Our review on measuring habitat complexity in ecology has just been published in Ecology Letters. The review is led by Lynette Loke, a post-doctoral fellow at Macquarie University and previous collaborator of our lab. Ecologists have theorised that habitats with higher complexity have higher diversity, and there is some empirical evidence to support this. But generalisations are difficult, partly because complexity is not measured in a standardised way. We review frequently used metrics of habitat complexity and identify qualities that an ideal metric of complexity should possess.
We find that fractal dimension, one of the most commonly used metrics of complexity, is fraught with problems: fractal dimension is hard to measure accurately (see second figure below); most real ecological habitats may not have fractal properties; and fractal dimension is often poorly correlated with diversity. Rugosity, or surface roughness, is another commonly used metric that is easier to measure and better correlated with diversity, but it may not capture important aspects of habitat complexity. We see promise in information-based metrics of complexity, such as entropy, which are more holistic.
Our recently graduated Honours student Annabel Lim has been awarded valedictorian of the 2021–2022 undergraduate class in Biological Sciences. Annabel specialised in Environmental Biology and completed a minor in Communications and New Media. She was also awarded the Navjot Sodhi prize and the Malayan Nature Society Silver Medal for Academic Excellence. Her Honours thesis explored conservation planning for dipterocarp forests in the Philippines under future climate-change scenarios. She plans to go on to post-graduate study at the NUS Centre for Nature-based Climate Solutions. Her valedictory speech can be viewed here at 1:48:00. Congratulations, Annabel!
Discoveries of new species continue to be reported regularly in scientific journals and the popular media, but how distinct are these new species from known species? In a new paper led by Deon Lum, a former research assistant in our lab, we explored this question for birds. We drew on multiple existing sources to build a robust avian phylogenetic tree, and explored how much each discovery of a new bird species over the last 250 years has added to our knowledge of phylogenetic diversity.
Our main finding is that newly discovered species are increasingly similar to known species. Around the turn of the 19th century, novel species discoveries included the Tawny Frogmouth and the Australian Owlet-nightjar, each of which represented a new taxonomic order and contributed roughly 60 Myr to known phylogenetic diversity at the time. In contrast, the most novel discoveries in recent decades have contributed only about 10% as much to our knowledge. One exception proves the rule: the discovery of the Udzungwa Forest Partridge in the 1990s was by far the most novel species in recent decades, but its novel contribution to known phylogenetic diversity was only about 20 Myr—one third that of the earliest decades’ most novel species. We conclude that our knowledge of the avian tree of life is mostly complete.
According to the stress-gradient hypothesis, mutualistic interactions between species can become stronger in harsher environments. For example, mosquito larvae in water-filled tree hollows can benefit from the presence of beetle larvae—the beetle larvae shred leaves into smaller pieces that the mosquito larvae can consume—and the benefit is greater when the environment is harsher, i.e., when there is less overall leaf litter available. In a previous study, we failed to find theoretical support for the stress-gradient hypothesis in a mathematical consumer–resource model. But in a new study, we show that the stress-gradient hypothesis can hold if there is a sufficient leakage rate of the downstream resource (the small pieces of leaves in the mosquito larvae example). We additionally show that our previous results are robust to the precise mathematical assumptions about how a species’ consumption rate increases with resource availability.
The new study was led by undergraduate student Sim Hong Jhun, and came out of his Honours project, which was supervised by Lam Weng Ngai (now at Nanyang Technological University) and Chong Kwek Yan (now at the National Parks Board, Singapore). The paper has just been published in the Journal of Theoretical Biology:
Three new students have joined the lab this month. Angelica See is a new PhD student, who recently completed her undergraduate degree in ecology at Nanyang Technological University. Her thesis research will focus on the effects of habitat fragmentation on tropical ecosystems. Kong Fanhua is a visiting PhD student from East China Normal University, where she is supervised by He Fangliang. She is studying species coexistence using empirical and theoretical approaches, and will be with us for two years. Nicholas Foong is an undergraduate student who will be doing his Honours project on global patterns of mangrove diversity. Welcome all!
Ecological communities are universally subject to temporal environmental stochasticity—random variation in environmental conditions over time that cause species’ vital rates to change. This can have both positive and negative effects on species richness. For example, negative effects can arise because stochasticity makes a species more vulnerable to extinction, whereas positive effects can arise because stochasticity reduces the strength of competition between species. Is the net effect on species richness typically negative or positive, and what mechanisms typically dominate? Answering this question is of paramount importance for our understanding of the effects of temporally varying environments on biodiversity. In a study just published in Oikos, Tak and Ryan together with James O’Dwyer from the University of Illinois tackled this topic using a novel mathematical approach that unified past models of temporal environmental stochasticity and generated new insights.
Our novel approach was to focus on one species and approximate the rest of the multispecies community as a single entity, thus converting a many-body problem into a one-body problem and greatly simplifying the mathematics. We developed methods for separating the effects of temporal environmental stochasticity into population-level effects and community-level effects, and found that the strength and direction of these effects varied in complex and surprising ways as a function of temporal correlation in the environmental stochasticity.
Our most tantalising result was that there is a threshold value of temporal correlation above which the net effect of temporal environmental stochasticity on species richness switches from positive to negative. The threshold is approximately twice the reciprocal of the rate at which new species enter the community. This points the way to a practical method for assessing whether temporal environmental stochasticity has a net positive or negative effect on biodiversity in reality, although further theoretical work is needed to test the generality of our result.
MacArthur & Wilson’s (1967) classic theory of island biogeography predicts that island species richness arises from an equilibrium between immigration and extinction, with islands further from a mainland receiving fewer immigrants and thus having lower species richness. A twist on the basic paradigm occurs for land-bridge islands, i.e., islands that were previously connected to a mainland. Such islands may exhibit more species than expected if insufficient time has passed for the immigration–extinction equilibrium to be reached. For example, in Baja California more lizard species are found on islands that have become isolated more recently in geological time (Wilcox 1978).
In a paper just published in the Journal of Biogeography, we explored whether this effect exists for birds on the islands of Sundaland. The project was led by Keita Sin, a former Honours student in Frank Rheindt’s lab, who created a database of bird diversity on 94 of these islands, from both published inventories and his own field work. We tested whether shelf islands, which have been connected to major landmasses at various points over the last 20,000 years, have higher bird diversity than do oceanic islands. Surprisingly, we found that they do not. Our explanation is that both immigration and extinction rates for birds on islands are higher than for most other taxa, such as lizards and plants, and thus the immigration–extinction equilibrium is reached faster. Immigration rates are higher because birds are effective dispersers; extinction rates may be higher because bird species’ population sizes on islands are typically small. Our results help shed light on the processes that structure species diversity on islands.
What forces structure the diversity of ecological communities on local scales? One perspective is that local niche factors determine how many species can coexist locally. A very different perspective is that local diversity is mainly a function of immigration and regional diversity. Pioneering ecologist Robert MacArthur (1930–1972) made seminal contributions to theory embodying both perspectives. The dichotomy between the two perspectives has thus been termed “MacArthur’s paradox”.
In previous work, we built a unified mathematical model of island biodiversity and showed that increasing immigration with island area drives a transition from niche-structured to immigration-structured communities. The model predicts a biphasic island species–area curve that is commonly seen in nature. In a new paper just out in Theoretical Ecology, we explore this transition in more detail. We modify MacArthur & Wilson’s (1967) classical graphical paradigm of island biogeography to include niches and show that it leads to a biphasic species–area relationship (SAR), consistent with our previous mathematical model (see graph below). We show that three classic mathematical niche models predict a similar SAR when immigration is added. We also reconcile this biphasic island SAR with the classic triphasic SAR seen on mainlands, and predict a tetraphasic SAR in low-diversity mainland systems, with a sampling phase at very small spatial scales and an elusive niche-structured phase at intermediate spatial scales.
By continuing the unification of the niche and immigration perspectives, our work helps resolve MacArthur’s paradox. We propose experiments that manipulate immigration directly to explore the transition between the niche- and immigration-structured regimes. And we propose large-scale data analyses to find the elusive niche-structured phase of the SAR on mainlands.