BLOG 7 – FOOD WEBS AND FOOD CHAINS

Food web vs. food chain

A food web is a graphical description of feeding relationships among species in an ecological community, that is, of who eats whom. It is also a means of showing how energy and materials flow through a community of species as a result of these feeding relationships. On The other hand, food chain is a succession of organisms in an ecological community that constitutes a continuation of food energy from one organism to another as each consumes a lower member and in turn is preyed upon by a higher member.

                                                           Fig. 1 A Food Web

The trophic-dynamic model of ecosystem structure

In ecology, trophic dynamics is the system of trophic levels, which describes the position that an organism occupies in a food chain: what an organism eats, and what eats the organism, for every level there is an increase in trophic level but a decrease in energy because of the absorption of biomass, thermodynamics and the law of conservation of energy in every trophic level. All trophic systems start with an autotroph that can be either photoautotroph or litoautotroph. All trophic-dynamic systems have four main parts: the abiotic environment, producers, consumers, and decomposers.

                                                  Fig. 2 An Ecological Pyramid

Top-down vs. bottom-up control of trophic levels

Both are theories of control of ecosystems but they have different functions.
Bottom-up control states that an ecosystem’s function is ultimately controlled by the nutrient supply to the primary producers and if the nutrient supply is increased, the resulting increase in production of autotrophs is propagated through the food web and all of the other trophic levels will respond to the increased availability of food (energy and materials will enter the cycle faster). 

Top-down control states that an ecosystem’s function is ultimately controlled by predation and grazing by higher trophic levels on lower trophic levels and if there is an increase in predators, that increase will result in fewer grazers, and that decrease in grazers will result in turn in more primary producers because fewer of them are being eaten by the grazers. Thus the control of population numbers and overall productivity "cascades" from the top levels of the food chain down to the bottom trophic levels.

Relationship between food web/food chains and biogeochemical cycles

            The movement or circulation of biogenetic nutrients through the living and non-living components of the biosphere or of any ecosystem is called biogeochemical cycling. Thus, it involves both biotic and abiotic components of organisms. There are two types of biogeochemical cycles, also called nutrient cycles, first is the closed system wherein nutrients such as oxygen is recycled instead of being lost and replenished constantly and is the complete opposite of the second type, the open system, where all energy comes finitely but within a long-lasting source like the sun but always lost. These two types are important in an ecosystem because they help producers and consumers live. Therefore, are needed for a productive food web/chain.

                                                                Fig. 3 Water Cycle

Applications of food web-food chain concepts

Absorption of Solar Radiation: autotrophs primarily absorb solar radiation.

Energy Transfer: By each trophic level consuming another, energy from the consumed level is transferred to the next. However, energy transfer is not efficient. In fact, only about 10 percent of the energy available at one level is actually passed onto the next. That means a massive level of life is needed at the base in order to sustain the life forms at the highest point. For example, if 100,000 calories are produced by a group of certain plants, only 1,000 calories will be transferred to the consumers that eat them. This is partly because not all plants or animals are consumed at every trophic level, nor are all the parts, such as beaks, shells, certain roots, leaves and poisonous fruits.

Interdependence: This interdependence of the populations within a food chain helps to maintain the balance of plant and animal populations within a community. For example, when there are too many carabaos; there will be insufficient grasses for all of them to eat. Many carabaos will starve and die. Fewer carabaos mean more time for grass and shrubs to grow to maturity and multiply. Fewer carabaos  also mean less food is available for the cow to eat and some carabaos will starve to death. When there are fewer cow, the carabao population will increase.

For example, many shark populations have been heavily overfished by people, further decreasing their predatory effects on otters. 

          In many places still without otters, commercial divers intensively harvest sea urchins for international seafood markets (see slide). Though the harvesting patterns and therefore the community effects of humans and otters may be quite different, to some extent, human harvests may simulate the controlling effect of otters. 

How have humans affected the food chain? 

    When we spray pesticides, we put the food chain in danger.  By breaking one link on the chain means all of the organisms above that link are in threat of extinction (like the domino effect).  By hunting animals nearly to extinction, everything above the animal in the food chain is put in danger.  A 'chain reaction' in the food chain can be perilous!  Since the food chain provides energy that all living things must have in order to survive, it is imperative that we protect it.

 IRRI is an autonomous international institute based in Los Banos, Laguna.  The Philippines is one of the foundation institutions of the CGIAR (Consultative Group on International Agricultural Research), and is dedicated to improving the lives and livelihoods of resource poor rice producers and consumers worldwide. It is a nonprofit organization doing research and training on agriculture.

IRRI has been at the forefront of rice research for almost thirty years (has been in the Philippines since 1960), delivering new rice varieties and practices to rice farmers throughout Asia and the developing world. Together, farmers and consumers find solutions to world hunger.

Through research, IRRI has been able to help almost half of the people all over the world who eat rice. It is doing research to help farmers grow MORE rice by using FEWER resources (less land; less water; less work and less chemicals. When there is more rice, there will be enough food for the 3 billion people who eat rice in the world!

Since 1960, IRRI has been able to grow rice plants and grains that grow faster; grow in different kinds of places; need for fewer chemicals; fight against harmful insects and are strong against plant diseases.

However, the use of pesticides can lead to extinction of some important species. Before learning to drive at the Jamboree route, during traffic, I take the Maahas road in going to UPLB. I remember there was a time when my daughter and I were covering our nose and breathing through our mouth when we passed the area near the railroad crossing. The foul smell? It was because of the smell coming from the pesticide that was previously applied by the IRRI people. Our respiratory system was affected since we would cough and sneeze due to the smell. Even if we did not see the species that have been killed, I knew that they were really affected. I knew that the participants in the food chain, for example, insects, and aquatic organisms (since the area is near a creek) were killed due to the pesticide. And this effect might put the food chain in chaos! If I could only have the authority to stop this application of pesticide, I know the extinction can be prevented. Perhaps, the least that I can do is to write a complaint to the Mayor so that the problem may be addressed properly.

BLOG 6 – FORAGING BEHAVIOR AND THE NATURE AROUND ME

The foraging theory studies how animals forage or search for their food to be able to survive. It is a branch of behavioral ecology or ethoecology which is the study of the evolutionary and ecological basis for the animal’s behavior and how it adapts to its environment.

Foraging Behavior, as stated earlier, is the manner on how animals search for their food. There are two groups where an animal can belong according to how it forages, namely foraging specialists and the foraging generalists. As the name implies, foraging specialists are selective in the food they consume. Meaning, they only eat a certain species like most herbivores do, except those which consume a variety of plants. Some examples of food specialists are koalas which only eat eucalyptus leaves, lynx which eats snowshoe hares and some insect species that eat a single plants species. On the other hand, the other category is the opposite of the food specialists. This group eats a variety of food, from different kinds of plants to meat or any type of either plant or meat. Omnivores, those who eat both plants and animals, are usually generalists. Some examples of the animals belonging to this group are opossums which eat from insects and berries to garbage, humans who eat a variety of plants and animals, raccoons which eat berries, insects, eggs and small animals and many more.

The Optimal Foraging Theory

Natural selection favored the maximized consumption of energy through foraging in the shortest time possible. This is referred to as Optimal Foraging. The Optimal Foraging Theory (OFT) predicts the foraging behavior to determine the optimal forager’s behavior. An optimal forager is an organism that can efficiently maximize food intake. It was first proposed by Robert MacArthur together with Eric Pianka and another paper proposed by J. Meritt Emten. OFT is an idea in ecology based on the study of foraging behavior that states the organism’s forage in a manner of acquiring much calories or any nutrient needed in the least time possible. The theory also explains how animals decide what to eat and why they choose it.

Fig. 1. Model of Optimal Foraging Theory

Fig.2 An Example of Optimal Foraging Theory

One Youtube video taken by David Attenborough shows how a certain animal managed to cope with its environment. The video contained how the crow efficiently used its surrounding, a city, to be able to lessen the energy spent to acquire the crow’s needed nutrients. Carrying a nut, the crow dropped it on the road and waited for vehicles to pass by and eventually crack it open; doing so, the crow was successful in preventing loss of too much energy trying to crack the shell of the nut. Now the nest challenge, how can the crow safely consume the cracked nut? Well, our little crow had that in min, too. It made sure that the nut will be dropped in a pedestrian lane, so that when the green light flashes, it would go towards the crack nut and eats it. A very fascinating way on how the crow adapted to the city life of Japan.

Another video, also found in Youtube, is about the BBC’s Planet Earth─Unique Dolphin Hunting Technique. The dolphins were able to reach the shore where food is easier to find by riding the waves. Thus, saves them energy rather than swimming in the vast ocean in search of food. Tail-slapping is the technique dolphins use to stun their pray and consume it. But due to the shallow waters, it doesn’t seem to work. Instead it resorted to gaining some speed and hiding and waiting for its prey. The momentum of its speed enabled them to get close to its prey and eventually consumes it. 

  

Assumptions of OFT

Predators usually select the most fruitful prey; however it is not always abundant to the environment so they resort in eating other prey types that may be easier to find. Energy is not the only nutritional requirement after all. Variation allows a more sufficient diet for the predators themselves.

There are some assumptions offered by the OFT. First, due to Darwins’ theory of evolution, the individual’s contribution to the next generation is based on how it searches for food. Second, there should be inheritance of foraging behavior from parent to offspring. Lastly, the currency of fitness which is the relationships between foraging behavior and fitness.

Models associated with OFT

There are 3 main models based on the rate of maximization explained by the OFT. The first one is the Marginal Value Theory or Patch choice theory. Animals encounter patches of food when they travel. The longer the time the animal spends in the patch, the lesser energy or nutrients it gains. This results to the animal moving to a new patch when the rate of gain on a patch equals the maximum rate of gain. Thus, the more the animal moves from patch to patch the higher the nutrients it gains. When the profitability of patch equals the profitability of the average patch and time for search or travels to a new patch this is the time the animal should leave the patch and search for another. The formula for this is stated below:

dE (h) / dh = E (h) / (s+h)

wherein:

·         energy (E) à acquired net calories by consuming prey

• Handling time (h) time consumed form capturing to digesting
• Search time (s) encounters of items of same food type; ease of locating it.

The second model is the Contingency theory or Prey Choice Model which is the selective way of acquiring food. As stated earlier, a predator does not always eat what’s in front of it. For example, shorecrabs eat muscles that are small instead of trying to crack a large muscle with hard shells. Thus, saves energy and increase their net food intake. The variables used in this model are the same as the ones used in the patch use model although there is an added variable. That variable is T which corresponds to the total time foraging (sum of searching and handling times). The formula for the model is:

IF  E₂/h₂ > E₁/(s₁+h₁)

Then the animal should eat food type no. 2 (E2). When there is abundance in food, the animal can be choosier on what it eats. In nature, the model does not exactly apply but is close.

The last model is the Central Place Foraging which means that the food the animal finds is being brought to its storage site or to its offspring. This is commonly observed in birds which deliver worms to their young. Examples of animals that deliver it to the storage site are bees that store collected nectar in their hive.

 Foraging tactics

            Organisms in nature have developed different foraging tactics to more efficiently gather and consume the nutrients they need for survival. There are three major tactics used by animals depending of the tropic level and food type. The different animals are grouped under croppers, active hunters and sit-and-wait hunters. In croppers, the most common foraging tactic makes use of the density of food supply; time and energy devoted to ingesting and digesting the food; less effort in search or capture; ensures high quantity of low quality foods and animals mostly under this group are herbivores and filter feeders. The second group are active hunters. Unlike croppers they use food in low density, patchy or difficult to catch; time and energy devoted to capture or search; less energy use in ingesting and digesting; low quantity of high quality and they mostly compose of mammalian carnivores and frugivorous primates. Last but not the least, the sit-and-wait hunter, which is the least common among the three, uses dense and mobile food; little energy to any component of foraging except capture and digestion; very low quantity of high quality food offset by very low costs as to croppers and active hunters and those organisms included in this group are some predatory fish, ant lions and web spiders.

The foraging tactics in video 1 are active hunters. (Note: Video 2-4 are not working).

Functional Responses

            Functional responses explain how the consumption rate of an individual changes with respect to resource density. Holling identified three types of functional response namely type 1, type 2 and type 3. Type I (linear) response in which the attack rate of the individual consumer increases linearly with prey density but then suddenly reaches a constant value when the consumer is satiated. Type II (cyrtoid) functional response in which the attack rate increases at a decreasing rate with prey density until it becomes constant at satiation. Cyrtoid behavioral responses are typical of predators that specialize on one or a few prey. For example, small mammals destroy most of gypsy moth pupae in sparse populations of gypsy moth. Type III (sigmoid) functional response in which the attack rate accelerates at first and then decelerates towards satiation. Sigmoid functional responses are typical of generalists natural enemies which readily switch from one food species to another and/or which concentrate their feeding in areas where certain resources are most abundant. For example, many predators respond to kairomones (chemicals emitted by prey) and increase their activity. Polyphagous vertebrate predators (e.g., birds) can switch to the most abundant prey species by learning to recognize it visually. 

                Holling (1959) suggested a model of functional response which remains most popular among ecologists. This model is often called "disc equation" because Holling used paper discs to simulate the area examined by predators. This model illustrates the principal of time budget in behavioral ecology. It assumes that a predator spends its time on 2 kinds of activities namely searching for prey and prey handling which includes chasing, killing, eating and digesting.

            Numerical response means that predators become more abundant as prey density increases. The most simple model of predator's numerical response is based on the assumption that reproduction rate of predators is proportional to the number of prey consumed. This is like conversion of prey into new predators. For example, as 10 prey species are consumed, a new predator is born. Aggregational response is better than "numerical response" because it is not ambiguous. Aggregational response was shown to be very important for several predator-prey systems. Predators selected for biological control of insect pests should have a strong aggregational response. Otherwise they would not be able to suppress prey populations. Also, aggregational response increases the stability of the spatially-distributed predator-prey (or host-parasite) system.

THE NATURE AROUND ME

For an organism to survive, it needs to acquire its lost nutrients through the process of food consumption. Nature offered us with fascinating ways on how various organisms search, capture and consume their prey. Because of man’s curiosity, we have developed different concepts regarding them. I remember once I was watching my son and playmates play with the spiders when about 3 years ago. I recall they the spiders on a stick and the rest was up to them. Of course, in a match there is always a winner or a loser, too bad for the loser because it had to be tied up and enclosed in its foe’s web. It’s a way how spiders adapt to the environment so that they won’t lose much of their energy in immobilizing their prey simply by wrapping them up. My son placed the winning spider and the losing one in the matchbox and he told me that the spider will drink up all that is inside the spider and only the skin will be left. After a few time passed, we checked it out and indeed the web and the skin only remained, though I did not bother to touch it.

Another usual experience I always observe is the case of a lizard waiting for something on the ceiling. Different kinds of insects, especially small ones, tend to gather round in any source of light such as light bulbs. The lizard patiently waits for the right moment to stick its tongue out and have itself a meal. Eventually, of course, it did because there are many insects along the area. I noticed that it decided to go to a place where it would acquire most of the nutrients it needed. Being cautious and making its best efforts not to alarm the insects so that they won’t fly away, it was able to take in nutrients with it expending not too much energy. Same goes when we take our lunch at around 1 pm during weekends that we just see the lizards coming out and looking at us while we eat. Being pitiful about it, we would try some unusual things to happen- pinch off a small part of our food and give it to the lizard. This was possible when the lizard fell down from the ceiling to catch for some food in our table. Trying to place the food very near the lizard caused them to crawl fast, so, we decided to throw it near them. Sometimes only one would eat, sometimes the lizard would bring the food to its companion and eat together. There is one though, which has become accustomed to living with humans. It can actually come near us or near our plates. Every time we see it trying to find the remains of our food in the plate, we will all wait and then laugh. Why? It is because the plate is really close to us, yet, the lizard conquers its fear. The lizard’s nature, as I see it, comes to a place where it can find food mostly in large amounts.

In both experiences, I’ve learned that organisms have different ways of foraging techniques that help them acquire the nutrients they need. They choose the best way possible to consume and catch their prey by expending the least energy possible, as explained by the optimal foraging theory.  Because of this, the science of behavioral ecology grows more and more as time passes by due to the adaptive mechanisms every organism develops to survive their environment.