Four Questions for EnviSci

Hi sophies… I have here four questions for you… I also have provided the answers which you can reflect on as you prepare for your next exam in envisci…

1. Discuss the fate of energy in terrestrial communities. Discuss it on the perspective of the laws of thermodynamics.

My Answer

The first law of thermodynamics is also referred to as the Law of Conservation of Energy. This law purports that energy is neither created nor destroyed. Implicit to this is the principle that energy is simply transferred from one system to another through various forms. As such, the total energy in the universe is constant and will remain constant ad infinitum.

Ecologically, life on this planet is given to be ultimately driven by radiant energy. The energy coming from the sun is trapped in the chlorophyll pigments of chloroplasts prior to getting converted to chemical energy in the form of ATP’s or adenosine triphosphates. This process takes on the complementary roles of both thylakoidal and stromal reactions of photosynthesis among photoautotrophic organisms. Utilization and assimilation of this energy decreases invariably from one trophic level in the food chain to another trophic level.

Among heterotrophs, chemical energy is released from the food and assimilated in the system through aerobic cellular respiration. This process involves the cytoplasmic reactions of the glycolytic pathway where sugar is broken down into pyruvate molecules and the mitochondrial reactions of the tricarboxylic acid cycle and the electron transport chain. Much of the energy assimilated by organisms are released in the form of heat during cellular respiration. A certain fraction also gets locked in the soil during decomposition. It is noted that only about 10% of the total energy in the organism is passed on to the succeeding organisms occupying higher trophic levels in the food chain. This means that the highest amount of energy is assimilated by the photoautotrophs or chemoautotrophs. This assertion is better understood using the following illustrations.

The energy pyramid is an illustration of energy transfer that is representative of the fate of energy as it flows from one system to another. And in the energy flow, one form of energy gets converted to another form as one organism makes use of it to carry out its metabolic requirements.

Implicit to the First Law of Thermodynamics, the energy that gets passed on from one organism to another is the same energy that originated from the sun. They just have assumed different configurations like heat and come in diminishing concentrations like the 10% Rule illustrated above. No new energy is incorporated in the succeeding trophic levels after the direct assimilation of radiant energy by photosynthetic organisms.

Energy and mass (materials) are related concepts. In Einstein’s famous equation E=mc2, a direct relationship between energy and mass is noted. Implicit to this is the fact that the more mass a certain body has, the more energy it possesses. Unlike energy however which is linear, mass can go through cycles. At a certain point, the material that makes up an abiotic system may, at another point make up a living system.
Geographically, different topologies and latitudinal locations on the surface of the earth are invariably heated up. This implies that some areas receive more sunlight than do other areas. This uneven distribution of sunlight effects to a certain extent, different climatic conditions that bring about different effects on the other factors that affect climate like rainfall, humidity, and wind systems. Such differences in the abiotic component of the environment entail differences in primary productivity in different ecosystems. Here is another way of looking at energy flow as pertaining to the amount of primary productivity in ecosystems.

From the premises raised, it can be inferred that the ecosystems with highest average net Primary Productivity are the estuaries and the tropical rainforests. Logically, they are also the ecosystems that exhibit the highest species diversity.

2. Explain the top-down and bottom-up controls of food webs. Identify the factors related to each of these controls.

My Answer

The top-down and bottom-up controls of food webs pertain to the interaction of consumer (top-down) and resource (bottom-up) effects on species composition and abundance. The objectives of identifying these controls are to investigate how species and populations are distributed within food webs and what factors determine biomass and productivity within a trophic level.

It is noted that the biomass of organisms in food webs based on primary productivity is controlled simultaneously by resources (bottom-up) and consumers (top-down).

It is a matter of consequence that the population of certain species is checked either by predatory feeding relationships with other organisms that they share a common functional group with or by the availability (abundance or scarcity) of resources in their respective ecospaces.

In a study conducted by Power and Dietrich of the UC Berkeley titled, “Food Webs in River Networks”, they described food webs as complex adaptive systems with diverse components that are linked by flows and interactions. It was Robert Paine however, who pointed out that “energy flows from more basal resources up to consumers at higher trophic positions (bottom-up) while chains of population control link consumers to the resource populations they regulate or limit (top-down), if these consumers are not suppressed by their own predators.

Given these assertions, how then does a food web illustrate the flow of energy across trophic levels? Power and Dietrich offered that conditions, resource fluxes and biotic interactions all play pivotal role in the determination of functional food chains in food web systems, the length of energy flow paths and the controls on both. They have identified these controls as dependent on two environmental factors: Productivity/Efficiency and Disturbance/Stability; and one evolutionary factor – that of design constraints.

Power and Dietrich argued that in terms of productivity or efficiency, functional food chains should “lengthen as fluxes of limiting resources or energy to food webs increase, or as consumers increase their efficiency of resource capture or conversion”. In terms however, of disturbance or stability, it should follow that functional food chains are shorter in situations that are characterized by frequent disturbance. Conversely, a more stable situation should be able to host a longer functional food chain. Lastly, on the evolutionary factor of design constraints, they argued that “it is impossible for evolution to build a Pterodactyl predator”, for example, because an organism large enough to subdue one could not fly to catch it. While at this, it is important to note that the length of functional food chains in complex adaptive systems is directly related to the length of energy flow paths.

3. What is a “keystone species”? How does its absence affect the stability of the community where it is once found?

My Answer

Robert D. Davic in his correspondence with the Ohio Environmental Protection Agency titled, “Linking Keystone Species and Functional Groups: A New Operational Definition of the Keystone Species Concept”, redefined the concept of keystone species as one that “is held to be a strongly interacting species whose top-down effect on species diversity and competition is large relative to its biomass dominance within a functional group.” The premise behind this redefinition is anchored on the fact that from 1969, when the term “keystone species” was first defined by Paine up to the present, the concept has gone through linguistic metamorphosis to a point of controversy.

At issue in the keystone species controversy is whether abundance of the keystone relates significantly to impact within its functional group. This is on top of the controversy on how to measure abundance and impact, and where to delineate between abundance and impact.

In 1969 when the concept was first coined, Paine defined keystone species as “a species of high trophic status whose activities exert a disproportionate influence on the pattern of species diversity in a community”. But because it is a given ecological tenet that species in a keystone set are invariably interrelated to each other, it becomes difficult to tell which species is the keystone species. In the case for example of the mycorrhizae and certain species of trees, it is clear that the keystone species is the mycorrhiza – in spite of its low trophic status – for upon whose death depends the survival of the trees.

In Davic’s redefinition of the concept of keystone species, he noted the narrow food-web context that Paine used in his definition of keystone species and provided a divergent line of thought. His redefinition presupposes that the influence of a keystone species should be considered in light of the species’ biomass relative to its functional group. This implies that a keystone species is not necessarily the dominant species or the species of higher trophic status within a biotic community. Further, this also implies that a keystone species may be considered from across a wider width of feeding niches. But while at this, Davic’s premise however, does not preclude nor deviate from the predator-prey relationship model of Paine.

Regardless however of the blurry line that divides between biomass and impact, it is clear that for a species to qualify as a keystone species, it’s presence within a functional group in an ecospace must have an influence on the population of other species that belong to the same functional group and that such influence is taken to mean as a large top-down effect on biodiversity and competition relative to the keystone species’ biomass within a functional group.

Thus said, it is for the same argument that the impact of a keystone species’ absence to the stability of a community must be gleaned.

In a study conducted by the University of Washington on the delicate balance between and among marine organisms inhabiting the coastal areas extending from Baja, California to Alaska, they have figured a species of starfish as a prospective keystone species whose removal from the community may lead to disruption of ecological equilibrium there. The starfish Pisaster ochracues is said to influence negatively on the population of other marine organisms that it shares a functional group with through predatory relationships. If the starfish is taken out of its functional group, it is noted that a skewed population growth of other species, some of which are secondary predators, ensues as a consequence. Such trend in the other species is not noted as a result of their absence.

In Costa Rica, a species of shore crabs is noted to feed primarily on tree saplings. There is however, a tree sapling that is distasteful to the shore crabs. This tree sapling is what eventually grows in number and dominates the entire landscape. The proliferation of these trees makes the landscape an open-space forest that caters to a host of local animals like howler monkeys, tapirs, and coatis. If the shore crabs were to be taken out of this community, the open space forest will revert back to the level of heterogeneity of the off-coast forest and may threaten the extinction of the said local animals with particular preference to the open-type forest.

The examples cited above illustrate two typical influence of keystone species on other species within a functional group. The case of the starfish typifies what a top predator does to skew the population trend negatively. The case of the shore crabs on the other hand typifies an indirect check on the population of the other organisms within its functional group through drastic changes in the community.

4. Discuss the two ecological theories of island communities. Explain supporting evidences of these theories.

My Answer

The concept of “Island” can be broadly defined as patches of land that are, to some extent, isolated from the mainland. In the classical sense, islands are terrestrial habitats that are isolated from continental habitats either by freshwater or marine areas that represent a certain degree of geologic barrier to dispersal between the island and the mainland.

There are two ecological theories of island communities: the Habitat Diversity Theory and the Equilibrium Theory.

The Equilibrium Theory centers on the balance between the rate at which allochthonous organisms (species new to the island) colonize the islands and the rate at which autochthonous organisms (species that are residents/native to the island) go extinct on the islands.

There are three basic characteristics to the Equilibrium Theory of Island Biogeography: species-area relationship; species-isolation relationship; and species turnover. It is given that a larger area works to the advantage of resident species because there will be more resources available for them and therefore, intraspecific competition is less likely. But eventually, when other organisms migrate to the place, the resident species will have to compete with the new species for available resources. To mitigate the effects of intraspecific competition among the resident species, either dispersal ensues among them or niche differentiation is resorted to. In the case of turnover, a study conducted by Jared Diamond on the turnover of birds on California Channel Islands, established that “turnover tends to be lower on larger islands and increases with generation time of the organisms”. This follows the logic that a larger area will play host to a more diverse collection of resources that can be utilized by a wider array of species. Moreover, in an experimental defaunation conducted on arthropods by Wilson and Simberloff, where they exterminated an entire population of arthropods, it was established that turnover tends to increase with rapid colonization that followed the killing of the arthropods. This illustrates the likely scenario that when populations leave a place either for reasons of dispersal or extinction, they leave behind their respective niches which eventually gets assumed by colonizing species that share the same environmental niche.

The Habitat Diversity Theory on the other hand, hubs on the “suitability of islands as habitats for various species”.

In a study conducted by Ricklefs and Lovette titled, “The Roles of Island Area, per se and Habitat Diversity in the Species-Area Relationships of Four Lesser Antillean Faunal Groups”, they have established strong positive correlations between area and habitat diversity and between elevation and habitat diversity. It was observed that large elevated islands tend to host a more diverse habitat that matches species richness among faunal groups. These correlations however applied differently to other species included in their study like certain species of butterflies and bats. Species richness observed in some species of butterflies and certain groups of bats tend to be correlated to strong habitat-diversity effects. These species are said to exhibit “high degrees of habitat specialization”, large population sizes, high fitness rate, life-cycles that include a resistant resting stage that reduces vulnerability to catastrophic extinction and such other adaptive biological traits.

In other words, habitat diversity is rendered appropriate to a certain group of species only if it translates to species richness. But studies point out that while there may exist strong correlations between habitat diversity and habitat area, and between habitat area and species richness on account of resource availability, the same strong correlation cannot be applied to habitat diversity and species richness. Certain match between geographic features and biological traits must first be established as indeed resulting to or may result to species richness.

Ricklef and Lovette have established that these “taxon-specific differences demonstrate that both biological characteristics of organisms and geographical features of island groups” control the relative influence of island area and habitat diversity to differences in species richness.

References

Bak, P. 1996. How nature works: the science of self-organized criticality.
Springer-Verlag, New York, New York, USA.
Berger, W. H., and F. L. Parker. 1970. Diversity of planktonic Foraminifera in
deep sea sediments. Science 168:1345-1347
Primack, Richard B. Essentials of Conservation Biology
Ricklefs, R. E. et al., Island Biology Illustrated by the Land Birds of Jamaica
The Keystone-Species Concept in Ecology and Conservation," L. Scott Mills,
Michael E. Soule, and Daniel F. Doak, BioScience
"The Keystone Cops Meet in Hilo," Mary E. Power and L. Scott Mills, TREE