```r
t.test(d$residuals~d$site)
```

```
## 
## 	Welch Two Sample t-test
## 
## data:  d$residuals by d$site
## t = -5.884, df = 248.55, p-value = 1.289e-08
## alternative hypothesis: true difference in means is not equal to 0
## 95 percent confidence interval:
##  -18.556135  -9.248877
## sample estimates:
## mean in group 1 mean in group 2 
##       -6.842640        7.059866
```