. Shrubs and trees would not be

able to exist in areas without this preparation of the soil.

2. Tolerance: Some species show greater tolerance of extremes in variation in

abiotic factors involving light, temperature, wind and dryness. Later

succession stages show greater tolerance of a variety of abiotic factors, and

are thus better competitors for resources. For example, shrubs are more

tolerant of shade than herbs. Pioneer trees are more tolerant of shade than

shrubs are, etc.

3. Inhibition: early succession plants are inhibited by later succession plants in a

mature community (late succession species are more tolerant of subdued light

(as in the shade example in #2 above)). Also, some herbaceous species inhibit

the growth of shrubs by outcompeting young shrubs for resources.

4. There has been some debate about which of the 3 mechanisms is the primary

driver of succession. However, generally, all three play a part in successional

change in most habitats.

III. Stability in Communities

Stability in a community refers to the ability of the community to resist abrupt

changes in species diversity and richness. Generally, a stable community is one that

is thought to be healthy. Abrupt changes here include the addition of an invasive,

foreign species into, or the extinction of a species from, the community.

A. Co-evolutionary relationships between species tend to add stability, because each

buffers the other from the adverse effect of many biotic and abiotic factors in a

community.

B. Keystone species are species that have a greater impact on the diversity and species

richness in their community than you would predict strictly based on their biomass.

For example see Sea stars.

C. Species richness: the greater the number of species in a community, the greater the

number of checks and balances on species numbers, and the less likely it is that the

community will experience any change in species richness or composition.

Primary Succession

• Results from the creation of new substrates (e.g., volcanic eruptions, glacial movement) where no original soil remains.

• Begins with bare rock or rock fragments.

• Pioneer plants (like lichens and mosses) colonize the area, eventually leading to soil formation through decomposition.

Secondary Succession

• Occurs in areas where a community has been disturbed but where soil and some organisms still remain (e.g., after fire, windstorms, or human activities like logging).

• Starts with exposed soil and moves into herbaceous species, eventually leading to a mature community.

• Example: Following a forest fire, the area may quickly regrow with herbaceous plants and later successively with shrubs and trees.