Figure 2. The correlation between mean annual temperature and
predator presence (a), and anole species richness (b). The trendlines
are modeled from linear regressions.
To address the hypothesis that predation pressure is a significant
driver of diversity, we tested whether anole abundance, species
richness, diversity (Shannon index), or evenness correlated with total
avian predator pressure at a site. We first examined whether predator
occurrence predicted these variables, while controlling for climate and
allowing for differences in relationship between island and between
forest and human-modified environments. We found a positive relationship
between predation and lizard abundance only when climate was not
included in the model (predation effect = 5.85, p-value<0.01,
Fig 3a). When mean annual temperature and precipitation were included,
this apparent relationship disappeared and more intense predation
regimes had no correlation with greater numbers of individual lizards
(p-value=0.11). Instead, lizard abundance simply increased with warmer
temperatures (figure 3b), regardless of predation.
In contrast to the lack of effect of predation on abundance, areas with
greater predator presence were associated with more anole species
(predator effect = 1.72, r2 = 0.33; Figure 4a.) even
while controlling for climate. While this trend was apparent on both
islands, it was more extreme on Jamaica, which experienced a steeper
increase in richness with predator presence (predator×Jamaica effect =
2.22, p-value= 0.04). This was also true when assessing diversity as the
Shannon Diversity Index, for which predation pressure was generally
correlated with more diverse communities, but for which this effect was
strongest on Jamaica (predator×Jamaica effect = 0.36, p-value= 0.04).
Areas with a higher diversity of predator species also had more even
anole communities (predator effect = 0.31, p-value = 0.01; Figure 4b;
see table 1 for all model outputs).
Specialized predators may have particularly strong effects on community
composition of their prey. To assess this possibility, we repeated the
analyses summing occurrence of cuckoos on both islands. Cuckoo presence
on Jamaica ranged from 0.05 - 1.83, and was on average higher (mean =
0.81) than the Dominican Republic (mean = 0.41,
p-value<0.001), where cuckoo presence ranged from 0.04 - 1.04.
In contrast to our findings considering all predator birds, cuckoo
presence positively correlated with lizard abundance (cuckoo effect =
2.18, p-value<0.001). We did, however, find that areas with
the greatest cuckoo presence had more anole species (cuckoo effect on
lizard richness = 0.90, p-value<0.001), higher values of anole
diversity (cuckoo effect on h = 0.31, p-value<0.001), and
greater anole community evenness (cuckoo effect on evenness = 0.17,
p-value<0.001). In fact, cuckoo presence was a better
predictor of abundance, evenness, and diversity than was climate, which
dropped out of the models (Table 1).
When we repeated these analyses with pigeons (our non-predatory
“control” group), we found no relationship with the abundance or
species richness of anole communities (p-value=0.99 and 0.79
respectively. However, we found that the correlation with pigeons and
anole diversity varied between islands: Jamaican anole communities
exhibited a positive relationship with pigeon occurrence (Jamaica effect
= 0.16), whereas the correlation was slightly negative in the Dominican
Republic (DR effect = -0.03, interaction-term p-value=0.01). Finally,
our results showed a significant relationship between pigeon presence
and community evenness, however areas with more pigeons tended to have
(very slightly) less even communities (effect = -0.04, p-value = 0.04),
whereas for the true predators presence positively correlated with more
even communities.