Spatial Turnover and Beta Diversity:

What causes species turnover and why is this important from a conservation

standpoint?

Amanda SenftAmanda Senft

Bio 255Bio 255

Beta Diversity (species turnover Beta Diversity (species turnover through space)through space)

Previously, we discussed how to measure beta Previously, we discussed how to measure beta diversity…. but what processes are we diversity…. but what processes are we measuring? In other words, what causes beta measuring? In other words, what causes beta diversity?diversity?

When we conserve areas of high beta diversity When we conserve areas of high beta diversity (biogeographic crossroads), are we conserving:(biogeographic crossroads), are we conserving: Areas of high species turnover?Areas of high species turnover?

OROR

The The processesprocesses causing this high species turnover? causing this high species turnover?

β=γ/α

Whittaker (1960)

Half changesSimilarity v. distance

NON - GRADIENT GRADIENT

Harrison

Wilson & Schmida

Cowling

Willig & Sandlin

Blackburn & Gaston

Oliver

Young

Pharo…..

Cowling

Rey Benayas

Porembski

Harrison

Nekola & White

Cody

Syuzo

Givnish…..

Slide stolen from Meghan with an H

What causes beta diversity?What causes beta diversity?

From Duivenvoorden et al 2002

Dispersal limitationDispersal limitation

From Janzen 1970

I= # seeds per area

P= probability that a seed predator won’t get the seed

PRC=population recruitment curve: distance from adult that its offspring is likely to appear

Climatic and Edaphic factors?Climatic and Edaphic factors?

How did Condit et al consider beta How did Condit et al consider beta diversity?diversity?

Similarity in species composition as a Similarity in species composition as a function of distance. function of distance.

All pairs of plots considered together. All pairs of plots considered together.

Spatial heterogenity (e.g. ecotones) not Spatial heterogenity (e.g. ecotones) not considered in null modelconsidered in null model

Tested Dispersal Limitation as a major Tested Dispersal Limitation as a major driving forcedriving force

How did Spector consider beta How did Spector consider beta diversity?diversity?

Caused by environmental factors: Caused by environmental factors: geological, climatic, geometric featuresgeological, climatic, geometric featuresPreserved by evolutionary factors: Preserved by evolutionary factors: Ecotones are “evolutionarily active zones”…Ecotones are “evolutionarily active zones”…

see Enserink 1992see Enserink 1992

and by “underlying processes”and by “underlying processes” Fire or another ecological gradientFire or another ecological gradient

But…considered only a single scale But…considered only a single scale

Today’s papers…Today’s papers…

Condit, R., Pitman, N., Leigh, E. G., Chave, J., Terborgh, J., Foster, R. B., Nunez, P., Aguilar, S., Valencia, R., Villa, G., Muller-Landau, H. C., Losos, E., and S.P Hubbell 2002. Beta-Diversity in Tropical Forest Trees. Science 295(5555): 666-669.

Spector, S. 2002. Biogeographic Crossroads as Priority Areas for Biodiversity Conservation. Conservation Biology. 16(6): 1480-1487.

How do you measure ecological How do you measure ecological distance between 2 sites? distance between 2 sites?

A Bw

Distance metrics: Similarity between plots

Sorenson 2w / A+B : Shared abundance over total abundance

Jaccard w / A+B-w: Proportion of combined abundance not shared

Other distance measures they could have used: Euclidean, Correlation distance, Mahalenobis, Chi-square

Probability metric using null model:

Probability: F=Σfi1 f j2 The sum of the abundance of a spp at site 1 times abundance at site 2 for all species. Probability that two trees chosen randomly from 2 plots are of the same species. Takes into account relative abundance of species.

Condit et al. Figure 1Condit et al. Figure 1

In Panama, plots within 1 km very similar, but In Panama, plots within 1 km very similar, but very different from plots 50 km awayvery different from plots 50 km away

In Peru and Ecuador, plots within 1 km nearly In Peru and Ecuador, plots within 1 km nearly as similar as plots 50 km awayas similar as plots 50 km away

Panama: Species turnover due to Panama: Species turnover due to environmental factorsenvironmental factors: climate, geology, soil: climate, geology, soil

Peru and Ecuador: Species turnover due to Peru and Ecuador: Species turnover due to dispersal limitationdispersal limitation

Null model based on a Bessel Null model based on a Bessel function of decay: function of decay: Null hypothesis: Only dispersal and Null hypothesis: Only dispersal and speciation affect spp distributions. speciation affect spp distributions. Used Hubbell’s null model: all spp Used Hubbell’s null model: all spp identical in death, reproduction, identical in death, reproduction, dispersal, speciation=extinction, etc. dispersal, speciation=extinction, etc. Probability that 2 individuals r Probability that 2 individuals r kilometers apart are the same species kilometers apart are the same species is a function of distance, speciation, is a function of distance, speciation, dispersal distance and tree density. dispersal distance and tree density.

Condit et al Figure 2Condit et al Figure 2Graphs showing experimental (points) vs. theoretical decay of species similarity with distance

- Variance is higher for Panama

-Decay in similarity from 0-100 m not well simulated by model

Condit et al Figure 2Condit et al Figure 2Graphs showing experimental (points) vs. theoretical decay of species similarity with distance

- Variance is higher for Panama

-Decay in similarity from 0-100 m not well simulated by model

Condit et al Figure 2Condit et al Figure 2Graphs showing experimental (points) vs. theoretical decay of species similarity with distance

- Variance is higher for Panama

-Decay in similarity from 0-100 m not well simulated by model

Conclusions from Condit paper:Conclusions from Condit paper:

Species more aggregated than dispersal Species more aggregated than dispersal theory predictstheory predicts

Abundant species not well modeled with Abundant species not well modeled with dispersal theory (they’re just everywhere, dispersal theory (they’re just everywhere, regardless of distance)regardless of distance)

Null model of dispersal works well for Null model of dispersal works well for distances .2-50 km, but other factors distances .2-50 km, but other factors influence beta diversity at different scalesinfluence beta diversity at different scales

Duivenvoorden et al: Variance Duivenvoorden et al: Variance partitioningpartitioning

Using Condit’s data, found that (linear) distance, Using Condit’s data, found that (linear) distance, elevation, precipitation, stand age and bedrock elevation, precipitation, stand age and bedrock type were all significant predictors of species type were all significant predictors of species similarity. similarity. Still, distance and environment together Still, distance and environment together explained left 59% of floristic variation explained left 59% of floristic variation unexplainedunexplainedA re-reanalysis using presence-absence data A re-reanalysis using presence-absence data instead of abundances, and log distances, was instead of abundances, and log distances, was able to leave only 41% of data unexplained able to leave only 41% of data unexplained (Ruoklainen and Tuomisto, 2002).(Ruoklainen and Tuomisto, 2002).

Spector paperSpector paper

We should conserve We should conserve “biogeographic crossroads” where “biogeographic crossroads” where there is high species turnover:there is high species turnover: Areas meet goals of Areas meet goals of

representativeness and representativeness and complementaritycomplementarity

Conserve evolutionary processes: Conserve evolutionary processes: speciation and coevolutionspeciation and coevolution

Congruence assumed to be high:Congruence assumed to be high: Environmental factors act on most Environmental factors act on most

taxa to create zones of high turnovertaxa to create zones of high turnover

Scarab beetle

How have others considered How have others considered biogeographic crossroads?biogeographic crossroads?

Clements 1905: Tension zones where Clements 1905: Tension zones where principle species from adjacent principle species from adjacent communities meetcommunities meet

Delcourts (numerous): shifting ecoclinesDelcourts (numerous): shifting ecoclines

Sky Islands (stacked biotic Sky Islands (stacked biotic communities)…communities)…

Marshall 1957 cited by Warshall 2003

Spector’s CaveatsSpector’s Caveats

Ecotones harbor species at the Ecotones harbor species at the edges of their ranges, which are edges of their ranges, which are often more fragmented, more often more fragmented, more extinction prone (is this true or extinction prone (is this true or not?)not?)Park size should equal the size of Park size should equal the size of the ecological gradient and the ecological gradient and processes maintaining the ecotone. processes maintaining the ecotone. How does one quantify this? How does one quantify this?

Discussion questions…Discussion questions…

Do “biogeographic crossroads” exist? At what scale? Do “biogeographic crossroads” exist? At what scale? Given that ecotones often mark the edge of a given Given that ecotones often mark the edge of a given species’ range, should we mark areas of high species species’ range, should we mark areas of high species turnover for conservation? Consider also that ecotones turnover for conservation? Consider also that ecotones are predicted to be highly sensitive to climate change are predicted to be highly sensitive to climate change (Turner, Gardner, and O’Neill 1991). (Turner, Gardner, and O’Neill 1991). What caused the differences in diversity patterns What caused the differences in diversity patterns between Panama and the two South American sites? between Panama and the two South American sites? Which variables predicted beta diversity in the Tropics? Which variables predicted beta diversity in the Tropics? How transferable are those results?How transferable are those results?

Are these “biogeographic Are these “biogeographic crossroads?”crossroads?”