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Transcript
Community ecology – interactions between individuals of different species. Community ecology – interactions between individuals of different species. Competition – who does this and why? Niche concept: if individuals of two species rely on the same resources… -- and we presume that the two species are not identical in their ability to exploit those resources -- then one species will out-compete the other -- and the one who acquires resources less well will be driven to extinction (competitive exclusion) 1. Competition leads to local extinction Competitive exclusion in two species of Paramecium Number of individuals 400 300 Paramecium aurelia 200 100 0 Paramecium caudatum 0 5 10 15 Time (days) Figure 53-3a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. 20 25 Number of individuals using resource When niches overlap completely Species 1: Strong competitor Species 2: Weak competitor, driven to extinction Niche range Figure 53-3c Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Caveats: • What if one species is better at obtaining resource A and the other at resource B? • Of all the resources needed, for how many is there direct competition? • For a given species, the most critical resources for population 1 may not be the most critical for population 2. • What does it mean to rely on the same resources (how similar must the resources be such that they are “the same”)? When there is partial niche overlap 2. Competition leads to niche partitioning Suppose the resource is a place to attach on an intertidal zone rock. Who gets that resource? Find two barnacle species, but distributed in distinct zones. Chthamalus in upper intertidal zone – it’s realized niche. Mean tide level Balanus in lower intertidal zone – competitively excludes Chthamalus The lower intertidal zone is better. If you remove Balanus, Chthamalus will occupy its fundamental niche. Percent survival Remove-Balanus results: 80 Chthamalus survival is higher when Balanus is absent 60 40 20 0 Competitor absent Competitor present Figure 53-6b part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Number of individuals using resource If niches do not overlap completely, weaker competitors use non-overlapping resources. Species 1 (strong competitor) Species 2 (weak competitor) Fundamental niche Realized niche Niche range Figure 53-4a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. 3. Competition leads to evolutionary change There is also the possibility of longer-term evolutionary change in populations where there is competition and partial niche overlap Sympatric vs allopatric populations of Darwin’s finches…. (Work by Grant & Grant) Number of seeds consumed For any species, there is variability in each population. In this case, beak size determines preferred seed size. Some individuals in the population are born with larger beaks, others with smaller beaks. Seed size Figure 53-2a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. When there are sympatric populations of two different species, and partial niche overlap for preferred seed size, competition is most intense for seeds of intermediate size. Number of seeds consumed Species 2 Species 1 Seed size Medium-beak birds of species 1 and medium-beak birds of species 2 have lowest fitness, and the distributions skew. Over time, competition can cause niche differentiation, or niche partitioning. Species 1 Species 2 Number of individuals using resource Time 1 Niche range Natural selection favors individuals Species 1 that do not compete. Number of individuals using resource Species 2 Time 2 Niche range Figure 53-4b Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Niche partitioning can also lead to speciation What happens if the 1b population “knows” that it should not mate with an individual from population A. Why not? Consider the Wombo… Species 1, population A Species 1, population B Niche partitioning can also lead to speciation Geographic range A B In sympatric zone, pop A matings with pop B – get some offspring in pop B with small wombos. These offspring get outcompeted. Species 1, population A Size of wombos Species 1, population B Size of wombos So it is an advantage to pop A and pop B individuals to recognize the difference and avoid matings between populations. To help with recognition, small differences (often unrelated to function) can develop quickly into large differences so each can tell who is from which population. This leads to “character displacement” and the development of a “prezygotic isolating mechanism”. Speciation (i.e., evolution) has been observed over a period of some years on the Galapagos islands. Rapid development of species isolating mechanisms occurs in zones of sympatry where it is likely that intermediate offspring can be produced. An incubator for this sort of speciation occurs on islands, where you have successive waves (say once every 300 years!) of mainland immigrants of the same species. One reason islands have so many endemic species Immigration 1 Population expands Immigration 2 Character Displacement & “Adaptive Radiation” S. America: Iguana fills large lizard niche Anolis fills small lizard niche BUT… Anolis is a much better colonizer! What happens when colonists raft from South American to lizard-unpopulated islands in the Antilles? Only Anolis, the better colonizer, survives the raft trip. The scenario... 1. Anolis immigrants arrive. After 100's of years, small Anolis everywhere. 2. By genetic drift and adaptation, the Anolis distribution becomes broader than that of their friends left behind on the mainland. In particular, some larger individuals do well (there is no competition with iguanas). The scenario... 3. New wave of small Anolis arrives and establishes a population “beach head”. They compete with old Anolis inhabitants. The larger members of the old Anolis population have less competition, though, because the new immigrants are all small. 4. The large members of the old Anolis population undergo rapid character displacement and develop species isolating mechanisms to not breed with the newcomers. And this is what you end up with… Types of Competition There are a number of variations on the competition theme: Consumptive – take all nutrient resources Pre-emptive – get there first and occupy Overgrowth – overwhelm other species with your numbers Chemical – make a space unsuitable for other species Territorial – defend a space Encounter – opportunistic defense of transient resource Consumptive competition: Organisms consume the same resources. These trees are competing for water and nutrients. Preemptive competition: Individuals occupy space, preventing access to resources. Space preempted by these barnacles is unavailable to competitors. Overgrowth competition: One organism grows over another. The large fern overgrows other individuals and shades them Chemical competition: One species produces toxins that negatively affect another. No grass next to or under the Salvia shrubs. Territorial competition: Mobile organisms protect a feeding or breeding territory. Grizzly bears drive off black bears. Encounter competition: Organisms interfere directly for access to specific resources. Spotted hyenas and vultures fight over a kill. Other interactions… 1. Neutral 2. Competition✔ 3. Predator-prey 4. Parasite-host 5. Commensalism 6. Mutualism 7. Symbiosis Often evolution proceeds in this direction. Neal Smith experiments in Panama: Involves -- Parasitism, mutualism, predation Cast of characters • • • • oropendola cow bird bot fly wasp Oropendolas and their nests The Cow bird is a brood parasite: o Lays eggs in host nest o Cow bird eggs develop faster o Cow bird chicks are larger and more aggressive, and outcompete Oropendola chicks Natural Selection should favor Oropendolas that recognize Cow bird eggs and throw them out of the nest. Cow Bird Eggs Cow bird Find shell fragments under Oropendola nests. Some CB eggs are good Orop. mimics, others not so much. Q1: Why don’t all CBs make mimetic eggs? Smith expts: Pole, ladder, clip – mess with Orop. Nests. Two more examples of mutualism (next two slides; from your text) Mutualism between ants and acacia trees Plant makes goodies for ants; ants defend tree from other insects and vertebrate herbivores. Mutualism between cleaner shrimp and fish Figure 53-15b Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Mutualisms – Human skin microbiome 1. 1st line defense against some pathogens. 2. Many health effects. Changes in floral composition associated with… • Obesity • Dermititis • Periodontal disease • Bacterial vagninosis • IBS • Autoimmune diseases Microbiome seems to be personal: Only 13% of bacterial species on the palm of your hand are shared between individuals. Community is variable over time. There are more transient and more permanent species. Compared to gut flora, skin is a more diverse community with less long-term stability. In “primitive” cultures (Amazonia) people have 2x the variability re: N. Americans. Ant – Tree mutualisms About ants 13000 species! 15% of terrestrial biomass!! Ants provide defense of trees. Chafer beetle landed on a wild cotton plant (U.S. Southwest). Mutualist ants took care of herbivore. Acacia trees have hollow thorns that provide food and shelter for defensive ants. Why don’t cheater ant species take Acacia tree food and not bother providing defense? Or what about herbivorous insects that evolve rapid strike techniques to get food before the defensive ants can mobilize? Acacia food pods full of protease inhibitors. Completely shuts down gut of herbiverous insects. The Acacia guard ants (Pseudomyrmex ferruginea) make the protease chymotrypsin-1, which is not affected by the protease inhibitors! Mutualisms can change quickly Long term study in Africa: Put fence around trees to preclude herbivore predation. In 10 years the host trees stopped making food resources for defensive ants. Relative to predator-prey interactions… How come herbivores or other predators don’t eat everything? 1. Predators and prey keep each other in check (muscles & crabs, lynx & hares, starfish & mussels) 2. Plants are just not that nutritious. 3. Plant-defense hypothesis (beavers & cottonwoods) Correlation between predation rate and prey defense: Mussels vs crabs keep each other in check. Mussel attachment strength is higher when predation is high Shell Thickness (mm) Attachment strength (N) 40 Mussel have thicker shells when predation is high 30 20 10 1.8 1.2 0.6 0.0 0 Low High predation predation Site type Low High predation predation Site type As before: The hare-lynx populations cycle every 11 years, on average. The peak in the lynx population lags behind that of the hares. 4000 25 2000 15 500 10 100 5 0 0 0 Hare 10 20 30 Years Lynx per 100 km2 Hares per km2 20 40 Lynx Experimental manipulations imply that ”release from predation” is a major contributor to population control. Results: Hare density relative to controls 15 Hare population doubles when predators are excluded 10 Hare population triples when food is added 5 0 Control Predators excluded Food added Predators excluded, food added Hare population increases over 16 times when predation is low and food high Conclusion: Hare populations are limited by both predation and food availability. When predation and food limitation occur together, they have a greater effect than either factor does independently. Pisaster clobbers muscles, but without the starfish, muscles take over everything (& species diversity goes down). Number of species present 20 Keystone predator present 15 10 5 Keystone predator absent 0 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year Figure 53-17 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Adaptive responses to predation: Resprouted trees make more defensive compounds. Increased toxicity to vertebrate herbivore comes at an energetic cost to the plant, though. 20 Salicortin concentration (mg/g dry mass) 15 10 5 0 Control trees Cut and resprouted trees BUT… …herbivorous insect larvae actually like the toxins, and survive longer on resprouted trees. So it is not so simple! Larval survival time (sec) 400 300 200 100 0 Larvae from control trees Figure 53-12c Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Larvae from cut and resprouted trees Question: Do identical communities develop in identical environments? Clements: Identical environments = identical communities Gleason: Identical environments +/- identical communities Start with 12 sterile ponds At end, same communities of plankton in each? Outcome: more Gleason than Clements Figure 53-18 part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. The composition of communities can, to some extent, be predicted based on abiotic factors. BUT…. there are equally powerful factors making things more random: Who are the first colonists to arrive after a disturbance? Has some unpredictable event influenced a keystone species? Historical record shows that different species compositions occur over time in the same habitat. So “the” community is not the same over time. Of course, not just any group of organisms can fit together! Often there are predictable succession stages leading to a climax community. plowed field – no plants Weeds colonize 1st Weeds Pioneering species Bigger weeds replace Bigger weeds Early successional community Shrubs replace Shrubs Figure 53-20 part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Mid-successional community Shrubs Mid-successional community Small tree invasion Short lifespan trees Latesuccessional community Bigger trees invade Cedar/hemlock in the PNW. Figure 53-20 part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Climax community Primary succession: major disturbance wipes out everything in a region, including soil. Secondary succession: at least soil is left, so seeds blown into the area can germinate right away. 35 years ago last week! Mt. St. Helens succession 1980 sterile pumice plain 1982 first lupine plant 1986 16,000 lupine plants 1992 first insect lupine herbivores 2001 first alders 2005 all small mammals originally present have returned. Some small mammals like deer mice survived eruption in burrows, and 12/14 species of amphibians survived at bottom of Spirit Lake & stream bottoms. Succession has not been exactly the same everywhere. The lupine predators lead to a dynamic change in the lupine population, but this is different for upland and wetter regions. At the outset, how sterile was it? 1 billion cubic meters of material ejected, plus avalanche (the largest recorded in human history) covered 13 miles to a depth of 300 feet. On top of this, pyroclastic flow at 800 °C. Not only completely sterile, but NO SOIL! After a few years wind and run-off born dirt enough soil for the 1st lupine plant. Why a lupine? They are among the very best colonizers. Dispersal filter: Who can travel & survive the trip? Environmental filter: Who can tolerate primitive conditions at new spot? Stable (+/-) population Community interaction filter: Who can resist predators? Plants with wind-born seeds are great at dispersal, but lupines unique in passing the environmental filter. 1. Very drought resistant: They make extremely long tap roots. 2. Need almost no soil nutrients: a) They have nitrogen-fixing root nodules. b) The roots secrete carboxylic acids which erode elemental phosphorous from rock. The only plants to start growing after the lupines were some grasses that could grow on dead lupine plants! The lupines provided the only carbon source for other plants. BUT… 9 years after the 1st lupine plant establishes, the population starts going through crashrebound cycles every 3-5 years. Before crash After crash Crash due to lupine-specific insect herbivores. Largest crashes are in lowest-density lupine patches! High-density lupines compete with each other, so any one plant has fewer nutrients. Insects pick sparse plants with higher concentration of nutrients. When lupines have made enough soil, willows can start. Willows critical in succession – they add a vertical component to habitat. Vertical habitat: cover, shade, bird perches, nest sites. … but willows attract a host-specific weevil, which kills willows in upland habitat, where the trees are more water-stressed. (Much less mortality in wetter areas). surprisingly… More rapid succession where willows are the most impacted: Weevil attack willow drops stems more carbon goes into the soil! In a primary succession environment, with a small number of species, the community interactions are few and direct. Interaction Strength (I.S.) = per capita effect of one species on another. Primary succession: high I.S. unstable interactions. Succession greatly slowed with low-density highcrash lupine sites, and increased in drier willow sites. Bottom line: Community interactions (consumer effects) affect succession. To what extent is succession predictable? 1. Some species are better colonizers. a) Fewer nutrient/shade requirements b) Faster growing c) Drought tolerant d) r-selected 1. After the pioneer species live for some generations, the physical environment is changed by those species. a) Soil stabilization b) Nutrients added c) Habitat suitable for different sorts of decomposers, pollinators 1. Climax cedar/hemlock: most shade tolerant, life longer & grow taller than Doug fir! Natural selection has produced some species who are specifically adapted to be colonizers! Such species are experts at dealing with “disturbed” habitats. Chaparral – seeds only germinate after a fire, and fire is part of the natural ecosystem in the SW deserts. (fire scars the seed coat and allows water to enter) Reconstructing history from fire scars Number of fires per century 50 40 30 20 10 0 0 400 800 1200 1600 2000 Years A.D. Natural selection can operate on regular, predictable abiotic events. Note that predictability not on a human life-span scale, is often overlooked. So…. Clements and Gleason are both right! Community composition, given abiotic factors, is somewhat predictable and stable – but there are many factors that influence the details thus there is no one community that is guaranteed. Three distinct successional pathways occur in Glacier Bay. PATHWAY 1 (Lower Bay) Alder patches; extensive spruce Early-mid successional PATHWAY 2 (Upper Bay) Extensive alder; some spruce, cottonwood Mature hemlock, spruce Spruce Hemlock arrives Cottonwood Alder Mid-late successional Climax Mature spruce; some alder Alder dies back No hemlock? ? Early successional Mid-successional PATHWAY 3 (Uppermost Bay) Extensive alder; scattered spruce Alder dies back Late successional Mature spruce, cottonwood; some alder Climax No hemlock? ? Early successional Mid-successional Late successional Figure 53-21b Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. Climax Hemlock Some communities are more diverse than others. large number of species High diversity = (richness), but not a great deal of uneveness. Diversity highly correlated with stability (relative to factors that lead to disturbance). Agricultural monocultures are the most vulnerable! Desert & high latitude communities are very fragile. Number of vascular plant species per 10,000 km2 Diversity declines with latitude 10,000 8000 6000 ! 4000 2000 0 0 10 20 30 40 50 Latitude (degrees North or South) Figure 53-23 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc. 60