Predator Prey’s Density

Junior (College 3rd year) ・Biology ・APA ・5 Sources

A predator controls its prey’s density (number in a given area) by using feeding on them thus limiting them from growing in number. Most of the time. Predators have a tendency to attack and feed on the old, sick, weak and the younger prey animals. This way, they eliminate those animal preys that are not going to survive leaving the stronger animals to stay and breed. The prey animals that survive to reproduce are those that are robust and thus produce offspring that are strong to suffer the environment. The same happens for the predator animals; the speedy and strong survive when the prey animal populace dwindles and produce offspring that can survive the conditions. Eventually, the predator-prey relationship maintains the population of both the species in balance.

How a Predator Controls Prey Diversity

Predators can control prey diversity (number of different species) where keystone species are involved. A keystone species are species that play a very critical role in its community (CK-12). Any significant changes in their number affect the population of several other species in the community. For instance, in the coral communities, some of the sea star species are keystone species. The sea stars prey on sea urchins and mussels, both of which have no any other natural predators. If the sea stars move from the coral reef community, then there would be an explosive growth in the number of sea urchins and mussels (CK-12). Eventually, the community would be destroyed since most of the species will be driven out.

Differences Between Orientation and Navigation

Orientation is a movement towards a given direction while navigation is directed movement towards a goal(Gould). Almost every animal moves in an oriented way. Navigation entails more since it’s not just an oriented movement and thus not all animals are capable of navigation. Navigation involves determining the direction and maybe distance through neutral processing of the sensory inputs. For example, if a honey bee were to look for food north of its hive then it would leave home with the sun to its right and vice versa. In going to look for the food, animals such as the honey bee may use time-compensated sun compass to set its goal (Gould). For proper orientation and navigation, animals utilize various reference cues such as smells, visual objects and geomagnetic field.

Differences and Similarities Between Migration and Short Distance Habitat Selection

Migration refers to a seasonal and directional movement of animals from one place or region to another. It enables animals to take advantage of different habitats that are favorable in particular seasons or which are favorable for specific stages of the lifecycle of an animal. On the other hand, short distance habitat selection refers to the hierarchical behavioral responses process that may create excessive use of habitats to influence individuals’ fitness and survival (Rice and Owsley). In both migration and short distance, habitat selection animals may move into an area that seems to he have lots of benefits even when in reality it damages their fitness.

Polyandry and Its Benefits and Costs

Polyandry is when one female mates with two or more different males (Boulton and Shuker). The male equivalent of polyandry, that is where one male mates with multiple females is referred to as polygyny while monandry is where a female only mates with just one male. Polyandry has both benefits and costs and these vary across environments and across different individual females. Its benefits can be grouped into two, that is, those that directly and indirectly increase the female’s fitness.

Benefits of Polyandry

The most apparent direct benefit is getting enough sperms to fertilize every ova of the females. During copulation, other material gains got include different types of nuptial gifts. Such nuptial gifts include things like food items acquired through the nutritious spermatophores. During mating, the females receive the spermatophores from the males which they then use to enhance their fecundity. In many insect species, nuptial gifts significantly enhance their fecundity by multiplying the mated females since on average they get more offspring than the females that have been singly mated (Boulton and Shuker).

Polyandry can enhance the level of male parental care for offspring or access to territories rich of resources that are controlled by the males. For example, such increased care and access can be witnessed in birds such as the superb fairy-wren. Polyandry helps keep offspring from harm, for instance, female may adopt polyandry as a strategy to reduce the risk of infanticide. A female may ‘hide’ the paternity of an offspring by mating with many males since in some species, the males do not hesitate to kill an unrelated offspring (Boulton and Shuker). Indirect benefits of polyandry come when females mate with many males when genetic mechanisms increase the offspring fitness. Having many males fertilize a female’s egg could give a more genetically diverse offspring with high chances of their survival in changing environments. In males, polyandry increases the level of male-male competition leading to selection on the males to secure their paternity (Boulton and Shuker).

Costs of Polyandry

Even though polyandry has multiple benefits, it can also be costly to the females in terms of less foraging efficiency and increased disease transmission or predation (Boulton and Shuker). It may also result in less paternal care males put in when they are unsure of the paternity. For example, male birds invest less in offspring feeding in situations where the rate of extra-pair copulations was high. Also, most of the time the males transfer some substances to the females which lower their lifetime fitness.


Trade-off refers to a situation where one trait cannot increase minus a decrease in another or cannot decrease minus an increase in another (Garland). It may be a result of various physical and biological mechanisms. One mechanism is what is referred to as the ‘Y-model’. According to this model, for a given amount of resources such as time, two traits cannot be increased at once. An example is a trade-off between number and size of eggs a bird can produce in a clutch. The trade-off can be brought about by a limitation in the energy, time and space depending on the organism. Also, trade-offs happen where characteristics that increase an aspect of performance necessarily reduce another kind of performance. In humans, since resources are finite, fundamental trade-offs must be navigated like survival versus reproduction, offspring quality versus quantity and between parental investment versus investing in mating efforts (Garland). For instance, people suffering from sickle-cell anemia have hemoglobin gene mutation that also confers malaria resistance. Plants such mouse-ear cress trade-off its size with its resistance to disease.

Works Cited

Boulton, Rebecca A and David M Shuker. "Polyandry." Science Direct (2013): 1080-1081. Web. <>.

CK-12. Predation. 2017. Web. 15 June 2017. <>.

Garland, Theodore. "Current Biology." Science Direct (2014): 60-61. Web. <>.

Gould, James. "Animal Navigation." Science Direct (2014): 221-224. Web.

Rice, Justin and Bret Owsley. Habitat Selection. n.d. Web. 15 June 2017. <>.

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