Question

Time in years since the re-introduction of gray wolves to Yellowstone National Park (x) and total living biomass of willow plants along the high-risk Blacktail Creek (y).

a.	Principles of inheritance – is a trait heritable?
b.	Life history tradeoffs
c.	Abiotic factors governing primary production
d.	Size-selective predation and/or predator-prey refugia
e.	Trophic cascades

Answer 1

a) Traits are heritable only if the similarity arises from shared genotypes. The understanding of how inherited traits are passed between generations come from principles of Gregor Mendel, who worked on pea plants, but his principles apply to traits in plants and animals. The following are Mendel's principle of inheritence.

1. Fundamental theory of Herdity: Inheritance involves the passing of discrete units of inheritance, or genes, from parents to offspring. Mendel found that paired pea traits were either dominant or recessive. When pure-bred parent plants were cross-bred, dominant traits were always seen in the progeny, whereas recessive traits were hidden until the first-generation (F1) hybrid plants were left to self-pollinate. Mendel counted the number of second-generation (F2) progeny with dominant or recessive traits and found a 3:1 ratio of dominant to recessive traits. He concluded that traits were not blended but remained distinct in subsequent generations, which was contrary to scientific opinion at the time. Now we know that Mendel’s inheritance factors are genes, or more specifically alleles, which is different variants of the same gene.
2. Principle of Independant assortment: Genes located on different chromosomes will be inherited independently of each other. Mendel cross-bred pea plants with round, yellow seeds and plants with wrinkled, green seeds. Only the dominant traits (yellow and round) appeared in the F1 progeny, but all combinations of trait were seen in the self-pollinated F2 progeny. The traits were present in a 9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green).
3. Principle of segregation: During reproduction, the inherited factors (now called alleles) that determine traits are separated into reproductive cells by a process called meiosis and randomly reunite during fertilisation. Mendel observed that by allowing hybrid pea plants to self-pollinate resulted in progeny that looked different from their parents. Separation occurs during meiosis when the alleles of each gene segregate into individual reproductive cells (eggs and sperm in animals, or pollen and ova in plants).

b) Life-history theory attempts to explain intra- and interspecific variation in the survival, growth, and reproductive traits of organisms. Because these variables affect both individual fitness and population dynamics, life-history patterns naturally fall at the intersection of evolution and ecology. Life-history theory is based on the premise that organisms face trade-offs arising from energetic, physiological, developmental, or genetic constraints and that these trade-offs affect the patterns we observe in nature.  A trade-off exists when an increase in one life history trait (improving fitness) is coupled to a decrease in another life history trait (reducing fitness), so that the fitness benefit through increasing trait 1 is balanced against a fitness cost through decreasing trait 2. Trade-offs can also involve more than two traits. Trade-offs are typically described by negative phenotypic or genetic correlations between fitness components among individuals in a population. If the relationship is genetic, a negative genetic correlation is predicted to limit (i.e. to slow down or prevent) the evolution of the traits involved. Thus, a genetic trade-off exists in a population when an evolutionary change in a trait that increases fitness is linked to an evolutionary change in another trait that decreases fitness. For example, direct artificial selection for extended lifespan in genetically variable laboratory populations of fruit flies (Drosophila melanogaster) causes the evolution of increased adult lifespan (sometimes in 10 or fewer generations), but this evolutionary increase in longevity is coupled to decreased early reproduction. This suggests that lifespan and early reproduction are genetically negatively coupled.  At the physiological level, trade-offs are caused by competitive allocation of limited resources to one life history trait versus the other within a single individual, for example when individuals with higher reproductive effort have a shorter lifespan or vice versa. A helpful way to think resource allocation trade-offs is to imagine a life history as being a finite pie, with the different slices representing how an organism divides its resources among growth, storage, maintenance, survival, and reproduction. The essential problem is this: given the ecological circumstances, and the fact that making one slice larger means making another one smaller, what is the best way to split the pie? Note that since resource allocation trade-offs might have a genetic basis, and since different genotypes may differ in aspects of resource allocation, the genetic and physiological views of trade-offs are not necessarily incompatible.

c) The factors in an ecosystem that are considered abiotic, or nonliving (e.g., precipitation and temperature) have traditionally been viewed as the main drivers of ecosystem primary production. Those factors are both chemical and physical, and include light (radiation), temperature, water, atmospheric gases, and soil. Climate generally influences all the abiotic factors. The ecosystems that are in and around the equator generally have the greatest primary productivity, because of the moisture and conducive temperature. All the major cycles , such as the nitrogen, carbon, and water, occur with higher regularity because there is more direct radiation from the sun at these locations. As we go towards the poles, there is less direct radiation, so the degree of primary production starts to lessen and by the time we get to the ninety degrees, north and south, these areas receive the least amounts of direct radiation, causing a marked decrease in the amount of primay production. Therefore, temperature and moisture are important influences on plant production (primary productivity) and the amount of organic matter available as food (net primary productivity). Net primary productivity is an estimation of all of the organic matter available as food; it is calculated as the total amount of carbon fixed per year minus the amount that is oxidized during cellular respiration.

d) Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. Under the pressure of natural selection, predators have evolved a variety of physical adaptations for detecting, catching, killing, and digesting prey. These include speed, agility, stealth, sharp senses, claws, teeth, filters, and suitable digestive systems. In size-selective predation, predators select prey of a certain size. Large prey may prove troublesome for a predator, while small prey might prove hard to find and in any case provide less of a reward. This has led to a correlation between the size of predators and their prey. Refuge use can be broadly defined to include any strategy that decreases predation risk, e.g., spatial or temporal refuges, prey aggregations, or reduced search activity by prey. The existence of refuges can clearly have important effects on the coexistence of predators and prey. For a stabilizing effect, the proportion of prey in refuges must either: (1) decrease with increasing prey density, or (2) increase with both increasing predator density and increasing predation pressure. Refugia can affect predator-prey dynamics via movements between refuge and non-refuge areas.

e) Trophic cascade, an ecological phenomenon triggered by the addition or removal of top predators and involving reciprocal changes in the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling. During the 1980s and ’90s a series of experiments demonstrated trophic cascades by adding or removing top carnivores, such as bass (Micropterus) and yellow perch (Perca flavescens), to or from freshwater lakes. Those experiments showed that trophic cascades controlled biomass and production of phytoplankton, recycling rates of nutrients, the ratio of nitrogen to phosphorus available to phytoplankton, activity of bacteria, and sedimentation rates. Because trophic cascades affected the rates of primary production and respiration by the lake as a whole, they affected rates of exchange of carbon dioxide and oxygen between the lake and the atmosphere. The removal of top carnivores triggers significant effects on prey populations, primary producers, and ecosystem processes. Therefore, the conservation of top carnivores helps to preserve the structure and processes of ecosystems in which these predators live. The normal functioning of ecosystems provides many services used by people, including food, fibre, and freshwater supplies as well as processes that maintain the quality of air, water, and soil.