Theoretical models predict that effects of dispersal on local biodiversity are influenced by the size and composition of the species pool, as well as ecological filters that limit local species ...membership. We tested these predictions by conducting a meta-analysis of 28 studies encompassing 62 experiments examining effects of propagule supply (seed arrival) on plant species richness under contrasting intensities of ecological filters (owing to disturbance and resource availability). Seed arrival increased local species richness in a wide range of communities (forest, grassland, montane, savanna, wetland), resulting in a positive mean effect size across experiments. Mean effect size was 70% higher in disturbed relative to undisturbed communities, suggesting that disturbance increases recruitment opportunities for immigrating species. In contrast, effect size was not significantly influenced by nutrient or water availability. Among seed-addition experiments, effect size was positively correlated with species and functional diversity within the pool of added seeds (species evenness and seed-size diversity), primarily in disturbed communities. Our analysis provides experimental support for the general hypothesis that species pools and local environmental heterogeneity interactively structure plant communities. We highlight empirical gaps that can be addressed by future experiments and discuss implications for community assembly, species coexistence, and the maintenance of biodiversity.
A variety of ecological processes influence diversity and species composition in natural communities. Most of these processes, whether abiotic or biotic, differentially filter individuals from birth ...to death, thereby altering species’ relative abundances. Nonrandom outcomes could accrue throughout ontogeny, or the processes that generate them could be particularly influential at certain stages. One long-standing paradigm in tropical forest ecology holds that patterns of relative abundance among mature trees are largely set by processes operating at the earliest life cycle stages. Several studies confirm filtering processes at some stages, but the longevity of large trees makes a rigorous comparison across size classes impossible without long-term demographic data. Here, we use one of the world’s longest-running, plot-based forest dynamics projects to compare nonrandom outcomes across stage classes. We considered a cohort of 7,977 individuals in 186 species that were alive in 1971 and monitored in 13 mortality censuses over 42 y to 2013. Nonrandom mortality with respect to species identity occurred more often in the smaller rather than the larger size classes. Furthermore, observed nonrandom mortality in the smaller size classes had a diversifying influence; species richness of the survivors was up to 30% greater than expected in the two smallest size classes, but not greater than expected in the larger size classes. These results highlight the importance of early life cycle stages in tropical forest community dynamics. More generally, they add to an accumulating body of evidence for the importance of early-stage nonrandom outcomes to community structure in marine and terrestrial environments.
Significance A long-standing paradigm in forest ecology holds that patterns of relative abundance among a forest’s mature trees are largely set by biotic and abiotic processes that operate most strongly at the earliest life-cycle stages. Although support for this model is implicit in the vast body of research on tropical seedling dynamics, the paradigm has never formally been tested in any forest. Using multidecadal demographic data, we present quantitative evidence for the primacy of early-stage, nonrandom dynamics in forest ecology. This finding corroborates the importance that tropical ecologists have traditionally assigned to early-stage dynamics, and calls for greater and more widespread inclusion of the smallest size classes into large, plot-based tropical forest dynamics projects worldwide.
Edge effects are major drivers of change in many fragmented landscapes, but are often highly variable in space and time. Here we assess variability in edge effects altering Amazon forest dynamics, ...plant community composition, invading species, and carbon storage, in the world's largest and longest-running experimental study of habitat fragmentation. Despite detailed knowledge of local landscape conditions, spatial variability in edge effects was only partially foreseeable: relatively predictable effects were caused by the differing proximity of plots to forest edge and varying matrix vegetation, but windstorms generated much random variability. Temporal variability in edge phenomena was also only partially predictable: forest dynamics varied somewhat with fragment age, but also fluctuated markedly over time, evidently because of sporadic droughts and windstorms. Given the acute sensitivity of habitat fragments to local landscape and weather dynamics, we predict that fragments within the same landscape will tend to converge in species composition, whereas those in different landscapes will diverge in composition. This 'landscape-divergence hypothesis', if generally valid, will have key implications for biodiversity-conservation strategies and for understanding the dynamics of fragmented ecosystems.
Pollen placement on petals Harms, Kyle E
Frontiers in ecology and the environment,
March 2020, 2020-03-00, 20200301, Volume:
18, Issue:
2
Journal Article
Peer reviewed
Open access
Many flowers attract pollinators with traits that stimulate animals' visual, olfactory, acoustic, gustatory, or tactile senses. Attractiveness often changes as floral traits develop, at times owing ...to the activities of interacting animals (eg floral herbivory, nectar robbery, pollen deposition or removal, etc). Hibiscus aculeatus is one such flower. Various events could cause pollen to appear on these petals, including wind, visits by pollinators, or anything else that shakes the flower. If starkly contrasting pollen on petals makes flowers more attractive to pollinators, the sensitivity of pollen release may be adaptive. Alternatively, if pollen is deposited on petals by visitors that have already removed some nectar, then freckled petals may dissuade subsequent pollinators seeking virgin flowers.
A trade-off between growth and mortality rates characterizes tree species in closed canopy forests. This trade-off is maintained by inherent differences among species and spatial variation in light ...availability caused by canopy-opening disturbances. We evaluated conditions under which the trade-off is expressed and relationships with four key functional traits for 103 tree species from Barro Colorado Island, Panama. The trade-off is strongest for saplings for growth rates of the fastest growing individuals and mortality rates of the slowest growing individuals (
r
2
= 0.69), intermediate for saplings for average growth rates and overall mortality rates (
r
2
= 0.46), and much weaker for large trees (
r
2
≤ 0.10). This parallels likely levels of spatial variation in light availability, which is greatest for fast- vs. slow-growing saplings and least for large trees with foliage in the forest canopy. Inherent attributes of species contributing to the trade-off include abilities to disperse, acquire resources, grow rapidly, and tolerate shade and other stresses. There is growing interest in the possibility that functional traits might provide insight into such ecological differences and a growing consensus that seed mass (SM), leaf mass per area (LMA), wood density (WD), and maximum height (
H
max
) are key traits among forest trees. Seed mass, LMA, WD, and
H
max
are predicted to be small for light-demanding species with rapid growth and mortality and large for shade-tolerant species with slow growth and mortality. Six of these trait-demographic rate predictions were realized for saplings; however, with the exception of WD, the relationships were weak (
r
2
< 0.1 for three and
r
2
< 0.2 for five of the six remaining relationships). The four traits together explained 43-44% of interspecific variation in species positions on the growth-mortality trade-off; however, WD alone accounted for >80% of the explained variation and, after WD was included, LMA and
H
max
made insignificant contributions. Virtually the full range of values of SM, LMA, and
H
max
occurred at all positions on the growth-mortality trade-off. Although WD provides a promising start, a successful trait-based ecology of tropical forest trees will require consideration of additional traits.
We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate ...potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 ×× 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-–10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (
P
== 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.
We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level ...analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.
Two prominent mechanisms proposed to structure biodiversity are niche-based ecological filtering and chance arrival of propagules from the species pool. Seed arrival is hypothesized to play a ...particularly strong role in high-diversity plant communities with large potential species pools and many rare species, but few studies have explored how seed arrival and local ecological filters interactively assemble species-rich communities in space and time. We experimentally manipulated seed arrival and multiple ecological filters in high-diversity, herbaceous-dominated groundcover communities in longleaf pine savannas, which contain the highest small-scale species richness in North America (up to >40 species/m
2
). We tested three hypotheses: (1) local communities constitute relatively open-membership assemblages, in which increased seed arrival from the species pool strongly increases species richness; (2) ecological filters imposed by local fire intensity and soil moisture influence recruitment and richness of immigrating species; and (3) ecological filters increase similarity in the composition of immigrating species. In a two-year factorial field experiment, we manipulated local fire intensity by increasing pre-fire fuel loads, soil moisture using rain shelters and irrigation, and seed arrival by adding seeds from the local species pool. Seed arrival increased species richness regardless of fire intensity and soil moisture but interacted with both ecological filters to influence community assembly. High-intensity fire decreased richness of resident species, suggesting an important abiotic filter. In contrast, high-intensity fire increased recruitment and richness of immigrating species, presumably by decreasing effects of other ecological filters (competition and resource limitation) in postfire environments. Drought decreased recruitment and richness of immigrating species, whereas wet soil conditions increased recruitment but decreased or had little effect on richness. Moreover, some ecological filters (wet soil conditions and, to a lesser extent, high-intensity fire) increased similarity in the composition of immigrating species, illustrating conditions that influence deterministic community assembly in species-rich communities. Our experiment provides insights into how dispersal-assembly mechanisms may interact with niche-assembly mechanisms in space (spatial variation in disturbance) and time (temporal variation in resource availability) to structure high-diversity communities and can help guide conservation of threatened longleaf pine ecosystems in the face of habitat fragmentation and environmental change.
1. Nutrients are a critical resource for plant growth, but the elements limiting growth in tropical forests have rarely been determined. 2. We investigated the influence of nitrogen (N), phosphorus ...(P), potassium (K) and micronutrients on seedling biomass and nutrient allocation in a factorial nutrient fertilization experiment in lowland tropical forest at the Barro Colorado Nature Monument, Panama. We also measured 8 years of herbivory and growth for 1800 seedlings. We sought to determine the identity of limiting elements and possible nutrient interactions. 3. The five study species were Alseis blackiana, Desmopsis panamensis, Heisteria concinna, Sorocea affinis and Tetragastris panamensis. Plants grew in deeply shaded understorey with a mean canopy openness of 4.9% (±0.7%; 1 SE). 4. Tissue N concentration increased by 11% with N addition. Tissue P concentration increased by 16% with P addition. Tissue K increased by 4% with K addition. K addition reduced root‐to‐shoot biomass ratio. There was no significant effect of fertilization on specific leaf area or leaf area ratio. 5. The proportion of leaves damaged and the mean level of damage by herbivory increased with P and K addition and showed a significant P × K interaction. 6.7ensp;Across all species and years, relative growth rate of height increased with K addition and with N and P in combination. Relative growth rate of leaf count trended 8.5% higher with K addition (P = 0.076). 7. We also added micronutrients in a parallel experiment. There was no effect of micronutrient addition on any seedling parameter. 8. Synthesis. K addition affected seedlings by enhancing tissue nutrient concentration, increasing herbivory, reducing root‐to‐shoot biomass ratio and increasing height growth. Additional effects of N or P on tissue chemistry, herbivory and growth offer support for the multiple limiting resources hypothesis. Our results suggest that seedling growth is limited by nutrients, especially K, even under highly shaded conditions in this lowland tropical forest.
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