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An outline and study guide for morehouse college bio 320 ecology course, focusing on interactions between species through competition and predation. It includes competition models, predation models, and experimental data. Students will learn about competition intensity, predator-prey relationships, and species responses.
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Section III Outline Interactions Between Species, Competition Competition Models Competition in Nature Competition Experiments Interactions Between Species, Predation Predation Models
Competition Intraspecific Competition: Competition within a species. Interaction between individuals for a limited resource, leading to reduced survivorship, growth, and/or reproduction Intraspecific competition is competition between individuals of one species. Consequences: Decreased resource uptake/individual Decreased individual rates of growth, or development Decreased stored reserves Increased predation risk Leading to: Decreased survivorship and/or decreased fecundity Resulting in: Decreased reproductive output/individual These outcomes are not necessarily distributed evenly among competing individuals. Forms of Competition Exploitation Competition Resource use by one individual results in less resource available for another individual when a resource is in limited supply (can occur without direct interaction between individuals) Interference Competition Resource use by one individual limits access to that resource for other individuals (direct interaction between individuals) Intensity of Competition Increases with density of competitors Expect density dependent effects on deaths, births, growth
Density dependent fecundity in fingernail clams, Musculium securis (data from Begon, Harper and Townsend, 1996, p 220, Fig. 6.6). Density dependent seed production in an annual dune plant, Vulpia fasciculata (after Begon, Harper and Townsend, 1996, p 220, Fig. 6.6).
If both birth rates (fecundity) and death rates (mortality) respond to density (A), then recruitment rates may respond to density (B). As density approaches carrying capacity, net recruitment decreases from its maximum level, and population growth will have a characteristic logistic (sigmoid) form (C) (after Begon, Harper and Townsend, 1996, p 225, Fig. 6.10).
Types of Interspecific (Between Species) Interactions Types of Interactions Benefit Harm No Effect Benefit Mutualism Predation Commensalism Harm Predation Competition Amensalism No Effect Commensalism Amensalism Neutralism Interspecific Competition Individuals of one or both species suffer reduced fecundity, survivorship, or growth as a result of resource exploitation or resource interference by individuals of each species. Examples: Terrestrial salamanders, Plethodon glutinosus and P. jordani , were studied in the southern Appalachian Mountains. Hairston (1980) conducted an experiment at two sites where both species naturally coexist. Treatment Result after five years Controls (3) no manipulation P. jordani more abundant than P. glutinosus Experimental Removals (2) removal of all P. jordani P. glutinosus abundance increases Experimental Removals (2) removal of all P. glutinosus P. jordani abundance does not change but year classes 1 and 2 increase In the absence of P. glutinosus , there may be an increased fecundity or increased early survival in P. jordani. However, total abundance of P. jordani did not change. Bedstraw plants, Galium , studied by Tansley (1917) are found in two soil environments in Britain. G. hercynicum is naturally found on acidic soils, and G. pumilum is naturally found on calcareous soils.
Experiment: Grown alone, both species of Galium thrive on the acidic soils from G. hercynicum sites and on the calcareous soils from G. pumilum sites. Therefore, soil condition alone cannot explain the non-overlapping distributions of these two species. Grown together: Soil Type Result when grown together Acidic only G. hercynicum survives Calcareous only G. pumilum survives This is an example of competitive exclusion, when these two species are together, only one species will persist. The outcome of competition depends on the soil type in which the plants are located. Barnacles on the rocky shores of Scotland were studies by Connell (1961). Chthamalus stellatus adults are found only in the upper intertidal, and Balanus balanoides adults are only found in the lower intertidal. Experiment: If an area of rock surface in the lower intertidal is cleared of all barnacles, larvae of both Chthamalus and Balanus will settle and successfully grow. However, Balanus will smother, undercut, or crush Chthamalus in the lower intertidal over time. Chthamalus mortality is greatest at the time of maximum Balanus growth. Balanus has a lower desiccation tolerance than does Chthamalus , so Balanus is limited to the lower intertidal and competition between these two species is limited to the lower intertidal. In this case, competition is direct, interference, which results in competitive exclusion of Chthamalus from the lower intertidal. Three species of the ciliate Paramecium were studied by Gause (1934, 1935) in laboratory cultures. When grown alone, P. auralia and P. caudatum have very similar logistic growth curves. P. bursaria reaches a stable population size of approximately two-thirds of that seen in the other two species when grown alone under the same culture conditions. When grown together, P. auralia and P. caudatum both exhibit growth suppression, but P. caudatum , is eventually driven to extinction in these combined cultures. When P. bursaria is grown with P. caudatum , both species have lower stable population densities, but both persist over time and coexist.
Two species of freshwater diatom, photosynthetic protista (single celled algae), Asterionella formosa and Synedra ulna both use silica to make their test (Tilman, Mattson and Langer, 1981). When grown in culture alone, both species deplete available silicate from liquid medium, but Synedra reduces silicate to a lower level than does Asterionella. When grown together, Synedra competitively excludes Asterionella , even when Asterionella starts with a 10-fold greater population density than Synedra. Growth curves for Asterionella and Synedra when grown alone and together (after Begon, Harper and Townsend, 1996, p 269, Fig. 7.3). Competition in this case is indirect, it is an example of exploitation competition. As in situations of intraspecific competition, interspecific competition can be either interference or exploitation. Results may be asymmetrical, two species do not necessarily have reciprocal inhibitory affects on each other. Competitive exclusion is, of course, an asymmetrical competitive outcome. In some cases, only one species is negatively affected by the competitive interaction.
The cattail species, Typha latifolia and T. angustifolia have separate water depth distributions in naturally occurring populations. Individual stems of T. latifolia are found in shallow waters and those of T. angustifolia are found in deeper waters (Grace and Wetzel, 1981). Transplant experiments (in the absence of competition) indicate that T. angustifolia can grown over a much wider range of water depths than it does in the presence of T. latifolia. The growth of T. latifolia at different water depths is the same in the absence of potential competition as it is in the presence of T. angustifolia in natural populations. Standing crop biomass per unit area for T. latifolia and T. angustifolia in natural populations containing both species show the separate distributions of these species with water depth. Standing crop biomass per unit area for T. latifolia and T. angustifolia as a function of water depth in competition-free transplants shows the response of T. angustifolia (after Begon, Harper and Townsend, 1996, p 272, Fig. 7.4)
Logistic Model of Interspecific Competition Lotka-Volterra Model Lotka (1932) and Volterra (1926) Logistic Equation € dN dt = rm N
Let: € N 1 = number of individuals of species 1 € N 2 = number of individuals of species 2 € K 1 and € K 2 = respective carrying capacities € r 1 and € r 2 = respective intrinsic rates of increase If: Ten individuals of species 2 have collectively the same competitive-inhibitory effect on species 1 as one individual of species 1 Then: The total effect on species 1 of intra- and inter-specific competition will be equal to the effect of €
species 1 individuals. The one-tenth factor = competition coefficient = α 12 “The competitive effect on species 1 by species 2” When α 12 < 1, each individual of species 2 has less inhibitory effect on each individual of species 1 than each individual of species 1 has on other species 1 individuals. In other words, when α 12 < 1, intra-specific competition in species 1 is more intense than inter- specific competition. For species 1: €
so: € dN 1 dt = r 1 N 1
For species 2: € dN 2 dt = r 2 N 2
How does population growth of one species respond to the presence of another species? When a population is stable: € dN dt = 0 (zero growth condition) If € dN 1 dt = 0 then €
This is true if: € r 1 = 0 or € N 1 = 0 (trivial examples) or if: € K 1 − N 1 − α 12 N 2 = 0 then € K 1 − α 12 N 2 = N 1 (a straight line, zero growth isocline) In species 1 terms: when € N 1 = 0 then €
α 12 and when € N 2 = 0 then €
Species 1 responses to the numbers of species 1 and numbers of competing species 2 (after Begon, Harper and Townsend, 1996, p 275, Fig. 7.6). Similarly for species 2: If € dN 2 dt = 0 then €
and € K 2 − N 2 − α 21 N 1 = 0 then € K 2 − α 21 N 1 = N 2
The opposite outcome occurs when the inhibitory effects on species 2 by itself (intraspecific) are greater than the inhibitory effects on species 2 by species 1 (interspecific), and species 2 has a greater inhibitory effect on species 1 than species 1 has on itself: Then species 2 drives species 1 to extinction, species 2 wins [see graph (b), p 85]. €
α 12 and €
α 21
K 1 so € K 2 α 12 > K 1 and € K 2 > K 1 α 21 Outcome: Competitive Exclusion Species 2 goes to carrying capacity € K 2 and species 1 is excluded. An unstable “equilibrium” occurs when the inhibitory effect on species 1 by species 2 is greater than the inhibitory effect on species 1 by itself, and the inhibitory effect on species 2 by species 1 is greater than the inhibitory effect on species 2 by itself [see graph (c), p 85]. €
α 12 and €
α 21 so € K 2 α 12 > K 1 and € K 1 α 21 > K 2 Outcome depends on the starting densities. The species with the initial advantage wins. Outcome: Competitive Exclusion (ultimately) Either: Species 1 goes to € K 1 and species 2 is excluded, or species 2 goes to € K 2 and species 1 is excluded. A stable equilibrium will occur when the inhibitory effect on species 1 by itself is greater than the inhibitory effect on species 2 by species 1, and the inhibitory effect on species 2 by itself is greater than the inhibitory effect on species 1 by species 2 [see graph (d), p 85]. €
α 12
K 2 and €
α 21
K 1 so € K 1 > K 2 α 12 and € K 2 > K 1 α 21 Outcome: Stable Coexistence Both species will persist (ultimately) at the equilibrium densities where their zero growth isoclines cross.
Outcomes of competition between two species (after Begon, Harper and Townsend, 1996, p 277, Fig. 7.8).
Wild Oats and Flax (Bell and Nalewaja, 1968) Flax is an agricultural seed crop and wild oats is a weed species in the northern Great Plains of the United States. If these two species are competing, we would predict that as the density of the wild oats increases, the yield of flaxseed would decrease. Flaxseed yields were evaluated on fields with varying densities of wild oats. The application of fertilizer was also a variable. Comparisons were made to plots containing no wild oats. Flaxseed yield reductions occur with increased wild oat densities (after Krebs, 1994, p 253). Wild Oats Density Flaxseed Yield (Bu/acre) number/m^2 Fertilized Unfertilized Mean Reduction% 0 19.5 17.9 -- 10 13.4 14.3 26% 40 6.7 8.0 60% 70 4.3 6.3 72% 100 3.5 4.2 80% Flour Beetles (Park, 1962) Two species of flour beetle, Tribolium confusum and T. castaneum have very similar habitats. Competition occurs along with cannibalism and reciprocal predation by adults and larvae on eggs and pupae. Climate and chance influence the outcomes of competition between these two species, but one species always excludes the other in culture experiments. This is very suggestive of a Lotka-Volterra unstable equilibrium with ultimately leads to competitive exclusion (after Begon, Harper, and Townsend, 1996, p 281, Table 7.2). Culture Conditions Competitive Exclusion (percentage) by T. castaneum Competitive Exclusion (percentage) by T. confusum Cold-dry 0 100 Temperate-dry 13 87 Hot-dry 10 90 Cold-moist 29 71 Temperate-moist 86 14 Hot-moist 100 0
Complete Competitors Cannot Coexist Organisms with similar requirements in nature compete most severely. Exceptions:
