Discussion
Our findings suggest that climate plays a major role in dictating anole
community diversity, and that bottom-up forces predominate, such that
both bird and anole richness is strongly correlated with mean annual
temperature (a proxy for ecosystem productivity). Further, more
lizard-specialized predator occurrence was positively correlated with
anole community metrics, even more than climate effects, suggesting that
these specialist predators may be exerting a stronger top-down effect.
Nevertheless, top-down effects from predators seem to leave some
signatures in anole communities. We found that areas with higher
predator presence also tended towards higher evenness and greater
overall diversity in anoles, whereas in general these relationships were
not observed with pigeons, our non-predatory “control” group. Our
finding that anole species richness was highest where bird predation
pressure was highest could be interpreted either as support for
bottom-up forces controlling bird occurrence, or as greater top-down
pressure on dominant competitors facilitating higher lizard diversity
(although, interestingly, without reducing total lizard abundance).
Either way, it demonstrates no evidence that greater predator pressure
results in a net loss of species richness, as would be expected in the
case of extreme top-down forcing inducing local extirpation of prey
species.
Our observation that more lizards and more bird species are present in
areas where there is higher resource abundance is consistent with the
idea that bottom-up forces drive diversity. Favorable environmental
conditions promote abundance at lower trophic levels, which thus allow
for greater abundance and diversity of consumer species. Indeed,
productivity has been used as an indicator of resource availability
(Evans et al. 2005, Novosolov et al. 2016). Higher temperature and
precipitation are tied to ecosystem productivity and are shown to drive
an increase in plant biomass, and can also sustain a larger arthropod
population (Siemann 1998, Haddad et al. 2001, Wenninger and Inouye 2008,
Bragazza et al. 2015). Lizards rely on vegetation for microhabitat and
arthropods for food, and a greater abundance of these would therefore
support a greater abundance of lizards in these areas. Under bottom-up
control, this higher resource availability would also allow for greater
abundances of bird species (both lizard predators and insectivorous
competitors, as well as granivores and frugivores that do not directly
interact with lizards).
Our findings show that there are more avian predators and anole prey
species in warmer climates. In these cases, predator presence and anole
community richness appear to be dictated by resource availability and
environmental suitability. This is especially true for cuckoo
occurrence, which even when controlling for climate shows a strong
positive correlation with anole community abundance. It is more likely
that we could detect a direct bottom-up effect for these more
specialized predators, since lizard abundance will be more limiting for
them than for other predators that may also be consuming mammals,
insects, amphibians, and birds in addition to lizards.
While our findings suggest that diversity is driven by bottom-up forces,
some of our results can be interpreted as showing signatures of top-down
effects that could be operating in conjunction. On both islands, we saw
that areas with more intense predation also had lizard communities with
higher evenness (Figure 3b). Interestingly, there did not appear to be
any relationship between predation pressure and lizard abundance,
despite there being a positive correlation with community evenness. This
suggests that while predator presence seems to track with prey presence,
there may be some feedback where predators alleviate competition between
anole species, potentially by reducing the abundance of the numerically
dominant competitor, and thereby granting more resources for other anole
species. This would then result in both minimal changes in community
wide abundance, while also increasing both anole species richness and
community evenness. Prey switching has been noted in birds, and this
would especially serve to increase evenness such that any common
competitor eliminated can be replaced by another single less common one
(Murdoch 1969, Fitzpatrick et al. 2009). The presence of specialist
predators seems to have particularly accentuated effects. We found that
while cuckoo presence had a positive relationship with lizard abundance
and richness, it was also a significant predictor of community evenness
– more so than temperature or precipitation. It is thus possible that
cuckoos specifically exert some top-down control of island lizard
communities.
While our results may be indicative of predators playing some role
regulating diversity, we also found some significant relationships when
considering our non-predatory guild: pigeons. Higher pigeon presence was
weakly associated with less even lizard communities on both islands and
alternatively lower and higher diversity on Hispaniola and Jamaica
respectively. Because there is no direct causal link by which pigeons
(which are not-insectivores, and therefore neither consume nor compete
with anoles) we suspect that pigeon presence captures some other aspect
of environmental variation that is not completely accounted for by our
climate variables. Importantly however, the significant relationship
between pigeons and anole evenness was negative (beta = -0.04),
suggesting that the positive relationship found with predatory species
is not a characteristic found between birds and anoles generally.
Similarly, the effect of pigeons on anole community diversity was
generally modest and differed in sign and magnitude on the two islands
– its effect was weak and negative on Hispaniola (beta = -0.03), and
positive of moderate size on Jamaica (beta = 0.16; Table 1). This was in
contrast to all other analyses of predator effects, which, even when a
significant interaction effect between island identity and predation
pressure existed, was always of the same sign and order of magnitude in
strength on both islands. Together, these patterns suggest that the
inferred signature of avian predation on these islands is likely real,
rather than purely driven by unmeasured features of the environment that
affect anoles and birds in tandem.
In addition to birds, snakes, bats, and introduced mammals such as rats
and mongoose also prey on lizards, and were not considered in this
study. Other potential predators include large spiders, andSolenopsis ants which feed on anole eggs (Reyes-Olivares et al.
2020, Andrews and Rand 2022). While lists of mammal and snake occurrence
on these islands as a whole exist, our study focused on birds because
the eBird platform provides timed and tracked presence-absence surveys
of bird communities at a local scale. As a result, our measures of bird
predation are lower overall than what lizards likely experience from all
predator sources. Hispaniola has more species of predator snakes than
Jamaica (4 and 2 respectively), but fine-scale data on local coexistence
is limited, so this this does not necessarily guarantee that snake
occurrence is greater at individual sites. Regardless, our measurements
consequently may have underestimated the effect of total predation in
shaping lizard communities, though we have no a priori expectation that
these other forms of predation would have different consequences than
those of birds. Our analytical approach represents an improvement over
previous whole-island predator lists in that it estimates probability of
predator occurrence at the local scale. However, the ideal metric of
predation pressure would incorporate local predator density, and then
further integrate information on per capita predator effects on prey.
Unfortunately, our preliminary examinations of eBird data with
abundance-based models resulted in convergence issues, or extreme
predictions, likely due to high variance in observed abundance in some
checklists (e.g. many hundreds or thousands of individuals, presumably
when distant flocks were observed). As such eBird and similar databases
at best represent a imperfect substitute for broad-scale time and
area-standardized ecological surveys.
Although previous studies generally report negative correlations between
anole abundance and predation pressure, thereby implicating predators
presence as important drivers of Anolis community structure
(Waide and Reagan 1983, Buckley and Jetz 2007, Calsbeek and Cox 2010,
Pringle et al. 2019), our results show no evidence that predation
pressure either reduces overall anole abundance or leads to local
extirpation and declines in species richness. The reasons for this
divergence is likely in part due to the differences in focal scale
between past studies and our own analyses. Many past studies focused on
small, bounded experimental islands, which often found that predation
may quickly lead prey to go locally extinct. On much larger islands,
however, there are more opportunities for species to find refuge, which
may mitigate a predator’s impact on prey abundance, and therefore
extinction. On the other end of the spectrum, studies comparing lizard
abundances versus predator richness across multiple islands account for
multiple species of both prey and consumer, but they rarely consider
which species on an island actually co-exist with one another on a local
scale. Buckley and Jetz (2007), for instance, conclude that sharp
reductions of predators are a major driver of increased lizard
densities. However, in such cross-island comparisons larger islands will
nearly always have more predator species, even if such predator species
do not occur everywhere on the islands. As such, actual local predator
pressure may be unrelated to total number of predators on the island as
a whole. If some other aspect of island size correlates with average
lizard density, then a negative relationship between island-wide
predator number and locally measured lizard density may be uncovered,
even if the mechanisms driving predator number and lizard density are
not directly related.