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.