Thomas W. Schoener

Surprisingly little is known about the evolutionary impacts of rare but extreme black swan events, like tropical cyclones. By intercepting three cyclones in fall 2018, we evaluated cyclone-induced selection on collective behavior in a group-living spider. We further examined whether historic frequencies of cyclone landfalls are correlated with geographic variation in group behavior. Cyclones consistently selected for more aggressive spider societies. Furthermore, sites where cyclones have historically been more common also harbor more aggressive groups. Thus, two corroborative lines of evidence convey that that cyclone-induced selection can drive the evolution of colony behavior, and suggest that extreme black swan events can shape within-species diversity and local adaptation. One Sentence Summary: Tropical cyclones drive the evolution of more aggressive spider societies.

Only a handful of multi-generational experiments in natural systems of eco-evolutionary dynamics currently exist, despite Fussmann et al.'s call for more such studies nearly a decade ago. To perform such a study, in 2008 we introduced the lizard Leiocephalus carinatus, a predator (and possible food competitor) of the lizard Anolis sagrei, to seven islands having A. sagrei, with seven unmanipulated islands having A. sagrei as controls. Almost immediately, L. carinatus, which is larger and more terrestrial than A. sagrei, caused a major habitat shift in A. sagrei away from the ground and toward higher and thinner perches; focal behavioral surveys showed that on islands where its predator was introduced, A. sagrei had less conspicuous visual displays. The expected pattern for density of A. sagrei is that it would decrease markedly at first via predation from the larger lizard, but then it would increase as the habitat shift selected for individuals better able to live in higher vegetation. Data through 2015 ...

Significance: We report for island populations of the termite-eating common gecko speciesGymnodactylus amaralirapid parallel morphological and ecological change in response to human-caused environmental disturbance. The islands were formerly part of an extensive terrestrial ecosystem; in 1997, the area was flooded to construct a reservoir, fragmenting the higher portions into separate islands. Populations on all five islands studied have proportionally larger heads than populations at five nearby mainland sites. The new island morphology is accompanied by an increase in dietary niche breadth, mainly via expansion toward larger prey. This expansion is likely due to the greater availability of such prey on the newly formed islands after the extinction there of four larger lizard species that typically also included termites in their diets.

Aim: We ask empirically how the strength of the three major interactions – predation, competition and mutualism – changes with increasing island area. We review and expand current theory concerning these relationships.Location: Data for evaluating this question come from several Bahamian archipelagos and involve both experiments and observations. The latter can be especially long term; in one case data were collected over a 17‐year period.Methods: We analyse the effect size of the following interactions across a range of island areas: (1) predation by lizards on spiders, (2) competition between two lizard species, (3) competition between two spider species, (4) ant–plant mutualism, and (5) plant–pollinator mutualism.Results: Effect sizes for predation and competition mostly show a hump‐shaped relationship with island area. Effect sizes for ant–plant mutualism were reduced on large islands compared with smaller islands. Germination rate showed a steady increase with island area which we infer to be caused by an increase in pollinator limitation on smaller islands.Main conclusions: We argue that the effect size–area relation has rather similar aetiologies for predation, competition and to a somewhat lesser extent ant–plant mutualism. Specifically, we suggest that high species diversity, top predators, plant defences and (for predation and competition) spatial heterogeneity with respect to refuges or resource use reduce effect size on large islands, while harsh abiotic conditions, marine subsidies and stochastic events reduce effect size on small islands. Thus, for these interaction types, the greatest effect sizes are observed on intermediate‐sized islands. For plant–pollinator mutualism we suggest that the monotonic increase in effect size with increasing island area is the result of interaction strength being enhanced, rather than weakened, by diversity.

Signaling individuals must effectively capture and hold the attention of intended conspecific receivers while limiting eavesdropping by potential predators. A possible mechanism for achieving this balance is for individuals to modulate the physical properties of their signals or to alter the proportion of time spent signaling, depending upon local levels of predation pressure. We test the hypothesis that prey can alter their visual signaling behavior to decrease conspicuousness and potentially limit predation risk via modulation of signal properties or display rate. To do so, we conducted a manipulative experiment in nature to evaluate the possible effect of predation pressure on the physical properties of movement-based signals and on the proportion of time spent signaling by using a well-understood predator-prey system in the Bahamas, the semiarboreal lizard Anolis sagrei, and one of its main predators, the curly-tailed lizard Leiocephalus carinatus. We find that on islands onto which the predator was introduced, male anoles reduce the maximum amplitude of head-bob displays but not the proportion of time spent signaling, in comparison with control islands lacking the predator. This reduction of amplitude also decreases signal active space, which might alter the reproductive success of signaling individuals. We suggest that future studies of predator-prey interactions consider the risk effects generated by changes in signals or signaling behavior to fully determine the influence of predation pressure on the dynamics of prey populations.

The effect of ecological change on evolution has long been a focus of scientific research. The reverse—how evolutionary dynamics affect ecological traits—has only recently captured our attention, however, with the realization that evolution can occur over ecological time scales. This newly highlighted causal direction and the implied feedback loop—eco-evolutionary dynamics—is invigorating both ecologists and evolutionists and blurring the distinction between them. Despite some recent relevant studies, the importance of the evolution-to-ecology pathway across systems is still unknown. Only an extensive research effort involving multiple experimental approaches—particularly long-term field experiments—over a variety of ecological communities will provide the answer.

Many animals modify their environments, apparently to reduce predation risk, but the success of such endeavors, and their impact on the density and distribution of populations, are rarely rigorously demonstrated. We staged a manipulative experiment to assess the effectiveness of self-made shelters by web spiders as protection from natural enemies. Scincid lizards were included or excluded from 21 replicated 200-m ² plots, and spiders therein were classified as exposed or sheltered, depending on whether they were uncovered in their web or hidden in cocoons, leaves/debris, or burrows. We found that exposed spiders were greatly affected by the presence of predatory scincid lizards, whereas sheltered spiders were not. More specifically, lizards, which forage close to the ground, reduced the abundance of exposed spiders by two-thirds but had no effect on the abundance of sheltered spiders. Sheltered spiders were able to avoid predation and share space with lizards, suggesting that shelter construction is a mechanism for reducing predation risk and has important population consequences.

We monitored spiders on 41 Bahamian islands for 4 years before and then 4 years after the catastrophic Hurricane Floyd passed directly over the site, inundating the study islands with its storm surge. The respective recoveries of major community properties after this annihilation were far from synchronous. Before the hurricane, the species-area relation was generally strong and the slope showed no temporal trend. After the hurricane, the slope increased from near zero (7 months later) to a value about equal to its prehurricane state. The lizard effect (difference in spider abundance or species richness between islands with and without the lizard Anolis sagrei ) was generally strong before the hurricane; 7 months after, the lizard effect on abundance was weak and the effect on richness had vanished. In subsequent years, the lizard effect on abundance became strong again, but the effect on species richness remained weak. The strength of the lizard effect on both abundance and richness over the 8 years was strongly positively related to the density of lizards measured on a subset of the study islands. Twelve months after the hurricane, species richness averaged over all islands rebounded to the last prehurricane value, but abundance attained only about half that value; this finding was remarkably similar to results found in an earlier study of spiders impacted by Hurricane Lili (1996) in a different Bahamian region. Nonetheless, in the next 3 years, species richness failed to increase further, part of its long-term decline at the study site.

Population phenomena, which provide much of the underlying basis for the theoretical structure of island biogeography, have received little direct study. We determined a key population trait—survival—in the Bahamian lizard Anolis sagrei on islands with an experimentally introduced predatory lizard and on neighboring unmanipulated islands. On unmanipulated islands, survival declined with several variables, most notably vegetation height: The island with the shortest vegetation had nearly the highest survival recorded for any lizard. On islands with the introduced predator, which forages mostly on the ground, A. sagrei shifted to taller vegetation; unlike on unmanipulated islands, its survival was very low on islands with the shortest vegetation but was higher on the others. Thus, species introduction radically changed a resident species' relation of survival to a key island-biogeographical variable.

Two recent hurricanes passed directly over the northern Bahamas 2 years apart, allowing a comparison of their effects on lizard populations inhabiting exactly the same islands. The hurricanes differed in two ways: one struck during the reproductive season and was relatively severe; the other struck after most reproduction had taken place and was milder. The late-season hurricane produced a significant relation between population reduction and lowness of the island that lasted at least through two seasons; the earlier hurricane produced no such relationship. The late-season hurricane wiped out populations of lizards on two islands (two of the three lowest) that the earlier hurricane failed to exterminate even though it was stronger. We relate these effects to the fact that the study lizards regenerated from the earlier hurricane only via the egg stage, whereas eggs were unavailable when the later storm struck and regeneration was via hatched lizards. We discriminate and illustrate four kinds of hurricanes, cross-classified by two contrasts: earlier vs. later and stronger vs. weaker. A later, stronger hurricane completely exterminated lizard populations at a second Bahamian site, whereas an earlier, weaker hurricane had no detectable effect at a third Bahamian site. We suggest that, in addition to severity, the timing of a hurricane as it coincides with reproductive scheduling or other phenological aspects may determine the magnitude of its effect on a variety of organisms.