APES Chapter 8 Class Notes Community Ecology Community Structure:
3 characteristics to describe a community
1. physical appearance – sizes, stratification and distribution of populations and species
2. Species diversity – a combination of numbers of different species (richness) and abundance of individuals within each species (species evenness)
3. Niche structure: the number of niches, their resemblance or difference from each other and interaction of species with each other.
Location of World’s Biodiversity:
Species rich: trop rain forests, coral reefs, deep sea, and large tropical lakes. Generally the communities such as rain forests with large species richness tend to have low species evenness – only have a few members of each species. 3 major factors affecting diversity of species in a community
1. latitude in terrestrial communities - generally as you move away from the equator toward either pole the species diversity decreases. Increases with increases solar radiation, increasing precipitation, decreasing elevation and pronounced seasonal variation
2. Pollution in aquatic communities – both species diversity and species abundance decrease with pollution
3. Depth in aquatic systems – species diversity increases from the surface to a depth of 2,000 meters and then begins to decline with depth until the deep sea bottom is reached, where species diversity is very high.
Island biogeography theory ( species equilibrium model).
Basically tells us that the number of species on an island is determined by how fast new species arrive, how fast old species become extinct, the size of the island, and the distance from the mainland.
2 features of an island affect it immigration and extinction rates :
1. size – small islands have fewer species because it is a smaller target for potential colonizers. Also has a higher extinction rate because it has fewer resources and less diverse habitats.
2. Island’s distance from the mainland. An island closer to the mainland should have a greater immigration rate than one farther away.
Recently scientists have used this model to help protect wildlife in habitat island such as national parks surrounded by a sea of developed and fragmented land.
Types of Species:
1. Native species are those whose original home is in this particular ecosystem.
2. Other that have evolved in a different ecosystem and have migrated or were introduced to a new ecosystem are called non native, invasive or alien species.. Cane toads, kudzu, killer bees.
Usually thought of as detrimental but some are beneficial – domesticated animals and some crop plants. 3. Indicator species alert us to harmful changes taking place in the community.
a. Example presence or absence of trout in a water system. Indicators of water quality – specifically DO and temp. b. Birds and butterflies are excellent indicator species – they are everywhere and quickly affected by environmental change such as introduction of chemical pesticides and loss of habitat .
c. some amphibians (frogs, toads, and salamanders) are also considered indictor species
Frogs are good indicators because they are especially vulnerable to environmental changes at various points in their life cycle.
4. Keystone species help ecological communities run smoothly – they determine the type and number of species in a community. They play critical roles in the community.
Examples: pollination of flowering plant species by bees, hummingbirds, bats, and other species Top predator species play a role in regulating populations of other species. Wolf, leopard, lion Dung beetles – remove, bury and recycle animal waste – would be in a mess without them
Loss of a keystone species can lead to population crashes and extinction of other species in the community that depend on the keystone species for certain services.
5. Foundation species – play a role in shape communities by creating and enhancing habitat that benefits other species. Ex: elephants push over, break, or uproot tress, creating openings in the savanna grasslands and woodlands of Africa. Promotes growth of grasses that benefit animals such as antelope.
Species Interactions:
Intraspecific competition: Interactions within a species
Territoriality – a way members of the same species interact, mark and defend their territory. Ex: Robins will chase other robins away from their nesting sites.
5 basic types of interactions between species in a community:
1. Interspecific competition – most common – interaction between species for shared or scarce resources such as food, space, water, soil, sunlight.
In interspecific competition parts of the two (or more) species fundamental niches overlap.
Fundamental niche – the full potential range of resources a species would use if there were no competition The more the niches overlap, the more the species compete with each other.
4 outcomes:
1. One species could migrate
2. Shift of feeding habits and behavior through natural selection and evolution 3. Depletion of population
4. extinction of species Species compete with other by:
Interference competition – defense mechanisms Ex: plants release chemicals into the soil which inhibits or prevents growth of other competing species
Exploitation competition – equal access to resource, but one species can use it more quickly and efficiently
Principle of competitive exclusion: tells us that 2 species can not occupy exactly the same niche in a habitat where there is not enough of a particular resource to meet the needs of both species.
Species reduce or avoid competition by:
Resource partitioning – dividing up of a resource so that the different species use the resources at different times, in different ways, or in different places. Share the wealth.
Involves the difference between realized niche and fundamental niche.
Realized niche is simply the part of the fundamental niche that a species actually uses. Species becomes more specialized as to what part of the niche it uses.
Fig. 8-10
2. predation –members of one species (predators) feed directly on all or part of another species (prey). Prey may or may not die. One benefits, the other is harmed.
On an individual level – prey is harmed.
On a population level – species may benefit. Sick , weak, or old are eliminated from the pop which serves the remaining by reducing competition for food and improving the genetic makeup of the pop.
Strategies of predators include: speed (cheetahs), keen eyesight (hawks), hunting in packs (wolves), camouflage ( snowy owls in snow-covered areas).
Prey strategies for defense include: speed, ability to fly, keen sense of smell or sight which alert them to the predator, protective shells (turtles and armadillos), thick bark, spines or thorns, camouflage or ability to change color (chamelions), chemical warfare like poisons, irritants, four smells. Defensive chemicals made by plants include: cocaine, caffeine, nicotine, cyanide, opium.
Warning coloration: predators will avoid these prey
Mimicry – some prey species which have no other defense strategies will take on the appearance of ones that do. Thereby, the predators are tricked into avoiding them. Nonpoisonous viceroy butterfly looks like the poisonous monarch.
Behavioral strategies: blowfish puffup, peacocks spread their wings. Other live in groups which offers some protection.
Next 3 types of species interaction fall under the term
Symbiosis: relationships in which species live together in an intimate relationship.
3. Parasitism: one species (parasite) feeds on another species (host) by living on or in it. A bit like predator/prey but the parasite is smaller than the host, stays with the host and usually weakens it but rarely kills it.
Ex: Tapeworms live inside hosts, Ticks, fleas, fungi live outside host
4. Mutualism: two species act in a relationship that benefits both species Ex: Pollination mutualism – between flowering plants and insects, bird, and bats Nutritional mutualism – Lichens consist of algae and fungi living together Fungi provide shelter (structure), hold moisture and collect nutrients Algae use photosynthesis to provide food
Legume family – root nodules have bacteria that convert atmospheric nitrogen into a form usable by the plants (nitrogen fixation) and the plants provide the bact. With simple sugars.
Coral reefs – algae and polyps
5. Commensalism –benefits one species but does not harm or benefit the other. Ex: remora on sharks
Epiphytes on trees – orchids attach to trees, benefit from having a base on which to grow and living in an elevated spot but does not harm the tree.
Ecological Succession:
With new environmental conditions, community structures can change; ones groups of species is replaced by another. Ecological succession is the gradual change in species composition of a given area.
Two types of ecological succession based on the conditions present at the beginning of the process.
1. Primary succession – Fig 8-14 establishment of biotic communities on lifeless ground – no soil in a terrestrial
ecosystem, no bottom sediment in an aquatic system. Specific ex: lava flow, abandoned highway or parking lot, retreating glacier.
Long process – have to have soil – may take hundreds or thousands of years.
Soil formation begins with pioneer species – these species attach themselves to bare rock.
Ex: lichens and mosses work by trapping windblown soil particles, secreting acids to break down rock, and produce organic materials. Uses chemical and physical weathering.
As soil is produced tiny annuals replace the lichen and mosses. What are annuals? Live a year but produce seeds that can germinate for the next year.
These are replaced by small perennial grasses. What are perennials? Plants that can live for >2 years without having to reseed. These are called Early successional Plants.
Grow low to the ground, can live under harsh conditions, and have short lives. Can help break up the rock into more soil particles. Also add more nutrients to the soil.
Mid successional plant species – herbs, grasses, low shrubs
These are replaced by trees that need sunlight pine, black spruce and aspen
As these trees grow and produce shade they are replace by late successional plants – shade tolerant trees. Birch and white spruce
Bare rocklichens and mosses small herbs and perennial grassesherbs, grasses,low shrubs Trees needing sunlightShade tolerant trees
Primary succession can also take place in newly created small ponds marsh dry land
2. Secondary succession: Fig 8-15 takes place where there is soil or bottom sediment. Has been a biotic community previously. Includes abandoned farmlands, burned or cut forests, land that has been dammed or flooded, heavily polluted streams.
Fig 8-15 shows secondary succ in NC. Abandoned farmlands
Starts with annual weeds perennial weeds and grasses shrubs pine forest oak-hickory forest Succession generally discusses plants, but with the plant changes you also see changes in the animal community. See fig. 8-16
How do species replace one another in succession. 3 factors that affect how and at what rate succession occurs:
1. facilitation – one set of species makes the area suitable for another lichens build up soil which makes the habitat suitable for grasses. 2. Inhibition – a species will hinder the establishment of another species.
3. Tolerance – late successional plants are largely unaffected by plants at earlier stages of succession. How do disturbance affect succession and species diversity?
Changes in environmental conditions that disrupt a community can set back succession.
Ex: Fire, drought, mining, plowing climate change can move succession back to an earlier stage. Can be natural or anthropogenic. Not all disturbances are detrimental – some can create unfilled niches or release nutrients.
Predictability of Succession:
Under the old Balance-of- nature view, a large terrestrial community undergoing succession eventually became covered with and expected type of climax community.
Ecologists have changed their views about this orderly, expected sequence. They now view a community as in continuous change, with instability.
Say that succession reflects a struggle for each species to obtain food, light, nutrients, and space to gain an advantage by occupying as much of its fundamental niche as possible.
Ecological Stability and Sustainability:
Living organisms maintain some degree of stability or sustainability through constant change in response to changing enviro conditions. 3 aspects of stability:
1. Inertia or persistance – the ability of a living system to resist beig disturbed or latered
2. Constancy – ability of a living system such as a population to keep its numbers within the limits imposed by available resources 3. Resilience – ability to bounce back after a disturbance.
Species diversity and Stability:
We know that there is a minimum threshold of diversity – no community can survive without some producers and decomposers Some communities obtain stability by having many different species present.
This is not always true.
Ex: compare the destruction of a rainforest with high diversity to the destruction of grasslands with lower diversity. Rainforest may not be able to recover when severely degraded (nutrients are tied up in the vegetation)j
The grassland however may recover if the root systems are not destroyed.
But beyond this general knowledge, ecologists do not know whether simple ecosystems are less stable than complex ones. Don’t really know how much biodiversity is needed to maintain stability.
Part of the difficulty in knowing this lies in the fact that nature is in a constant state of change. Protecting Natural Systems:
Argument ongoing between developers and ecologists:
Developers say if nature is unpredictable then there is no point in trying to maintain and preserve. Don’t see a problem with converting grasslands to croplands, draining and filling wetlands, etc.
Ecologists say that human disturbances are disrupting ecosystem services that sustain life. They say that because we don’t know everything about the natural world around us that we should be careful to prevent harm to our environment. Called the precautionary principle.
Another way to put it is “better safe than sorry”.
In summary , in this chapter we’ve learned that the interconnectedness of species, communities, and ecosystems are vital processes for life on Earth.
Final Notes for Chapter 8
How do disturbance affect succession and species diversity?
Changes in environmental conditions that disrupt a community can set back succession.
Ex: Fire, drought, mining, plowing climate change can move succession back to an earlier stage. Can be natural or anthropogenic. Not all disturbances are detrimental – some can create unfilled niches or release nutrients.
According to the intermediate disturbance hypothesis communities that experience fairly frequent but moderate disturbances have the greatest species diversity.
Predictability of Succession:
Under the old Balance-of- nature view, a large terrestrial community undergoing succession eventually became covered with an expected type of climax community.
Ecologists have changed their views about this orderly, expected sequence. They now view a community as in continuous change, with instability.
Say that succession reflects a struggle for each species to obtain food, light, nutrients, and space to gain an advantage by occupying as much of its fundamental niche as possible.
Ecological Stability and Sustainability:
Living organisms maintain some degree of stability or sustainability through constant change in response to changing enviro. conditions. 3 aspects of stability:
1. Inertia or persistence – the ability of a living system to resist being disturbed or altered
2. Constancy – ability of a living system such as a population to keep its numbers within the limits imposed by available resources 3. Resilience – ability to bounce back after a disturbance.
Species diversity and Stability:
We know that there is a minimum threshold of diversity – no community can survive without some producers and decomposers Some communities obtain stability by having many different species present.
This is not always true.
Ex: compare the destruction of a rainforest with high diversity to the destruction of grasslands with lower diversity. Rainforest may not be able to recover when severely degraded (nutrients are tied up in the vegetation)
The grassland however may recover if the root systems are not destroyed.
But beyond this general knowledge, ecologists do not know whether simple ecosystems are less stable than complex ones. Don’t really know how much biodiversity is needed to maintain stability.
Part of the difficulty in knowing this lies in the fact that nature is in a constant state of change. Protecting Natural Systems:
Developers say if nature is unpredictable then there is no point in trying to maintain and preserve. Don’t see a problem with converting grasslands to croplands, draining and filling wetlands, etc.
Ecologists say that human disturbances are disrupting ecosystem services that sustain life. They say that because we don’t know everything about the natural world around us that we should be careful to prevent harm to our environment. Called the precautionary principle.
Another way to put it is “better safe than sorry”.
In summary , in this chapter we’ve learned that the interconnectedness of species, communities, and ecosystems are vital processes for life on Earth.
