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7.014 Lectures 33,34,35 Species Interactions May 5, 7, 10 2004 Types of Competition Consumptive competition Preemptive competition Overgrowth competition Chemical competition Territorial competition Encounter competition 1 The Fundamental Ecological Niche: “An n-dimensional hyper-volume every point on which a species can humidity survive and reproduce indefinitely in the absence of other species” (Hutchinson) tem per atu re od Fo e siz The Realized Ecological Niche: the niche actually occupied in the presence of other species humidity niche overlap leads to competition tem per atu re od Fo e siz 2 Figure 49.13a Number consumed One species eats seeds of one size range Seed size (one niche dimension Figure 49.13b Partial niche overlap: competition for seeds of intermediate size Partial niche overlap can lead to Niche Partitioning and Competitive Coexistence Species 2 Number consumed Species 1 Seed size 3 Figure 49.13c Complete niche overlap Strong nice overlap can lead to Competitive Exclusion Number consumed Species 1: Strong competitor Species 2: Weak competitor, driven to extinction Seed size The Spread of Introduced Zebra Mussels in the United States (1998 – 2001) Extensive niche overlap can lead to competitive exclusion 4 Figure 49.15a Observed Distributions Chthamalus in upper intertidal zone Mean tidal level Balanus in lower intertidal zone Figure 49.15b COMPETITION EXPERIMENT Upper intertidal 1. Transplant rocks containing young Chthamalus to lower intertidal. Lower intertidal 2. Let Balanus colonize the rocks. Chthamalus Balanus 3. Remove Balanus from one-half of each rock. Monitor survival of Chthamalus on both sides. On which side of the rocks do Chthamalus survive better? 5 Chthamalus survives better without competition. 100 Competitor present Percent mortality 80 Competitor present 60 Competitor absent 40 Competitor absent 20 0 Young Chthamalus Older Chthamalus Figure 49.15c Larval Settlement Zone Larval Adult Settlement Distribution Zone Spring high tide Adult Fundamental Distribution Niche Neap high tide Could not become adults due to competition Barnacles subject to desiccation Realized Niche Chthamalus Mean tidal level Balanus Neap low tide Spring low tide Distribution of Two Barnacle Species – Balanus and Chthamalus 6 Niche Partitioning can lead to Adaptive Radiation ANCESTOR FINCH Blue-back grassquit Sharp-billed ground finch Vegetarian finch Mangrove finch Small tree finch Large cactus finch Cactus finch Large tree finch Medium tree finch Large ground finch Woodpecker finch Medium ground finch Warbler finch Insect Eaters Bud Eaters Seed Eaters Galapagos Finches Hawaiian Honeycreepers Adaptive Radiation to Exploit a Variety of Food Sources Results in Speciation and a variety of Beak Shapes 7 Stabilizing Selection Medium-sized individuals favored Number of Individuals with phenotype Reduces Variation 0 10 20 30 40 50 Peak gets higher and narrower Selection Mean stays the same 0 10 20 30 40 50 Directional Selection Larger individuals favored Number of Individuals with phenotype 0 10 20 30 40 50 Peak shifts in one direction Selection Mean Shifts 0 10 20 30 40 50 8 Directional Selection Large and small individuals favored Number of Individuals with phenotype 0 10 20 30 40 50 Two Peaks Two populations forming Selection 30 40 50 African Seedcracker (Pyrenestes ostrinus) Birds with smaller bills can consume soft seeds more efficiently Number of Birds 100 Birds with intermediate beak size survive poorly 50 0 10 15 20 Birds with large bills can crack hard seeds Competition can lead to character displacement Width of lower bill (mm) 9 Predation Snowshoe Hare and Lynx Population Population Cycles Number of Pelts (thousands) Lepus americaus Lynx canadensis Hudson Bay Company Trapping Records 150 (1840 –1940) Hare 100 Lynx 11 years 50 1840 1870 1900 1930 What drives the oscillation? 10 Field Experiments - Lloyd Keith (1960’s to 1980’s) What accounts for population cycles? Food Supply or Predation? 5 Food Supply Increased Ratio of Hare Density to Controls 3 1 5 Predators Excluded 3 1 15 13 Food Supply Increased & Predators Excluded 11 CONCLUSION: 9 5 3 Food Supply & Predation both affect population cycles. 1 Control Phase of Hare Population Predatory mite Prey mite prey Source: Ricklefs, R. The Economy of Nature 3rd Ed., p.372 11 Evidence for predation as an evolutionary agent: - Cryptic Coloration - Physical and Chemical Defenses - Mimicry Figure 49.12b 20 Resprouted trees have more defense compounds. Salicortin concentration (mg/g dry mass) 15 10 5 0 Control trees Browsed and resprouted trees 12 Figure 49.12c 400 Herbivorous larvae have higher survivorship in the face of their own predators (ants) when they have been feeding on resprouted trees. Thus --- Leaf beetle larvae sequester antibeaver compounds and use them as a defense against ants Larval survival time (sec) 300 200 100 0 Larvae from browsed and resprouted trees Larvae from control trees Figure 49.8b Shell mass (g) Attachment strength (N) Correlation between predation rate and prey defense Low predation High predation Site type Low predation High predation Site type 13 Is prey defense induced by presence of predator? Figure 49.8c Are mussel defenses induced by the presence of crabs? Are mussel defenses induced by the presence of broken mussel shells? Seawater Crab (fed fish, not mussels) Mussels Shell thickness HIGH Seawater No crab Broken mussel shells Intact mussel shells Mussels Mussels Mussels Shell thickness LOW Shell thickness HIGH Yes Shell thickness LOW Yes Mutualism • Mutualism is a type of interaction that is beneficial to both species involved. • It does not involve altruism. The benefits are a by-product of each species’ own selfinterest. • The costs and benefits of mutualism vary widely between partners, over time, and from one area to the next. (Fig. 49.16a–c) 14