Table 1 Estimated models coefficients for the two response variables, δ13C and δ15N.

From: Nitrogen and carbon stable isotope analysis sheds light on trophic competition between two syntopic land iguana species from Galápagos

 

Estimate

SE

χ2

Z

df

P

Model 1: (δ13C, δ15N) ~ Species + Sex

δ13C

Intercept

 − 25.483

0.109

–

–

–

–

Species (C. subcristatus)

0.499

0.114

19.130

4.374

1

 << 0.001

Sex (male)

0.325

0.117

7.780

2.789

1

0.005

δ15N

Intercept

4.903

0.255

–

–

–

–

Species (C. subcristatus)

 − 0.549

0.348

–

 − 1.580

–

–

Sex (male)

 − 0.223

0.319

–

 − 0.700

–

–

Species (C. subcristatus): Sex (male)

1.126

0.444

6.423

2.530

1

0.011

Model 2: (δ13C, δ15N) ~ Species + Sex + SVL + NDVI + te (X, Y)

δ13C

Intercept

 − 25.266

0.103

–

–

–

–

Species (C. subcristatus)

0.243

0.119

4.167

2.041

1

0.041

Sex (male)

0.174

0.105

2.733

1.653

1

0.098

SVL

0.307

0.055

31.089

5.576

1

 << 0.001

NDVI

 − 0.120

0.056

4.578

 − 2.140

1

0.032

te (X, Y)

–

–

4.171

–

3

0.339

δ15N

Intercept

5.011

0.239

–

–

–

–

Species (C. subcristatus)

 − 0.612

0.348

–

 − 1.760

–

–

Sex (male)

 − 0.166

0.284

–

 − 0.590

–

–

Species (C. subcristatus): Sex (male)

0.729

0.408

3.188

1.790

1

0.074

SVL

0.356

0.103

11.906

3.450

1

0.001

NDVI

 − 0.234

0.107

4.833

 − 2.200

1

0.028

te (X, Y)

–

–

25.833

–

3

 << 0.001

Model 3: (δ13C, δ15N) ~ SVL + NDVI + te (X, Y)

δ13C

Intercept

 − 25.041

0.051

–

–

–

–

SVL

0.365

0.051

50.472

7.100

1

 << 0.001

NDVI

 − 0.135

0.057

5.683

 − 2.380

1

0.017

te (X, Y)

–

–

5.725

 

3

0.126

δ15N

Intercept

4.814

0.094

–

–

–

–

SVL

0.340

0.095

12.756

3.570

1

 < 0.001

NDVI

 − 0.258

0.105

6.033

 − 2.460

1

0.014

te (X, Y)

–

–

30.947

–

3

 << 0.001

  1. The table shows the output of the three fitted models: Model 1 is the bivariate linear regression with species and sex as predictors; Model 2 is the bivariate generalized additive model (GAM) with species, sex, body size (scaled SVL), productivity at the capture points (scaled NDVI) and a spatial tensor product smooth (te (X, Y)) as predictors; Model 3 is the bivariate generalized additive model (GAM) including only body size, productivity of the capture point and the spatial tensor product smooth as predictors. The table reports the estimated model value (Estimate), standard error (SE), degrees of freedom (df), the χ2 statistic for LRT with the corresponding P-values (P) and the z-score (z). The interaction between Species and Sex was not a significant predictor of δ13C (χ2 = 0.386, P = 0.535) and was therefore removed from the final models. P-values for intercept and for main effects included in an interaction do not have a meaningful interpretation and are not shown.
Back to article page