Inter-Specific Competition's Role in Structuring Communities: A Review, Slides of Ecology and Environment

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The role of inter-specific competition IMPORTANT – YES BUT……… HOW IMPORTANT In a stable, homogeneous environment, where species compete with each other on an ongoing basis – competitive interactions will reach equilibrium: IMPORTANT But if other factors prevent equilibrium being reached, competition may not be so important E.g. phytoplankton communities: diverse despite limited scope for resource partitioning Hutchinson (1961: American Naturalist 95: 137-145) suggested this reflected short term fluctuations in conditions and resources that prevented competitive exclusion Docsity.com Spread-sheet Exercise r1 r2 k1 k2 alpha12 alpha21 0.9 0.9 800 400 1.1 0.6 N2,t+1 = N2,t + r2.N2,t K2 – N2,t – α21N1,t K2 N1,t+1 = N1,t + r1.N1,t K1 – N1,t – α12N2,t K1 Start populations of both species off with 100 individuals Plot population changes over 25 time units Calculate and plot changes in H’ over 25 time units Create disturbance column using random numbers Set disturbance criteria as 0.1 If system disturbed then N (both species) reset to 100 Calculate and plot changes in H’ Play around with disturbance criteria Docsity.com One approach to examining the role of competition in structuring communities is to predict what they should look like if inter-specific competition played/plays a part in shaping them and then to compare the results with real communities. Predictions: Potential competitors that coexist should exhibit niche differentiation Niche differentiation by species may take form of morphological differentiation Within a community, potential competitors with little differentiation would be unlikely to coexist – negative associations Taking each of these in turn…………………….. Docsity.com Potential competitors that coexist should exhibit niche differentiation – evidence from community patterns Anemonefish - Amphiprion Nine species off PNG – each typically associated with just one species of anemone that is aggressively defended Anemones limiting resource – results of translocation experiments Surveys at three sites – replicated Surveys in four depth zones - nearshore (N) - mid-lagoon (M) - outer barrier (O) - offshore (OS) Results show each anemonefish associated with particular anemone AND characteristic preference for a particular zone 1 – niche complimentarity E llio tt & M a ris c a l (2 0 0 1 ) M a rin e B io lo g y 1 3 8 : 2 3 -3 6 Docsity.com This example suggests that WITHIN A GUILD niche differentiation involves several dimensions – and species that occupy a similar position along one dimension (anemone species) tend to differ along another dimension (zone occupied) Docsity.com 3 – Are patterns real or not? The NULL MODEL approach There is a temptation to interpret differences as confirming the existence of competition BUT… are the differences big or regular enough to be different from those expected by chance? Need to construct a null model A null model is like a null hypothesis – it provides a set of “random” data that can be used to test observations against. A null model of a community must retain certain characteristics of the community under investigation but reassemble components at random – specifically excluding the consequences of biological interactions. Docsity.com Lizard communities in North America Lawlor (1980) examined the dietary overlap between lizards in ten communities and then asked if these differed from those that would be expected by chance alone. HOW? 1) Calculated electivities for each diet item for each species in each community (range from 0-1) Species A Species B Species A Species B 1 0 4 0 0.16 2 5 4 0.2 0.16 3 9 3 0.36 0.12 4 3 0 0.12 0 5 0 5 0 0.2 6 0 9 0 0.36 7 8 0 0.32 0 Diet Item ELECTIVITYNUMBER 2) Calculated dietary overlap between every pair of species in each community 3) Calculated mean dietary overlap between species in each community Community No No Lizards in Community Mean Dietary Overlap 1 4 0.065 2 5 0.30 3 5 0.29 4 6 0.12 5 6 0.16 6 7 0.11 7 8 0.28 8 9 0.19 9 9 0.21 10 9 0.20 L a w lo r (1 9 8 0 ) A m e ric a n N a tu ra lis t 1 1 6 : 3 9 4 -4 0 8 Docsity.com Four null models used that retained different aspects of the food environment Model 1 Minimal amount of initial structure retained Only original number of species and number of dietary items retained. Otherwise, all electivities, including zeros, assigned a random number between 0-1. Repeated 100 times Species A Species B Species A Species B 1 0.65 0.38 0.18 0.16 2 0.13 0.06 0.04 0.02 3 0.57 0.23 0.16 0.10 4 0.48 0.80 0.13 0.33 5 0.87 0.06 0.24 0.03 6 0.44 0.32 0.12 0.13 7 0.52 0.54 0.14 0.23 Diet Item ELECTIVITYNUMBERIndividual overlap between species in a community then calculated, as too mean overlap per community No species in community M e a n d ie ta ry o v e rla p Null Model data Real data Niche breadth and overlap increased wrt observed Docsity.com Original number of species, number of dietary items and electivities retained: non-zero values just randomly reassigned to other non-zero diet items. Repeated 100 times Species A Species B Species A Species B 1 0 4 0 0.16 2 5 4 0.2 0.16 3 9 3 0.36 0.12 4 3 0 0.12 0 5 0 5 0 0.2 6 0 9 0 0.36 7 8 0 0.32 0 Diet Item ELECTIVITYNUMBER Original Data Null Model Data Species A Species B Species A Species B 1 0 3 0.00 0.12 2 8 9 0.32 0.36 3 3 4 0.12 0.16 4 9 0 0.36 0.00 5 0 5 0.00 0.20 6 0 4 0.00 0.16 7 5 0 0.20 0.00 Diet Item ELECTIVITYNUMBER Individual overlap between species in a community then calculated, as too mean overlap per community Model 4 This model retains the greatest amount of the original structure in the system Docsity.com Taken overall then – there is a significant difference between the observed patterns and those simulated by the null models – implying that biological interactions (in this case interspecific competition) have played a part in structuring the communities observed Docsity.com Niche differentiation may take form of morphological differentiation: evidence from community patterns Where niche differentiation results in morphological differentiation, niche spacing should be reflected in morphological differences between species in a guild In animal guilds, adjacent species tend to show regular differences in body size or in size of feeding structures Ratios of 2.0 for mass and 1.3 for length Cuckoo doves – 1.9 (mass) Bumblebees – 1.32 (proboscis length) Weasels – 1.23 (canine length) Fossil brachiopods – 1.48-1.57 (body outline length) Docsity.com How do you test if patterns real? Construct a null model…….. Compare the pattern of species co-occurrences at a suite of locations with that which would be expected by chance Index of association between all pairs of native and (separately) alien plant species found on 23 islands in Lake Manapouri (New Zealand) constructed: dik = (Oik – Eik) / SDik dik = Association Index Oik = Observed number of island shared by species I and k Eik = Expected number of islands shared by species I and k SDik = Standard deviation of expected number For example……… Wilson (1988) J of Ecology 76: 1030-1042 Docsity.com The distribution of these values was then compared to the distribution of values obtained if the species were randomly distributed amongst the islands (keeping number of species on each island, and the total number of islands occupied by a species, at the observed level) Native Species Significantly more negative associations than expected by chance – competitive exclusion Significantly more positive associations than expected by chance – probably based on common microhabitats Docsity.com The distribution of these values was then compared to the distribution of values obtained if the species were randomly distributed amongst the islands (keeping number of species on each island, and the total number of islands occupied by a species, at the observed level) Alien Species No significant differences between observed and expected distribution of associations – interactions have not yet reached equilibrium OR generally weedy and generalist nature Docsity.com