Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ecology of Banksia wikipedia , lookup
Plant morphology wikipedia , lookup
Plant physiology wikipedia , lookup
Plant nutrition wikipedia , lookup
Plant defense against herbivory wikipedia , lookup
Plant reproduction wikipedia , lookup
Glossary of plant morphology wikipedia , lookup
Ornamental bulbous plant wikipedia , lookup
Plant use of endophytic fungi in defense wikipedia , lookup
Sustainable landscaping wikipedia , lookup
This weeks schedule Wednesday, Exam. – – – – – – Open book, take home. 70 questions. Review of the material to date through Lesson 10. 70 short answer review questions (70 questions, 1.43 points each, 100 points total.) The exam will likely take 2 hours (possibly more) to complete. You can use the lecture period on Wednesday if you wish to complete the exam. The exam is due Wed, Feb 11 at 9 am in class. I will deduct 5 points if the exam is late that day, and I will not accept it beyond 5 pm,, Feb 11.! I will email the exam to everyone today and you can email the answers back. 1 Field Trip • Will only go if temperature is above -10û F. • Equipment: • Snow shoes or skis. Does everyone have these? • Hat, gloves, long underwear, jacket, daypack, hand lens, notepad, pencil, scotch tape, large garbage sack for collecting plants. • Objectives: – – – – – Learn some of the common local plants in their winter condition. Examine a snow pit to examine characteristics of the snowpack. Look at the plants beneath the snowpack. Look at some wintertime plant-animal interactions. Enjoy the boreal forest in winter. 2 Resource competition: Effect of competition between species for a single resource, R. Tilman model (1982) • Curves are the population growth rates for species A and B. • ma and mb are the mortality rates for species A and B. • The intersection of the curves with the m lines represent the minimum amount of the resource R needed to sustain the population. • Best competitor is the one with the lower R* for the limiting resource. Recall from the last lesson the work of Tilman, who showed the consequences of competition where more than one resource is limiting. Here, R* is the minimum level of a resource required to sustain a population. Under this assumption, R* represents the amount of a resource that is the minimum required for an individual to maintain a positive net growth rate. Competition between individuals can then be viewed as mediated by the R*’s of different species. In this case, the better competitor is defined as the species with the lower R* for a limiting resource because this species may continue to thrive while drawing the resource below the minimum level for the other species. Above, curves A and B depict population growth curves dN/dt for two species versus availability of resource, R. The two curves show the resourcedependent populations growth curves for species A and B. The dashed lines (mA and mB) represent mortality rates for A and B. The intersection of the curved solid and dashed lines represents the minimum resource level R* to support and equilibrium population (represented by the vertical solid lines). In this case, species B survives at lower resource levels than species A, and hence is regarded as the superior resource competitor. From Tilman, D. 1982. Resource competition and community structure. Princeton University Press, Princeton, NJ. 3 Tilman’s resource-ratio model (1982: How 2 species can coexist competing for the same resource (1) Lines A and B are Zero Net Growth Isoclines (ZNGIs) of species A and B for resources R1 and R2. These show the availabilities of the two resources for which the reproductive rate of the species match the mortality rate. CA and CB are resource consumption vectors for each species. The slope of each vector is the ratio of consumption of resource R 2 divided by the consumption of resource R 1. Tilman’s resource-ratio model. This model demonstrates how two or more species may differ in R* for the same resource and still coexist in nature. The figure shows Zero Net Growth Isolines (ZNGIs) for species A and B along two resource gradients R1 and R2. These lines show the minimum level of reource 1 and 2 required to support each species. In (a) species A is superior competitor for both resources because it can exist at a lower level for both resources, and will draw the resource down below the critical level required to support species B. In (b) the opposite situation occurs, with B being the superior competitor. In c, A is the superior competitor for resource 1, and B is the superior competitor for resource 2. The vectors CA and CB are resource consuption vectors that show the relative consumption of resource 1 and 2 for each species. The slopes of the lines are proportional to the relative amount each resource is limiting to each species. (For CA in c, resource 2 is more limiting than resource 1 for species A, and resource 1 is more limiting for species B.) The intersection of the ZNGIs is stable in this case because each species consumes relatively more of the resource that is limiting to its growth at equilibrium. In zone 1, neither species can exist because resource levels are too low for either species. In zone 2 and 3, species A will dominate, and in zones 5 and 6, species B will dominate (These are situations where one resource is much more abundant than the other, and the other is more limiting, and one species will prevail. Both species can coexist in zone 4 and will reduce the resources 1 and 2 to the equilibrium point. In (d), the equilibrium point is unstable, because each species uses more of the resource that primarily limits the other species. In this case, either species A or B can dominate, depending on the initial conditions. From Tilman, D. 1982. Resource competition and community structure. Princeton University Press, Princeton, NJ. 4 Tilman’s resource-ratio model (1982): How 2 species can coexist competing for the same resource (2) a) b) (a) (b) (c) (d) Think of R1 and R2 as light and water. Species A has a lower ZNGI for both resources, and is the superior competitor. B has lower ZNGI for both resources and is the superior competitor. ZNGIs cross, and Species A is superior competitor in areas 2 and 3, and species B is superior competitor in areas 5 and 6. Resource consumption vectors, CA and CB, show that species A consumes more of resource that is limiting to itself, and vice versa for species B. Hence the species can coexist in area 4. Resource consumption vectors show that each species consumes more of the resource that is limiting to the other species. Hence the species are unable to coexist in area 4. 5 Implications of Tilman’s Resource-ratio hypothesis Differences in the relative supply rates of limiting resources should lead to differences in the composition of plant communities. • Species allocation patterns: Species with allocation patterns focusing on shoots are assumed to be relatively effective competitors for light, and those allocating more heavily to roots are assumed to be good competitor for below-ground resources (water, nutrients). • Succession implications: Resource supply ratios also vary systematically through successional series to first favor root specialists (because soil nutrition is more limiting than light in primary succession) and then shoot specialists (because light is more limiting in later stages of succession. • Landscape implications: Various habitats within landscapes differ in their level of key resources, and hence will favor either root or shoot specialists depending on the local resource supply. Resource-ratio hypothesis Plants are unusual because they have organs for maintenance in two different environments. The organs for acquiring water and nutrients are located belowground (roots), and the organs for acquiring energy and CO2 for photosynthesis are located above ground (leaves and stems). Some plants are shoot specialists and are good competitors for light. Some plants are root specialists and respond are competitors for water and nutrients. Although somewhat intuitive, this hypothesis is experimentally difficult to demonstrate. • Plants appear to be more flexible in the root-shoot allocation patterns than suggested. • There is wide variation in the spatial and temporal supply of resources. 6 Examples of situations where plants use environmental tolerance to avoid competition • Serpentine soils, – Low in essential nutrients, extreme pH, high in toxic elements (e.g., Ni, Cr) – Support unusual plants, often highly endemic floras – Experimental evidence (e.g., Kruckenberg 1954) indicate that although serpentine plant species often can grow better in nonserpentine soils if grown without other species, they are poor competitors when grown with other species. • Saline soils – Halophytes can grow in soil with > 0.2-0.25% salt. – Many have special structure whereby they secrete excess salts. – Examples include mangroves, coastal salt marsh species, beach plants, desert herbs. Examples of situations where plants use environmental tolerance to avoid competition Serpentine soils, Low in essential nutrients, extreme pH, high in toxic elements (e.g., Ni, Cr) Support unusual plants, often highly endemic floras Experimental evidence (e.g., Kruckeberg 1954) indicate that although serpentine plant species often can grow better in nonserpentine soils if grown without other species, they are poor competitors when grown with other species. Saline soils Halophytes can grow in soil with > 0.2-0.25% salt. Many have special structure whereby they secrete excess salts. Examples include mangroves, coastal salt marsh species, beach plants, desert herbs. 7 Amensalism • Interaction which depresses one plant population while the other species remains unaffected. • A good example is the strongly negative effect that a large species such as a tree might have on some small ground cover species. Amensalism Amensalism is an interaction which depresses one plant population while the other species remains unaffected. A good example is the strongly negative effect that a large species such as a tree might have on some small ground -cover species. 8 Allelopathy • A negative biochemical influence of higher plants upon another species (usually inhibition of germination or growth) that is caused by the release of metabolic substances under natural conditions. Examples: several lichens, alders, Artemisia (sagebrush), Larrea (creosote bush). 9 Allelopathy: Salvia leucophylla-grassland interface, Santa Barbara, CA (Muller 1966) • Light bands around soft chapperal (Salvia) are devoid of plants. • Salvia emits volatile oils (cineole and camphor). Allelopathy Many plants release chemicals that inhibit the growth of other species. These allelochemicals are selectively toxic to some species of plants. In competitive situations plants are depriving other plants of a critical resource. In allelopathic interactions a plant adds a substance to the environment. This can be viewed as interference competition, i.e. direct competition. Studies of C.H. Muller Some of the best documented examples of allelopathy come from the work of C.H. Muller and his students. The above photos show the nearly bare zones between soft chaparral and annual grassland near Santa Barbara, California. (a) Aerial photograph of Salvia leucophylla shrubs and adjoining grassland. The light bands beneath and next to the shrubs are devoid of all but a few species of small herbs. (b) Ground view of the same situation. A is Salvia; B is the bare zone, and C is the grassland. Muller showed that Salvia emits a number of volatile oils (e.g. cineole and camphor) from their leaves that are toxic to the annuals. The story, however, is more complex than it might appear. For example, Kaminsky (1981) has shown that in chamise (Adenostoma fasciculata) shrublands, the chemicals are released in a nontoxic form, and require soil microbes to convert the chemicals to allelochemicals. Some have argued that the effect could be due to animal seed predators that keep the halo areas around the shrubs free of any seedlings. 10 Effect of overstory and understory plants on soil properties (Tappeiner and Alm 1975) Effects of plants on soil properties Plants can have a very inhibitory effect on growth of many organisms including soil microbes. For example, pines create very acidic soils that are toxic to many species of plants and soil organisms, including worms, and many bacteria. Fungi tend to dominate the microflora in these soils, whereas bacteria dominate the more neutral soils beneath deciduous forests. The above table shows the difference in some key soil properties of pine and birch forests. The pine forest have lower pH, lower bulk density, lower soil nutrients, and slower turnover times. There is also some variation due to understory species, but this effect is relatively minor. From Tappeiner and A.A. Alm. Undergrowth vegetation effects on the nutrient conten of litterfall and soils in red pine and birch stands in northern Minnesota. Ecology 56: 1193-1200. 11 Effect of canopy water throughfall on soil chemistry Carlisle et al. 1966 Effect of canopy throughfall Difference in the chemistry of water reaching the soil surface can also greatly modify the soil chemistry. This table shows the dramatic difference in chemistry of rainfall versus the chemistry of water that falls through an oak (Quercus petraea) forest overstory. Note that N is slightly reduced beneath the trees because of direct absorption of N into the tree leaves. Other nutrients however, are leached out of the tree leaves. 12 Summary • • • • • • Major types of competition: (1) interference competition (species directly interfere with each other, e.g. allelopathy), (2) exploitation competition (mediated by exploitation for a shared resource, most plant competition is of this type), (3) apparent competition (mediated through a third species such as an herbivore). Gause’s competitive exclusion principle for animals can be applied to plants in modeling situations, but in the real world, plants do coexist because natural populations may not come into equilibrium very often, or other interactions may limit the full competitive interaction between species. Spatial and temporal variation in resource availability allows for the coexistence of several species. This can be inferred using differences in dispersal abilities, or differences in above- and below-ground allocation. Tilman focused on resource competition as the basis for most competitive interactions. His resource-ratio models are based on species’ relative abilities to compete for resources. Grime’s models predict the strongest competition in high resource environments. Plants able to convert resources to high growth rates are the best competitors in these situations. Allelopathy is an example of an amensal (0,-) interaction (or interference competition). Many plants release allelochemicals that are inhibitory to the growth of other species. Summary: Major types of competition: (1) interference competition (species directly interfere with each other, e.g. allelopathy), (2) exploitation competition (mediated by exploitation for a shared resource, most plant competition is of this type), (3) apparent competition (mediated through a third species such as an herbivore). Gause’s competitive exclusion principle for animals can be applied to plants in modeling situations, but in the real world plants do coexist because natural populations may not come into equilibrium very often, or other interactions may limit the full competitive interaction between species. Spatial and temporal variation in resource availability allows for the coexistence of several species. This can be inferred using differences in dispersal abilities, or differences in above- and belowground allocation. Tilman focused on resource competition as the basis for most competitive interactions. His resource-ratio models are based on species’ relative abilities to compete for resources. Allelopathy is an example of an amensal (0,-) interaction (or interference competition). Many plants release allelochemicals that are inhibitory to the growth of other species. 13 Literature for Lesson 8 • Everyone please read the following paper: • Grace, J.B. 1991. A clarification of the debate between Grime and Tilman. Functional Ecology 5: 583-587. • Becky, Esther, Brandon, Sandra also read: 1. Grime, J.P. 1977. Evidence for the existence of three primary stategies in plants and its relevance to ecological and evolutionary theory. The American Naturalist,! 111:1169-1194. Brandon will present: it to the class. • Darcy, Joe, Dan, Daniel, Brad will also read: • 2. Tilman, D. 1985. The resource ratio hypothesis of plant succession. The American Naturalist,! 125: 827-852. Brad will present it to the class: 14 Lesson 10: Species interactions: Commensalism, mutualism, and herbivory Commensalism Examples: Epiphytes, Nurse plants, Protocooperation Examples: Root grafts, Transfer of nutrients through mycorrhizal fungi Mutualism Examples: Mycorrhizae, Symbiotic N-fixation, Pollination Herbivory Effect on plant communities Limits to herbivory Plant defenses against herbivory Introduction In the last lesson we examined the pairs of negative interactions of amensalism, and competition. Here we will examine three generally positive interactions (commensalism, protocooperation and mutualism) and one more negative interaction (herbivory). 15 Commensalism • Definition: an interaction that stimulates one organism but has no effect on the other. • Examples: – Epiphytes – Nurse plants Commensalism Definition: an interaction that stimulates one organism but has no effect on the other. Examples: Epiphytes Nurse plants Clear examples of true commensal relationships are difficult to come by. It is not always clear that a host species is not affected negatively be the other. And as we mentioned earlier, the relationship can change and grade into one of parasitism or competition. These two examples, are often given, but they are not always true commensalism. 16 Examples of epiphytes Spanish Moss (Tillandsia usneoides), a Bromeliad Spanish Moss, closeup Arboreal lichen, grandfather’s beard, (Ramalina reticulata) •23,000 vascykar-plant epiphyte species (not counting mosses, lichens, liverworts), in 879 plant groups. Epiphytes There are a wide variety of epiphytes that include vascular plants such as the bromeliad, Tillandsia usneoides (a and b, Spanish Moss) or the arboreal lichen Ramalina reticulata (grandfather’s beard). Over 23,000 epiphyte species are distributed in 879 vascular plant groups. Epiphytes have commensal relationships only as long as they do not harm the host. Some are autotrophic and use the host only for support to gain access to sunlight. Others are parasites (e.g., mistletoe, Arceuthobium), so not all epiphytes have a commensal relationship. Sometimes a mutualistic relationship can occur if the lichen produces nutrients that are leached to the tree roots. For example, Forman (1975) found that most lichens in the upper canopy of a Columbian rain forest contain a blue-green algae, Nostoc, that fixes carbon equivalent to the amount of carbon provided by rainwater. This N is probably redistributed through leaching and decomposition. Some epiphytes have special leaf or root structures to trap water. Others such as tree lichens can get all their water needs from the atmosphere. 17 Microhabitats of epiphytes • Zone 1: Small epiphytes. 86% of these contain Nostoc, a blue-green algae that fixes nitrogen (Forman 1975), • Zone 2: Large epiphytes (e.g. vines) • Zone 3: Crustose lichens • Zones 4 and 5: Bryophytes Longman & Jenik 1974 Microhabitats of epiphytes Competition for space in the tree canopy can be intense. Censuses in tropical trees have shown that relatively distinct parts of the canopy are associated with different epiphyte species (Longman and Jenik 1974, Janzen 1975). The diagram shows an emergent tree in the tropical rain forest. Small epiphytes are common in the Zone 1. 86% of these contain Nostoc, a blue-green algae that fixes nitrogen in the amount of about 1.5-8.0 kg N ha-1yr-1, about the equivalent of that fixed in rainwater by lightning. (Forman 1975), large epiphytes in zone 2, crustos lichens in zone 3, and bryophytes in zones 4 and 5. From Longman, K.A. and J. Jenik. 1974. Tropical Rainforest and Its Environment. Longman, London. 18 Mutualistic epiphytes: Trees that produce canopy roots The mass of epiphytes is also a great source of water and nutrients to the trees themselves. Several tree species are thought to tap these nutrient sources by producing adventitious roots in their canopies that penetrate the mass of humus associated with the epiphytes. Thus the epiphytes can produce a positive effect for their host. Nadkarni 1994 Trees that produce canopy roots to tap epiphyte nutrient resource The mass of epiphytes is also a great source of water and nutrients to the trees themselves. Several tree species are thought to tap these nutrient sources by producing adventitious roots in their canopies that penetrate the mass of humus associated with the epiphytes. Thus the epiphytes can produce a positive effect for their host, (a mutualistic relationship, +, +). Epiphytes, however, more commonly act as parasites (+,- interaction): Several epiphytes’ roots (haustoria) penetrate the bark of the tree and tap the phloem and xylem. Hemiparasite: a species able to live facultatively as a parasite or on its own (e.g., Phoradendron, a species of green mistletoe). True parasite: a species that relies on the photosynthate and/or other resources of its host; e.g. Arceuthobium). 19 Parasitic epiphytes: Hemiparasite vs. true parasite • Haustoria: Epiphyte roots that penetrate the bark of the tree and tap the phloem and xylem. • Hemiparasite: a species able to live facultatively as a parasite or on its own (e.g., Phoradendron, a species of green mistletoe). • True parasite: a species that relies on the photosynthate and/or other resources of its host; e.g. Arceuthobium). Phoradendron californicum; Mistletoe. Arceuthobium campylopodum;Western Dwarf Mistletoe, Photos Alfred Brousseau, Saint Mary's College 20 Parasitism for light: Strangler fig (Ficus leprieuri) on palm (Elaeis quineensis) Longman & Jenik 1974 Parasitism: Perhaps the ultimate case of an epiphyte as a parasite is the strangler fig (Ficus leprieuri). (a) This plant begins its life as in typical epiphyte in the crown of a tree. (b) As the strangler fig grows, aerial roots grow toward the soil. (c ) Eventually these aerial roots reach the ground and and introduce a new source of nutrients to the fig. At this point, the fig is no longer an epiphyte. These roots thicken, engulfing the host trunk and preventing further growth of the host tree. (d) At the same time the canopy of the fig enlarge to overtop the host and deprive it of light. And eventually the host dies, but the fig remains. In this case the epiphyte parasitizes and competes with its host. From Longman and Jenik. (1974) Tropical Rainforest and Its Environment. London: Longman. 21 Commensalism: Nurse plants • • Nurse plants are plants that afford seedlings protection from a harsh environment while they grow large enough to establish. Positive effects: 1. Reduce soils temperature and rate of soil drying. 2. Hide the young cactus from rodent herbivores. 3. Protection from frost. • Examples: – – – – Palo verde (Cercidium floridum), for saguaro cactus (Cereus gigantea). Dead palo verde plants are often found in close association with mature saguaros, indicating that the relationship may have shifted from a commensal one to a competitive one for water (Vandermeer, 1980). Desert annuals. For example, Malacothrix and Chaenactis are positively associated with the canopies of burro bush and turpentine broom. These plants have dense canopies that trap debris that it is a better substrate for the annuals. The seeds are also trapped in abundance (Went 1942, Muller 1953, Muller and Muller 1956). Desert shrubs such as bitterbrush, Purshia tridentata, shadscale, Atriplex confertifolia, and winter fat, Eruotia lanata, also require nurse plants. And many bunchgrasses require the shade of mesquite, Prosopsis juliflora. (Yavit and Smith 1983). Blue oak, Quercus douglassii, has a positive effect on surrounding herbaceous plants if the tree has tapped its roots into groundwater. However, if it hasn’t, it will deplete the soil surface of soil moisture for herbaceous plant . Malacothrix californica Nurse plants: Nurse plants are plants that afford seedlings protection from a harsh environment while they grow large enough to withstand the travails of the environment on their own (Muller 1953, Niering et al. 1963, Lowe 1969). A good example is provided by the saguaro cactus (Cereus gigantea). Sauguaro seedlings are nearly always found close to a shade producing object, such as the palo verde tree in the above photo. Turmer et al. Studied these relationships and found 14 other species that also act as nurse plants in southern Arizona. The nurse plants have the following positive effects: 1. Reduce soils temperature and rate of soil drying. 2. Hide the young cactus from rodent herbivores. 3. Protection from frost. Vandermeer (1980) showed that dead palo verde plants are often found in close association with mature saguaros, indicating that the relationship may have shifted from a commensal one to a competitive one for water. Other examples of nurse plants are: 1. Desert annuals. For example, Malacothrix and Chaenactis are positively associated with the canopies of burro bush and turpentine broom. These plants have dense canopies that trap debris that it is a better substarte for the annuals. The seeds are also trapped in abundance (Went 1942, Muller 1953, Muller and Muller 1956). 2. Many other desert shrubs such as bitterbrush, Purshia tridentata, shadscale, Atriplex confertifolia, and winter fat, Eruotia lanata, also require nurse plants. And many bunchgrasses require the shade of mesquite, Prosopsis juliflora. (Yavit and Smith 1983). 3, Blue oak, Quercus douglassii, has a positive effect on surrounding herbaceous plants if the tree has tapped its roots into groundwater. However, if it hasn’t, it will deplete the soil surface of soil moisture for herbaceous plants. 22 A physiological perspective: Commensalism between maples (Acer saccharum) and herb layer through nighttime hydraulic lift Numbers on lines are horizontal distance from the tree. X-axis numbers are dates at noon. Emmerman & Dawson 1996 • Herbaceous species within 2 m of the base of the trees were larger and more vigorous because of the additional water. • Trees take up deep ground water and pass it out through the stomates during the day. At night, there is water pressure gradient upward from the deep roots to the stem and back out through the near surface roots to upper soil surface. • The graph shows higher soil water potential during each night at the soil surface. The effect is diminshed at greater distance from the tree. The effect is swamped after a rain event. Another example of commensalism from a physiological perspective: Emermon and Dawson observed that herbaceous species within 2 m of the base of the trees were larger and more vigorous because of the additional water. Emermon and Dawson (1996) showed observed that sugar maples (Acer saccharum) take up deep ground water and pass it out through the stomates during the day during the day. During the night, there is a water pressure gradient upward from the deep roots to the stem and back out through the near surface roots to the upper soil surface (hydraulic lift). The above graph shows higher soil water potential occurring each night at the soil surface. This was due to the hydraulic lift during the night, which was most pronounced near the tree. This effect is diminished at greater distances from the tree. Also after a rain event. 23 Protocooperation through root grafts • • • Protocooperation: An interaction that stimulates both partners (+,+) but is not obligatory. Growth and survivorship is possible in the absence of the interaction. Example: two trees are connected by root grafts or unions between the same or different species. (About 160 species of tree species can form grafts and 20% of these form interspecific or intergeneric grafts.) When one species is much smaller as in (b), then the relationship is one of parasitism. Example of protocooperation through a root graft (left) In this example, two trees are connected by root grafts or unions between the same or different species. About 160 species of tree species can form grafts and 20% of these form interspecific or intergeneric grafts. When one species is much smaller as in (b), then the relationship is one of parasitism. 24 Protocooperation through soil mycorrhizae (Woods and Brock 1964) •Within 8 days 45% of the trees within a 7.3 radius of the stump showed radioactivity. Woods and Brock’s concluded that the labeled nutrients had moved to the surrounding plants through michorrhizal connections. •They felt that the root mass of a forest often has such connections and can be viewed as a single functional unit. Protocooperation through mycorrhizal hyphae: Woods and Brock (1964) put labeled 45Ca and 32P in bottle on top of a freshlycut maple stump and sealed it so it wouldn’t escape into the surrounding soil or air. Within 8 days 45% of the trees within a 7.3 radius of the stump showed radioactivity. Woods and Brock’s concluded that the labeled nutrients had moved to the surrounding plants through michorrhizal connections. They felt that the root mass of a forest often has such connections and can be viewed as a single functional unit. 25 Mutualism • A symbiotic relationship that is essential to the survival of both species. • Common examples: – – – – – Lichen (algae for photosynthate and fungus for nutrients) Mycorrhizal fungus Symbiotic nitrogen-fixing bacteria Pollinating insects, birds, mammals Zoochory, animal dispersal of propagules Mutualism; A symbiotic relationship that is essential to the survival of both species. Common examples: Lichen (algae for photosynthate and fungus for nutrients, or structural support). Ahmadjian and Jacobs (1982), however have shown that this relationship is actually one of a controlled parasitism where the fungus is actually an obligatory parasite of the alga. Mycorrhizal fungus Symbiotic nitrogen-fixing bacteria Pollinating insects, birds, mammals Zoochory, animal dispersal of propagules (for example, ants dispersing seeds, while consuming nutritious appendage (eliasome) also frugivory of birds) Also example of ants in relation to epiphytes. Ants use the plants for protection and nesting sites. The ants also pack feces around the rhizomes and adventitious roots of the epiphytes. They also may reduce herbivory and help in dispersal of seeds. 26 Mutualism: Mycorrhizae: Ecto- vs. endomycorrhizae Fungal hyphae Fungal mantel (haustoria) Root cells with hyphae penetrating between the cells •Mycorrhizae are fungal associations with the roots of higher plants. •Mycorrhizae transfer nutrients and metabolites in both directions between the vascular plant and the fungus. They exude nutrients, which are absorbed by the fungus. And the mycorrhizae help the plants, which are somehow stimulated to take up greater amounts of nutrients (Ca, P, K). •Endomycorrhizae are those are those that penetrate the cell walls. • Ectomycorrhizae do not. Mycorrhizae: Mycorrhizae are fungal associations with the roots of higher plants. Endomycorrhizae are those penetrate the cell walls. Ectomycorrhizae do not. Endomycorrhizal associations are the most common and appear to affect nearly all higher plants with the exceptions of aquatic vascular plants, and members of the Brassicacea, Cyperaceae, and Juncaceae. There are also few members of the Poaceae that have mycorrhizae. 1. (a) of the above figure shows a root cross section with the long hair of a fungus hypha. The area labled (2) is the fungal mantal (haustoria) covering the root. (3) shows the fungus hyphae penetrating between the cortex cells of the root. (b) and (C) show some of the forms that mycorrhize can take. In both cases the fungus covers short club-shaped lateral roots. 2. Mycorrhizae transfer nutrients and metabolites in both directions between the vascular plant and the fungus. They exude nutrients, which are absorbed by the fungus. And the mycorrhizae help the plants, which are somehow stimulated to take up greater amounts of nutrients (Ca, P, K). 3. 27 Mutualism: Symbiotic N-fixation • Nitrogen fixation is the conversion of atmospheric N into organic ammonium NH3+. • Usually a nitrogen fixing bacteria fixes N on a host in return for carbon-based resources. • Examples include: – Rhizobium bacteria in root nodules of legumes – Blue green algae Nostoc and Anabaena in association with bryophyte gametophytes, root nodules of cycads, or the leaf tissues of the fern Azolla. – Soil actinomycetes (nodule forming filamentous bacteria) Symbiotic N-fixation: There are also numerous species of bacteria and algae that form symbiotic relationships with plants provide nitrogen to the plant. These organisms convert atmospheric N to organic ammonium NH3+. 4N2 + 6H20 = 4 NH3 + 3O2 (also require ATP and nitrogenase in a reducing environment; see equation p. 163) The plant in return, provides metabolites to the bacteria or algae. Some examples include: Rhizobium bacteria in root nodules of legumes This association is well known and is the basis for interplanting peas or other legumes with high nitrogen users such as corn. Also hay fields are often planted with alfalfa. The alfalfa grows slower than the grasses is harvested late in year after the grasses and adds to the nitrogen content of the soil. Blue green algae Nostoc and Anabaena in association with bryophyte gametophytes, root nodules of cycads, or the leaf tissues of the fern Azolla. Some lichens also have blue-green algal associations, such as Peltigera aphthosa. 3/4 of the N requirements of rice can be met with Azollai in rice paddies. Soil actinomycetes (nodule forming filamentous bacteria) These organism resemble the fungi micorrhizae associations. They invade the roots causing elongate nodules, and can fix nitrogen in rates comparable to legume nodules. Some 285 species of woody plants possess Actinorhizal associations. Many are pioneering species and occur on N-poor, acidic, saline, or sandy soils. 28 Mutualism between insects and plants: Pollination: some characteristics of adapted plants • Pollination is a special form of mutualism that is the key to much evolution in flowering plants, and is responsible for specialized morphology of many flowers of angiosperms. • Provides a food source for the animals. • Advantages to the plant: – Increased pollination results in increased seed production in about 62% of species examined (Burd 1994). – Possibility of accurate pollen dispersal far from the host anther, allowing for outcrossing and genetic varibility. 29 Pollination (cont’) • Plant adaptations to attract pollinators: – Attractive petals, sepals, or inflorescences (either visually or olfactorily). – Sculpted or sticky pollen grains, sometimes massed together. – Nutritious nectar, pollen or starch bodies. – Attractants that are available at pollinaton time. Pollination: 1. Pollination is a special form of mutualism that is the key to much evolution in flowering plants, and is responsible for specialized morphology of many flowers of angiosperms. 2. It provides an obvious advantage for the possibility of accurate pollen dispersal far from the host anther, allowing for outcrossing and genetic varibility. 3. It provides a food source for the animals. 4. Increased pollination results in increased seed production in about 62% of species examined (Burd 1994). 30 Adaptations of bee-pollinated flowers and pollinators Flowers: – – – – – – Bilateral symmetry. Mechanically strong flowers, often with sexual organs concealed. Bright blue or yellow colors (bees can’t see red). Nectar guides along a landing platform. Moderate quantities of nectar that is sometimes concealed. Many ovules per ovary, few stamens. Pollinators: - Good color discrimination. – High degree of intelligence, long memory. – Long proboscis capable of probing for nectar. Adaptations of bee-pollinated flowers: Bilateral symmetry. Mechanically strong flowers, often with sexual organs concealed. Bright blue or yellow colors (bees can’t see red). Nectar guides along a landing platform. Moderate quantities of nectar that is sometimes concealed. Many ovules per ovary, few stamens. Adaptations of pollinators: Good color discrimination. High degree of intelligence, long memory. Long proboscis capable of probing for nectar. 31 Herbivory • The consumption of all or part of a living plant by a consumer. • Includes: – – – – Parasitic and phytophagous microbes (e.g., some fungi and algae) Phytophagous invertebrates Browzing and grazing vertebrates Seed predators. Herbivory: The consumption of all or part of a living plant by a consumer. Includes: Parasitic and phytophagous microbes (e.g., some fungi and algae) Phytophagous invertebrates Browzing and grazing vertebrates Seed predators 32 Herbivorous insects in 9 out of 29 orders of insects Numbers of species ( 80% of macroscopic plants and animals are plants, herbivores, or species that prey on herbivores. Strong, Lawton & Southwood 1984 Although only 27 of 97 order of animals and 9 of 29 orders of insects contain herbivores, these groups are very diverse. The shaded bars in the above diagram are the orders that contain herbivores. Note the log scale showing the large number of species in the order containing herbivores. Strong et al. (1984) estimate that 80% of all macroscopic species of plants and animals are plants, herbivores, or species that prey on herbivores. 33 Effects of herbivory • Herbivores typically consume about 10% of net primary production (NPP). (Deserts and tundra: 2-3%; Forests: 47%; Temperature grasslands: 10-15%; African grasslands 30-60%). • Seedlings are most vulnerable • Mature plants can withstand huge losses due to herbivory. Typically, wood production is not affected until about 50% of the leaf surface is consumed. • Seed consumption is much higher than 10% and may reach 100%. Effects of herbivory Herbivores typically consume about 10% of net primary production (NPP). (Deserts and tundra: 2-3%; Forests: 4-7%; Temperature grasslands: 10-15%; African grasslands 30-60%). Seedling are most vulnerable Mature plants can withstand huge losses due to herbivory. Typically, wood production is not affected until about 50% of the leaf surface is consumed. Seed consumption is much higher than 10% and may reach 100%. 34 Escape hypothesis: Seed dispersal is mainly a mechanism to escape from seed predators • Escape from seed predators may be the biggest factor governing dispersal and plant establishment, particularly in tropical systems (Janzen 1970 and Connell 1971). • Optimal dispersal distance from parent plant is one where survivorship from predators is balanced by liklihood of finding a favorable habitat. Augsberger 1983 Escape hypothesis: Janzen and Connel have hypothesized that the driving mechanism behind much seed dispersal is escape from predators and pathogens. Seed predators and pathogens associate parent plants with nearby food sources. The closer the seed falls to the parent, the more likely it is to be consumed by seed predators. In the above model by Augsberger, the optimum survival distance for dispersal is portrayed to be an intermediate distance from the parent where the density of seeds is moderate and the probability of survivorship in a favorable environment is also moderate. 35 Why is the world still green? Limits to herbivory • Top-down limits (predator control of herbivores) • Bottom-up limits (poor nutritional quality of plants) – Plant proteins are different from animal proteins and must be digested and resynthesized by the herbivore. – Protein content of plants is low. – Carbohydrate content is high, but mostly in the form of poorly digestible forms (lignin and cellulose). – N is often bound in relatively inaccessible forms such as secondary metabolites. Limits to herbivory: Although about 25% of the multicelled species are herbivores, only about 1020% of the aboveground green biomass is consumed annually by herbivores. There are two primary reasons for the limited effect of herbivory: 1. Top-down limits (predator control of herbivores) 2. Bottom-up limits (poor nutritional quality of plants) Plant proteins are different from animal proteins and must be digested and resynthesized by the herbivore. Protein content of plants is low. Carbohydrate content is high, but mostly in the form of poorly digestible forms (lignin and cellulose). N is often bound in relatively inaccessible forms such as secondary metabolites. 36 Plant defenses • Tolerate herbivory – Cheap plant parts, rapid growth rates (typical in resource-rich environments) – Some plants may actually be stimulated to greater production and reproduction through herbivory (e.g., scarlet gilia, Ipomopsis aggregata, Paige 1992). Ipompopsis aggregata • Constitutive (physical) defenses – Those that are a fixed part of plant allocation – Generally more expensive for the plant – Examples include hairy stems and leaves, spines, or chemicals that are not induced. • Inducible defenses – Preformed inducible chemical defenses that are stored in the plant but are transported and become active under stimulation from attack (e.g., Furanocoumarin in cow parsnips, Pastinaca sativa). – Induced chemical defenses that are produced after stimulation (e.g., nicotine production in tobacco is stimulated by early herbivory to the seedlings.) Fouquieria splendens, Ocotillo, Photos St. Marys of California Heracleum lanatum, cow parsnip, Photos Charles Webber, California Acad. Sci. Plant defenses: Plants have a wide variety ways of defending themselves from attack. 1. Tolerate herbivory Cheap plant parts, rapid growth rates (typical in resource-rich environments) 2. Physical defenses (constitutive defenses) Examples include hairy stems and leaves, spines 3. Chemical defenses (secondary plant compounds) Preformed inducible chemical defenses that are stored in the plant but are transported and become active under stimulation from attack (e.g., Furanocoumarin in cow parsnips, Pastinaca sativa). Induced chemical defenses that are produced after stimulation (e.g., nicotine production in tobacco is stimulated by early herbivory to the seedlings.) 37 Major classes of secondary plant compounds Ledum decumbens, an evergreeen shrub with abundant phenols Major classes of secondary plant compounds: This table shows some of major classes of secondary plant compounds involved in plant-animal interactions: 38 Hypothetical relationship between type of defense and probability of attack (Bazzaz 1992) • Optimal defense theory of Rhoades (1979) states that a plant should neither overallocate nor underallocate to its defenses. • Plants that grow fast are usually poorly defended. • Predictability of attack should be correlated with the allocaiton to constitutive and induced defences, i.e., if plants are not likely to be eaten they will preserve their resources for defense until they are under attach (inducible defences, see left, diagram). • Apparency theory: long-lived plants are apparent to herbivores and require more heavy defenses, i. e., high levels of Zangerl & Bazzaz 1992 constitutive defenses throughout their green tissues including tannins, resins, and lignin, also spines and tough leaves. . Numerous theories have developed that try to explain allocation of plant resources to defense. Optimal defense theory of Rhoades (1979) states that a plant should neither overallocate nor underallocate to its defenses. Plants that grow fast are generally poorly defended. In the above figure, the predictability of attack should be correlated with the allocation to constitutive and induced defenses (Zangerl and Bazzaz 1992) . Apparency theory states that long-lived plants are apparent to herbivores and require more heavy defenses. Easy to find perennials require high levels of constitutive defenses throughout their green tissues because they are constantly attacked by herbivores. This is why many tree leaves contain high levels of tannins, resins, and lignin, also spines and tough leaves. 39 Summary • Commensalism is an interaction that stimulates one organism but has no effect on the other (+,0). Examples include epiphytes and nurse plants. • Protocooperation is an interaction that stimulates both partners (+,+) but is not obligatory (e.g., root grafts in large trees).) • Mutualism is a symbiotic relationship that is essential to the survival of both species (e.g., lichens, mycorrhizae, symbiotic N-fixers, pollination, zoochory) • Herbivory is the consumption of all or part of a living plant by a consumer (e.g. Parasitic and phytophagous microbes, phytophagous invertebrates, browzing and grazing vertebrates, seed predators). • Top-down limits to herbivory relate to predator control of herbivores • Bottom-up limits are those associated with poor nutritional quality of plant • Secondary plant compounds are a primary method of defense against herbivory in many plant species. • Constitutive controls on herbivory are those that are produced without stimulation from herbivores and are expensive. Induced controls are activated or produced by stimulation from herbivores. 40 Literature for Lesson 10 Bertness, M.D. and S.M. Yeh. 1994. Cooperative and competitive interactions in the recruitment of marsh elders. Ecology 75: 2416-2429. **Bryant, J. P., F. D. Provenza, et al. 1991. Interactions between woody plants and browsing mammals mediated by secondary metabolites. Annual Review of Ecology and Systematics 22: 431-446. Bryant, J. P., J. Tahvanainen, et al. 1989. Biogeographic evidence for the evolution of chemical defense by boreal birch and willow against mammalian browsing. American Naturalist 134: 20-34. Kielland, K. and J.P. Bryant. 1998. Moose herbivory in taiga: Effects on biogeochemistry and vegetation dynamics in primary succession . Oikos, 82: 377-383. **Mulder, C.P.H. 1999. Vertebrate herbivores and plants in the Arctic and subarctic: effects on individuals, populations, communities and ecosystems. Perspectives in plant ecology, evolution, and systematics, 2: 29-55. Ruess, R. W., R. L. Hendrick, and J. P. Bryant. 1998. Regulation of fine root dynamics by mammalian browsers in early successional Alaskan taiga forests. Ecology 79:2706-2720. 41 Economic model of mutualism (Schwartz and Hoeksema 1998) Economic model: This example by Schwartz and Hoeksema demonstrates why it is adventageous for two species to enter into a mutualistic relationship based on trade of resources. We won’t go into the details of the trade, but it demonstrates why both species can benefit. For those who do want to go into the details: Species A is equally efficient at obtaining R1 and R2. Species B is 3 times more efficient at obtaining resource R2. Both species require both resources in equal amounts. Before the trade, Species A uses 24 units to consume 24 combined units of R1 and R2. Species B uses 9 units to consume 3 units of R1 and 3 units to consume 3 units of R2. After the trade, A specializes in R1 and B specializes in R2. A produces 24 units of R1 at a cost of 24 units, consuming 16 and trading 8 to B. B produces 12 units of R2 at a cost of 12 units, consuming 4, and trading 8. For the same cost as before the trade, A gains 4 units of both resources (33% gain) and B gains 1 unit of both resources (33% gain). 42 Theories to explain types of defensive compounds used (Karban and Baldwin 1997) • Supply side theories (secondary metabolites are waste products of the plant) – Carbon-nutrient balance theory. – Substrate-enzyme imbalance theory – Growth-differentiation balance theory • Part of a multifaceted strategy of plant allocation – Generalized stress-response theory – Active defense reponse theory Supply side theories (secondary metabolites are waste products of the plant) Carbon-nutrient balance theory. The types of defensive compounds will be determined by whether the plant is limited by carbon or nitrogen. Nitrogen limited plants will invest in carbon-based defenses (e.g., phenolics, terpenoids, tannins, lignin). Carbon limited plants will invest in nitrogen-based defenses (e.g., alkaloids) Substrate-enzyme imbalance theory Either C-based or N-based defensive compounds are produced as a result of excess metabolic activity. Growth-differentiation balance theory Plant growth and differentiation of different allocation pathways: plants do not differentiate tissues when maximizing growth and vice versa. If growth and differentiation processes are negatively correlated, then plants may produce secondary metabolites for use as anti-herbivore defenses only when not maximally allocating toward growth. Part of a multifaceted strategy of plant allocation Generalized stress-response theory (Chapin 1991) Plants have a centralized mechanism that allows them to simultaneously and interactively respond to diverse stresses. Active defense reponse theory (Chesin and Zipf 1990) The signaling system that plants use to induce the consturction of defensive compounds is very specific. 43 Nutrient flux gradients (Huston and De Angelis 1994) (a) High nutrient flux: Plants can coexist because each has access to only a small portion of the total available resource. (b) Low nutrient flux: Plants deplete nutrients over a much broader area. Effects of soil nutrient flux gradients Tilman later modified his resource ratio model to incorporate allocation patterns to roots and shoots (Tilman 1988). And expanded the model to predict species life histories, diversity, and competitive effects of communities at different successional stages (Tilman 1988, Tilman and Pacala 1993). Huston and De Angelis examined the effects of local soil resource depletion and soil nutrient transport rates on competition and coexistence. Because nutrient transport rates in soil are low, plants have a very limited ability to affect resource availability outside their root zones. In the above figure, the lightly shaded regions represent portions of the root zones where an individual plant has depleted local resources. Species with similar resource requirements, but restricted rooting zones (as in a) can coexist because each can access only a small portion of the of the total resources available. If soil resource depletion zones extend into the rooting zones of neighboring individuals, then competitive effects become important. Figure (a) represents the situation with high nutrient flux, where plant plants deplete resources in a narrow region. Figure (b) represents the situation with low nutrient flux, where plants deplete nutrients over a much broader area. CR is the regional concentration of soil nutrient. Cp# is the soil nutrient depletion zone created by each plant. From M.A. Huston and D.L. DeAngelis. 1994 American Naturalist 140: 539572. 44 Productivity vs. species richness (Tilman and Pacala 1993) • • • Habitats intermediate in resources (and productivity) tend to support the most species. Extremely poor soils are likely to dominated by a few species that can compete will for a single limiting resource. Extremely rich soils support high biomass production and are dominated by the few species that compete the most effectively for light. Sites with intermediate resources tend to have the highest species diversity. Extreme habitats (too rich or too poor in resources) generally support relatively low biological diversity. Habitats intermediate in resources (and productivity) tend to support the most species. Extremely poor soils are likely to dominated by a few species that can compete will for a single limiting resource. Extremely rich soils support high biomass production and are dominated by the few species that compete the most effectively for light. From D. Tilman and S. Pacala. 1993. The maintenance of species richness in plant communities. In: R.E. Ricklefs, and D. Schluter (eds.) Species diversity in ecological communities. Historical and geographical perspective. University of Chicago Press. 45 Salt marsh commensalism between Iva frutescens and Juncus Bertness & Yeh 1994 • Marsh elder, Iva frutescens, is inhibited by dense perrenial turfs of plants, and is typically found in disturbed bare patches. • These sites, however, have hot soil conditions, high salinity, and have few recruits. • Iva that germinates under adults or in clumps of Juncus has higher success rates. Salt-marsh example 4. In the above example, Bertness and colleagues observed that in the Rhode Island salt marshes, marsh elder, Iva frutescens, is inhibited by dense perrenial turfs of plants, and is typically found in disturbed bare patches. These sites, however, have hot soil conditions, high salinity, and have few recruits. Iva that germinates under adults or in clumps of Juncus has higher success rates. Caption for the above figure: Dry mass of surviving seedlings of Iva frutescens, when grown solitarily or with other Iva seedlings or with Juncus seedling neighbors. Three treatments included watering to prevent saline buildup, shade to prevent high evaporation, and other nurse plants. Dry mass of the surviving seedlings are shown. In the control there is little surviving in any of the situations. Plants that are grown with nurse plants, do as well as those in the watered or shade experiments, and much better than the control. Each bar (and 1 standard error) represents the mean dry mass of 4-100 seedlinigs. Means with same letter are not significantly different at the p< 0.05 level. From Bertness, M.D. and S.M. Yeh. 1994. Cooperative and competitive interactions in the recruitment of marsh elders. Ecology 75: 2416-2429. 46 Bird Pollination Faegri & van der Pijl 1971 Pollination: In tropical regions, birds are much more important pollinators. Examples in include sunbirds of Africa, honey creepers of Hawaii, and hummingbirds of North and South America. There has also been a great deal of coevolution between flowers and bird pollinators. This topic of coevolution between birds and flowers has received a lot of attention of plant evolutionary ecologists. 47