origins & maintenance of diversity a hierarchy of processes

Origins & Maintenance of DiversityA Hierarchy of Processes

Processes operate on a variety of temporal & spatial scales to influence diversity

The levels are nested

Unique (chance) events may occur at any level

Figure 30.2 from Ricklefs & Schluter (1993)

Regionaldiversity

Localdiversity

Speciesproduction

Habitatselection

Competitiveexclusion

Biotalinterchange

Stochasticextinction

Massextinction

Predatoryexclusion

A major challenge confronting advances to a research program that investigates this multi-scale set of processes is the reconstruction of historical events

So, community ecologists have added phylogenetic & biogeographic analyses (e.g., phylogeography) to their toolkits…

Redrawn from figure 30.1, Ricklefs & Schluter (1993)

Origins & Maintenance of DiversityPhylogenetic Perspectives

Losos (1996)

“If species adapted rapidly and without constraint, and if any lineage could occur in any community, then we would expect differences in community structure to be indicative of environmental differences. Because lineages differ in their evolutionary potential and are geographically restricted, however, comparisons of community structure must take account of communities’ histories…

“…differences in the structure of communities may result as much from the differences in the lineages that occur in different communities as from differences in environmental conditions”

“Phylogenetic information about the constituent lineages in a community can allow lineage effects to be factored out, thus allowing an assessment of environmental determinants of community structure”

Ricklefs & Schluter (1993)

An example from passerine birds compared between Panama and Illinois: Why do different areas vary in species richness of particular clades?

“The approximately twofold difference in average age of the taxa matches the twofold difference in diversity, although age and diversity are not well correlated among individual clades.”

Table 30.2 from Ricklefs & Schluter (1993)

Losos (1990, 1996)

An example from Anolis lizards inhabiting Caribbean islands: Where & under what community conditions did patterns of niche-use originate?

27 islands of the Lesser Antilles contain either one or two species of Anolis lizards; on 9 of the 10 two-species islands, the species differ considerably in size (one large & one small), whereas on 16 of 17 one-species islands, the species are intermediate in size

Figure from Losos (1996)

Hypotheses:

(1) “Size adjustment (character displacement): Species of similar size colonize an island & evolve in opposite directions in situ to minimize resource competition”

From Losos (1990)

Large2

Small2

(Int. 1)

(Int. 1)(Int. 1) (Int. 1)

Size adjustment

(Int. 1)

Increase in size

Decrease in size

Switch to 2-species island

Redrawn from Losos (1990)

Hypotheses:

(1) “Size adjustment (character displacement): Species of similar size colonize an island & evolve in opposite directions in situ to minimize resource competition”

(2) “Size assortment: Competitive exclusion or other processes prevent similar-sized species from colonizing the same island; only species that are already dissimilar in size can successfully colonize and coexist”

From Losos (1990)

Large2

Small2

(Large 1)

(Int. 1)

(Small 1)

(Small 1)(Int. 1) (Int. 1)

Size assortment

(Int. 1)

Increase in size

Decrease in size

Large2

Small2

(Large 2)

(Int. 1)

(Small 2)

Size assortment & size adjustment

(Int. 1)

Increase in size

Decrease in size

Large2

Small2

Small1

(Large 2)

(Int. 2)

(Int. 1)

(Small 1)

No evidence for size assortment or size adjustment

(Int. 1)

Increase in size

Decrease in size

“In the northern Lesser Antilles, large & small size appear to have evolved simultaneously when two lineages came into sympatry… exactly the prediction of a hypothesis of character displacement…”

Dominica

Figure from Losos (1996); quote from Losos (1990)

“By contrast, in the southern Lesser Antilles, evolutionary change in body size appears to have been unrelated to whether a species occurred in sympatry with congeners…

Consequently, the existence of size patterns must have resulted from a process of ecological sorting in which only dissimilar-sized species can successfully colonize and coexist on the same island…”

Martinique

Dominica

“Thus, the relative importance of co-evolutionary processes in determining community structure differs between the northern and southern Lesser Antilles, a result that is only apparent when these lizards are studied in a historical [phylogenetic] context”

St. MaartenSt. Eustatius

Martinique

Dominica

McPeek (1995)

An example from damselflies inhabiting freshwater lakes: Where and under what community conditions did patterns of niche-use originate?

0.00.0

+0.9+0.4

Numbers represent the amount of evolutionary change in limb length between ancestor & descendant, as deduced using parsimony methods for

reconstruction of ancestral character states

Redrawn from Losos (1996), based on McPeek (1995)

0.00.0

+0.9+0.4

The common ancestor inhabited lakes occupied by fish,where its best strategy to avoid predation was to hide

0.00.0

+0.9+0.4

Bars represent transitions to fishless lakes, where a damselfly’s best strategy to avoid predation by dragonflies is to swim or run away

Cadle & Greene (1993)

An example from Neotropical snake assemblages: For areas that differ in the species richness of particular clades, do contrasting characteristics of those clades contribute to emergent community-level properties?

Observations:

Three main lineages of Neotropical snakes within the family Colubridae

“Goo-eaters”

Observations:

The xenodontine lineages originated & diversified relatively isolated from one another, even though the current distributions overlap substantially

Colubrines radiated relatively recently

worldwide

Principal zones of radiation

Observations:

The xenodontine lineages originated & diversified relatively isolated from one another, even though the current distributions overlap substantially

The lineages differ in mean body size, diet, and etc.

Observation: Sites in Central & South America differ in body size distribution, diet & etc. of their snakes, but those differences are largely due to differences in the dominant clades occupying those sites

Conclusion:The overall size distribution, diet & etc. of a community bears a direct relationship to the clade composition (i.e., history matters)

Suggested a reinterpretation of patterns from Brazilian caatinga…

In contrast to Vitt and Vangilder (1983), “we do not need to ‘explain’ the absence of invertebrate-eating snakes [in the caatinga community] in terms of present-day ecological factors if historical events resulted in the absence of appropriate lineages from the community”

2. Species that colonized a community but did not lead to ecological shifts in residents (new associations of lineages, but ancestral ecologies were maintained)

Losos (1996), after Brooks & McLennan (1991, 1993)

“Four components contributing to community structure”:

1. Species that interact in the same way that their ancestors did in other communities

3. Species that arose in situ and evolved different ecological requirements

4. Species that colonized a community and interacted with species already present, leading to ecological shifts in the colonizer, the resident, or both

Webb et al. (2002)

“The differences among species that co-occur in an ecological community are the result of modifications to a common ancestor that the species all ultimately share”

Phylogenetic relationships provide “a new dimension of information… with which to make sense of these differences among species”

Differences among species are due to divergence (either in sympatry or allopatry), which may have occurred recently due to changes in the focal species, or in the distant past due to changes in the ancestors of the focal species

Similarities among species are either due to shared ancestry, or convergence (either in sympatry or allopatry)

Webb et al. (2002)

Webb (2000)

An example from rainforest trees: Is the distribution of species among habitats (or samples) in a community nonrandom with respect to phylogeny?

“The demonstration of nonrandom spatial association of species with habitats is a necessary but not sufficient criterion in demonstrating that habitat partitioning is important in enabling many species to coexist…”

“However, to be able to show that co-occurring species in different habitats are more ecologically similar than expected by chance would support the case that species are partitioning habitat according to their autecology”

“On the other hand, if species in local communities were less ecologically similar than expected by chance, this would suggest that negative neighborhood interactions [e.g., competition] were causing increased mortality among ecologically similar species…”

Webb (2000)

Specific question: Are the tree species in 0.16-ha plots more or less related than expected if such communities were formed from a random sampling of available species in the larger area (150 ha)?

“…because of the conservatism of many species traits in the evolution of a lineage, we expect, in general, a positive relationship between a measure of the phylogenetic relatedness of two species and a measure of their overall life-history and ecological similarity…”

Webb (2000)

What about character displacement?

“Under certain circumstances, where new niches are encountered (e.g., on islands) or where competition with an ecologically similar species is strong and predictable, particular ecological traits of an organism may change adaptively over time… leading to a breakdown of the correlation between phylogenetic relatedness and ecological similarity for some traits… However, even in these organisms we would still expect that a measure of ‘overall’ ecological similarity, taking many traits into account, would be correlated with phylogenetic relatedness”

Webb (2000)

Data: 28 tree plots of 40 x 40 m (0.16 ha) scattered throughout 150 ha of lowland tropical forest, Indonesia

How to efficiently estimate relatedness?

Webb (2000)

Data: 28 tree plots of 40 x 40 m (0.16 ha) scattered throughout 150 ha of lowland tropical forest, Indonesia

How to efficiently estimate relatedness?

Created “supertrees” from published phylogenies andthen counted nodes separatingterminal taxa

A B C D E F

Phylogeny

Community 1: A, B, C, D Community 2: A, B, E, F

A B C D

Nodal distances:

A B E F

Nodal distances:

Greatest possible mean pairwise nodal distance for a community of 4 taxa (given this phylogeny) = 3.66 nodes (for A, B, E, F)

Mean pairwise nodal dist. =(1+2+4+2+4+3)/6 = 2.66

Mean pairwise nodal dist. =(1+5+5+5+5+1)/6 = 3.66

Net Relatedness Index = 1 - (2.66/3.66) = 0.273

Net Relatedness Index = 1 - (3.66/3.66) = 0.0

From Webb (2000)

A B C D E F

Phylogeny

Community 1: A, B, C, D Community 2: A, B, E, F

A B C D

Nodal distances:

A B E F

Nodal distances:

Nearest Taxa Index = 1 - (1.75/2.0) = 0.125

Mean nearest nodal dist. =(1+1+1+1)/4 = 1.0

Greatest possible mean nearest nodal distance for a community of 4 taxa (given this phylogeny) = 2.00 nodes (for A, C, D, F)

Mean nearest nodal dist. =(1+1+2+3)/4 = 1.75

Nearest Taxa Index = 1 - (1.0/2.0) = 0.5

From Webb (2000)

Methods: Webb (2000) created 1000 sets of plots with randomized species membership, following two simple rules: (1) each species occurred in the same total number of plots as observed; (2) each plot contained the same total number of species as observed

Randomized plots represent expected distributions of species if membership in plots occurs at random with respect to the overall species pool of 324 observed species throughout the 150-ha area

Compared the observed Net Relatedness Index (NRI) and the observed Nearest Taxa Index (NTI) to the appropriate distributions of expected valuesResults: The mean NRI did not differ significantly from the expectation of the null hypothesis, but the mean NTI was significantly greater than expected

Conclusion:

Overall, species in the 0.16-ha plots were more likely to be found with species separated by fewer nodes than expected by chance (e.g., congeners)

We can therefore reject the null hypothesis that species are assembled into local communities at random; there is evidence that species occur with closely related species more than we expect by chance

origins & maintenance of diversity a hierarchy of processes

diversitythe levels

nestedunique chance

community ecologists

structure of communities

different communities

multiscale set of processes

twofold difference

constituent lineages

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