Matthew D. Potts

HABITAT PATTERNS IN TROPICAL RAIN FORESTS: A COMPARISON OF 105 PLOTS IN NORTHWEST BORNEO

Understanding the maintenance of high tropical tree species diversity requires disentangling the effects of habitat vs. geographic distance. Using floristic, topographic, and soil nutrient data from 105 0.6-ha plots in mixed dipterocarp forest throughout Sarawak, Malaysian Borneo, we explore the degree to which floristic patterns are habitat-driven from local to landscape scales. We assess how the floristic influence of geographic distance vs. abiotic factors varies from local to regional scales. We employ several multivariate analytical techniques and perform a hierarchical clustering of the research plots using the Steinhaus index of floristic dissimilarity, as well as Mantel analyses on matrices of floristic, habitat, and geographic distance. These analyses indicate that floristic variation is more strongly correlated with habitat than with geographic distance on the regional scale. On the local- landscape to community scale, we find evidence of a resource threshold above which habitat effects weaken; that is, below the resource threshold floristic similarity between sites is dominated by habitat effects, while above the threshold floristic similarity between sites is dominated by geographic-distance effects. We also find evidence that topography and soil nutrients correlate in part independently with floristics. These results, together with previous studies in the Neotropics, emphasize that tree species distribution and community com- position are variously influenced by the interplay of both habitat and dispersal-driven effects.

Inferring the dynamics of diversification: a coalescent approach.

A novel approach to infer diversification dynamics shows that biodiversity is still expanding but at a slower rate than in the past.

The effects of human movement on the persistence of vector-borne diseases

With the recent resurgence of vector-borne diseases due to urbanization and development there is an urgent need to understand the dynamics of vector-borne diseases in rapidly changing urban environments. For example, many empirical studies have produced the disturbing finding that diseases continue to persist in modern city centers with zero or low rates of transmission. We develop spatial models of vector-borne disease dynamics on a network of patches to examine how the movement of humans in heterogeneous environments affects transmission. We show that the movement of humans between patches is sufficient to maintain disease persistence in patches with zero transmission. We construct two classes of models using different approaches: (i) Lagrangian models that mimic human commuting behavior and (ii) Eulerian models that mimic human migration. We determine the basic reproduction number R(0) for both modeling approaches. We show that for both approaches that if the disease-free equilibrium is stable (R(0)<1) then it is globally stable and if the disease-free equilibrium is unstable (R(0)>1) then there exists a unique positive (endemic) equilibrium that is globally stable among positive solutions. Finally, we prove in general that Lagrangian and Eulerian modeling approaches are not equivalent. The modeling approaches presented provide a framework to explore spatial vector-borne disease dynamics and control in heterogeneous environments. As an example, we consider two patches in which the disease dies out in both patches when there is no movement between them. Numerical simulations demonstrate that the disease becomes endemic in both patches when humans move between the two patches.

Pseudoreplication in Tropical Forests and the Resulting Effects on Biodiversity Conservation

Tropical forest ecosystems are threatened by habitat conversion and other anthropogenic actions. Timber production forests can augment the conservation value of primary forest reserves, but studies of logging effects often yield contradictory findings and thus inhibit efforts to develop clear conservation strategies. We hypothesized that much of this variability reflects a common methodological flaw, simple pseudoreplication, that confounds logging effects with preexisting spatial variation. We reviewed recent studies of the effects of logging on biodiversity in tropical forests (n = 77) and found that 68% were definitively pseudoreplicated while only 7% were definitively free of pseudoreplication. The remaining proportion could not be clearly categorized. In addition, we collected compositional data on 7 taxa in 24 primary forest research plots and systematically analyzed subsets of these plots to calculate the probability that a pseudoreplicated comparison would incorrectly identify a treatment effect. Rates of false inference (i.e., the spurious detection of a treatment effect) were >0.5 for 2 taxa, 0.3-0.5 for 2 taxa, and <0.3 for 3 taxa. Our findings demonstrate that tropical conservation strategies are being informed by a body of literature that is rife with unwarranted inferences. Addressing pseudoreplication is essential for accurately assessing biodiversity in logged forests, identifying the relative merits of specific management practices and landscape configurations, and effectively balancing conservation with timber production in tropical forests.

Competition Drives Specialization in Pollination Systems through Costs to Male Fitness

Specialization in pollination systems played a central role in angiosperm diversification, yet the evolution of specialization remains poorly understood. Competition through interspecific pollen transfer may select for specialization through costs to male fitness (pollen lost to heterospecific flowers) or female fitness (heterospecific pollen deposited on stigmas). Previous theoretical treatments of pollination focused solely on seed set, thus overlooking male fitness. Here we use individual‐based models that explicitly track pollen fates to explore how competition affects the evolution of specialization. Results show that plants specialize on different pollinators when visit rates are high enough to remove most pollen from anthers; this increases male fitness by minimizing pollen loss to foreign flowers. At low visitation, plants generalize, which minimizes pollen left undispersed in anthers. A model variant in which plants can also evolve differences in sex allocation (pollen/ovule production) produces similar patterns of specialization. At low visitation, plants generalize and allocate more to female function. At high visitation, plants specialize and allocate equally to both sexes (in line with sex‐allocation theory). This study demonstrates that floral specialization can be driven by selection through male function alone and more generally highlights the importance of community context in the ecology and evolution of pollination systems.

Soil-Related Floristic Variation in a Hyperdiverse Dipterocarp Forest

A 52 hectare permanent research plot was established in Lambir Hills National Park to enable long-term study of factors controlling the origin and maintenance of tree diversity. In this chapter we summarize some of our recent work on the relationships between floristic variation and edaphic heterogeneity in the Lambir forest. First, we provide a general description of the floristic composition of this hyperdiverse forest. Second, we use a detailed survey of soil chemistry to test whether floristic composition changes in relation to edaphic characteristics. We also assess the extent to which individual species have non-random distributions in the forest with respect to edaphic heterogeneity. Finally, to investigate the influence of habitat variation on floristic diversity we compare our results from the heterogeneous forest at Lambir Hills with a more homogeneous forest in Peninsular Malaysia.

Tropical countries may be willing to pay more to protect their forests

Significance Tropical forests, especially the primary tropical forests that are globally important for biodiversity conservation and carbon storage, are increasingly concentrated in relatively wealthier developing countries. This creates an opportunity for domestic funding by these countries to play a larger role in (i) closing the funding gap for tropical forest conservation, and (ii) paying for supplementary conservation actions linked to international payments for reduced greenhouse gas emissions from deforestation and forest degradation. Inadequate funding from developed countries has hampered international efforts to conserve biodiversity in tropical forests. We present two complementary research approaches that reveal a significant increase in public demand for conservation within tropical developing countries as those countries reach upper-middle-income (UMI) status. We highlight UMI tropical countries because they contain nearly four-fifths of tropical primary forests, which are rich in biodiversity and stored carbon. The first approach is a set of statistical analyses of various cross-country conservation indicators, which suggests that protective government policies have lagged behind the increase in public demand in these countries. The second approach is a case study from Malaysia, which reveals in a more integrated fashion the linkages from rising household income to increased household willingness to pay for conservation, nongovernmental organization activity, and delayed government action. Our findings suggest that domestic funding in UMI tropical countries can play a larger role in (i) closing the funding gap for tropical forest conservation, and (ii) paying for supplementary conservation actions linked to international payments for reduced greenhouse gas emissions from deforestation and forest degradation in tropical countries.

Linking size-dependent growth and mortality with architectural traits across 145 co-occurring tropical tree species

Tree architecture, growth, and mortality change with increasing tree size and associated light conditions. To date, few studies have quantified how size-dependent changes in growth and mortality rates co-vary with architectural traits, and how such size-dependent changes differ across species and possible light capture strategies. We applied a hierarchical Bayesian model to quantify size-dependent changes in demographic rates and correlated demographic rates and architectural traits for 145 co-occurring Malaysian rain-forest tree species covering a wide range of tree sizes. Demographic rates were estimated using relative growth rate in stem diameter (RGR) and mortality rate as a function of stem diameter. Architectural traits examined were adult stature measured as the 95-percentile of the maximum stem diameter (upper diameter), wood density, and three tree architectural variables: tree height, foliage height, and crown width. Correlations between demographic rates and architectural traits were examined for stem diameters ranging from 1 to 47 cm. As a result, RGR and mortality varied significantly with increasing stem diameter across species. At smaller stem diameters, RGR was higher for tall trees with wide crowns, large upper diameter, and low wood density. Increased mortality was associated with low wood density at small diameters, and associated with small upper diameter and wide crowns over a wide range of stem diameters. Positive correlations between RGR and mortality were found over the whole range of stem diameters, but they were significant only at small stem diameters. Associations between architectural traits and demographic rates were strongest at small stem diameters. In the dark understory of tropical rain forests, the limiting amount of light is likely to make the interspecific difference in the effects of functional traits on demography more clear. Demographic performance is therefore tightly linked with architectural traits such as adult stature, wood density, and capacity for horizontal crown expansion. The enhancement of a demographic trade-off due to interspecific variation in functional traits in the understory helps to explain species coexistence in diverse rain forests.

Maximizing biodiversity co-benefits under REDD+: a decoupled approach

Current debates on biodiversity co-benefits under REDD+ are marked by considerable ambiguity and contention. Nevertheless, REDD+ continues to represent one of the most important opportunities for global biodiversity conservation, and the question of how best to achieve biodiversity co-benefits remains an important one. Thus far, most biodiversity conservation in the context of REDD+ is predicated on the notion that services are co-located on a landscape. In contrast, this letter argues that decoupling biodiversity and carbon services on a landscape through national-level planning is a better approach to biodiversity conservation under REDD+. We discuss the fundamental ecological differences between the two services and use principles of resource economics to demonstrate that a decoupled approach will be more efficient, more flexible, and better able to mobilize sufficient finance for biodiversity conservation than a coupled approach.

Mortality rate estimation when inter-census intervals vary

The probability of death during a unit time period, or mortality rate, is an essential demographic parameter characterizing population turnover. There are several problems in estimating mortality of tree populations from censuses of marked trees in permanent plots (Sheil 1995). Besides these problems, a practical problem exists that is common with large-sized plots; namely, variable return intervals among trees between censuses. It usually takes a long time for establishment of large plots, as well as remeasurement. The duration of a single census takes a long time, and the return interval for each tree in each species population therefore becomes variable. Therefore, we cannot apply, for example, such a conventional estimation of instantaneous mortality rate X (y-) between the two sequential censuses (e.g. Sheil et al. 1995):