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CHAPTER 57 LECTURE SLIDES Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Interspecific Interactions and the Ecology of Communities Chapter 57 2 Biological Communities • Community – Species that occur at any particular locality – Characterized by • Species richness –Number of species present • Primary productivity –Amount of energy produced – Interactions among members govern many ecological and evolutionary processes 3 4 Biological Communities • Two views of structure and functioning of communities – Individualistic concept: a community is nothing more than an aggregation of species that happen to occur together at one place – Holistic concept: a community is an integrated unit; superorganism – more than the sum of its parts 5 Biological Communities • Most ecologists today favor the individualistic concept • In communities, species respond independently to changing environmental conditions • Community composition changes gradually across landscapes 6 • Abundance of tree species along a moisture gradient in the Santa Catalina Mountains of Southeastern Arizona • Each line represents the abundance of a different tree species • Community composition changes continually along the 7 gradient Biological Communities • Sometimes the abundance of species in a community does change geographically in a synchronous pattern • Ecotones: places where the environment changes abruptly 8 Ecological Niche • Niche: the total of all the ways an organism uses the resources of its environment – Space utilization – Food consumption – Temperature range – Appropriate conditions for mating – Requirements for moisture and more 9 Ecological Niche • Interspecific competition – Occurs when two species attempt to use the same resource and there is not enough resource to satisfy both • Interference competition – Physical interactions over access to resources • Exploitative competition – Consuming the same resources 10 Ecological Niche • Fundamental niche – Entire niche that a species is capable of using, based on physiological tolerance limits and resource needs • Realized niche – Actual set of environmental conditions, presence or absence of other species, in which the species can establish a stable population • Other causes of niche restriction – Predator absence or presence – Absence of pollinators – Presence of herbivores 11 Ecological Niche J.H. Connell’s classical study of barnacles 12 Ecological Niche • Principle of competitive exclusion • If two species are competing for a limited resource, the species that uses the resource more efficiently will eventually eliminate the other locally • G.F. Gause’s classic experiment on competitive exclusion using three Paramecium species shows this principle in action 13 14 Ecological Niche • Paramecium caudatum and P. bursaria – Expected same results – one winner – Both species survived by dividing resources • Realized niche did not overlap too much 15 • Resource partitioning among sympatric lizard species • Subdivided niche to avoid direct competition 16 • Resource partitioning is often seen in similar species that occupy the same geographic area – Thought to result from the process of natural selection • Character displacement – Differences in morphology evident between sympatric species – May play a role in adaptive radiation 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Question: Does interspecific interaction occur between rodent species? Hypothesis: The larger kangaroo rat will have a negative effect on other species. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Question: Does interspecific interaction occur between rodent species? Hypothesis: The larger kangaroo rat will have a negative effect on other species. Experiment: Build large cages in desert areas. Remove kangaroo rats from some cages, leaving them present in others. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING Number of Captures Of Other Rodents Question: Does interspecific interaction occur between rodent species? Hypothesis: The larger kangaroo rat will have a negative effect on other species. Experiment: Build large cages in desert areas. Remove kangaroo rats from some cages, leaving them present in others. Result: In the absence of kangaroo rats, the number of other rodents increases quickly and remains higher than in the control cages throughout the course of the experiment. kangaroo rats removed kangaroo rats present 15 10 5 0 1988 1989 1990 Ecological Niche Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SCIENTIFIC THINKING • Seed-eating rodents and Kangaroo rats – 50 m x 50 m enclosures – Enclosures had openings large enough for seedeating rodents but not the Kangaroo rats – Monitor the number of small rodents • Results indicate that kangaroo rats compete with the other rodents and limit their population sizes 21 Number of Captures Of Other Rodents Question: Does interspecific interaction occur between rodent species? Hypothesis: The larger kangaroo rat will have a negative effect on other species. Experiment: Build large cages in desert areas. Remove kangaroo rats from some cages, leaving them present in others. Result: In the absence of kangaroo rats, the number of other rodents increases quickly and remains higher than in the control cagesthroughout the course of the experiment. kangaroo rats removed kangaroo rats present 15 10 5 0 1988 1989 1990 Interpretation: Why do you think population sizes rise and fall in synchrony in the two cages? Predator–Prey • Predation – Consuming of one organism by another • Predation strongly influences prey populations 22 Predator–Prey • Prey populations can have explosions and crashes – White-tailed deer in Eastern U.S. – Introduction of rats, dogs, cats on islands – New Zealand: Stephen Island wren extinct because of a single cat • Predation and coevolution – Predation provides strong selective pressure on the prey population – Features that decrease the probability of capture are strongly favored – Coevolution race may ensue 23 Predator–Prey • Plants adapt to predation (herbivory) by evolving mechanisms to defend themselves – Chemical defenses: secondary compounds • Oils, chemicals to attract predators to eat the herbivores, poison milky sap, and others – Herbivores coevolve to continue eating the plants 24 • Chemical defenses in animals – Monarch butterfly caterpillars feed on milkweed and dogbane families – Monarchs incorporate cardiac glycosides from the plants for protection from predation – Butterflies are eaten by birds, but the Monarch contains the chemical from the milkweed that makes the birds sick 25 Predator–Prey Poison-dart frogs of the family Dendrobatidae produce toxic alkaloids in the mucus that covers their brightly colored skin 26 • Defensive coloration – Insects and other animals that are poisonous use warning coloration – Organisms that lack specific chemical defenses are seldom brightly colored • Camouflage or cryptic coloration help nonpoisonous animals blend with their surroundings • Camouflaged animals do not usually live together in groups 27 Predator–Prey • Mimicry allows one species to capitalize on defensive strategies of another – Resemble distasteful species that exhibit warning coloration – Mimic gains an advantage by looking like the distasteful model – Batesian mimicry • Mimics look like distasteful species – Müllerian mimicry • Several unrelated but poisonous species come to resemble one another 28 29 Species Interactions • Symbiosis – 2 or more kinds of organisms interact in more-or-less permanent relationships – Potential for coevolution – Three major types of symbiosis • Commensalism • Mutualism • Parasitism 30 Species Interactions • Commensalism benefits one species and is neutral to the other – Spanish moss: an epiphyte hangs from trees 31 Species Interactions • When commensalism may not be commensalism • Oxpeckers and grazing animals – Oxpeckers eat parasites off of grazers – Sometimes pick scabs and drink blood – Grazers could be unharmed by the insects the oxpeckers eat 32 Species Interactions • Mutualism benefits both species – Coevolution: flowering plants and insects • Ants and acacias –Acacias provide hollow thorns and food –Ants provide protection from herbivores 33 Species Interactions • Not all ant and acacia relationships are mutualism – In Kenya, several species of ants live on acacias • One species clips the acacia branches to prevent other ants from living in the tree • Clipping branches sterilizes the tree • A parasitic relationship 34 Species Interactions • Parasitism benefits one species at the expense of another – External parasites • Ectoparasites: feed on exterior surface of an organism • Parasitoids: insects that lay eggs on living hosts –Wasp, whose larvae feed on the body of the host, killing it 35 External parasite: the yellow vines are the flowering plant dodder; it is a parasite that obtains its food from the host plant it grows on 36 Species Interactions • Internal parasites – Endoparasites live inside the host – Extreme specialization by the parasite as to which host it invades – Structure of the parasite may be simplified because of where it lives in its host – Many parasites have complex life cycles involving more than one host 37 Species Interactions • Dicrocoelium dendriticum is a flatworm that lives in ants as an intermediate host with cattle as its definitive host • To go from the ant to a cow, it changes the behavior of the ant • Causes the ant to climb to the top of a blade of grass to be eaten with the grass 38 Species Interactions • Ecological processes have interactive effects – Predation reduces competition • Predators choice depends partly on relative abundance of the prey options • Superior competitors may be reduced in number by predation • This allows other species to survive when they could have been outcompeted 39 Species Interactions Starfish eat barnacles, allowing other species to thrive instead of being crowded out by the explosive population of barnacles 40 Species Interactions • Parasitism may counter competition – Parasites may affect sympatric species differently, changing the outcome of interspecific interactions – Flour beetles and a competition experiment • Without a parasite T. castaneum is dominant • With the parasite: T. confusum is dominant 41 Species Interactions • Indirect effects: presence of one species may affect a second by way of interactions with a third species 42 Species Interactions • Keystone species – Species whose effects on the composition of communities are greater than one might expect based on their abundance – Sea star predation on barnacles greatly alters the species richness of the marine community – Keystone species can manipulate the environment in ways that create new habitats for other species • Beavers 43 Beavers construct dams and transform flowing streams into ponds, creating new habitats for many plants and animals 44 Succession and Disturbance • Succession – Communities have a tendency to change from simple to complex • Primary succession occurs on bare, lifeless substrate –Open water –Rocks – Organisms gradually move into an area and change its nature 45 46 Succession and Disturbance • Secondary succession – Occurs in areas where an existing community has been disturbed but organisms still remain • Field left uncultivated • Forest after a fire • Succession happens because species alter the habitat and the resources available in ways that favor other species entering the habitat 47 Succession and Disturbance • Three dynamic concepts in the process – Tolerance: early successional species are characterized by r-selected species tolerant of harsh conditions – Facilitation: early successional species introduce local changes in the habitat. Kselected species replace r-selected species – Inhibition: changes in the habitat caused by one species inhibits the growth of the original species 48 Succession and Disturbance • Animal species in a community can also change over time – Krakatau island • Volcanic eruption • Fauna changed in synchrony with the vegetation • Changes in animals affect plant occurrences; pollination, animal dispersion 49 Succession and Disturbance Succession after a volcanic eruption 50 Succession and Disturbance • Communities are constantly changing as a result of – Climatic changes – Species invasions – Disturbance events • Nonequilibrium models that emphasize change rather than stability are used to study communities and ecosystems 51 Succession and Disturbance • Intermediate disturbance hypothesis – Communities experiencing moderate amounts of disturbance will have higher levels of species richness than communities experiencing either little or great amounts of disturbance • Patches of habitat will exist at different successional stages • May prevent communities from reaching the final stages of succession 52 Succession and Disturbance • Disturbance is common, rather than exceptional in many communities • Understanding the role that disturbances play in structuring communities is an important area of ecology 53