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ES101 Study Guide - Final Guide: Keystone Species, Insular Biogeography, Species Evenness

Environmental Studies
Course Code
Edmund Okoree
Study Guide

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Lesson 6/Chapter 8
Ecologists use three characteristics to describe a biological community:
Physical appearance – relative sizes, stratification, and distribution of its
populations and species for various terrestrial communities
There are also differences in the physical structures and zones of
communities in aquatic life zones (such as oceans, rocky shores, sandy
beaches, wetlands)
Physical structure within a particular type of community/ecosystem can vary
(such as in size)
Community structure also varies around its edges where one type of
community makes a transition to a different type of community
E.g. the edge area between a forest and an open field may be
sunnier/warmer/drier than the forest interior and have a different
combination of species
Increased edge area from habitat fragmentation makes many species
more vulnerable to stresses such as predators and fire, and also creates
barriers that can prevent some species from colonizing new areas and
finding food/mates
Species diversity – combination of its number of different species (species
richness) and the abundance of individuals within each of its species (species
E.g. 2 communities each with a total of 20 different species and 200
individuals (same species diversity). But they could differ in species richness
and species evenness (Community A has 10 individuals in each of its 20
species, and community B has 10 species with 2 individuals each and 10
other species each with 18 individuals)
Niche structure – number of ecological niches, how they resemble or differ from
each other, and how species interact with one another
Most species-rich environments are tropical rain forests, coral reefs, deep
sea, and large tropical lakes
Communities with a larger number of different species (high species richness
such as rainforests) generally only have a few members of each species (low
species evenness)
Several factors affect the species diversity in communities:

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Latitude – distance from the equator in terrestrial communities (for most
plants/animals, species diversity is highest in the tropics and declines from the
equator to the poles)
Pollution (in aquatic systems)
Habitat diversity, NPP, habitat disturbance, time
In 1960s, Robert MacArthur and Edward O. Wilson studied communities on islands to
discover why large islands tend to have more species of a certain category than do
small islands
Species equilibrium model (or theory of island biogeography) – the number of species
on an island is determined by how fast new species arrive and old species become
extinct, the island’s size, and how far it is from the mainland
Native species – those that normally live and thrive in a particular community
Non-native (or exotic), invasive, or alien species – others that evolve somewhere else
and then migrate into or are deliberately or accidentally introduced in a community
Sometimes a non-native species can thrive and crowd out native species (e.g. Brazil
imported wild African bees to help increase honey production but they displaced
domestic honeybees and reduced honey supply – killer bees)
Indicator species – species have exacting requirements or specific sensitivities that
make them useful indicators in terms of the presence or absence of those conditions
(strong associations, absence/presence can indicate boreal forests, water quality etc)
Amphibians (frogs, toads, salamanders) vanishing due to habitat loss, pollution, fungus,
radiation, overhunting, etc – their extinction suggests that environmental quality is
deteriorating in parts of the world because they’re sensitive biological indicators of
changes in environment
Keystone species – remove, bury, and recycle animal wastes or dung - top predator
keystone species feed on and help regulate populations of other species (such as
pollination of flowering plants by trees etc)
Foundation species – create and enhance habitats that can benefit other species in a
community (e.g. elephants push over, break, uproot trees to create forest openings)
Five basic types of interactions between species:
Interspecific competition – most common interaction between species:
competition for shared or scarce resources such as space and food
Resource partitioning – some species competing for the same resources
evolve adaptations that reduce or avoid competition – evolve more
specialized traits that allow them to use shared resources at different times,
ways, and places (lions take the larger animals while leopards take smaller
ones; hawks hunt during the day and owls at night)

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Predation – members of one species (the predator) feed directly on all or part of
a living organism of another species (the prey)
Predator-prey relationship
At the individual level, prey species are harmed, but at the population level,
predation plays a role in evolution by natural selection
Predation can benefit the prey species because predators often kill the
sick/weak/aged/least fit of a population (giving the remaining better access to
food supplies and prevents excessive population growth)
Some shark species eat and remove sick and injured ocean animals and
some can help us learn how to fight cancer and immune system disorders
Herbivores can walk, swim, or fly up to the plants they feed on
Carnivores feeding on prey have 3 main options: ambushing, stalking,
Ambushing: liying in wait for prey to come within range (least energy
Stalking: slowly gaining close proximity to the prey, followed by quick rush
Pursuing: chasing down (most energy)
Some use predators use chemical warfare – using venom to paralyze
Prey escape their predators or have protective shells or thorns, some
camouflage, and some use chemicals to repel or poison predators (chemical
Parasitism – when one species (the parasite) feed on part of another organism
(the host), usually by living on or in the host
Can be viewed as special form of predation
Tapeworms and other endoparasites live inside their hosts
Ectoparasites attach themselves outside hosts (fleas, mosquitoes)
Parasites harm the hosts but promote biodiversity by helping keep some
species from becoming plentiful that they eliminate others
Mutualism – 2 species interact in a way that benefits both
Pollination mutualism between flowering plants and animals
Nutiritional mutualism – hardy species that can grow on trees or barren rocks
Gut inhabitant mutualism
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