Numerous studies have shown reduced performance in plants that are surrounded by neighbours of the same species1,2, a phenomenon known as conspecific negative density dependence (CNDD)3. A long-held ecological hypothesis posits that CNDD is more pronounced in tropical than in temperate forests4,5, which increases community stabilization, species coexistence and the diversity of local tree species6,7. Previous analyses supporting such a latitudinal gradient in CNDD8,9have suffered from methodological limitations related to the use of static data10–12. Here we present a comprehensive assessment of latitudinal CNDD patterns using dynamic mortality data to estimate species-site-specific CNDD across 23 sites. Averaged across species, we found that stabilizing CNDD was present at all except one site, but that average stabilizing CNDD was not stronger toward the tropics. However, in tropical tree communities, rare and intermediate abundant species experienced stronger stabilizing CNDD than did common species. This pattern was absent in temperate forests, which suggests that CNDD influences species abundances more strongly in tropical forests than it does in temperate ones13. We also found that interspecific variation in CNDD, which might attenuate its stabilizing effect on species diversity14,15, was high but not significantly different across latitudes. Although the consequences of these patterns for latitudinal diversity gradients are difficult to evaluate, we speculate that a more effective regulation of population abundances could translate into greater stabilization of tropical tree communities and thus contribute to the high local diversity of tropical forests.
Intra‐specific negative density dependence promotes species coexistence by regulating population sizes. Patterns consistent with such density dependence are frequently reported in diverse tropical tree communities. Empirical evidence demonstrating whether intra‐specific variation is related to these patterns, however, is lacking. The present study addresses this important knowledge gap by genotyping all individuals of a tropical tree in a long‐term forest dynamics plot in tropical China. We show that related individuals are often spatially clustered, but having closely related neighbours reduces the growth performance of focal trees. We infer from the evidence that dispersal limitation and negative density dependence are operating simultaneously to impact the spatial distributions of genotypes in a natural population. Furthermore, dispersal limitation decreases local intra‐specific genetic diversity and increases negative density dependence thereby promoting niche differences and species coexistence as predicted by theory.
more » « less- NSF-PAR ID:
- 10064034
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Ecology Letters
- Volume:
- 21
- Issue:
- 8
- ISSN:
- 1461-023X
- Page Range / eLocation ID:
- p. 1174-1181
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Aim The “sexy shrimp”
Thor amboinensis is currently considered a single circumtropical species. However, the tropical oceans are partitioned by hard and soft barriers to dispersal, providing ample opportunity for allopatric speciation. Herein, we test the null hypothesis thatT. amboinensis is a single global species, reconstruct its global biogeographical history, and comment on population‐level patterns throughout the Tropical Western Atlantic.Location Coral reefs in all tropical oceans.
Methods Specimens of
Thor amboinensis were obtained through field collection and museum holdings. We used one mitochondrial (COI ) and two nuclear (NaK, enolase) gene fragments for global species delimitation and phylogenetic analyses (n = 83 individuals, 30 sample localities), while phylogeographical reconstruction in theTWA was based onCOI only (n = 303 individuals, 10 sample localities).Results We found evidence for at least five cryptic lineages (9%–22%
COI pairwise sequence divergence): four in the Indo‐West Pacific and one in the Tropical Western Atlantic. Phylogenetic reconstruction revealed that endemic lineages from Japan and the South Central Pacific are more closely related to the Tropical Western Atlantic lineage than to a co‐occurring lineage that is widespread throughout the Indo‐West Pacific. Concatenated and species tree phylogenetic analyses differ in the placement of an endemic Red Sea lineage and suggest alternate dispersal pathways into the Atlantic. Phylogeographical reconstruction throughout the Tropical Western Atlantic reveals little genetic structure over more than 3,000 km.Main conclusions Thor amboinensis is a species complex that has undergone a series of allopatric speciation events and whose members are in secondary contact in the Indo‐West Pacific. Nuclear‐ and mitochondrial‐ gene phylogenies show evidence of introgression between lineages inferred to have been separated more than 20 Ma. Phylogenetic discordance between multi‐locus analyses suggest thatT. amboinensis originated in the Tethys sea and dispersed into the Atlantic and Indo‐West Pacific through the Tethys seaway or, alternatively, originated in the Indo‐West Pacific and dispersed into the Atlantic around South Africa. Population‐level patterns in the Caribbean indicate extensive gene flow across the region. -
Yavitt, Joseph B. (Ed.)Conspecific negative density dependence (CNDD) promotes tree species diversity by reducing recruitment near conspecific adults due to biotic feedbacks from herbivores, pathogens, or competitors. While this process is well-described in tropical forests, tests of temperate tree species range from strong positive to strong negative density dependence. To explain this, several studies have suggested that tree species traits may help predict the strength and direction of density dependence: for example, ectomycorrhizal-associated tree species typically exhibit either positive or weaker negative conspecific density dependence. More generally, the strength of density dependence may be predictably related to other species-specific ecological attributes such as shade tolerance, or the relative local abundance of a species. To test the strength of density dependence and whether it affects seedling community diversity in a temperate forest, we tracked the survival of seedlings of three ectomycorrhizal-associated species experimentally planted beneath conspecific and heterospecific adults on the Prospect Hill tract of the Harvard Forest, in Massachusetts, USA. Experimental seedling survival was always lower under conspecific adults, which increased seedling community diversity in one of six treatments. We compared these results to evidence of CNDD from observed sapling survival patterns of 28 species over approximately 8 years in an adjacent 35-ha forest plot. We tested whether species-specific estimates of CNDD were associated with mycorrhizal association, shade tolerance, and local abundance. We found evidence of significant, negative conspecific density dependence (CNDD) in 23 of 28 species, and positive conspecific density dependence in two species. Contrary to our expectations, ectomycorrhizal-associated species generally exhibited stronger (e.g., more negative) CNDD than arbuscular mycorrhizal-associated species. CNDD was also stronger in more shade-tolerant species but was not associated with local abundance. Conspecific adult trees often have a negative influence on seedling survival in temperate forests, particularly for tree species with certain traits. Here we found strong experimental and observational evidence that ectomycorrhizal-associating species consistently exhibit CNDD. Moreover, similarities in the relative strength of density dependence from experiments and observations of sapling mortality suggest a mechanistic link between negative effects of conspecific adults on seedling and sapling survival and local tree species distributions.more » « less
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Abstract Many studies identify fungal and oomycete phytopathogens as natural enemies capable of influencing plant species composition and promoting diversity in plant communities. However, little is known about how plant‐pathogen interactions vary along regional abiotic gradients or with tree species characteristics, which limits our understanding of the causes of variation in tree species richness.
We surveyed 10,756 seedlings from 272 tree species for disease symptoms along a mean annual precipitation gradient in the tropical wet forests of Central Panama for 3 months in the early wet season (June–August) and 2 months in the following dry season (March–April). Over 99% of observed disease symptoms were caused by necrotrophic foliar pathogens, while less than 1% of symptoms were attributed to soilborne pathogens. Foliar disease incidence was inversely related to mean annual precipitation, a pattern which may be due to greater disease susceptibility among dry forest species.
Foliar disease incidence increased with conspecific seedling density but did not respond to the proximity of conspecific adults. Although foliar disease incidence decreased as mean annual precipitation increased, the strength of conspecific density‐ or distance‐dependence was independent of the precipitation gradient.
Seedlings of common tree species and species dispersed by non‐flying mammals had a higher risk of foliar pathogen incidence. Increased disease in common species may help reduce their dominance.
Synthesis . The increases in foliar pathogen incidence with conspecific seedling density, species abundance, and dispersal mechanism indicate that foliar disease incidence is non‐random and may contribute to the regulation of tropical plant communities and species coexistence. Furthermore, the relationships between foliar disease incidence, dispersal mechanism and precipitation suggest plant‐pathogen interactions could shift as a response to climate change and disruption of the disperser community. -
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Abstract One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure.
