Tropical forests are important reservoirs of biodiversity, but the processes that maintain this diversity remain poorly understood. The Janzen-Connell hypothesis suggests that specialized natural enemies such as insect herbivores and fungal pathogens maintain high diversity by elevating mortality when plant species occur at high density (negative density dependence; NDD). NDD has been detected widely in tropical forests, but the prediction that NDD caused by insects and pathogens has a community-wide role in maintaining tropical plant diversity remains untested. We show experimentally that changes in plant diversity and species composition are caused by fungal pathogens and insect herbivores. Effective plant species richness increased across the seed-to-seedling transition, corresponding to large changes in species composition. Treating seeds and young seedlings with fungicides significantly reduced the diversity of the seedling assemblage, consistent with the Janzen-Connell hypothesis. Although suppressing insect herbivores using insecticides did not alter species diversity, it greatly increased seedling recruitment and caused a marked shift in seedling species composition. Overall, seedling recruitment was significantly reduced at high conspecific seed densities and this NDD was greatest for the species that were most abundant as seeds. Suppressing fungi reduced the negative effects of density on recruitment, confirming that the diversity-enhancing effect of fungi is mediated by NDD. Our study provides an overall test of the Janzen-Connell hypothesis and demonstrates the crucial role that insects and pathogens have both in structuring tropical plant communities and in maintaining their remarkable diversity.

/pdf/pathogens-and-insect-herbivores-drive-rainforest-plant-1atqf66dz8.pdf

Pathogens and insect herbivores drive rainforest plant diversity and composition

Summary 1. 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. 2. 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. 3. 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. 4. 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. 5. 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.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2745.12232

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

Seed persistence is the survival of seeds in the environment once they have reached maturity. Seed persistence allows a species, population or genotype to survive long after the death of parent plants, thus distributing genetic diversity through time. The ability to predict seed persistence accurately is critical to inform long-term weed management and flora rehabilitation programs, as well as to allow a greater understanding of plant community dynamics. Indeed, each of the 420000 seed-bearing plant species has a unique set of seed characteristics that determine its propensity to develop a persistent soil seed bank. The duration of seed persistence varies among species and populations, and depends on the physical and physiological characteristics of seeds and how they are affected by the biotic and abiotic environment. An integrated understanding of the ecophysiological mechanisms of seed persistence is essential if we are to improve our ability to predict how long seeds can survive in soils, both now and under future climatic conditions. In this review we present an holistic overview of the seed, species, climate, soil, and other site factors that contribute mechanistically to seed persistence, incorporating physiological, biochemical and ecological perspectives. We focus on current knowledge of the seed and species traits that influence seed longevity under ex situ controlled storage conditions, and explore how this inherent longevity is moderated by changeable biotic and abiotic conditions in situ, both before and after seeds are dispersed. We argue that the persistence of a given seed population in any environment depends on its resistance to exiting the seed bank via germination or death, and on its exposure to environmental conditions that are conducive to those fates. By synthesising knowledge of how the environment affects seeds to determine when and how they leave the soil seed bank into a resistance–exposure model, we provide a new framework for developing experimental and modelling approaches to predict how long seeds will persist in a range of environments.

The ecophysiology of seed persistence: a mechanistic view of the journey to germination or demise

Southeast Asia (SE Asia) is a known global hotspot of biodiversity and endemism, yet the region is also one of the most biotically threatened. Ecosystems across the region are threatened by an array of drivers, each of which increases the probability of extinction of species in a variety of ecosystems. These issues are symptomatic of the issues that face the global tropics; however, with around 4 billion people in the wider region and the associated pressures on biodiversity, this region may be under some of the greatest levels of biotic threat. Deforestation rates in SE Asia are some of the highest globally, additionally it has the highest rate of mining in the tropics, around the greatest number of hydropower dams under construction, and a consumption of species for traditional medicines which is a threat to biodiversity globally. In this review, the greatest threats to regional biodiversity in the SE Asian region are discussed. Tree-plantations and deforestation represent one of the most imminent threats, and some countries have already lost over half their original forest cover (i.e., the Philippines, parts of Indonesia), with projections of as much as 98% loss for some regions in the coming decade. Hunting and trade represent a significant threat as demand stems not only for food, but also for medicine, for ornamentation, and as a status symbol. Mining represents a frequently overlooked threat, as the Asian region is one of the greatest exporters of limestone and various minerals globally, and the cost of this to biodiversity is not only through the direct loss of areas for mines, but also through the development of roads that further fragment the landscape, the leakage of heavy metals, and the destruction of limestone karsts, which represent global endemicity hotspots. Reservoir construction, wetland drainage, fires, pollution, invasive species, disease, and finally climate change are also considered. Once each issue has been discussed, the overall prognosis of regional biodiversity and priority actions to protect SE Asian biodiversity in the future is discussed.

https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1002/ecs2.1624

Understanding the drivers of Southeast Asian biodiversity loss

Despite recent progress in understanding mechanisms of tree species coexistence in tropical forests, a simple explanation for the even more extensive diversity of insects feeding on these plants has been missing. We compared folivorous insects from temperate and tropical trees to test the hypothesis that herbivore species coexistence in more diverse communities could reflect narrow host specificity relative to less diverse communities. Temperate and tropical tree species of comparable phylogenetic distribution supported similar numbers of folivorous insect species, 29.0 ± 2.2 and 23.5 ± 1.8 per 100 square meters of foliage, respectively. Host specificity did not differ significantly between community samples, indicating that food resources are not more finely partitioned among folivorous insects in tropical than in temperate forests. These findings suggest that the latitudinal gradient in insect species richness could be a direct function of plant diversity, which increased sevenfold from our temperate to tropical study sites.

There So Many Species of Herbivorous Insects in Tropical Rainforests

Much of the forest remaining in South East Asia has been selectively logged. The processes promoting species coexistence may be the key to the recovery and maintenance of diversity in these forests. One such process is the Janzen–Connell mechanism, where specialized natural enemies such as seed predators maintain diversity by inhibiting regeneration near conspecifics. In Neotropical forests, anthropogenic disturbance can disrupt the Janzen–Connell mechanism, but similar data are unavailable for South East Asia. We investigated the effects of conspecific density (two spatial scales) and distance from fruiting trees on seed and seedling survival of the canopy tree Parashorea malaanonan in unlogged and logged forests in Sabah, Malaysia. The production of mature seeds was higher in unlogged forest, perhaps because high adult densities facilitate pollination or satiate pre-dispersal predators. In both forest types, post-dispersal survival was reduced by small-scale (1 m2) conspecific density, but not by proximity to the nearest fruiting tree. Large-scale conspecific density (seeds per fruiting tree) reduced predation, probably by satiating predators. Higher seed production in unlogged forest, in combination with slightly higher survival, meant that recruitment was almost entirely limited to unlogged forest. Thus, while logging might not affect the Janzen–Connell mechanism at this site, it may influence the recruitment of particular species.

https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2011.0034

Impacts of logging on density-dependent predation of dipterocarp seeds in a South East Asian rainforest

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Impacts of logging on density-dependent predation of dipterocarp seeds in a South East Asian rainforest