Meghna Krishnadas

Testing predictions of the Janzen–Connell hypothesis: a meta‐analysis of experimental evidence for distance‐ and density‐dependent seed and seedling survival

The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats. We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis. Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias. The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage. Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.

Conflict of human–wildlife coexistence

Carter et al. (1) used data on spatial overlap of tigers and people to conclude that human–tiger coexistence is possible at fine spatial scales. The question then is whether spatial overlap suggests that human–tiger coexistence is in fact a viable strategy for their mutual well-being in the long run. Coexistence, or rather its absence between large carnivores and humans, is an oft-repeated theme in conservation biology. The results presented by Carter et al. (1) support this recognition; tigers were detected less frequently near human populations and the probability of their detection increased with distance from human settlement. Although this finding is indicative of a scenario where tigers are being pushed into areas of low human activity, the authors argue that it is a mechanism by which tigers coexist with people. We contend that the same result could just as easily be interpreted as increasing anthropogenic pressures hemming in a tiger population that is deprived of alternative habitats.

Above‐ground biomass is driven by mass‐ratio effects and stand structural attributes in a temperate deciduous forest

Summary 1.Forest ecosystems are critical for the global regulation of carbon (C), a substantial portion of which is stored in aboveground biomass (AGB). While it is well understood that taxonomic and functional composition, stand structure, and environmental gradients influence spatial variation in AGB, the relative strengths of these drivers at landscape-scales has not been investigated in temperate forests. Furthermore, when biodiversity enhances C storage, it is unclear whether it is through mass-ratio effects (i.e., the dominant trait in communities regulates AGB) or through niche complementarity (i.e., increased AGB due to interspecific resource partitioning). 2.To address these mechanisms, we analyzed data from a census of 28,262 adult trees sampled across 900 ha of temperate deciduous forest in southwestern Pennsylvania. We used data on four key plant functional traits to determine if (1) there is a positive relationship between species diversity and AGB and (2) whether this is due to mass-ratio effects or niche complementarity. We also sought to (3) identify the physical stand structural attributes and topographic variables that influence AGB across this landscape. 3.We found AGB was positively related to species richness and negatively related to species evenness, albeit weakly, while functional diversity indices had neutral effects. AGB was enhanced in communities dominated by traits related to greater maximum tree height, deeper minimum rooting depths and larger seeds. Most importantly, areas with high AGB were dominated by Acer saccharum and Liriodendron tulipifera. Overall, these results support mass-ratio effects, with little evidence for niche complementarity. 4.Synthesis: Stand structure, topography, and species and functional composition, but not taxonomic or functional diversity, were found to be key drivers of AGB at landscape-scales (<900 ha) in this temperate deciduous forest. Our findings suggest that simultaneously managing for both high diversity and for aboveground C storage may prove challenging in some forest systems. Our results further indicate that the impact of tree biodiversity loss on aboveground C stocks will depend greatly on the identity of the species that are lost. This article is protected by copyright. All rights reserved.

Equality in Conservation: Comment on Bawa et al. 2011

MEGHNA KRISHNADAS,∗† TARUN NAIR,†‡ AND DIVYA KARNAD†§ ∗Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, OH 43210-1219, U.S.A. †National Centre for Biological Sciences, GKVK Campus, Bangalore 560065, India ‡Gharial Conservation Alliance, Centre for Herpetology – Madras Crocodile Bank Trust, P.O. Box 4, Mamallapuram, Tamil Nadu, 603 104, India §Department of Geography, Rutgers University, New Jersey, 08854-8045, U.S.A. email divyakarnad@hotmail.com

Weaker plant-enemy interactions decrease tree seedling diversity with edge-effects in a fragmented tropical forest

In fragmented forests, tree diversity declines near edges but the ecological processes underlying this loss of diversity remain poorly understood. Theory predicts that top-down regulation of seedling recruitment by insect herbivores and fungal pathogens contributes to maintaining tree diversity in forests, but it is unknown whether proximity to forest edges compromises these diversity-enhancing biotic interactions. Here we experimentally demonstrate that weakened activity of fungal pathogens and insect herbivores reduced seedling diversity, despite similar diversity of seed rain, during recruitment near forest edges in a human-modified tropical landscape. Only at sites farthest from forest edges (90–100 m) did the application of pesticides lower seedling diversity relative to control plots. Notably, lower seedling diversity corresponded with weaker density-dependent mortality attributable to insects and fungi during the seed-to-seedling transition. We provide mechanistic evidence that edge-effects can manifest as cryptic losses of crucial biotic interactions that maintain diversity.Tree diversity decreases at the edges of fragmented forests. Here, Krishnadas et al. find that weaker top-down regulation by insects and fungal pathogens during seedling recruitment contributes to reduced tree seedling diversity near forest edges in a human-modified landscape.