Patch Size and Isolation Predict Plant Species Density in a Naturally Fragmented Forest

<div><p>Studies of the effects of patch size and isolation on plant species density have yielded contrasting results. However, much of the available evidence comes from relatively recent anthropogenic forest fragments which have not reached equilibrium between extinction and immigration. This is a critical issue because the theory clearly states that only when equilibrium has been reached can the number of species be accurately predicted by habitat size and isolation. Therefore, species density could be better predicted by patch size and isolation in an ecosystem that has been fragmented for a very long time. We tested whether patch area, isolation and other spatial variables explain variation among forest patches in plant species density in an ecosystem where the forest has been naturally fragmented for long periods of time on a geological scale. Our main predictions were that plant species density will be positively correlated with patch size, and negatively correlated with isolation (distance to the nearest patch, connectivity, and distance to the continuous forest). We surveyed the vascular flora (except lianas and epiphytes) of 19 forest patches using five belt transects (50×4 m each) per patch (area sampled per patch = 0.1 ha). As predicted, plant species density was positively associated (logarithmically) with patch size and negatively associated (linearly) with patch isolation (distance to the nearest patch). Other spatial variables such as patch elevation and perimeter, did not explain among-patch variability in plant species density. The power of patch area and isolation as predictors of plant species density was moderate (together they explain 43% of the variation), however, a larger sample size may improve the explanatory power of these variables. Patch size and isolation may be suitable predictors of long-term plant species density in terrestrial ecosystems that are naturally and anthropogenically fragmented.</p></div

Log linear models proposed to explain the variation in plant species density (S) in 19 forest patches on the Yucatan Peninsula.

<p>Log linear models proposed to explain the variation in plant species density (S) in 19 forest patches on the Yucatan Peninsula.</p

Relationship between plant species density (number of species in 0.1 ha) and patch size (Size) (A) and between plant species density and the distance to the nearest patch (B) for a group of 19 forest patches on the Yucatan Peninsula.

<p>Regression lines for A and B are also shown. The scale on the “x” axis for A is logarithmic.</p

Number of species observed in 0.1 ha samples (S) and predicted number of plant species using three nonparametric estimators (Chao1, ACE and Bootstrap) for 19 forest patches on the Yucatan Peninsula.

<p>Number of species observed in 0.1 ha samples (S) and predicted number of plant species using three nonparametric estimators (Chao1, ACE and Bootstrap) for 19 forest patches on the Yucatan Peninsula.</p

The Effect of Pollen Source vs. Flower Type on Progeny Performance and Seed Predation under Contrasting Light Environments in a Cleistogamous Herb

<div><p>Dimorphic cleistogamy is a specialized form of mixed mating system where a single plant produces both open, potentially outcrossed chasmogamous (CH) and closed, obligately self-pollinated cleistogamous (CL) flowers. Typically, CH flowers and seeds are bigger and energetically more costly than those of CL. Although the effects of inbreeding and floral dimorphism are critical to understanding the evolution and maintenance of cleistogamy, these effects have been repeatedly confounded. In an attempt to separate these effects, we compared the performance of progeny derived from the two floral morphs while controlling for the source of pollen. That is, flower type and pollen source effects were assessed by comparing the performance of progeny derived from selfed CH vs. CL and outcrossed CH vs. selfed CH flowers, respectively. The experiment was carried out with the herb <i>Ruellia nudiflora</i> under two contrasting light environments. Outcrossed progeny generally performed better than selfed progeny. However, inbreeding depression ranges from low (1%) to moderate (36%), with the greatest value detected under shaded conditions when cumulative fitness was used. Although flower type generally had less of an effect on progeny performance than pollen source did, the progeny derived from selfed CH flowers largely outperformed the progeny from CL flowers, but only under shaded conditions and when cumulative fitness was taken into account. On the other hand, the source of pollen and flower type influenced seed predation, with selfed CH progeny the most heavily attacked by predators. Therefore, the effects of pollen source and flower type are environment-dependant and seed predators may increase the genetic differences between progeny derived from CH and CL flowers. Inbreeding depression alone cannot account for the maintenance of a mixed mating system in <i>R. nudiflora</i> and other unidentified mechanisms must thus be involved.</p> </div

Abundance herbivores of tropical trees, effects of diversity

Data is collected in the field,on the first column Species diversity with two levels monoculture and polyculture. Second column Genotypic diversity with two levels one maternal family and four maternal families. Rest of columns Ants, Sap feeders, leaf chewers (abundance) , H. grandella y P. meliacella (attacks as proxy of abundance

Abundance of sap-feeding generalist herbivores on mahogany (<i>Swietenia macrophylla</i>) plants with artificially reduced or ambient (control) ant abundances, across two levels of tree species diversity (mahogany monocultures and polycultures of four tree species, including mahogany), in a tree diversity experiment in southern Mexico.

<p>Values are means ± S.E. We found negative effects of ants on sap feeders at low diversity (F_{1, 17} = 5.7, P = 0.01), but not at high diversity (F_{1, 17} = 0.63, P = 0.42), whereas effects of diversity were significant under ambient (control) ant densities (F_{1, 17} = 10.23, P = 0.001) but not under ant-reduced conditions (F_{1, 17} = 1.52, P = 0.21). Statistics are based upon a posteriori contrasts.</p

Effects of mahogany (<i>Swietenia macrophylla</i>) genotypic diversity and tree species diversity on the abundance of ants (A, B), generalist leaf-chewing herbivores (C, D), generalist sap-feeding herbivores (E, F), specialist stem-boring herbivores (<i>Hypsipyla grandella</i>) (G, H), and specialist leaf-mining herbivores (<i>Phyllocnistis meliacella</i>) (I, J) on mahogany saplings in a tree diversity experiment in southern Mexico (bars are means ± S.E.).

<p>Values are means ± S.E. *P<0.05, **P<0.01, ***P<0.001, n.s. = not significant. G1 = plots with one mahogany maternal family, G4 = plots with four mahogany maternal families; M = monocultures of mahogany, P = polycultures of four species (including mahogany).</p

Results from generalized linear mixed models testing for the effects of an ant reduction treatment (A), tree species diversity (SD), mahogany (<i>Swietenia macrophylla</i>) genotypic diversity (GD), and their interactions on the abundance of generalist (G) and specialist (S) herbivores on mahogany.

<p>The model for ant abundance tested for the effects of SD, GD, and their interaction, but did not test for the ant reduction treatment or its interactions with SD or GD as it only considered control (ambient ant abundances) plants. All models include survey and plot as random effects. Significant effects (P < 0.05) are in bold.</p