The joint effects of forest habitat area and fragmentation on Dung beetles

Habitat loss and habitat fragmentation usually occur together, at the same
time and place.  However, while there is consensus that habitat
loss is the preeminent threat to biodiversity, the effects of
fragmentation are contentious.  Some argue that habitat
fragmentation is not bad for biodiversity, and even that it is
good.  Generally, the studies that find no harm or positive
outcomes of fragmentation invariably assume that it is independent of
habitat loss.  However, dissociating the effects of habitat
fragmentation from habitat loss is questionable because the two are
essentially coupled.  Accordingly, we evaluated how forest area
and fragmentation (via edge effects) influenced dung beetles per se, and
through their effects on the abundance of mammals, using structural
equation modeling (SEM).  Dung beetles are very sensitive to
forest habitat loss and fragmentation, and to changes in the abundance of
mammals on which they depend for dung.  Our study area was in the
Tana River, Kenya, where forest fragments are depauperated of mammals
except for two endemic species of monkeys.  We mapped 12 forests,
counted the resident monkeys, and sampled 113,955 beetles from 288
plots.  Most of the 87 species of beetles found were small
tunnellers.  After implementing a fully latent Structural
Regression SEM, the optimal model explained a significant 26% of the
variance in abundance, and 89% of diversity.  The main drivers of
beetle abundance were positive, direct, effects of forest area and number
of monkeys, and negative edge effects.  The main drivers of
diversity were the direct effects of the beetle abundance, indirect
effects of forest area and abundance of mammals, and indirect negative
edge effects.  Thus forest area, fragmentation (via edge
effects), and the number of monkeys jointly influenced the abundance and
diversity of the beetles directly and indirectly.