Background on Ecosystems, Food Webs, and Ecological Regulation
Species interactions can be visualized through food webs, which denote which species eat each other. Rainforest food webs contain millions of connections between predators and prey, competitors, symbionts, and parasites and hosts. These interactions regulate populations by causing mortality from top-down predation and starvation via competition for limited bottom-up resources (Terborgh 2015). For example, healthy forests require some herbivores (like orangutans) to disperse tree seeds, and forests are also degraded when there is an overpopulation of seed and seedling consumers like deer or pigs (Luskin et al. 2017b). Disturbances can drive species to extinction through direct and indirect pathways. For example, hunting can directly eliminate a species, and this also creates imbalances in the food web, leading to secondary ‘cascading’ effects on other species.
A core question in ecology is how species populations are regulated to ensure ecosystems remain balanced. Regulation can occur via ‘top-down’ consumption (predators eating prey and herbivores eating plants) or ‘bottom-up’ food limitation. However, the relative contribution of top-down versus bottom-up processes in Asia remains largely unknown. This makes it difficult to establish the impact of disturbances like deforestation, hunting, and global warming. Human activities that alter these mechanisms trigger series of linked changes to species populations at different trophic levels (plants <--> prey <--> predators), called ecological cascades (knock-on effects).
What questions does our lab focus on?
The Ecological Cascades Lab studies these mechanisms in SE Asia. We use empirical data from a network of SE Asian forests to address the following key questions about how disturbances reshape forest function:
Does the loss of top-down control from predators trigger ecological cascades?
Is bottom-up fruit limitation the primary mechanism regulating herbivore populations?
Does hunting of large mammals free up resources that indirectly facilitate an increase in smaller mammals?
How will global change affect ecological regulation?
Top-down and bottom up ecological control in Asia
Top-down predation can drive species to extinction when its strength is altered. For example, the introduction of new predators to islands often drives native prey species to extinction. On the other hand, without top-down control by predators, abundant herbivores can decimate plant life and cause ecosystems to collapse. Today, the latter issue has played out in many ecosystems where predators have been killed and herbivores are released from top-down control (Estes et al. 2011). A famous example occurred in America: when wolves were exterminated and deer populations erupted, the deer over-consumed seedlings and devastated tree populations. These chain reactions are called ‘ecological cascades’ and have become a dominant theme in conservation. The wolves’ reintroduction to Parks like Yellowstone largely solved its problems, highlighting the importance of top-down control by predators for effective conservation. A key research question for Asia is whether tigers and leopards play the same role, or are bottom-up processes more important in regulating food webs (Q1)?
*for more information, check out our 2018 PNAS publication about top-down control of plants by herbivores
Plants are also bottom-up regulated by the availability of water, nutrients, and sunlight. For example, SE Asia generally has lower soil nutrients than other tropical regions and this may limit plant growth and reproduction. Herbivore populations positively on plant productivity and are negatively affected by plant adaptations to avoid being eaten, such as thorns or chemical defenses. Plant communities vary in their defensive traits and this creates spatial patterns in strength the bottom-up control of herbivores.
There is a remarkable reason to suspect SE Asian herbivores are more strongly bottom-up regulated than top-down: the trees have adapted a bizarre community-wide defensive strategy against herbivores by only reproducing every few years. This phenology strategy is called ‘masting’ and its purpose is to starve the herbivores that consume and kill their seeds. In fact, a majority of SE Asia’s trees spend years foregoing reproduction. Then, triggered by the droughts associated with El Nino climate variations, the trees produce copious seeds synchronously and there are relatively few animals left surviving to eat them. Bottom-up control by masting may explain why animal biomass in SE Asian forests is substantially lower than the Neo- or Afro-tropics where fruiting is more continuous (Q2). If animals are competing for limited food, then hunting that targets large herbivores may indirectly facilitate an increase in smaller animals (Q3). The ECL addresses these two hypotheses by testing whether sites with stronger masts have lower herbivore abundances and by comparing hunted and unhunted sites.
The ECL is also at the forefront of investigating how global change will reshape ecological regulation. For example, climate change is predicted to double the frequency of El Nino events over the next century (Cai et al 2014). Atmospheric fallout of important nutrients (phosphorus and nitrogen) is fertilizing forests nearby industry and urban centers. Climate change and atmospheric fallout may both increase forest productivity or fruiting, reducing bottom-up control. Will top-down regulation rise to stabilize these future ecosystems (Q4)? Predators may thus become crucial to limiting negative impacts of global change. Unfortunately, many of the region’s remaining forests have already lost predators, leaving them vulnerable. To address this, my lab is designing studies of predator reintroductions in Singapore (smaller felids) and neighbouring countries (larger felids).
From a scientific point of view, these gaps in our knowledge mean that food web ecology is a nascent field in SE Asia that is ripe for breakthroughs