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Chapter 16 –Competition
(328-335, A; 335-339, C; Connell’s barnacles and Paine’s Starfish, A)
Tansley’s experiment involving two types of plant – one flourishing in basic soil, the other in acidic soil –
found that both plants could establish themselves /adapt to both soil types, except in the presence of competition. i.e. when both were
planted in acidic soil, the plant more suited for that soil type outcompeted the other plant and vice versa.
Competition: use of defense of a resource by one individual (intraspecific) or species (interspecific) which subsequently makes the
resource less available to other individuals or species.
Intraspecific competition, in particular, regulates populations in a density-dependant manner. With interspecific competition, species
will regulate the size of the other population as well as their own. When resource is scarce, populations may reach a stable state and
stop growing; one speices may also outcompete the other, resulting in the other’s extinction.
Resource is not limited to food; things like shelter and space (particularly important for immotile/sessile animals) also function as
resources as they can be used or “consumed” and made unavailable to other individuals.
Darwin reasoned that competition would be more intense between closely related species as their similar structure creates a need for
similar resources. While there is generally correct, it is also true that distantly related species may utilize many of the same resources
and compete strongly.
Resources may either be unrenewable (e.g. space – it becomes available only when a consumer moves or dies) or renewable, in
which case, they may either be renewed regardless of consumption, e.g. the sun, or their rate of consumption may directly or indirectly
affect their rate of renewal, e.g. predator-prey relations and nitrogen cycling, respectively.
Liebig’s law of minimum offers that a population will increase until a particular resource – the limiting resource – is reduced to such
an extent that it impedes further growth, i.e. terrestrial animals require oxygen as a resource, but they won’t like deplete its supply;
food on the other hand, can be depleted, making it limiting as compared to oxygen. Often the interaction of two of more resources
serve as limiting: synergism.
Competitive exclusive principle: two species cannot coexist indefinitely when the same resource limits both of them. As these species
do, however, exist in nature, one concludes that there are subtle differences in habitat or diet preference which allow this coexistence.
One can extend standard Lotka-Volterra models to include competition by using the variable a1,2 and
a 2,1, i.e. the effect of population 2 on population 1 and vice versa, respectively). Generally, coexistence is most likely when these
variables are >1, i.e. species must limit themselves more than they limit other species. Competition coefficients (i.e. a1,2), are often >1
when species are specialized to rely on different resources.
When species have two resources as limiting (synergism), specific ratios of availability of each may allow for coexistence of species
which would otherwise compete two strongly in instances of a 1:1 ratio.
Asymmetric competition: each competitor has a specific advantage with regards to environmental factors, i.e. one may exploit
resource more efficiently; another may tolerate a broader range of conditions. Connell illustrated this with his experiment involving
barnacles. Barnacles have space as a limiting resource. He observed that Balanus established themselves in lower regions of rock
(always submerged in water) and Chtahamalus established themselves in upper region (exposed at low tide). By removing Balanus,
Chtahamalus could establish itself in lower regions; however, the converse is not true. He concluded then, that Balanus was
outcompeted by Chtahamalus for the lower rock region, but that Balanus could tolerate a broader range of conditions, allowing for
Exploitive competition: species will compete directly for resources; consumption by one makes it unavailable to the other.
Interference competition: species will compete indirectly for resources; defending by one makes it unavailable to the other.
Allelopathy: chemical competition; chemical defense of space is observed in certain plant species.
Paine illustrated that consumers/predators can shape biodiversity with this study involving starfish. He removed starfish from specific
intertidal zones. Preceding removal, the biodiversity was about 15, following it, biodiversity dropped to 8, leaving mostly mussels.
Paine concluded that, as the starfish was a natural predator for all of the species in the intertidal zone, it kept all populations, including
that of the mussel, in check, allowing for biodiversity. Lacking predation from starfish, the mussel, a superior competitor,
outcompeted many other species for space, lowering biodiversity.
This is called apparent competition: species are mediated not by resources such as food, but by their consumers.