Community Ecology

Chapter 8

COMMUNITY STRUCTURE AND SPECIES DIVERSITY

• Biological communities differ in their structure or spatial distribution.

Figure 7-2Figure 7-2

Community Structure has 4 characteristics:

• 1. Physical appearance: relative size, stratification & distribution of its populations and species

• 2. Species diversity or richness • 3. Species abundance• 4. Niche structure – number of ecological

niches, species interactions with similarities and differences

Species Diversity and Niche Structure: Different Species Playing

Different Roles• Biological communities differ in the types and

numbers of species they contain and the ecological roles those species play.– Species diversity: the number of different species it

contains (species richness) combined with the abundance of individuals within each of those species (species evenness).

Species Diversity and Niche Structure

• Niche structure: how many potential ecological niches occur, how they resemble or differ, and how the species occupying different niches interact.

• Geographic location: species diversity is highest in the tropics and declines as we move from the equator toward the poles.

What determines species diversity ?

• Edge Effect-the boundaries between two ecosystems (ecotones) may have a

different combination of species than those

found in the two ecosystems.• Forest-field ecotone and game species• The most species rich areas:

– Coral reefs– Large tropical lakes

Three main factors affect species diversity:

• 1. Latitude – decreases as moves away from equator

• 2. Depth in aquatic systems – increases from surface to 2000 m, then decreases until the sea bottom where diversity is usually high

• 3. Pollution in aquatic systems

Case Study:Species Diversity on Islands

• MacArthur and Wilson proposed the species equilibrium model or theory of island biogeography in the 1960’s.

• Model projects that at some point the rates of immigration and extinction should reach an equilibrium based on:– Island size– Distance to nearest mainland

TYPES OF SPECIES

• Native, nonnative, indicator, keystone, and foundation species play different ecological roles in communities.– Native: those that normally live and thrive in a

particular community.– Nonnative species: those that migrate,

deliberately or accidentally introduced into a community.

Indicator Species: Biological Smoke Alarms

• Species that serve as early warnings of damage to a community or an ecosystem.– Presence or absence of trout species because they

are sensitive to temperature and oxygen levels.– Canary in the Coal mine – they

sang untill increase in CO– Salmon and Stoneflies– Amphibians

Case Study: Why are Amphibians Vanishing?

• Frogs serve as indicator species because different parts of their life cycles can be easily disturbed.

Figure 7-3Figure 7-3

Case Study: Why are Amphibians Vanishing?

• Habitat loss and fragmentation.• Prolonged drought.• Pollution.• Increases in ultraviolet radiation.• Parasites.• Viral and Fungal diseases.• Overhunting.• Natural immigration or deliberate introduction

of nonnative predators and competitors.

Keystone Species-• Strong interaction with other species affect

the health and survival of the• other species. Significance is out of proportion

with their biomass…. In other words, a small biomass of these organisms have a large effect on the ecosystem.

• Pacific Northwest Millipede• They process material • out of proportion to • their numbers or• biomass

• Top Predators-• Interaction Between Species• Intraspecific Competition-• Territoriality-• Interspecific Competition-• Interference Competition-• Exploitation Competition

The Competitive Exclusion Principle• Says that two species sharing the same Resource

cannot coexist indefinitely in an ecosystem that does not have enough resources to meet the needs of both species.

Strategies that Reduce Competition• Resource Partitioning- share the wealth by

using a resource at• Different times• In different ways• In different places• Fundamental verses Realized • Niche

– Hawks and Owls– Lions and Leopards– Chum and Coho Salmon

Predator-Prey Relationships• On the population level predators are beneficial to prey• because the predator• Reduces the prey population giving remaining• prey greater access to the available food supply• Improve the genetic stock of the prey population• Examples of predator adaptations that maximize their• chances of getting a meal• Cheetahs-speed• Eagles• Wolves and Lions-teamwork• Snowy Owls

PREDATION• Some prey escape their

predators or have outer protection:

• Mimicry• Deceptive Looks• Deceptive Behavior• Warning Coloration• Camouflaged• Chemicals warfare to

repel predators.

Figure 7-8Figure 7-8

Symbiotic Relationships

• Parasitism- one benefits while the other is harmed (host) Usually draws nutrients from the host which weakens it, but seldom kills it.

• Some parasites live inside (tapeworm); others live outside (fleas and mosquitoes)

Mutualism: Win-Win Relationship• (a) Oxpeckers (or tickbirds) feed on

parasitic ticks that infest large, thick-skinned animals such as the endangered black rhinoceros.

(b) A clownfish gains protection

& food by living among deadly stinging sea anemones and helps

protect the anemones from some of their predators.

• (c) Beneficial effects of mycorrhizal fungi attached to roots of juniper seedlings on plant growth compared to

• (d) growth of such seedlings in sterilized soil without mycorrhizal fungi. Figure 7-9Figure 7-9

Mutualism-both benefit• Lichens are a relationship between a• fungus and an algae. The fungus supplies

_________ and receives___________ from the algae. The algae supplies ____________ and receives ___________ from the fungus– Rhizobium bacteria and the roots of legumes– Zooanthellae and coral polyps– Clownfish and sea anemones– Mycorrhizae fungi and the roots– of many trees

Commensalism: Using without Harming• Some species

interact in a way that helps one species but has little or no effect on the other.

Figure 7-10Figure 7-10

Ecological Succession –gradual change in species composition in a specific area.

• 1. Primary succession – establishing life on lifeless ground– Begins where there is no soil in terrestrial ecosystems – no bottom sediment in aquatic ecosystems

• Pioneer species start things off by getting a foothold on bare surfaces like rocks. Lichens and moss specialize at this.

• They secrete acids which begin to breakdown the rock and they trap wind blown soil and bits of organic matter.

Primary Succession: Starting from Scratch

• Primary succession begins with an essentially lifeless are where there is no soil in a terrestrial ecosystem

Figure 7-11Figure 7-11

Early successional plant species follow. Typically these:Grow close to the groundGrow quickly under harsh conditionsHave short lives Ex: include small perennial grasses, herbs, or ferns

Mid-successional species follow these which include less hardy species of grasses, herbs, and low shrubs

Late successional species are typically those that are adapted to the climate and soil type of the area and are typically trees

Secondary succession-begins in areas where an established natural community has been disturbedAbandoned farmlandsBurned or cut forestsHeavily polluted streamsLand that has been dammed or flooded

Secondary Succession: Starting Over with Some Help

• Secondary succession begins in an area where the natural community has been disturbed.

Figure 7-12Figure 7-12

Three species interactions involved in succession

1. Facilitation- one species makes an area suitable for another species with different niche requirements. Example… lichens, legumes & N 2. Inhibition- early species hinder the establishment and growth of otherspecies. Example… plants that release toxins 3. Tolerance- late successional stages are unaffected by earlier successionalstages. Example…

Disturbances and species diversity

Intermediate disturbance hypothesis - communities that experience fairly frequent,moderate disturbances have the greatest species diversity

Old View of SuccessionOrderly progression of successional stages building towards a stable community of a few long lived species known as a ______________ community.Instead, most disturbed communities result in ever-changing mosaic of vegetation patches at different successional stages.

ECOLOGICAL STABILITY AND SUSTAINABILITY

• Living systems maintain some degree of stability through constant change in response to environmental conditions through:– Inertia (persistence): the ability of a living system to

resist being disturbed or altered.– Constancy: the ability of a living system to keep its

numbers within the limits imposed by available resources.

– Resilience: the ability of a living system to bounce back and repair damage after (a not too drastic) disturbance.

ECOLOGICAL STABILITY AND SUSTAINABILITY

• Having many different species appears to increase the sustainability of many communities.

• Human activities are disrupting ecosystem services that support and sustain all life and all economies.

The Precautionary Principle• When evidence indicates that our actions may

harm the environment, even though all the “cause and effect” relationships have not been established

• between our actions and harm to the environment, it is better to take precautionary measures to prevent harm.– It is easier to prevent pollution than it is to clean it up

once the harm of that pollution has been established.– It is easier to protect ecosystems than it is to recreate

them once they have been destroyed.• How much time and effort should we put into

preventing Global Warming• when we are not even sure of the ultimate effects

of climate change ?