Does the loss of top-down control from predators trigger ecological cascades?
Rationale: Many of SE Asia’s forests that have lost apex predators and also have abundant herbivores (Luskin et al. 2014, 2017a). While this initially appeared to be a classic example of how the loss of top-down control can trigger ecological cascades (Ickes 2001), newer work highlights many forests have high densities of both predators and herbivores (Harrison et al. 2013; Luskin et al. 2017b; Love et al. 2018). This suggests that top-down control is not the key variable regulating herbivores. However, the current empirical evidence is disjointed and insufficient to make conclusions. A key task for the ECL is conducting robust tests of top-down control in SE Asian forests.
Hypothesis: The loss of predators has negligible effects on prey species or plants where there is mast fruiting.
Approach 1: THE ECL compares wildlife communities in forests with and without predators. The ECL has 9 sites across the region where we conduct standardized wildlife surveys (camera trapping and live-trapping). Five of these sites retain apex predators and four have lost apex predators (tigers, leopards, and clouded leopards). Six of the sites have already been surveyed and the ECL is conducting fieldwork at three new sites (see map). The project will soon engage one PhD student and one postdoc and include camera and live-trapping in Singaporean forests.
Approach 2: We use experimental predator reintroductions to assess the influence of regained top-down control (e.g. similar to the wolves in Yellowstone). The ECL is in conversations with Wilmar®, the largest oil palm grower, which is interested in predator reintroductions in their conservation easement forests. This project will engage a new postdoc.
Does masting (bottom-up fruit limitation)
suppress herbivore populations?
Rationale: Animals in temperate regions have adapted to annual seasonal fluctuations by hibernating through the winter. In SE Asia however, such adaptations are difficult because masting occurs approximately three times per decade (Figure 2). Herbivores may starve or refrain from reproduction in non-mast years.
Hypothesis 1: Herbivore population fluctuations are tied to mast fruiting.
Hypothesis 2: Stronger masting phenology is associated with lower seed and seedling mortality from wildlife.
Approach 1: The ECL uses longitudinal wildlife data to assess the degree to which herbivore abundances increase following masts and crash in non-mast years. Masting strength, or the percentage of trees that strictly limit their fruit production to masting years, is strongest in Borneo and declines with the distance outward. This creates a gradient for comparative analyses. We have successful pilot studies at 5 sites and one site where surveys have been repeated after a mast (with Jon Moore).
Approach 2: The ECL has established a network of experiments to exclude herbivores and assess differences in plant community dynamics across a gradient of mast fruiting conditions. There are currently exclosure experiments at 3 sites and ECL is adding 3 additional sites. This project will engage a PhD student and a postdoc that may be co-advised with Dr. Kelly Anderson or Dr. David Wardle (in the Asian School of the Environment at NTU). It will also include multiple exclosures in Singapore in collaborations with NParks.
Does hunting of large mammals free up food and indirectly
facilitate increases in smaller mammals?
Rationale: Most herbivorous mammals in Asian rainforest depend on fruit resources to survive and reproduce successfully. If herbivores are limited by resources, the removal of large herbivores by hunting should lead to the compensatory increase in other smaller competing species (e.g. shrews, squirrels, mice, and rats).
Hypothesis: Hunted sites will have fewer large herbivores and more small mammals than pristine sites.
Approaches: We have quantified mammal abundances through camera trapping and live-trapping at 4 sites and will add 5 more. We are comparing wildlife communities between hunted and non-hunted sites, and (b) areas where we experimentally exclude large herbivores with fencing and (c) areas where lethal management has been used to reduce pigs and macaques.
Lethal management is being arranged with Wilmar® as part of their experimental pest control program to reduce crop-raiding wild pigs and macaques in conservation easement forests.
Fenced megafauna exclosure experiments include multiple locations in SE Asia and Singapore, and this will also provide infrastructure for undergraduate theses on related topics and are also located at the Smithsonian’s permanent research sites.
4) How will global climate change affect ecological regulation in Asia?
Rationale 1: Climate change is predicted to double the frequency of El Nino and associated masting events. More fruit will reduce the strength of bottom-up control on herbivores. In a future without strong bottom-up control, predators may become more important to ecological regulation. The first question is whether retaining predators suppresses herbivore fluctuations?
Rational 2: Masting may also be an adaptation to conserve nutrients by reproducing less often, since fruits are high in nitrogen and phosphorus. For example, Singaporean forests have fewer nutrients and mast less often than nearby forests in Malaysia. Bottom-up nutrient limitation may disappear with high rates of atmospheric fertilization from nearby industry (described above). As fertilization accumulates, higher plant productivity may increase fruiting frequencies, easing ‘bottom-up’ regulation of herbivores.
Approach 1: We are monitoring predator and herbivore populations before and after mast fruiting events.
Approach 2: To test this hypothesis, the ECL is participating in a collaborative project with soil scientists and botanists at NTU to establish a soil fertilization experiment in Singapore and Malaysia. We hosted a workshop in 2017 to discuss the experimental design. We have begun conversations with Singaporean authorities (NParks) to establish the fertilization experiments. The ECL's role will be to evaluate if nutrient additions alter tree phenology and how animal populations respond. This project will engage multiple PhD and postdocs that will be co-advised with Dr. Kelly Anderson.