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Ecology 15.docx

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Biology 2483A
Mark Moscicki

Ecology-Lecture 15 Nov 5 2013 Introduction  Species are connected with one another, and their environment (study of ecology)  Although so far we have considered species interactions in two-way relationships, in reality, species experience multiple interactions that shape the communities in which they live. What are Communities?  Communities are groups of interacting species that occur together at the same place and time.  The existence of a community is dependent on the individual species that are present and how they interact with one another and their physical surroundings. These interactions are synergistic meaning that the effect of multiple interactions together is greater than the sum of their separate effects. Interactions can be positive, negative or indirect.  In practical terms, ecologists usually define communities based on physical or biological characteristics.  A physically defined community might encompass all the species in a sand dune, a mountain stream, or a desert. (defined by physical characteristics of environment)  desert, hot spring (biomes and aquatic biological zone are largely based on physical characteristics)  A biologically defined community might include all the species associated with a kelp forest, a freshwater bog, or a coral reef. This approach emphasizes the importance of an abundant species, such as trees.  tropical rainforest, coral reef  Ecologists often define a community somewhat arbitrarily, based on the questions they are posing. They restrict the definition of the community to that particular interaction.  A study of marine invertebrates in seagrasses might restrict the definition of the community to that interaction, and not include mussel-eating birds, etc. (only look at interaction between marine invertebrates and the sea grasses that they live in, not focusing on other species or other interactions in the same habitat that may have an effect) Using Subsets of Species to Define Communities  Counting all the species in a community is difficult to impossible, especially if small or unknown species are considered.  Ecologists usually consider a subset of species when they define and study communities.  Subsets of species can be defined by:  Taxonomic affinity —e.g., study of forest community might be limited to all the bird species in a community.  Guild—group of species that use the same resources, even if they are taxonomically different. For example, some birds, bees and bats all feed on flower pollen.  Functional group—species that function in similar ways, but do not necessarily use the same resources. For example, mosquitos and aphids both have stylet mouthparts, although one feeds on mammalian blood and the other feeds on plant phloem. (in the case of plants, some functional groups do use similar resources-nitrogen fixing plants) Food Webs  Food webs are another subset of communities. Almost all food webs have 2-5 trophic levels.  Food webs organize species based on trophic or energetic interactions. They are organized into trophic levels (groups of species with similar ways of obtaining energy).  Trophic levels:  The lowest trophic level is Primary producers (autotrophs)—plants and algae.  Primary consumers which feed on primary producers—herbivores.  Secondary consumers feed on primary consumers— carnivores.  Tertiary consumers feed on secondary consumers— carnivores.  Food webs tell little about the strength of interactions or their importance in the community. In addition, trophic levels create confusion.  Some species span two trophic levels (corals can be classified as herbivores and carnivores), and some species change feeding status as they mature (amphibians may be herbivores as tadpoles and carnivores as adults).  Some species are omnivores, feeding on more than one trophic level (some fish feed on both algae and invertebrates).  Food webs do not include nontrophic (non feeding) interactions (horizontal interactions, such as competition).  Interaction webs more accurately describe both the trophic (vertical) and non-trophic (horizontal) interactions than a traditional food web. Community Structure  Community structure is the set of characteristics that shape communities. Species diversity and species composition are important descriptors.  Species diversity is the most commonly used measure of community structure. It combines species richness and species evenness.  Species richness—the number of species in a community  Species evenness—relative abundances compared with one another. This tells us about the commonness or rarity of species.  Two mushroom communities, each containing 4 species. Both communities have the same species richness, but their species evenness differs. In community A, one species constitutes 85% of mushroom abundance, while the others constitute only 5% (low species evenness) In community B, mushroom abundances are evenly divided among 4 species (25%) (high species evenness thus higher species )  Community A has lower Shannon Index (H) of the two communities confirming that the community has lower species diversity.  Species diversity is an important measure of community structure  The most commonly used species diversity index is the Shannon index:  Lowest possible value of H is zero. The higher a communities H value, the greater its species diversity  To calculate the Shannon index (H) the natural logarithm (ln) is applied to pi for each species (i) Then, that value is multiplied by pi again. All the values are summed for all the species in the community and multiplied by -1 to get H. s H    piln pi i1  p = iroportion of individuals in the ith species  s = number of species in the community  Biodiversity describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the interconnectedness of individuals, populations, species, and even community level components of diversity.  Genetic diversity affects the viability (chance of persistence) of populations, which in turn affects species diversity in a community. Population viability in turn has important consequences for species persistence. For example, if a species goes extinct it will affect the species diversity. The number of community types in an area is critical to diversity at larger regional and latitudinal scales.  Graphical representations of species diversity can give an explicit view of commonness or rarity.  Rank abundance curves plot the proportional abundance of each species (pi relative to the others in rank order from most abundant to least abundant  Relative abundances can suggest what species interactions might be occurring.  In Community A, the dominant species might indicate that it has a strong have a strong negative effect on the three rare species. In community B, where all the species have the same abundance, their interactions  Experiments that add or remove species are used to explore these relationships.  Species diversity and rank abundance curves were determined for two soil bacteria communities in pastures. One pasture had been fertilized regularly. The other pasture was undisturbed.  Bacteria species can be identified quickly using DNA sequencing of 16S ribosomal DNA. Unique DNA sequences can then be combined into taxonomic groups of bacteria using phylogenetic analysis.  Each unique DNA sequence they identified can be thought of as an individual. Therefore, abundance can be determined for each of the 20 taxonomic groups. Both pastures had similar community structure. A few species were abundant; most species were rare (larger organisms were the focus). Species Composition  Species composition—identity of species in a community.  Two communities could have identical species diversity values, but have completely different species.  The identity of species is critical to understanding community structure.  Species diversity estimates vary with sampling effort and scale. Imagine you were sampling insect species in your backyard. The more samples you collect, the more species you are likely to find. However, eventually you reach a point in which additional sampling will reveal so few new species that you could stop sampling and still have a good notion of the species richness. This point of 'no return' is determined using a species accumulation curve.  Species accumulation curves—species richness is plotted as a function of the total number of individuals that have been counted (sampling effort). Each data point represents the total number of individuals and sampling effort up to that point.  These curves can help determine when most or all of the species in a community have been observed.  Note: In reality this never actually happens because new species are constantly being found.  Hughes et al. (2001) compared species accumulation curves for five different communities to see how communities differ in the relationship between species richness and sampling effort.  The communities varied greatly in the amount of sampling effort necessary to determine species richness.  The temperate forest and tropical bird community were adequately represented before half the individuals were counted.  For tropical soil bacteria, more effort was needed to sample this extremely diverse community (linear) Each new sample resulted in the observation of many new bacterial species.  Spatial scale is also important.  If we sampled bacteria in tropical soils at the same scale as Costa Rican moths, the bacterial diversity would be immense in comparison.  The study highlights how little we know about community structure of rarely studied assemblages, such as microbial communities. Interactions of Multiple Species  Communities can be characterized by complex networks of direct and indirect interactions that vary in strength and direction.  In a community, multiple species interactions generate a multitude of connections.  Direct interactions occur between two species (e.g., competition, predation, and facilitation). They include trophic and non trophic interactions.  Indirect interactions occur when the relationship between two species is mediated by a third (or more) species. The simple addition of a third species to a two species interaction creates many more effects, both direct and indirect, which have the pote
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