There are many relationships that occur in nature that contribute to the overall balance of the ecosystem. Whether they are positive or negative, an organism's survival depends on these relationships. Relationships like mutualism, commensalism, parasitism and predation occur with a dynamism that is constructed through many years of development and changes in the organisms that is driven by many factors in the environment. Many of the changes in an organism that share in a certain ecological relationship with another are also highly influenced by the changes that occur in the organisms that share this relationship with them. In a sense, the evolution of an organism as a member of an ecosystem is never solitary. As one organism changes, other organisms that are related to it also change accordingly to increase their chances of survival (Berryman, 1992).
Predation is one of the most dynamic ecological relationships as the organisms that are involved are constantly changing to eat or not be eaten. These organisms change morphologically, physiologically and behaviourally to effectively achieve advantage over their predator or prey. The survival of the predator depends on its ability to successfully capture its food, and the survival of the prey is dependent on its ability to evade the advances of its natural predator. Predator and prey are always trying to outdo each other in what many ecologists describe as a “biological arms race”.
This paper aims to describe the dynamics and changes of predator-prey relationships that occur in nature by discussing the principles that are involved in these changes on the morphological and behavioural level. The paper will also site a specific vertebrate predator-prey pair to illustrate these points.
II. Co-evolution: Changing together
Biologists over the years have observed that some species evolve in response to the changes in another. These changes can occur in different levels and in different intensities. Some species exhibit molecular changes such as the structures of macromolecules like proteins or lipids, while others show morphological and behavioural changes in the presence of other organisms. Some evolutionary changes that happen in a species can be a change that is most especially directed towards its adaptation in its relationship with another organism. This evolution that happens in response to the changes in the biotic factors of the ecosystem is called co-evolution. The habits or changes in one organism in an ecological relationship create a pressure that drives changes to another. Therefore, evolutionary changes in organisms due to changes in temperature, water and other abiotic factors are not co-evolution, even if changes happen simultaneously in organisms that are in a specific ecological relationship.
Easily observable co-evolution occurs in the level of two species interacting, but co-evolution can also be driven by a number of species interacting with each other. Co-evolutionary changes may affect interactions positively or negatively, depending on the type of relationship that drove it in the first place. For example, if co-evolution is to happen between two species of mutualistic organisms, an organism's evolution may be a response to the change that occurred in one of the interacting species to keep the mutualistic relationship running, which affects the relationship in a positive way. Co-evolutionary changes that happen in the prey which hamper the predator from successfully capturing the prey affect the predatory relationship negatively because they reduces the chances of the predatory relationship from continuing.
However, it is important to remember that the changes that are pertained to are genetic changes, the ones that can be passed on from generation to generation. Therefore, physical disabilities of an individual species that are purely phenotypic in nature that affects an ecological relationship or is caused by that relationship. For example, if a hairy rodent that lost a patch of fur from a previous attack from an eagle that it managed to escape gains advantage by making it invisible to other eagles is not a product of co-evolution. If, however, some genetic mutation makes a certain members of a rodent species lose a patch of hair so that it becomes safe from the eagle, its natural predator, this could be considered co-evolution (Yoshida, Jones, Ellner, Fussman, & Hairston, 2003).
It is also important to remember that the ecological relationship will remain even though co-evolution occurs. Changes in an organism might be driven by the other organism, but never will one outdo the other permanently. They will both change for the better so as both will always be almost on the same level of fitness (Langerhans, Layman, Shokrollahi, & DeWitt, 2004).
III. Case Study: Morphological Changes in Mosquito Fish (Gambusia affinis) found in Predator Populations
Success of predation in aquatic environments, as with any other environment, is also based on the ability of predators to move faster than their prey. Moving fast to capture prey requires certain morphological adaptations such as powerful muscles that can sustain sudden high-speed movement, streamlined bodies and excellent maneuverings capabilities. These features are common among predatory fish of all species. For fish that are considered prey of a vast species of predators, these predator characteristics are what they need to adapt to. The pressure created by living in a predatory environment will drive changes in the prey species, which can then be considered as co-evolution.
A study by Langerhans, et al. (2004) focused on the morphological changes in the western mosquito fish (Gambusia affinis), a common prey species many piscivorous fish like largemouth bass (Micropterus salmoides), green sunfish (Lepomis cyanellus), warmouth (L. gulosus), white crappie (Pomoxis annularis), longear sunfish (L. megalotis), and bluegill (L. macrochirus), underwent under predator pressure. The study compared the morphology and the kinematics of movement of mosquito fish found in predatory and predator-free habitats using morphometrics, statistical analysis and biomechanical models.
Measurements done on the bodies of males, females and juvenile fishes found in both predatory and non-predatory environments showed significant differences in morphometric analyses. Ten points of comparison were marked by the researchers, some of the points covering a certain area in the body of the fish.
The researchers found out that the mosquito fish collected from environments with predators are more streamlined than their counterparts living in predator-free environments. Moquito fish in predatory environments exhibited a more elongated body, larger caudal region and a smaller head. Their eyes exhibited a posterior-ventral position relative to the eyes of those found in predator-free populations.
The maximum burst-swimming speed of male mosquito fish was analyzed and it was found that those populations that were found in predatory habitats moved with a faster burst speed than those populations found in areas with no predators. These fish were faster than their counterparts living in non-predatory areas by 20%.
These data were then compared with the morphological data obtained using biomechanical models to produce a correlation between speed and morphology. The correlation produced suggested that the body shape differences between the two populations were responsible for the differences in burst-swimming movements in fish in predatory and predator-free areas. Furthermore, these morphological differences are found to be heritable as these characteristics remained even in the next generations grown in the laboratory.
These results agree with the hypothesis that predators do exert significant stress on prey populations that is enough to drive persistent morphological changes. This data obtained for mosquito fish can act as models for many aquatic taxa.
Organisms have certain characteristics that allow them to gain advantage in their habitat and these characteristics may have arisen as a product of their relationships with other organisms. Co-evolution occurs when some genetic characteristics are developed as a result of a pressure caused by another organism that is closely related to it in niche or in another ecological relationship. These changes may be morphological, physiological or behavioural.
Co-evolution in the predator-prey relationship is often viewed as a race to outdo each other. But inasmuch as we see that both species seem to overcome advantages that are developed by the other, it is important to remember that the changes that they get happen in such a manner that one cannot absolutely outdo the other to the point of eliminating the other completely.
The mongoose and the cobra have a very unusual predatory relationship as the prey is considered as one of the most venomous animals in the world. But these two organisms display the principles of morphological and physiological advantages when it comes to the predator-prey relationship.
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