Community Ecology BDC321 Mark J Gibbons, Room 4.102, BCB Department, UWC Tel: Image acknowledgements –

A more mechanistic approach – Tilman (1977, 1982) Two limiting resources – e.g. light and nutrients Amounts set limits to population growth ZNGI Amount of resource 1 Amount of resource 2 Limitation by resource 1 Limitation by resource 2 Population must decrease Population increases Rate of resource consumption important (differs between species) – influence consumption vectors Population A is limited by resource 1, whilst Population B is limited by resource 2 A B

In a, above, Species B needs more resources than species A and it is driven to extinction (Blue). Yellow is area where A naturally present and B absent. In b, above, species A needs more resources than species B and it is driven to extinction (Brown). Green is area where A naturally present and B absent. NO EQUILIBRIUM POINT WITH BOTH SPECIES Putting two species together…… Amount of resource 1 Amount of resource 2 ABAB ab

Here, the ZNGI overlap – equilibrium point: Stable or unstable? Need information on consumption rates. At the equilibrium point, Species A is limited by resource 2, and Species B by resource 1. IF, A consumes more resource 1 than B, unstable – extinction of B (blue) IF B consumes more of resource 2 than A, unstable – extinction of A (brown) Else – stable co-existence

What do the models tell us? 1) Competition can lead to winners and losers 2) Some competitive interactions lead to co-existence 3) Can understand competitive interaction ONLY with knowledge of resources and mechanisms Consumption rates variable

Lotka-Voltera models mimic reality in some respects: Outcomes of competition are predictable SaccharomycesSchizosaccharomyces K r

A word about models….. A balance between reality and understanding >

Here K1K1 K 2 α 21 > AND Interspecific effects of species 1, greater than intraspecific effects of species 2 Interspecific effects of species 2, greater than intraspecific effects of species 1 What is the OUTCOME? Unstable equilibrium – varies with N 0 K2K2 K 1 α 12 > Where inter-specific competition is greater than intra-specific competition for BOTH species Mutual Antagonism

Where inter-specific competition is greater than intra- specific competition for BOTH species For example…. Tribolium confusum T. castaneum Temperature Moisture SEPARATELY Both species used same flour resources – competition (as we shall see in just a moment) BUT they also preyed preferentially on the eggs, larvae and pupae of the other species – effects of each species on each other greater than on own Reciprocal predation – mutual antagonism Park et al. (1965) Physiological Zoology 38:

Depending upon the culture “conditions”, the outcome of competition between the two species varied At environmental extremes there were clear winners and losers – realised niches In intermediate environments, one species would win sometimes, but lose out at other times – no certainty – just probable – DEPENDS on N 0 Percentage Wins ClimateT. confusumT. castaneum Hot-Moist0100 Temperate-Moist1486 Cold-Moist7129 Hot-Dry9010 Temperate-Dry8713 Cold-Dry1000 Park (1954) Physiological Zoology 27:

Apparent Competition – Competition for enemy-free space Imagine a predator that eats two species of prey Both prey species suffer from the predation, and the predator (enemy) benefits from both species of prey Increases in the predator abundance caused by its consumption of one prey species, increase the harm done to the other prey species, indirectly. Therefore each prey species adversely affects the other prey species – even if no obvious limiting resource The limiting resource that the prey species compete for is “enemy-free” space – the persistence of one prey species will be favoured by avoiding attacks from the predator, which also attacks the other prey species. This can be achieved if the two species occupy a habitat that is sufficiently different from the other prey species – niche differentiation

Venturia canescens Plodia interpunctella Coupled Oscillations Ephestia kuehniella No Competition between hosts Bonsall and Hassell (1997) Nature 388:

Venturia canescens Plodia interpunctella Coupled Oscillations Low r Ephestia kuehniella Bonsall and Hassell (1997) Nature 388:

Much of our understanding of competition comes from laboratory experiments or field manipulations (additions and/or removals) Easy to conduct - controlled Mostly conducted on sessile organisms BUT……..Artifacts Natural experiments provided additional, evolutionary, information about competition Lack of controls Evidence for competition from natural experiments comes from niche expansion in the absence of a competitor (competitor release), and comparisons of the realised niche of a species in allopatry or sympatry (often involving character displacement): competitive exclusion and resource-partitioning.

Diamond (1975) In: Ecology and Evolution of Communities, Cody and Diamond (Eds) Harvard, Coastal Scrub Light Forest Inland Rain Forest Competitive Release Chalcophaps indica Chalcophaps stephani Gallicolumba rufigula Bagabag New Guinea New Britain Espiritu Santo

Character Displacement Length (mm) Hydrobia ventrosa Hydrobia ulvae Food size proportional to body size Percentage of diet Fenchel (1975) Oecologia 20, 19-32

Character Displacement Length (mm) Hydrobia ventrosa Hydrobia ulvae Food size proportional to body size Percentage of diet Fenchel (1975) Oecologia 20, 19-32

Hydrobia ventrosa Hydrobia ulvae Percentage of diet Food size proportional to body size Length (mm) Food Particle Size Fenchel (1975) Oecologia 20, 19-32

Niche Differentiation and how do you measure it? Species may split common resources in a number of ways By finely dividing resources – e.g. food size by birds & snails Percentage of diet Hydrobia Food Particle Size Despite problems of directly linking competition to niche differentiation, the latter clearly leads to co-existence

DEPTH (m) J F M A M J J A S O N D Antho- Lepto- Trachy- Narco- Scypho- By utilizing common resources at different times Buecher & Gibbons (1999) Marine Ecology Progress Series 189:

By utilizing common resources in different microhabitats Sharitz & McCormick (1973) Ecology 54:

Niches may be differentiated on the basis of conditions Two species utilize the same resource, but their ability to do so is determined by the environmental conditions, which favour one species over another Can result in microhabitat separation, temporal separation

How do you measure niche differentiation? Firstly…………Niche breadth (Levins Index - B) B = 1 ∑ p2ip2i p i = proportion of individuals using resource i B A = 4.65 = 1 / B B = 6.06 = 1 / Species B has a broader dietary niche than species A

Niche Overlap M jk = ∑ p 2 ij p ij p ik ∑ MacArthur & Levins (1967) Index M jk – overlap index of species k on species j p ij – proportion that resource i is of the total resource that species j utilises p ik – proportion that resource i is of the total resource that species k utilises M AB = = / M BA = = / Range: 0 - 1

NOTE – just because there may be strong overlap – doesn’t mean that competition takes place! M AB = = / M BA = = / Species B has less of an overlap in its niche with Species A than Species A has on Species B

THE END Image acknowledgements –