Tuesday, November 29, 2011

Interactions and Succession in Nose Hill Park’s Pond, by Katherine and Alice

On September 22, 2011, Sir Winston Churchill High School’s Biology 20 IB students went to Nose Hill Park in Calgary, Alberta, Canada to investigate the three major ecosystems present there – the pond, forest, and grassland. This blog post will be focusing on the pond (please view Figure 1. for a sketch of the pond and general information), and the symbiotic interactions of its organisms. It is important to note that no comparisons could be made between Edworthy Park and Nose Hill Park, as the park does not possess a still-standing body of water such as a pond with a comparable ecosystem. Now before we begin delving into the core of this post, let us first establish some context. Ecology is the study of organisms in relation to their environment – not merely the physical components of the environment but their effect on other organisms whose lives overlap theirs, particularly those on which they feed and for which they in turn provide sustenance. Symbiotic relationships (or symbiosis) describe the interactions between two organisms of different species, where these interactions tend to take place for most of the organisms' lives. There are a number of different types of interactions, each deserving an innumerable amount of research, however the ones we are going to explore include mutualism, parasitism, commensalism, predation, interspecific competition, intraspecific competition, Batesian mimicry, and Mullerian mimcry. In addition, we will discuss primary and secondary succession in relation to Nose Hill Park.
Let’s begin with predation, a form of symbiotic relationship where one species acts as a predator that captures and feeds on the other organism that serves as the prey. Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissues through consumption. (Freeman, 2008) The main predators found at the pond are the Pond Wolf Spider (Pardosa pseudoannulata), Harvestmen (Opiliones), damselfly and dragonfly nymphs and adults such as the Aeshnidae, Water Boatman (Corixidae), Predacious Diving Beetle (Hydaticus modestus), Flatworm (Dugesia polychroa), Wasp, Mallard duck, and Black-billed magpie. To further understand the trophic levels each predator is in, please view Figure 2 for a food web and the video below for pictures. Pond Wolf Spiders commonly capture prey such as damselflies and dragonflies, injecting them with fatal poison using its chelicerae, and then proceeding to consume them. (Clifford, 1991) In addition, damselfly and dragonfly nymphs will use their powerful jaws to kill and eat mollusks, other insects, crustaceans, worms, and small fish. One more example is the Hydatiscus Modestus (Predacious Diving Beetle) which will cling to grasses or pieces of wood along the bottom of the pond, hold perfectly still until prey passes by, then lunge, trapping their soon-to-be-food between their front legs and killing them by biting down with its pincers. Their usual prey includes tadpoles and glassworms, among dozens of other smaller water-dwelling creatures. (Clifford, 1991) These are all cases of true predation, where the prey lose their lives while the predators gain chemical energy from consuming the prey. Mathematical models of predation are amongst the oldest in ecology. The Italian mathematician Volterra developed his ideas about predation from observing Adriatic fishing fleets. When the number of fishermen increased due to a particularly successful fishing season, after a time, the fish population declined due to over-harvest, and thus the number of fishermen also declined. After some time, the cycle repeated. (Williams, Nichols & Conroy, 2002) Logic and mathematical theory suggest that when prey are abundant their predators also increase in numbers, reducing the prey population, which in turn causes the predator population to decline. The prey population sooner or later recovers because once the predator population declines, the prey can fuel a new round of population increase, consequently beginning a new cycle. Prey evolve behaviors, structural armor, and other defenses that reduce their vulnerability to predators every cycle. Predation, while not the only complex community interaction, has often had strong and long-lasting (although indirect) effects.
            Next let’s focus on mutualism – a relationship between two or more species where both receive mutual benefit from the interaction. The most common form of mutualism is when one resource that is needed by one participant is traded for a different resource needed by the other. The first example vibrantly present in the Nose Hill Park pond is the mutualistic association between a fungus and algae. The fungus provides a tough, waterproof body able to withstand extreme environments on rocks, being good at obtaining water and secreting acids to dissolve minerals from the rocks.  It also produces carbon dioxide.  All of these materials are then provided for the algae, which use them in photosynthesis to produce sugars, which are then shared with the fungus. Another example is between algae and freshwater snails, where the algae find substrates on the shells or carapaces of the snails, which in turn benefit from the camouflage. Camouflage is a method of crypsis (hiding), which allows an otherwise visible organism, or object to remain unnoticed, by blending with its environment. (Freeman, 2008) All of these examples found in Nose Hill Park have both participants gaining from their relationship with each other.
            Our next prevalent interaction under study is parasitism. Parasitism is a form of symbiosis in which one organism, the parasite, grows, feeds, and gains at the expense of another organism usually of a different species, the host, which harbors the other organism. The interaction may cause injury to the host thus, when there are more parasites, it means that there will be less hosts due to the nutrients the parasites are taking away. There are many cases of parasitism occurring in the pond, one of them being when a wasp stings and paralyzes a Pond Wolf Spider, afterwards taking it to a nest and laying an egg in it.  The larvae will consume the still-living spider; often from the inside out. This is a parasitic relationship as the host, the spider, loses by undergoing fatal harm while the parasite, the wasp larvae, gains as it cannot survive without the nutrients and shelter the spider’s body provides.
            Another vital symbiotic interaction is commensalism, a relationship between two organisms usually of different species, where one benefits from the other, the host, which remains unaffected. The organism that attaches itself onto the host will receive some type of profit such as food or shelter while the host doesn’t undergo a change at all (neither positive nor negative). One example found at the pond is between marsh wrens and cattail plants. The marsh wrens build their nests on the stalks of cattails, gaining shelter from predators, but do not affect the plant itself. This is an instance of commensalism because the organisms that are using primary producers as protection gain because they stay safe and the primary producers do not gain or lose anything. Another example of commensalism found is between damselfly and dragonfly nymphs and pondweeds. The nymphs hide at the bottom of the pond among the pondweeds, allowing them safety and camouflage from predators as well as the ability to surprise and attack prey. In turn, the nymphs affect the pondweeds neither positively nor negatively.
            Next let’s focus on intraspecific competition, which is conflict between individuals of the same species over a resource that is in short supply. One example of intraspecific competition is between leeches, fighting over food sources. Due to chemicals that humans use (like fertilizers) draining into the pond, plants will be able to grow but animals that may use the pond as drinking water will suffer from the toxicity. The fertilizers in the pond can also lead to algae blooms in the water, which will use up the dissolved oxygen, leaving less oxygen for other aquatic species. In addition, many vehicles drive by Nose Hill Park polluting the air, while visiting humans litter, introducing new or harmful substances to the pond ecosystem, which can affect the animals because they have not adapted to the new chemicals. Due to these factors, organisms in the pond such as snails, frogs, dragonfly and damselfly nymphs are easily put at risk. This is an example of intraspecific competition because leeches of the same species feed on these organisms in short supply, consequently resulting in conflict. Some leeches occasionally even eat other leeches as both a defense mechanism and as a way to gain sustenance. Both leeches lose from expend energy and both are usually harmed in the conflict, hence this is a solid example of intraspecific competition.
            Very similar to intraspecific competition is interspecific competition. This is the conflict between two or more different species for the same limited resources such as food, nutrients, spaces, or mates. The individuals of one species will likely experience a reduction in population or growth as a consequence of resource exploitation or interference by individuals of the other species. One example of interspecific competition is between dragonflies and damselflies for prey. Because of pollution in both the water and air, many organisms the two species prey on do not grow as large or live as long, thus being in short supply. Consequently, dragonflies and damselflies will compete for the prey, causing one species to eventually undergo a reduction in population either due to starvation or interference competition (direct physical confrontation). This is a solid example of interspecific competition as the species in competition are different and the resources in question are of limited supply. In addition, consequent conflict usually arises and the stronger species usually bests the other, weaker one, and gains the resource, however both lose energy in the process.
            Now let’s focus on Batesian mimicry. Batesian mimicry is where a harmless species, the mimic, has evolved to imitate the warning signals of a harmful species, the model, to a common predator. This form of mimicry can employ the deception of any of the senses and must depend on a disparity between unpalatable and edible species. The mimics must be smaller in population, while the models must be very abundant, with a high probability that the predator will try to eat the inedible model species first. In this type of mimicry, the mimic benefits as it gains protection without having to spend energy arming themselves, while the model loses because if there are a large number of mimics, the predator may begin to regard the model as harmless. This can also be detrimental towards the predator as it may begin to more frequently encounter toxic prey while initially thinking it was harmless. An example of Batesian mimicry present at the Nose Hill pond is when harmless hoverflies mimic bees’ and wasps’ distinctive bright striped coloring. As members of Diptera, all hoverflies have a single functional pair of wings, similar to a wasps’. (Clifford, 1991) They are also brightly colored, with spots, stripes, and bands of yellow or brown on their bodies. Due to this coloring, they are often mistaken for wasps or bees; thus exhibiting Batesian mimicry. This is also a form of protective coloration where the coloration or color pattern of an animal affords it protection from observation by its predators.
Finally, a look at Mullerian mimicry – where two or more harmful species that don’t have to be closely related and share one or more common predator, mimic each other’s warning signals. If a common predator confused two species with one another, individuals in both would be more likely to survive. In this form of mimicry, both parties serve as co-mimics or co-models, however if one species were to be more rare than the other, the more common species would be the model and the other the mimic. What is interesting about Mullerian mimicry is that the predator in question also is at an advantage because although it is not gaining sustenance, it is being deceived into evading potentially harmful encounters. An example of Mullerian mimicry present in the Nose Hill pond would be where flatworms, which excrete toxic body fluids when dying, mimic the poison-carrying nudibranchs. (Clifford, 1991) This turns potential predators such as fish, away from them, through visual recognition. In this form of mimicry, all parties benefit  - as predators learn to avoid the group as a whole.
Let’s move on to succession – the vital process that developed Nose Hill Park into what it is today. Hundreds of thousands of years ago, the land of which Nose Hill Park occupies today was covered by a large river. The large river was then transformed in to a sizeable glacier during the last ice age, its glacial movements shaping the distinct hills and valleys present today. After the ice age ended, a form of succession – primary succession – took place and began the long process of establishing life. Ecology succession is the process of change in the species structure of an ecological area while succession is the process of change in the environment over time. More specifically, there are two major types of succession, primary and secondary. Primary succession is the gradual colonization of a habitat of bare rock or gravel, usually after an environmental disturbance that removed all soil and previous organisms. (Freeman, 2008) In this case, Nose Hill Park had just been covered with glaciers for a couple thousands of years thus the succession over these areas would consequently require extra time and forces to prepare the land for further development.
            A pioneer community is a collection of species that colonize previously untouched land, usually leading to ecological succession. These species are the first organisms to begin the chain of events leading to a flourishing biosphere or ecosystem. (Freeman, 2008) Pioneer species that likely cropped up in Nose Hill Park include lichen, small ephemeral bunchgrasses and wildflowers. Winds likely carried seeds from other plants over and as a result, grasses would have begun to grow on sand dunes or dry areas, while lichens would have tended to grow in damp and rocky environments. In the pond, the pioneer community probably consisted of hardy, long rooted plants such as green algae, Pickleweed, and mosses. Pioneer species die creating plant litter, breaking down to make new soil for secondary succession or nutrients for small organisms in aquatic environments.
            However, the pond located in Nose Hill Park did not form from natural succession or natural forces. Although the pond was a man-made storage for storm run-off from surrounding communities, secondary succession still occurred within the pond itself – a gradual colonization of a habitat after a disturbance that removed some or all previous organisms but left the soil intact. Since soil is provided, secondary succession proceeds at a faster past. (Freeman, 2008). Before the pond became storage for storm run-off, the area was simply a depression in the land covered by grass, shrubs, and organisms adapted to that environment. After the reroute of water and over time, sturdier organisms adapted to the slightly toxic pond water began to appear, evolve and multiply. Since the environmental conditions were continually changing, new species were introduced and the initial ones evolved to adapt to the ever-changing ecosystem, cycling again and again.
            A climax community is the secure and final ecosystem that develops from ecological succession. (Freeman, 2008) A prediction of Nose Hill pond’s climax community is that the pond will have all the present organisms it has now, except with more fish species adapted to low oxygen levels. This prediction was made on the basis that despite all the pollution from car exhaust, litter, and street runoff, the pond is still teeming with life. The only organisms in rare existence are fish, which are extremely sensitive to pollution. Thus, the pond with evolved fish would be the climax community – the sere having reached equilibrium and adapted to the climax conditions of the environment. A sere is a transitional stage found in ecological succession in an ecosystem advancing towards its climax community. A seral community is the name given to each group of plants within the succession, an example being a polluted pond. During the first two years, algae, fungus, and moss would be abundant. After a few more years cattails and small organisms would start to appear; and about six to eight years after the clearing, the area would likely to be teeming with aquatic animals. Each of these stages can be referred to as a seral community.
            We have now explored both succession and the multitude of interactions prevalent in Nose Hill Park’s pond, discovering along the way the complexities of its organisms and its evolution from the past to present. There is so much still to be found, and we hope this inspires everyone to take a trip down to Nose Hill Park and enjoy the environment as much as we did.

Figure 1. Sketch of Nose Hill pond. (for larger version click http://www.flickr.com/photos/69278508@N02/6300199091/in/photostream/)
Figure 2. Food web of Nose Hill pond (for larger version click
For the bibliography, please click http://www.scribd.com/doc/71094495/Nose-Hill-Refs

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