Can latitudinal variation in plant height be explained by climate? This is a hypothesis testing question: we want to know if there is a relationship between height and latitude, after controlling for climate.
This research question now involves several variables: height (quantitative), latitude (quantitative) and climate. How climate is measured has not been specified here, but it typically involves quantitative measurements of temperature, precipitation and often related variables.
We will answer this question using multiple regression, a type of linear model, see Code Box 3.2.
height as a function of latitude only.
library(ecostats)
data(globalPlants)
ft_heightLat=lm(height~lat, data=globalPlants)
summary(ft_heightLat)
#>
#> Call:
#> lm(formula = height ~ lat, data = globalPlants)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -15.740 -7.905 -5.289 4.409 68.326
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 17.00815 2.61957 6.493 1.66e-09 ***
#> lat -0.20759 0.06818 -3.045 0.00282 **
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 13.59 on 129 degrees of freedom
#> Multiple R-squared: 0.06705, Adjusted R-squared: 0.05982
#> F-statistic: 9.271 on 1 and 129 DF, p-value: 0.002823plant height as a function of annual precipitation and latitude.
Note the code is almost the same as for simple linear regression – just add an extra predictor variable!
ft_heightRainLat=lm(height~rain+lat, data=globalPlants)
summary(ft_heightRainLat)
#>
#> Call:
#> lm(formula = height ~ rain + lat, data = globalPlants)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -16.965 -7.144 -3.529 2.317 65.434
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 5.223135 4.317148 1.210 0.22856
#> rain 0.005503 0.001637 3.363 0.00102 **
#> lat -0.052507 0.080197 -0.655 0.51381
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 13.08 on 128 degrees of freedom
#> Multiple R-squared: 0.1428, Adjusted R-squared: 0.1294
#> F-statistic: 10.66 on 2 and 128 DF, p-value: 5.226e-05library(car)
#> Loading required package: carData
avPlots(ft_heightLat, terms = ~lat, xlab="Latitude", ylab="Height",
grid=FALSE) ##left plot
avPlots(ft_heightRainLat, terms = ~lat, xlab="Latitude|rain",
ylab="Height|rain", grid=FALSE) ## right plot
ft_Lat=lm(height~lat,data=globalPlants)
ft_LatClim=lm(height~lat+rain+temp,data=globalPlants)
anova(ft_Lat,ft_LatClim)
#> Analysis of Variance Table
#>
#> Model 1: height ~ lat
#> Model 2: height ~ lat + rain + temp
#> Res.Df RSS Df Sum of Sq F Pr(>F)
#> 1 129 23818
#> 2 127 21794 2 2023.9 5.897 0.003556 **
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1Note that standard errors are larger and suddenly everything is non-significant.
ft_climproblems=lm(height~rain+rain.wetm+lat,data=globalPlants)
summary(ft_climproblems)
#>
#> Call:
#> lm(formula = height ~ rain + rain.wetm + lat, data = globalPlants)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -16.868 -7.286 -3.472 2.345 65.196
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 4.907914 4.607746 1.065 0.289
#> rain 0.004911 0.003370 1.457 0.148
#> rain.wetm 0.004686 0.023283 0.201 0.841
#> lat -0.046927 0.085140 -0.551 0.582
#>
#> Residual standard error: 13.12 on 127 degrees of freedom
#> Multiple R-squared: 0.143, Adjusted R-squared: 0.1228
#> F-statistic: 7.066 on 3 and 127 DF, p-value: 0.0001977 par(mfrow=c(1,2),mar=c(3,3,1,1),mgp=c(1.75,0.75,0))
globalPlants$loght = log(globalPlants$height)
ft_logheightRainLat=lm(loght~rain+lat, data=globalPlants)
plotenvelope(ft_logheightRainLat)
summary(ft_logheightRainLat)
#>
#> Call:
#> lm(formula = loght ~ rain + lat, data = globalPlants)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -3.3508 -1.0542 -0.0468 0.9755 3.2702
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 1.4974481 0.4752634 3.151 0.00203 **
#> rain 0.0005400 0.0001802 2.997 0.00328 **
#> lat -0.0273834 0.0088286 -3.102 0.00237 **
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 1.439 on 128 degrees of freedom
#> Multiple R-squared: 0.2548, Adjusted R-squared: 0.2432
#> F-statistic: 21.89 on 2 and 128 DF, p-value: 6.68e-09These diagnostic plots look a lot better.
But now results have changed – the effect of latitude, after controlling for rainfall, is significant! These results are more trustworthy than the previous ones because assumptions are looking reasonable now, whereas before we had strong right-skew.
Results changing is not entirely unexpected here. Least squares methods have low power when applied to strongly skewed data. What happened here is that there was a (fairly strong) pattern in the data, with latitudinal effects that are not explained by rainfall, but we couldn’t see this before because data were strongly skewed, which makes it hard to see anything.
par(mfrow=c(1,2),mar=c(3,3,1,1),mgp=c(1.75,0.75,0))
globalPlants$logRain = log(globalPlants$rain)
hist(globalPlants$rain)
hist(globalPlants$logRain)
ft_logheightlogRainLat=lm(loght~logRain+lat, data=globalPlants)
plotenvelope(ft_logheightlogRainLat)
summary(ft_logheightlogRainLat)
#>
#> Call:
#> lm(formula = loght ~ logRain + lat, data = globalPlants)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -3.3620 -1.0032 0.0473 0.9843 3.1241
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) -1.952796 1.485134 -1.315 0.19090
#> logRain 0.603392 0.191128 3.157 0.00199 **
#> lat -0.027740 0.008599 -3.226 0.00160 **
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 1.434 on 128 degrees of freedom
#> Multiple R-squared: 0.2601, Adjusted R-squared: 0.2486
#> F-statistic: 22.5 on 2 and 128 DF, p-value: 4.224e-09The histogram for rainfall is slightly right-skewed, although it’s not super-scary. Reanalysing data we still get pretty good-loking diagnostic plots and results look very similar – \(R^2\) is almost the same, \(P\)-values are still very significant.
require(car)
vif(ft_heightRainLat)
#> rain lat
#> 1.494158 1.494158
vif(ft_climproblems)
#> rain rain.wetm lat
#> 6.287835 7.031092 1.671396Clearly adding rain.wetm to the model has done some damage (but to rain only, not so much to lat).
X = data.frame(globalPlants$lat,globalPlants$rain,globalPlants$rain.wetm)
cor(X)
#> globalPlants.lat globalPlants.rain
#> globalPlants.lat 1.0000000 -0.5750884
#> globalPlants.rain -0.5750884 1.0000000
#> globalPlants.rain.wetm -0.6336210 0.9170080
#> globalPlants.rain.wetm
#> globalPlants.lat -0.633621
#> globalPlants.rain 0.917008
#> globalPlants.rain.wetm 1.000000
pairs(X)
Does invertebrate density change with isolation? This research question asks for a hypothesis test: is there are relationship between invertebrate density and isolation.
There are two variables – invertebrate density (quantitative) and distance of isolation, which was set experimentally at 0, 2 or 10 metres (categorical).
So I’m thinking ANOVA.
lm.
data(seaweed)
seaweed$Dist = factor(seaweed$Dist)
ft_seaweed=lm(Total~Dist,data=seaweed)
anova(ft_seaweed)
#> Analysis of Variance Table
#>
#> Response: Total
#> Df Sum Sq Mean Sq F value Pr(>F)
#> Dist 2 300.25 150.123 8.5596 0.0005902 ***
#> Residuals 54 947.08 17.539
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1There is strong evidence of a Dist effect.
want.
confint(ft_seaweed)
#> 2.5 % 97.5 %
#> (Intercept) 2.7785049 6.533423
#> Dist2 2.9211057 8.460686
#> Dist10 0.4107071 5.720963 library(multcomp)
#> Loading required package: mvtnorm
#> Loading required package: survival
#> Loading required package: TH.data
#>
#> Attaching package: 'TH.data'
#> The following object is masked from 'package:MASS':
#>
#> geyser
contDist = mcp(Dist="Tukey") # telling R to compare on the Dist factor
compDist = glht(ft_seaweed, linfct=contDist) # run multiple comparisions
summary(compDist)
#>
#> Simultaneous Tests for General Linear Hypotheses
#>
#> Multiple Comparisons of Means: Tukey Contrasts
#>
#>
#> Fit: lm(formula = Total ~ Dist, data = seaweed)
#>
#> Linear Hypotheses:
#> Estimate Std. Error t value Pr(>|t|)
#> 2 - 0 == 0 5.691 1.382 4.119 <0.001 ***
#> 10 - 0 == 0 3.066 1.324 2.315 0.0621 .
#> 10 - 2 == 0 -2.625 1.382 -1.900 0.1484
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> (Adjusted p values reported -- single-step method)
plot(compDist)
I will take climate to mean temperature (temp) and rainfall (rain).
So a good plan here would be to fit a multiple regression model, to appropriately transformed data, and see if latitude explains any variation not captured by climate.
par(mfrow=c(1,2),mar=c(3,3,1,1),mgp=c(1.75,0.75,0))
globalPlants$logRain = log(globalPlants$rain)
globalPlants$logHt = log(globalPlants$height)
ft_logheightClim=lm(loght~logRain+temp+lat, data=globalPlants)
plotenvelope(ft_logheightClim)
summary(ft_logheightClim)
#>
#> Call:
#> lm(formula = loght ~ logRain + temp + lat, data = globalPlants)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -3.2777 -1.0191 -0.0016 0.9328 3.2730
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) -3.45043 1.96520 -1.756 0.08154 .
#> logRain 0.64012 0.19347 3.309 0.00122 **
#> temp 0.03855 0.03319 1.162 0.24760
#> lat -0.00753 0.01940 -0.388 0.69859
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 1.432 on 127 degrees of freedom
#> Multiple R-squared: 0.2679, Adjusted R-squared: 0.2506
#> F-statistic: 15.49 on 3 and 127 DF, p-value: 1.2e-08In terms of minding our P’s and Q’s:
Q’s our research question suggests we are primarily interested in seeing if there is an effect of latitude after controlling for climate, hence primary interest is in the bottom row of the above summmary table.
P’s
After adding temp to the model, there is no longer any evidence of a latitudinal effect. So latitudinal variation in height can be explained by climate, in the sense that there is no evidence of a latitudinal effect after controlling for climate.
The main reason for this is that temperature and latitude are highly correlated (as below), hence we can think of latitude primarily as a surrogate for temperature. It is not immediately obvious however if plants tend to grow taller towards the equator because it is warmer (hence temp is driving this relationship) or because of reduced seasonality and more even day lengths (hence lat is the driver).
X = data.frame(globalPlants$lat,globalPlants$rain,globalPlants$temp)
cor(X)
#> globalPlants.lat globalPlants.rain globalPlants.temp
#> globalPlants.lat 1.0000000 -0.5750884 -0.9158796
#> globalPlants.rain -0.5750884 1.0000000 0.4786965
#> globalPlants.temp -0.9158796 0.4786965 1.0000000
pairs(X)
boxplot(Total~Dist,data=seaweed)
by(seaweed$Total,seaweed$Dist,sd)
#> seaweed$Dist: 0
#> [1] 2.108063
#> ------------------------------------------------------------
#> seaweed$Dist: 2
#> [1] 5.507271
#> ------------------------------------------------------------
#> seaweed$Dist: 10
#> [1] 4.456622Small means have small variances. There is a boundary at zero and
data are being “pushed up” against it, with Total tending
to take small values near zero for Dist=0, with less than
half the sd of Dist=2 or Dist=10.
There is also a suggestion of non-normality when checking assumptions on the fitted ANOVA model:
par(mfrow=c(1,3),mar=c(3,3,1,1),mgp=c(1.75,0.75,0))
plotenvelope(ft_seaweed,which=1:3)
Notice that in the quantile plot that points get close to the lower bound in the middle of the plot and get near the top of it at large values – this is mildly suggestive of skew. We similarly see an increasing trend in the scale-location plot, although it does stay within its simulation bounds. Notice these are only subtle patterns and not particularly strong.
(As an aside, note there is no envelope visible on the residual vs fits plot. This happens because we are fitting an ANOVA model, which ends up with a residuals mean exactly equal to zero for all treatments, hence no mean trend on the res vs fits plot for all fitted values, just a flat line at zero. Put another way, non-linearity isn’t an assumption to worry about in ANOVA models, the fitted model goes exactly through the mean of each treatment.)
We can try to address the suggestion of skew this using log-transformation:
seaweed$logTot = log(seaweed$Total)
boxplot(logTot~Dist,data=seaweed)
by(seaweed$logTot,seaweed$Dist,sd)
#> seaweed$Dist: 0
#> [1] 0.4423775
#> ------------------------------------------------------------
#> seaweed$Dist: 2
#> [1] 0.667999
#> ------------------------------------------------------------
#> seaweed$Dist: 10
#> [1] 0.5421633This is looking pretty good now, let’s run with that.
par(mfrow=c(1,2),mar=c(3,3,1,1),mgp=c(1.75,0.75,0))
ft_loghabconf=lm(logTot~Dist,data=seaweed)
plotenvelope(ft_loghabconf,which=1:2)
anova(ft_loghabconf)
#> Analysis of Variance Table
#>
#> Response: logTot
#> Df Sum Sq Mean Sq F value Pr(>F)
#> Dist 2 4.8786 2.43929 8.011 0.0008981 ***
#> Residuals 54 16.4427 0.30449
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
library(multcomp)
contDist = mcp(Dist="Tukey") # telling R to compare on the Dist factor
compDistlog = glht(ft_loghabconf, linfct=contDist) # run multiple comparisions
summary(compDistlog)
#>
#> Simultaneous Tests for General Linear Hypotheses
#>
#> Multiple Comparisons of Means: Tukey Contrasts
#>
#>
#> Fit: lm(formula = logTot ~ Dist, data = seaweed)
#>
#> Linear Hypotheses:
#> Estimate Std. Error t value Pr(>|t|)
#> 2 - 0 == 0 0.7117 0.1820 3.910 0.000749 ***
#> 10 - 0 == 0 0.4584 0.1745 2.627 0.029601 *
#> 10 - 2 == 0 -0.2533 0.1820 -1.392 0.352299
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> (Adjusted p values reported -- single-step method)
plot(compDistlog)
And the main change in results here is that there is now evidence
that mean logTot is different between Dist==0
and Dist==10 now as well as between Dist==0
and Dist==2. Again this is an example of being able to
better see patterns when assumptions are closer to being satisfied (in
particular, removing right-skew from response when fitting a linear
model).
habSmall=seaweed[seaweed$Size=="SMALL",]
ft_smhabconf=lm(logTot~Dist,data=habSmall)
plotenvelope(ft_smhabconf,which=1:2)
anova(ft_smhabconf)
#> Analysis of Variance Table
#>
#> Response: logTot
#> Df Sum Sq Mean Sq F value Pr(>F)
#> Dist 2 2.8974 1.44870 4.9358 0.0149 *
#> Residuals 27 7.9247 0.29351
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
library(multcomp)
contDist = mcp(Dist="Tukey") # telling R to compare on the Dist factor
compDistsm = glht(ft_smhabconf, linfct=contDist) # run multiple comparisons
summary(compDistsm)
#>
#> Simultaneous Tests for General Linear Hypotheses
#>
#> Multiple Comparisons of Means: Tukey Contrasts
#>
#>
#> Fit: lm(formula = logTot ~ Dist, data = habSmall)
#>
#> Linear Hypotheses:
#> Estimate Std. Error t value Pr(>|t|)
#> 2 - 0 == 0 0.7165 0.2423 2.957 0.0170 *
#> 10 - 0 == 0 0.5808 0.2423 2.397 0.0597 .
#> 10 - 2 == 0 -0.1357 0.2423 -0.560 0.8422
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> (Adjusted p values reported -- single-step method)
plot(compDistsm)
Results are now less significant, with only marginal evidence of a
difference in mean logTot between Dist==0 and
Dist==10. This is expected because we have almost halved
our sample size:
dim(seaweed)
#> [1] 57 23
dim(habSmall)
#> [1] 30 23By reducing our sample size, we are less able to see patterns in the data (because there is less data to look for patterns in!).