Inter Specific Competition

COMPETITION an interaction between individuals brought about by a shared requirement for a resource in limited supply, leading to a reduction in survivorship, growth, and/or reproduction of the individuals concerned.

A: minimum requirement C: an optimum level or level of maximum growth B: maximum tolerance

Fundamental Niche - the largest possible niche (widest range of conditions) a population could occupy in the absence of competition

Realized Niche - a smaller subset niche which a population actually occupies. Smaller presumably due to competition

INTERSPECIFIC COMPETITION • Competition for a given resource between individuals of different species • interactions due to sharing a limited resource • an interaction where both participants are disadvantaged by the relationships • interference competition: -direct interactions of competitors seeking the same resource - direct agressive encounters e.g. fighting, toxins, territory, overgrowth • exploitation competition: -indirect interactions; - A form of competition that revolves around the superior ability to gather resources rather than an active interaction among organisms for these resources - one individual affects the other’s uninterrupted use of resource through prior exploitation & affects the other’s use of that resource by diminishing the total amount available

• Its immediate effect on interacting populations is in suppression of population growth & vitality • interspecific reduction in abundance and/or fecundity and/or survivorship • asymmetric - the consequences are often not the same for both species • amensalism – an interaction in which one species adversely affects another, but the second species has no effect on the first • competition for one resource often affects the ability of an organism to exploit another resource, eg. interdependence between competition for space & for food.

•

if 2 species that occupy the same niche come together in space & time, there are 3 possible results:

iv. Extinction - One species becomes extinct because its competitor was more successful at monopolizing resources - E.g. competition between Paramecium caudatum & Paramecium aurelia by G. F. Gause - Paramecium aurelia was the more successful competitor because it was adapted to rapid growth & could trap a greater percentage

Each species grown alone

100

P. aurelia

Size of population

50

P. caudatum

0

2

4

6

8

10

12

14

16

18

Both species in mixed culture

100

P. aurelia 50

P. caudatum 0

2

4

6

8

10

Days

12

14

16

18

ii. Competitive exclusion (Gause’s principle) -One species is forced out of part of the habitat but it continues to survive in other adjacent portions of the habitat Law of Competitive exclusion : When two species compete for a limited resource one species always eventually wins and the losing species becomes excluded, unless: a. environmental heterogeneity (a range of resources in the natural habitat) allows one species to survive just beyond the niche limits of the other competing species so that at least in part of the habitat they're not competing at precisely the same time and space. -e.g vertical distribution of 2 species of barnacles Chthamalus stellatus & Balanus balanoides

Barnacles are mollusks which attach to rocks and other substrates. They reach out and comb the water with feathery "cirri" (combs) to pull in tiny organisms for food. When the tide recedes and they are exposed to the air they close plates to prevent drying out.

larger Balanus is the dominant species in a competition which produces a significantly restricted realized niche for the smaller Chthamalus without competition the Chthamalus spreads into all three zones, its fundamental niche.

“ no two species can occupy the exact same niche at the exact same time”

b. the losing species can re-colonize (reimmigrate or "hide") (=recruitment) c. the winning species is prevented from its monopoly by mild disturbances (unpredictable weather, predators, disease, parasites etc., ).

iii. Character displacement • Divergence of characters in areas of overlap e.g. 2 potentially competing species occur together, they have greater differences in their feeding adaptations • each species has evolved characteristics that lessen competition with other species for food resources & hence promote reproductive success

• prolonged coexistence promote genetic changes in competitive ability of the cooccurring species • the strongest selection pressures on the abundant species is derive from intraspecific competition • selection pressures on the rare species is through interspecific competition • over a longer time scale, such genetic changes may lead to major changes in a species’ ecology, physiology or morphology to establish a phenomena of ecological separation

How can coexistence of competiting species be explained? 1. Resource Partitioning/niche diversification In a variable environment, would resource partitioning arise due to coevolution of competitors or successful colonization of species that already differed in niche utilization? 2. Competitive Networks No single species is a better competitor than all other competitor species

3. Compensatory mortality Greatest mortality falls on the competitive dominant. Compensatory mortality keys on the idea of “competition for resources,” an ambiguous phrase that suggests sufficient “resources” (such as food or habitat) exist to support a finite number of organisms 4. Spatial / Temporal changes in the environment The 'paradox of the plankton' considered the high diversity of freshwater phytoplankton at any one time given that all species need essentially the same resources. Temporal changes in the physical environment were used by G.E. Hutchinson to explain how this diversity was maintained.

A LOGISTIC MODEL OF INTERSPECIFIC COMPETITION The Lotka-Volterra model of interspecific competition The logistic equation: dN dt

= rN K-N K

r : natural rate of increase (intrinsic rates of increase K: carrying capacity – a measure of the total population the environment can support at balance N: population size The total competitive effect on species 1 (intra- and interspecific) will be equivalent to the effect of (N1 + N2α12) species 1 individuals

α12 : competition coefficient, the competitive (inhibitory) effect of species 2 on species 1 N2α12 : N1 equivalents dN1 dt

= r1N1 (K1 – (N1 + α12N2) K1

dN1

= r1N1 (K1 - N1 - α12N2)

dt

K1

The total competitive effect on species 2: dN2 dt

= r2N2 (K2 – N2 – α21N1) K2

the rate of growth of one population is a function of: • its natural rate of increase (r), • its present size, how efficient it is in obtaining resources (K is essentially it's intraspecific ability), • the number of competitors of another populaton and • how strongly that that species affects it competitively.

Numbers Intraspecific competition only Intra & interspecific competition Time

Numbers

Intraspecific competition only Intra & interspecific competition Time

The behavior of the Lotka-Volterra model is investigated using ‘zero isocline’: a. The N1 (species 1) isocline:

K1

A

α 12

N2

B N1

K1

dN1

=0

r1N1 (K1 - N1 - α12N2) = 0

dt K1- N1- α 12N2 = 0

When N1 = 0,

When N2 = 0 ;

N1 = K1- α 12N2

N2 =

K1

(point A)

α12

N1 = K1

(point B)

b. The N2 (species 2) isocline:

K2

N2 N1

K2 α 21

α12N2 K1

and

α21N1 K2

determine the outcome of the competitive interaction Species 1 outcompete species 2: α12 relatively small; α21 large Species 2 outcompete species 1: α21 ≤ α12

Different ways in which the two zero isocline can be arranged: i. species 1 always outcompetes species 2, and is referred to as the competitive exclusion of species 2 by species 1 :

K1 α 12 K2 N2 K2 α 21

K1

N1

K1 α12

> K2

K1 > K2 α12

and

K1 > K2 α21

and

K1 α21 > K2

ii. Species 2 is a strong interspecific competitor & drive species 1 to extinction:

K2 K1 α 12 N2 K1

K2 α 21

N1

K2 >

K1 α12

K2 α12 > K1

and

and

K1 <

K2 α21

K1 α21 < K2

iii. Individuals of both species compete more strongly with individuals of the other species than they do among themselves; The outcome depends on the initial abundances of the two species

K2 K1 α 12 N2 K2 α 21

K1

N1

K2 >

K1 α 12

K2α 12

> K1

and

K1 >

K2 α 21

and K1α 21 > K2

iv. Both species are more strongly limited by intraspecific competition; Two species are able to coexist; The outcome regardless of the initial abundances. K1 α 12 K2 N2 K1

K2 α 21

N1

K1 α 12

> K2

K1 > K2α 12

and

K2 α 21

and

K2

> K1

> K1α 21

The assumptions of the model (e.g., there can be no migration and the carrying capacities and competition coefficients for both species are constants) may not be very realistic, but are necessary simplifications A variety of factors not included in the model can affect the outcome of competitive interactions by affecting the dynamics of one or both populations Environmental change, disease, and chance are just a few of these factors.

Indirect interaction: • 2 species with non-overlapping diets which are preyed upon by a common predator • 2 predatory species with little or no dietary overlap may influence each other indirectly, if their prey species compete