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Unformatted text preview: Community Ecology Community Communities: Assemblage Assemblage of species found in the same area and presumably interacting. same Includes: Includes: plants, animals, fungi, bacteria, protists etc. protists Community Ecology Community These These species interact and affect one another, often in subtle and unanticipated ways. ways. Structures Structures and behaviors of communities are complex and require distinct methodologies. distinct Community Ecology Community 1. 2. 3. 4. We are going to focus on four community We topics: topics: Dominance Changes with Time (Ecological Succession) Diversity Island Biogeography and Conservation Island Biology Biology Community Ecology Community Community Properties 1. Physiognomy = Physical structure and growth form of dominant organisms, usually growth the plant community. Usually used to describe the dominant biome describe Community Ecology Community Forests = Trees Chaparral = Chaparral Shrubs Shrubs Community Ecology Community Savannas Savannas and Prairies: Grasses Prairies: Tundra: Tundra: Lichens and Dwarf Shrubs and Community Ecology Community 2. Abundance = the number of individuals of a given species in the community; Density (number of individuals per unit Density area) = ni for species i. area) for The total number of individuals in the The community is: community ni ∑ Community Ecology Community 3. Relative Abundance = Proportion of a given species in the community = pi given ni pi = ∑ ni Community Ecology Community 4. Species Richness = the total number of different species in the community different 5. Species Diversity = relative abundance Species and species richness are combined into a single parameter, such as the Shannonsingle Weaver Index (H’). Community Ecology Community 6. This index reflects the relative diversity of a This sample of the community. Can be compared to samples taken in other Can communities. communities. Dominance = those species assumed to Dominance be most important ecologically. be Dominance Dominance Dominant species may be: 1. The greatest primary producer. 2. The most numerous. 3. Those with the greatest biomass. 4. Those with the most frequent Those occurrence across samples. occurrence 5. Those with the greatest impact on the Those ecosystem. ecosystem. Dominance Dominance For plant communities the following For plant are often used in combination as a measure of dominance. measure 1. Relative Abundance = Proportion of a given species in the community = pi given ni pi = ∑ ni Dominance Dominance 2. Coverage or Biomass Measure Canopy Coverage, or more Measure commonly measure tree diameter (Diameter at Breast Height or DBH). Convert the diameter to basal area. For Convert each tree the basal area = each d π 2 2 Dominance Dominance Take the sum of the above for each species per Take unit area. This is the absolute basal area/coverage for This each species. Then divide the species specific sum by the total Then of the basal areas of all individuals of all species. of This is the relative dominance based on This relative biomass. biomass. Dominance Dominance 2. That is: Relative Dominance = total dominance (basal area) per species ÷ total dominance (basal area) of trees of all species. Dominance Dominance 3. The third measure is based on relative The frequency. In this case you simply record frequency In the percentage of plots in which you find a given species. If you set up 100 ten meter square plots, you would then count how many of the 100 plots contains a given species, i. This is the Relative Frequency. This Relative Importance Value Importance The Importance Value of a Species is then The Importance measured as: measured Percent Relative Abundance Percent 2. Percent Relative Dominance 2. Percent 3. Percent Relative Frequency 3. 1. Importance Value Importance Importance value is therefore the sum of: Relative Abundance + Relative Dominance + Relative Relative Frequency. Since each is a number based on 100, the importance value is scaled to 300. to Dividing by 3 gives us an Importance Value Dividing based on a scale of 0 – 100. based Community Development or Ecological Succession Ecological Communities Communities change with time in relatively predictable ways. These These temporal patterns are most evident in the plant community, which again led to the importance of community studies by botanists. Community Development or Ecological Succession Ecological The The term “community development” reflects the old view that a community was much like a living individual organism and would go through “developmental stages” from egg to mature organism. mature Animal Community Succession Succession Note Note that animal communities change over time in parallel with the plant community. plant Community Development or Ecological Succession Ecological 1. Two different types of Two succession are recognized, depending on what produced the starting point. starting Primary Succession takes place on a habitat the never previously supported a biological community. Examples: Lava field, bare rock, sand dune. bare Community Development or Ecological Succession Ecological 2. Secondary Secondary Succession takes Succession place on habitats that have been disturbed, but which had previously supported a biological community. community. Ecological Succession Ecological Examples of Examples secondary succession include succession succession after: succession A fire A flood A landslide Human clearing of the Human land land Ecological Succession Ecological In secondary succession, there almost In secondary there always remains organic debris, a developed soil and a supply of seeds, undamaged roots and other remnants of the previous plant community. plant Ecological Succession Ecological Therefore, secondary succession usually Therefore, occurs much more rapidly than primary succession. In primary succession, early colonizing In organisms must deal with a severe organisms microclimate and a lack of soil. and Time Frames Time It is important to distinguish the time frame It of succession. For some successional sequences, like the For colonization and ultimate elimination of a dung pile, the time frame may be days, days weeks or months. weeks months Time Frames Time For most plant For communities, successional time from colonization to the climax community is anywhere from 50 to 500 years. to Time Frames Time By contrast, the time frame for climatic change is harder to specify, but we are change usually talking about hundreds of years to tens of thousand of years. Evolutionary time is also hard to specify, but we are usually talking thousands to millions of years for major evolutionary changes. of Time Frames Time Continental Drift takes place from 100 to several hundreds of millions of years millions And in Geological And Time, the numbers Time the are at least that large. large. Time Frames Time The The major point is that succession takes succession place in ecological time, which is shorter ecological which than these other time frames. than The The earliest stages of succession occur rapidly enough to be observed within a human lifetime. human Succession: Properties Succession: 1. There is often an orderly, predictable There pattern of species colonization and replacement. Early colonizers (pioneer or rreplacement. selected species) are replaced by species selected that arrive later. that Succession: Properties Succession: One community replaces another over time One until the final or climax community is climax achieved. This climax community perpetuates itself This until the next disturbance or until the climate changes. climate Example: Succession in the Piedmont Piedmont Years After Last Dominant Plant Cultivation Other Common Species 0 (fall) crabgrass 1 horseweed ragweed 2 aster ragweed 3 Broomsedge 5 – 15 shortleaf pine 50 – 150 (Loblolly Pine) Hardwoods (oaks) (Hickories) Properties of Succession 2. 2. Early successional species must have Early special properties that enable them to live special in the severe environment they encounter. These species almost always have rThese selected life histories. Characteristics of Early Successional and Late Seral Stages in Succession (From Odum 1969 and Bazzaz 1979) Attributes Early Stage Later Stage Seed dispersal Good Poor Plant efficiency at low light Low High Resource (nutrient) acquisition Fast Slow Biomass Small Large Stability Low High Diversity Low High Species life history r K Seed dispersal vector Wind Animals Seed longevity Long Short Properties of Succession Properties Their activities often ameliorate the Their environment, making it possible for later successional species to survive. successional This is called autogenic succession This autogenic because the changes are caused by the activities of the community itself. activities This process is also called the Facilitation This Model. Model Properties of Succession Properties Examples: A. Early successional species improve the soil and Early microclimate during both primary and secondary succession (Indiana Dunes, described by Cowles in 1899). in B. Shade produced by pines (shade-intolerant trees) Shade results in favorable conditions for the shaderesults tolerant tree species (oaks, hickories, beech) that tolerant replace them. replace Properties of Succession Properties C. D. Lakes and marshes trap nutrients and organic Lakes material over time, elevating the substrate and leading to a drier habitat. Wetland species are progressively replaced by more drought tolerant species. species. Degradative succession involves the progressive Degradative destruction of dung or leaf-litter leading to the demise of the original colonizing species. demise Properties of Succession Properties The activities of some species appear to The inhibit the colonization of other species. This is known as the Inhibition Model. Inhibition For example, the grass broomsedge is For thought to chemically inhibit the germination and growth of species that might replace it. Properties of Succession Properties Many plant species Many such as tree of heaven, Ailanthus altissima, and black walnut, produce what walnut, are known as allelopathic allelopathic chemicals that inhibit other species from growing in the soil around it. around Properties of Succession Properties 3. Succession can also Succession be driven by changes in the physical environment. environment. This is known as This Allogenic Succession. Succession Properties of Succession Properties Example: In lake or pond succession, silt and debris can enter the lake or pond due to breakdown of rocks, along with wind and water erosion of the terrestrial landscape. Therefore, lake or pond succession is driven by both allogenic and autogenic forces. forces. Properties of Succession Properties 4. Succession stops when there is no further Succession change in the plant community. This final community is known as the Climax Community. Community This implies that there is also no further This change in the environment. Properties of Succession Properties Deterministic Succession Models propose that there is one final climax community for a given prevailing environment. environment. Properties of Succession Properties This is known as the “climatic climax”. Most biome or life zone models adopt the Most climatic climax when they label a given geographic area. Climatic climax nomenclature usually uses Climatic the dominant species names. the Properties of Succession Properties Examples: Beech-Maple Forest Oak-Hickory Forest Life Zones by contrast are named: Temperate Deciduous Forest Properties of Succession Properties In In any case, ecologists now recognize that, although succession is relatively predictable, it can take alternative pathways and we may not always end up with the predicted climax. not In In addition to climate, soil may also play a soil role in succession. role Properties of Succession Properties Models Models recognizing the potential unpredictability of succession are stochastic models. stochastic Processes of Succession Processes 1. Colonization of an open habitat: Carried out by Colonizing, r-selected, or Carried “pioneer” species. “pioneer” 2. Site modification: Soil stabilized and improved. Microclimate Soil improved. improved. Processes of Succession Processes 3. Continuing colonization by new species. 4. Competition between earlier and later Competition colonizers. colonizers. Processes of Succession Processes 5. Community Changes: Earlier colonizers lose to later-colonizing Earlier species that are better competitors. better 6. Further Site Modification. Processes of Succession Processes 7. Further colonization and further community Further change. change. Finally, 8. Climax community when there are no Climax longer major changes in community. longer Primary and Secondary Successional Sequences Successional Examples: 1. Secondary Succession: North Carolina and Secondary Virginia Piedmont Virginia 2. Primary Succession: Indiana Dunes Example: Succession in the Piedmont Piedmont Years After Last Dominant Plant Cultivation Other Common Species 0 (fall) crabgrass 1 horseweed ragweed 2 aster ragweed 3 Broomsedge 5 – 15 shortleaf pine 50 – 150 (Loblolly Pine) Hardwoods (oaks) (Hickories) From Billings, 1938. Red Cedars invading Broomsedge field Broomsedge Pines invading Broomsedge field field Typical Summer Wildlife Typical Succession in North Carolina Dunes Dunes Successional Convergence Successional According According to the deterministic successional theorists, such as Clements, the end point of succession is the climatic climax, no matter what the starting point is. starting Successional Convergence Successional Thus, succession in bog lakes in Michigan (a hydric community) and dry dunes in hydric and Michigan and Indiana (a xeric community) Michigan should end in a forest dominated by beech and sugar maple (a mesic community). and Successional Convergence Successional This This is know as successional convergence. successional A xeric (dry) or hydric (wet) community xeric hydric (wet) undergoes succession resulting in a mesic mesic (medium moisture) community: the climatic climax. climax. In In this case a Beech-Sugar Maple dominated forest. forest. Predicted Trends during Succession Succession 1. Reproductive Reproductive strategies tend to be r-selected early in succession and Ksuccession selected later in selected succession. succession. Predicted Trends during Succession Succession There There is basically a life history trade-off between high reproductive potential and colonizing ability versus competitive ability. colonizing Known Known as the Competition-Colonization Trade-off. Trade-off Predicted Trends during Succession Succession 2. Species Richness/Diversity Increases Species with Succession. with As ecosystems develop they become more As productive and structurally more complex leading to a larger number of species. Over time species continue to colonize and species richness goes up. species Predicted Trends during Succession Succession However, maximum diversity is found prior However, to the Climax stage of succession, before competitive exclusion has eliminated some of the early colonizing species. of This is known as the “intermediate This disturbance hypothesis”. disturbance Diversity Diversity Ecologists have long been fascinated by Ecologists biological diversity. One of the basic facts of life is that the number of species found in a community shows an amazing amount of variability. The preservation of biodiversity has The become a rallying cry for environmentalists and even rock stars like Sting and Bono. and Diversity Diversity As mentioned previously, a simple idea of As diversity is to make simple counts of species. This is known as species richness. richness However, species counts depend on: A. B. C. Sample size and effort. Ability to capture or locate the species in Ability question. question. Taxonomic knowledge. Insect Sweep Net Insect Early Succession Insect Sweeps - Year 3 Early Succession Insect Sweeps - Years 1 - 2 350 400 300 350 300 Cumulative Species 200 150 y = 34.783x + 11.639 R2 = 0.9473 100 50 250 200 y = 36.167x + 9.1667 R2 = 0.9382 150 100 50 0 0 0 1 2 3 4 5 6 7 0 8 1 2 3 4 Early Succession Insect Sweeps - Year 4 350 300 250 200 150 100 y = 35.333x - 8.8889 R2 = 0.9634 50 0 0 5 Hundreds of Sweeps Hundreds of Sw eeps Cumulative Sw eeps Cumulative Species 250 1 2 3 4 5 Hundreds of Sw eeps 6 7 8 6 7 8 Diversity Diversity Therefore, biologists have searched for an Therefore, index, known as a species diversity index, species that will allow comparisons among similar samples, but which is somewhat independent of sample sizes. independent We will return to this topic later. Diversity Diversity When we are discussing diversity, what When forms of life contribute most significantly to this pool of species? this The answers: A. B. C. D. Arthropods, but especially insects. Arthropods, insects Mollusks. Flowering plants. Note that prokaryote diversity is mostly unknown. Trends in Diversity Trends 1. Environmental Harshness As As temperature, salinity, pH, moisture, light intensity and chemicals reach extremes, diversity declines. declines Biodiversity can be an indicator of Biodiversity pollution or other environmental degradation. degradation. Trends in Diversity Trends 2. Succession As As communities develop over time diversity increases, but reaches a peak prior to climax. Recall again the intermediate Recall disturbance concept. disturbance Trends in Diversity Trends 3. Latitude/ Altitude Diversity declines with increasing latitude Diversity and altitude, with some exceptions. and Fifty-percent of biodiversity of the Earth is Fifty-percent located in 7% of the Earth’s surface in the tropical latitudes. tropical Trends in Diversity Trends Examples Tree species per Tree hectare: hectare: Michigan = 10 – 15 S.E. US = 30 Tropical Peru = 300 Trends in Diversity Trends Examples Number of Flowering Plant Species Temperate North America = 17,000 Tropical/Subtropical Africa = 42,000 Tropical/Subtropical Asia = 50,000 Neotropics = 85,000 Land Bird Species Numbers Fruit Eating Bats Nectar Eating Bats Insect Eating Bats Carnivorous Bats Fishing Bat Video Vampire Bat Video Trends in Diversity Recent surveys have found that diversity hot spots are mostly found in three areas of the world. By hot spots we mean high numbers of endemic species (species found uniquely in these areas). Tropical areas near the equator Islands Mediterranean type ecosystems Caucasia Caucasia The The Caucasus or Caucasia is a region in Caucasus Caucasia Eurasia bordered on the north by Russia, on the southwest by Turkey, on the west by the Black Sea, on the east by the Caspian Sea, and on the south by Iran. The Caucasus includes the Caucasus The Mountains and surrounding lowlands. Caucasus Caucasus The Caucasus The Mountains are Mountains commonly reckoned as a dividing line between Asia and Europe, and is one of the most linguistically and culturally diverse regions on Earth. Caucasus Caucasus The Caucasus is an The area of great ecological significance. The Caucasus The harbors about 6400 species of higher plants, 1600 of which are endemic to the endemic region. region. Caucasus Caucasus Among Among its native animals are leopards, brown bears, wolves, European bison and golden eagles. Among the invertebrates approximately 1000 Among spider species are recorded from the Caucasus. The natural landscape of the Caucasus is The one of mixed forest, with substantial areas of rocky ground above the treeline. rocky Measuring Diversity Measuring 1. Species richness is the number of species in the community. in Alpha diversity = Within-community diversity. We specify a scale and count the number of species in a standard sample. For example, the number of insect species in 100 sweeps of a net or number of trees per hectare sample. per Measuring Diversity Measuring 2. Gamma Diversity = We record the number of species from a whole landscape made up of many different communities. We often record the number of species for some political boundary. For example, the number of mammal species in the state of Virginia or the number of bird species in Costa Rica. Costa Measuring Diversity Measuring The difficulty here is that we are lumping The together diversity from different functional communities. Still we sometimes find that while each Still community has few species, different communities have unique species and the entire landscape can be species rich (high gamma diversity), while each small community has few species (low alpha diversity). diversity). Measuring Diversity Measuring For example, the Mediterranean biome of For South Africa, known as the fynbos, has 8500 plant species on 89,000 km2. 8500 It is made up of a mosaic of different It communities, each of which has few species. But combining all of the mosaic results in But one of the most diverse areas of the world. one Fynbos in South Africa Measuring Diversity Measuring 3. Beta Diversity = Within a community, species replace each other because of microclimatic or topographic differences. For example, within a small area, different species live on the tops of hillsides (oak or hickory, for example) and others in valley bottoms (beech or tulip poplar). Species also change with successional time. also Measuring Diversity Measuring As discussed above, species richness is As affected by sample size and taxonomic errors. Generally speaking, as we increase sample Generally increase size we find more species. more If the sample size is large enough, the If gamma diversity problem comes into play. gamma Measuring Diversity Measuring But we also must But be concerned with the amount of effort necessary to increase the sample size. sample Early Succession Insect Sweeps - Year 3 Early Succession Insect Sweeps - Years 1 - 2 350 400 300 350 300 Cumulative Species 200 150 y = 34.783x + 11.639 R2 = 0.9473 100 50 250 200 y = 36.167x + 9.1667 R2 = 0.9382 150 100 50 0 0 0 1 2 3 4 5 6 7 0 8 1 2 3 4 Early Succession Insect Sweeps - Year 4 350 300 250 200 150 100 y = 35.333x - 8.8889 R2 = 0.9634 50 0 0 5 Hundreds of Sweeps Hundreds of Sw eeps Cumulative Sw eeps Cumulative Species 250 1 2 3 4 5 Hundreds of Sw eeps 6 7 8 6 7 8 Measuring Diversity Measuring If we are using standard samples, the If results are affected by the number of individuals encountered in a sample. individuals The results can be affected by differences The among communities in the amount of dominance. This so-called relative abundance can affect the results of our abundance sample. sample. Measuring Diversity Measuring Therefore ecologists developed diversity Therefore indices (s. index) with two components: indices 1. Species Richness 2. Evenness or Equitability To understand why, consider the following To two samples. two Measuring Diversity Measuring Measuring Diversity Measuring Measuring Diversity Measuring Richness and evenness are combined into evenness one value known as a heterogeneity index. Best know is the Shannon-Weaver Index Shannon-Weaver (H’). To To calculate this index you must produce a list containing the number of individuals for each species in the sample, as above. each Measuring Diversity Measuring N = the total number of individuals of all the species species n = the number of individuals of each species i p = the proportion of the sample belonging to species i species S = the total number of species in the the community community i Measuring Diversity Measuring pi = ni Shannon-Weaver N Index: S pi ln i H ' = −∑ pi Measuring Diversity Measuring Measuring Diversity Measuring Measuring Diversity Measuring Obviously Obviously the H’ is much greater for community 1 than for community 2, even though the species richness values both equal 10. equal Community 1 has the maximum diversity Community possible for a community with ten species. possible Maximum diversity = lnS = ln10 = 2.30 Maximum Measuring Diversity Measuring Another Another view of H’ is to take it as an exponent of e, the base of natural logs. exponent The H’ value of eH’ for community one = 10.0 and for community two = 1.65. These These numbers represent the equivalent number of equally represented species in the community. So for community one we have 10 equally represented species. 10 Measuring Diversity Measuring To To compare species diversity among different communities, we must compare comparable organisms and we must take comparable samples. When When communities show differences we must do statistical testing to differentiate minor from significantly different H’ values. minor Island Biogeography Theory and Conservation Biology and 1. 2. Basic issues in conservation biology Basic include: include: How can human dominated and natural How ecosystems be managed to preserve not only species diversity, but also ecosystem integrity? integrity? What can be done to mediate the threats What posed by wildlife diseases and by invasive species? species? Island Biogeography Theory and Conservation Biology and 3. 4. 5. Which ecosystems are most in need of Which preservation? preservation? What is the critical size necessary for What ecosystem integrity and maximum species preservation? preservation? How to resolve the old argument: A Siingle ngle How Large Or Several Small (SLOSS) arge everal preserves? Island Biogeography Theory and Conservation Biology and Many wildlife populations that were once: Numerous Widespread Occupied Contiguous Habitats Are now: Small Restricted in Distribution Isolated from each other Island Biogeography Theory and Conservation Biology and These These small, isolated populations are subject to stochastic processes. stochastic Chance Chance events can dominate long-term dynamics of small populations. dynamics Sources of Stochasticity in Small Populations Small Genetic Processes: 1. Inbreeding Depression 2. Genetic Drift and loss of genetic Genetic diversity diversity 3. Monopolization by a small number of Monopolization males in polygynous mating systems males Sources of Stochasticity in Small Populations Small Disruption of breeding systems. Allee effect (higher mortality and lower fertility in very small populations). very Sources of Stochasticity in Small Populations Small Demographic processes Mortality rate (new born, juveniles, adults) 2. Reproduction rate (litter size, number of litters per Reproduction year) year) 3. Sex ratio 1. Sources of Stochasticity in Small Populations Small Catastrophes Local or regional events of low probability Local with significant density independent effects (may affect survivorship, reproduction, or both), such as fire or drought. both), Sources of Stochasticity in Small Populations Small Examples: One of two remaining One whooping crane populations was decimated by a hurricane in 1940 and soon thereafter went extinct. went Sources of Stochasticity in Small Populations Small The only remaining population of the blackfooted ferret was being decimated by an footed outbreak of distemper when the last 18 distemper were captured by scientists for a captive breeding program. breeding Only 7 animals actually reproduced. Moreover, its prey, the prairie dog was Moreover, suffering from an attack of bubonic plague bubonic at the same time. at Sources of Stochasticity in Small Populations Small At the present time, the black footed ferret At population needs to be supplemented from captive populations. captive The animals also need to be vaccinated The against distemper and plague. against The males are suffering from reduced sperm The count as well as a high percentage of defective sperm. defective Any hope? Sources of Stochasticity in Small Populations Small Black Footed Ferret (Mustela nigripes) Prairie Dog (Cynomys ludovicianus) History of Krakatau On August 27, 1883 Krakatau, an island about the size of Manhattan located between Sumatra and Java, underwent a series of volcanic eruptions releasing as much energy as 100 megatons of TNT (Wilson 1992). Magma, ash and rock flew 5 km into the air and fell back into the sea, creating a tsunami 40 m in height, washing away villages in Java and Sumatra, killing 40,000 people. Waves were still a meter high when they came ashore in Sri Lanka. History of Krakatau History Krakatau A total of over 18 cubic kilometers of rock total and ash were thrown into the air with dust and sulfuric acid aerosol reaching 50 km in the stratosphere where its effects were seen as brilliant sunsets for several years thereafter. All of this airborne material produced All “darkness at noon” in areas near the former Krakatau. Krakatau. History of Krakatau History Krakatau Only Only the southern end of Krakatau remained. This island, which became known as Rakata, was covered by pumice 40m thick. The pumice had been heated to between 300 and 850oC been and all living things had been destroyed. destroyed. Rakata was a sterile island. History of Krakatau History Krakatau Yet living things soon Yet began colonizing this lifeless rock. Nine months after the Nine explosion a visitor found a small spider spinning its web. In the fall of 1884, a year after the eruption, biologists found a few shoots of grass. grass. History of Krakatau History Krakatau By By 1886 there were 15 species of grasses and shrubs; by 1897 there were 49 species; and in 1928 300 species of plants were found. In 1919 there were patches of forest; by In 1929 most of the island was forested, forcing the grasses into small pockets (Wilson 1992). (Wilson History of Krakatau History Krakatau What Wilson described in the preceding What paragraphs sounds like a typical successional sequence, proceeding from a community of colonizing species to a mature or “climax” forest. What we want to emphasize as we consider the What topic of metapopulations, is the two processes at work on Rakata. Obviously one of those processes is Obviously colonization. New species continuously arrive colonization on this island from the nearby mainlands. History of Krakatau History Krakatau The The other process is local extinction. local Many species that arrived on this island, and Many were recorded early in the 20th century, are no longer present. Again, this may not be surprising to students Again, of succession. So called climax species are supposed to outcompete and eliminate earlier successional species. History of Krakatau History Krakatau But But is that what happened? At least one animal species, the reticulated python, that we would associate with the more mature community was present as early as 1933, but was gone by the 1980s. The bird community is perhaps more to the The point. History of Krakatau History Krakatau In In 1908, 13 species of birds were recorded on Krakatau; by 1920 there were 31 species; and by 1933 30 species were found. Wilson (1992) believed that an equilibrium Wilson equilibrium number for Krakatau was approximately 30 species of birds and that number had been reached by about 36 years. reached Table 4. Number of species of land and fresh-water birds on Krakatau and Verlaten during three collection periods together with losses in the two intervals. (Based on data from Dammerman, 1948) History of Krakatau History Krakatau More importantly, however, is that the actual More composition of the bird community has not remained stable. During the interval between the 1920 and 1933 During surveys, five species of birds went extinct on Krakatau, to be replaced by four new species (MacArthur and Wilson 1967). For example, the bulbul (Pycnonotus aurigaster) and the gray-backed shrike (Lanius schach) had (Lanius gone extinct on Rakata between 1920 and 1933. gone History of Krakatau History Krakatau 1. 1. 2. 2. The history of Krakatau illustrates two major The points: Local populations are continuously subject Local to the twin processes of colonization and extinction. Communities are continuously changing. Communities Even when the number of species in the community is static, the composition of the community is not. community Paradigms for Conservation Biology Biology 1. The equilibrium theory of island The biogeography of MacArthur and Wilson biogeography 2. Metapopulation Ecology Both have similar assumptions. Paradigms for Conservation Biology Biology 1. 2. Local population Local extinctions are common events. common Movements of Movements individuals among populations are common. common. Paradigms for Conservation Biology Biology 3. 4. Populations on a given habitat patch Populations routinely undergo local extinction, but these extinctions are balanced by immigration from other local populations. from Species diversity on a habitat patch is Species therefore a balance between extinction and recolonization. recolonization. Paradigms for Conservation Biology Biology A major difference is that the MacArthur major and Wilson theory is now called a and mainland-island metapopulation, as as opposed to the more classical metapopulations. metapopulations. Paradigms for Conservation Biology Biology In a mainland-island metapopulation, the In source of species (the mainland) is never envisioned as suffering from extinction. envisioned Paradigms for Conservation Biology Biology The mainland is seen as a large population, The not subject to extinction processes and is therefore a constant source of species. therefore The mainland itself does not benefit from The the islands as sources of species. the Paradigms for Conservation Biology Biology In a metapopulation, all habitat patches can In undergo extinction, and all can receive immigrants from other populations. immigrants Patches Connected by Immigration and Emigration Immigration Paradigms for Conservation Biology Biology Immigration and emigration are two-way Immigration processes for all populations, whereas in the island-mainland metapopulation, the mainland is the only source. Movements among islands are not seen as Movements important. important. The MacArthur and Wilson Theory Theory Islands Islands and island examples have long been of importance to Biology. of The lessons learned from islands have been The applied to isolated habitat islands. habitat Examples include lakes, tidal pools, mountain Examples tops, caves, even host plants for herbivores or host animals for parasites or The MacArthur and Wilson Theory Theory The The world is increasingly fragmented and conservation biologists are aware that wildlife preserves are essentially islands in a sea of development. development. Most Most wildlife populations are highly fragmented. fragmented Changes in the distribution of forest due to human influence in the past century and a half. The total area in Wisconsin is about 10 kilometers on each side. Fragmentation of the forest has created many very small habitat islands. (From J.T. Curtis, 1956.) The MacArthur and Wilson Theory Theory A cornerstone of island biogeography theory cornerstone is the relationship between the number of species and the area of an island or habitat patch. This relationship is described as: patch. S = CAz Where Where A = area and S = the number of species on the island species The MacArthur and Wilson Theory Theory Taking Taking logs of both sides of this equation gives us: gives Log S = Log C + z LogA This is an equation for a straight line, with C as This the y-intercept. the The MacArthur and Wilson Theory Theory Z = the slope Z iis a constant that remains fairly s consistent depending on the type of taxonomic group we are examining and/or the type of island. and/or That is, true oceanic islands, versus That habitat islands etc. habitat The MacArthur and Wilson Theory Theory Log S = Log C + z LogA The The main point is that on a log-log scale there is an expected linear relationship between the number of species found on an island and the area of the island. island This This is known as the Species-Area Relationship. Relationship Island Biogeography Using Lichens Fall 2010 1.20 Log of Num ber of Species 1.00 0.80 0.60 0.40 0.20 0.00 0.00 y = 0.1482x + 0.2819 R2 = 0.3113 0.50 1.00 1.50 2.00 2.50 Log of Rock Area 3.00 3.50 4.00 The MacArthur and Wilson Theory Theory On On larger islands we find more species for any taxonomic group as compared to small islands. islands. Why? Some have suggested that as islands get Some bigger there are more habitats due to topographic complexity. topographic Hence, more species. The MacArthur and Wilson Theory Theory However, However, Simberloff and Wilson found that even on simple mangrove islands (islands consisting of one species of tree) the species area relationship still held. area As As discussed below, larger islands can have larger average population sizes, leading to a lower rate of stochastic extinction. The MacArthur and Wilson Theory Theory Larger Larger islands are also larger targets for targets immigrant species to find. immigrant This This leads us to the major point of the MacArthur and Wilson theory. MacArthur The MacArthur and Wilson Theory Theory The number of species on an island is due to The number the two contrasting processes: the 1. Immigration 2. Local Extinction The number of species on an island is due to The the equilibrium between these two processes. the Just as in our discussion of succession, we are Just talking about ecological time. In this case 1 – 25 years. 25 The MacArthur and Wilson Theory Theory Immigration The earliest immigrants to arrive on an The island, just as in succession, are r-selected species. That is, species with high dispersal rates, That small bodied, with fast growth and high reproductive rates. But by contrast, they are poor competitors. The MacArthur and Wilson Theory Theory 1. 2. As the number of species on the island increases, As the addition of new species decreases. This happens for two reasons: happens Many immigrants will belong to species already on Many the island. Therefore, they simply add their numbers to an already established species. numbers New r-selected species that arrive may lack the New competitive ability to succeed in an increasingly crowded environment. crowded The MacArthur and Wilson Theory Theory Finally, the immigration curve is Finally, expected to be concave since the most rapid dispersers will arrive first. rapid The MacArthur and Wilson Theory Theory Extinction The extinction rate will be low at first. The There will be many open habitats and little competition. competition. The MacArthur and Wilson Theory Theory As populations of established species grow, As competition will increase. Some species will be competitively excluded. There will also be lower average population sizes resulting in an increase in stochastic extinctions. in Finally, the more species there are, the Finally, larger the number than can go extinct. larger The MacArthur and Wilson Theory Theory The extinction curve The is expected to have an exponential shape due to competition and diminishing average population sizes. sizes. The MacArthur and Wilson Theory Theory The Equilibrium Number of Species The expected number of species on the The island is that portion of the graph where the two curves intersect. two The MacArthur and Wilson Theory Theory When When the “equilibrium” number of species is reached, we expect no further change in the number of species. number However, the constant is the number of However, number species, not the identity of the species. identity Since extinction is a locally common event, Since we actually expect a regular “turnover” in the identity of the species in the community. identity The MacArthur and Wilson Theory Theory Contrast Contrast this to the “Climatic Climax” theory of succession. of In the climatic climax, the expectation is a In fixed deterministic community and we do fixed not expect local extinctions or community changes. changes. The MacArthur and Wilson Theory Theory The The expected number of species is also influenced by the size of the island and its size distance from the mainland. distance Immigration Immigration rates are lower on islands that are far from the mainland, the source of the species. species. The MacArthur and Wilson Theory Theory Immigration Immigration rates may also be lower on small islands since they provide smaller targets for species dispersing from the mainland. species Extinction Extinction rates are higher on small islands. This is due to the lower average population sizes resulting in higher rates of stochastic extinctions. The MacArthur and Wilson Theory Theory We We therefore expect to find more species on large islands close to the mainland. large We We also expect to find fewer species on small islands far from the mainland. small The The other two combinations result in intermediate diversity. intermediate Ŝ for Large Far Islands Evidence for Island Biogeography Biogeography 1. Lines of evidence include: Rates of colonization of new habitats. Krakatau Krakatau As describe earlier, this volcanic island As exploded in 1883 burying the surface under 10 – 30 m of ash. The entire flora and fauna was sterilized, The setting primary succession into action. setting Evidence for Island Biogeography Biogeography Colonization rates were recorded. See next slide for plants. For Birds: Equilibrium reached in 25 – 36 years. The Equilibrium equilibrium was 30 species with a turnover rate of 1.15 species per year. rate Table 4. Number of species of land and fresh-water birds on Krakatau and Verlaten during three collection periods together with losses in the two intervals. (Based on data from Dammerman, 1948) Evidence for Island Biogeography Biogeography 2. Simberloff and Wilson tested the theory in Simberloff the Florida Keys. Seven red mangrove islands of various sizes were surveyed for arthropod species. They then killed all animals on the islands by fumigation. animals The pre-defaunation surveys found 20 – 50 The species on the islands out of 600 – 700 on the mainland. the Evidence for Island Biogeography Biogeography The study included control islands. Results: On all islands, the number of species On recovered due to immigration, rose above pre-experimental levels and then fell back to expected numbers. expected As predicted, the equilibrium was dynamic. 1. 2. Evidence for Island Biogeography Biogeography Species Species turnover was 0.5 – 1.0 per day or about 2% per day. about The The results confirmed that the number of species rose to an equilibrium number and stabilized. stabilized. Moreover, Moreover, the equilibrium was dynamic, with the species composition constantly changing. Island Biogeography, Metapopulations and Wildlife Preserves Preserves Metapopulation Metapopulation biology has replaced Island Biogeography as the dominant model for conservation biology. conservation We We do not have time to explore the differences between these two approaches in detail. detail. Island Biogeography, Metapopulations and Wildlife Preserves Preserves But But a major difference is that we do not now envision a permanent, extinction-free “mainland” as the source for dispersing species. dispersing Island Biogeography, Metapopulations and Wildlife Preserves Preserves We We are interested in the movement of individuals from one population to another and acknowledge that any population may go extinct. extinct. We We make efforts to prevent an entire metapopulation from going extinct. This would result in regional or even global extinction. extinction. Wildlife Preserves Wildlife Three main issues are: Size 2. Shape 3. Connectivity to other preserves 1. Wildlife Preserves Wildlife Size According to the species/area relationship, According the larger the preserve, the more species it can accommodate. In addition, we want our preserve to be able to accommodate populations of large plants, large animals and all trophic levels large Wildlife Preserves Wildlife Recall the trophic pyramid. There is less energy Recall flowing to the top trophic levels. These carnivores are also likely to be some of the largest animals in the ecosystem. largest Top Carnivores Top Therefore we need large preserves so that there Therefore is enough room and enough energy to support these top carnivores. these Wildlife Preserves Wildlife Large Large wildlife preserves would also allow larger population sizes for all of the plants and animals. This would reduce the probability of stochastic extinctions. Genetic Drift Genetic Finally, Finally, larger populations would reduce the problems of genetic drift and inbreeding depression since there would be larger gene pools. pools. Wildlife Preserves Wildlife Biologists once calculated Biologists that a viable population of grizzly bears should include at least 50 females. Each grizzly needs a very large home range. Only our very largest Only national parks, Yellowstone and Glacier, are large enough for a population of grizzlies. grizzlies. Single Large Preserve? Single Islabella, the largest of Islabella, the Galapagos islands, has 344 plant species. This is more species than any of the other islands support. However, a collection of smaller islands with the same total area has 609 species. 609 Single Large Preserve? Single Random Random extinctions occur on even the largest islands. If If there is only one preserve and there is no other source of an extinct species, then we have regional or global extinction. Wildlife Preserves Wildlife If If there is only one preserve and there is no other source of an extinct species, then we have regional or global extinction. Therefore, using the metapopulation model, Therefore, what we actually want is many preserves with enough connectivity to allow for reenough iimmigration to an island once a species goes mmigration extinct on that island. extinct Wildlife Preserves Wildlife Shape The The best shape for a wildlife preserve is roughly circular, rather than oblong. An An oblong shape maximizes the edge to area ratio (the opposite is true for circles). ratio Large edges bring the preserve into contact with human dominated and disturbed habitats. dominated Shape Shape The The r-selected species, parasites, diseases, and invasive species often dominate the climax species in these disturbed habitats, resulting in lower diversity. resulting Wildlife Preserves Wildlife Connections and Corridors Many species migrate seasonally from winter Many to summer ranges, or by following the rains in tropical habitats. tropical Wildlife Preserves Wildlife Therefore, Therefore, wildlife preserves must not only include both seasonal ranges, but must include the corridors that allow wildlife to move from one area to another. move As As stressed above, connectivity of one island or habitat patch to another is crucial for longor term metapopulation survival. Patch Connection and Migratory Routes Migratory Winter Range Summer Range Recommendations Recommendations 1. Large preserves are needed for Large preservation of genetic diversity, to minimize random extinctions and to maintain top trophic level feeders. maintain Recommendations Recommendations 2. Large preserves must have reasonable Large connections to a series of smaller preserves to maintain the integrity of the metapopulation. metapopulation. Recommendations Recommendations 3. Various shapes of preserves may be Various included, but the largest should have a low edge/area ratio to minimize “edge effects.” edge/area Recommendations Recommendations 4. Preserves should include climax stages of Preserves succession (“old growth”) to prevent the extinction of those plants and animals that depend exclusively on this successional stage. stage. Recommendations Recommendations 5. However, some regular, low level However, disturbances should be allowed to maintain mid- and even early-successional species in the preserve. Where fire is a natural process in the ecosystem is should not be completely suppressed. completely Recommendations Recommendations The The management of fire is, however, a complex issue. complex Conclusion Conclusion The The conservation and maintenance of biological diversity and, more importantly, of ecosystem functioning, is a complex task. Metapopulations, Islands and Biological Control That’s All Folks! ...
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