Community Ecology BCB331

Community Ecology BCB331 Mark J Gibbons, Room Z108, BCB Department, UWC Tel: 021 959 2475. Email: mgibbons@uwc.ac.za

Provide habitats Quercus Provide food resources Alter conditions Individual species can influence communities in a variety of ways

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 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

Floder et al. (2002) Oecologia 133: 395-401 Diversity higher in unstable environments – competitive exclusion prevented

In a literature survey of inter-specific competition, Schoener (1983) noted that approx equal numbers of studies had been published on marine organisms, terrestrial plants and terrestrial animals; paucity in freshwater Conclusions about generalities restricted to environments and taxa studied – BUT………….. 89% - Terrestrial 91% - Freshwater 94% - Marine Competition demonstrated In a similar study, Connell (1983) noted that in > 90% of studies of one species pairs, competition demonstrated but that this dropped to < 50% in studies of more species Connell (1983) American Naturalist 122: 661-696 Schoener (1983) American Naturalist 122: 240-285 Amongst terrestrial studies, most conducted in temperate areas – few dealt with phytophagous insects BEWARE – people don’t publish negative results, people tend to study species that hint at competition and journals don’t publish all papers submitted to them!

The case of phytophagous insects Southern England Lawton (1984) In: Ecological Communities, Strong et al. (Eds), Princeton, 67-100 Papua New Guinea New Mexico Competition rare – vacant niches: Implies competition unimportant Maybe widespread amongst herbivores WHY?

It may not be important for all species Whilst competition is obviously important for sessile species

At the same time – even if competition is not obvious at the time of observation, does not mean it is not, or was not, important in determining composition of community “Ghost of competition past” Species may compete rarely – when seasonally abundant Structuring power of competition Even if competition intense, species concerned may co-exist and competition need not determine species composition of community Unpredictable environments Patchy resources Aggregated distributions 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.

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……………………..

Surveys at three sites – replicated Surveys in four depth zones - nearshore (N) - mid-lagoon (M) - outer barrier (O) - offshore (OS) Potential competitors that coexist should exhibit niche differentiation – evidence from community patterns 1 – niche complimentarity 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 Elliott & Mariscal (2001) Marine Biology 138: 23-36 Results show each anemonefish associated with particular anemone AND characteristic preference for a particular zone

Amphiprion clarkii Amphiprion chrysopterus Amphiprion leucokranos Amphiprion melanopus Amphiprion sandaracinos Amphiprion chrysopterus Amphiprion leucokranos Amphiprion clarkii Amphiprion percula Amphiprion perideraion

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)

Species Marked differentiation in light requirements Shade tolerant species (e.g. K) small, persist in understorey, rare in new gaps High light species (G), pioneers of large new gaps Intermediate light specialists (T), small gap specialists Percentage of individuals in each of five crown illumination classes 2 – niche differentiation in space 11 species of Macaranga in Borneo

Species Species also differentiated along a second niche axis – soil type (moisture and/or nutrients) Evidence of complimentarity – species with similar light requirements differ in terms of soils – especially in the case of shade-tolerant species Davies et al (1998) J of Ecology 86: 662-673

Dickie et al (2002) New Phytologist 156: 527-535

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.

1) Calculated electivities for each diet item for each species in each community (range from 0-1) 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? 2) Calculated dietary overlap between every pair of species in each community 3) Calculated mean dietary overlap between species in each community Lawlor (1980) American Naturalist 116: 394-408

Null Model data Mean dietary overlap Four null models used that retained different aspects of the food environment Real data No species in community Minimal amount of initial structure retained Model 1 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 Individual overlap between species in a community then calculated, as too mean overlap per community Niche breadth and overlap increased wrt observed

Null Model Data Null Model data Mean dietary overlap Real data No species in community Model 2 Original number of species and number of dietary items retained: ONLY electivities > 0 assigned a random number between 0-1. Repeated 100 times Individual overlap between species in a community then calculated, as too mean overlap per community Niche breadth and overlap increased and different wrt observed

Original Data Null Model Data Null Model data Mean dietary overlap Real data No species in community Original number of species, number of dietary items and electivities retained: just randomly reassigned to different diet items. Repeated 100 times Model 3 Individual overlap between species in a community then calculated, as too mean overlap per community

Null Model data Mean dietary overlap Real data No species in community Model 4 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 Original Data Null Model Data Individual overlap between species in a community then calculated, as too mean overlap per community This model retains the greatest amount of the original structure in the system

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

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)

74 fossil brachiopod taxa – random sample of four drawn and size ratios between adjacent species calculated. Repeated 100 000 times. Results compared to actual observations – null hypothesis (observed ratios were a chance event) rejected, support idea of limiting similarity How do you test if patterns real? Construct a null model…….. Hermoyian et al (2002) Geology 30: 15-18

Inter-specific competition may often act through a process of selective extinction: too similar species fail to persist together Over the period 1860-1980, 18 pairs of introduced con-generic passerine birds were present on the same Hawaiian Island at the same time Serinus species Moulton & Pimm (1986) In: Community Ecology, Diamond & Case (Eds), Harper & Row, 80-97 Of these, six pairs persisted together, three pairs both went extinct, and in the other nine, one of the two species went extinct When one of the two species went extinct, the species pair were morphologically more similar to each other than when neither species went extinct: 9% difference in bill length as oppose to 22% difference.

Within a community, potential competitors with little differentiation would be unlikely to coexist – negative associations: evidence from community patterns Diamond (1975) In: Ecology & Evolution of Communities, Cody & Diamond (Eds), Belknap, 342-444 Checkerboard distribution of two cuckoo-doves in the Bismarck archipelago. NO island has both species!

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 For example……… 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 Wilson (1988) J of Ecology 76: 1030-1042

Native Introduced Significantly different; Not significantly different Significantly different : Not significantly different Neutral model results 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

Native Introduced Significantly different; Not significantly different Significantly different : Not significantly different Neutral model results 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

Role of interspecific competition – an appraisal Possible and plausible explanation for some organisation in some communities – BUT not all Why? Current competition not widely demonstrated “Ghost of Competition Past” too easy to uncritically invoke to explain patterns Communities chosen for study may not be typical – subjective. Studies in which competition not demonstrated may not have been published! Patterns may have alternative explanations Patterns may have arisen by chance! Role of competition will vary from community to community – important in species rich, stable vertebrate communities unimportant in phytophagous insects Other interactions may also play a role

THE END Image acknowledgements – http://www.google.com

Community Ecology BCB331