Modelling secondary succession of neotropical mangroves: Causes and consequences of growth reduction in pioneer species

Mangrove forest structure is the result of interactions between species responses to abiotic stress factors, disturbance, dispersal and competition. The combination of abiotic conditions and disturbance history may determine the growth potential of the species, whereas dispersal, competition and external or biogenic changes in abiotic conditions may tune their succession. Even in forests with only a few species, this set of factors can create multiple species compositions. For example, pure stands of the mangrove Laguncularia are common in newly colonized areas, but they may evolve into mixed stands dominated by Rhizophora or Avicennia. We demonstrate such an evolution with a field study of mangroves established in abandoned rice fields. We use an individual-based simulation model to understand how species-dependent dispersal, growth rate and shade tolerance lead to the observed patterns in horizontal and vertical forest structure. We find that the initially dominating species will be gradually replaced in the canopy. This alteration cannot be explained exclusively by shade tolerance of the succeeding species. Rather, it is essential to assume that the height growth rate of the pioneer species slows down relative to those of the later species. A decrease in nutrient availability may be responsible for these relative changes in the growth rates of different species.     

Asymmetric competition as a natural outcome of neighbour interactions among plants: results from the field-of-neighbourhood modelling approach

Numerous attempts have been made to infer the mode of competition from size or biomass distributions of plant cohorts. However, since the relationship between mode of competition and size distributions may be obscured by a variety of factors such as spatial configuration, density or resource level, empirical investigations often produce ambiguous results. Likewise, the findings of theoretical analyses of asymmetric competition are equivocal. In this paper, we analyse the mode of competition in an individual-based model which is based on the new field-of-neighbourhood approach. In this approach, plants have a zone of influence that determines the distance up to which neighbours are influenced. Additionally, a superimposed field within the zone of influence defines phenomenologically the strength of influence of an individual on neighbouring plants. We investigated competition at both individual and population level and characterised the influence of density and of the shape of the field-of-neighbourhood on occurrence and degree of competitive asymmetry. After finding asymmetric competition emerging in all scenarios, we argue that asymmetric competition is a natural consequence of local competition among neighbouring plants.     

The evolution of social philopatry in female primates

The transition from solitary life to sociality is considered one of the major transitions in evolution. In primates, this transition is currently not well understood. Traditional verbal models appear insufficient to unravel the complex interplay of environmental and demographic factors involved in the evolution of primate sociality, and recent phylogenetic reconstructions have produced conflicting results. We therefore analyze a theoretical model for the evolution of female social philopatry that sheds new light on the question why most primates live in groups. In individual-based simulations, we study the evolution of dispersal strategies of both resident females and their offspring. The model reveals that social philopatry can evolve through kin selection, even if retention of offspring is costly in terms of within-group resource competition and provides no direct benefits. Our model supports the role of predator avoidance as a selective pressure for group-living in primates, but it also suggests that a second benefit of group-living, communal resource defense, might be required to trigger the evolution of sizable groups. Lastly, our model reveals that seemingly small differences in demographic parameters can have profound effects on primate social evolution.     

Uptake of asialoglycophorin-liposomes by the perfused rat liver

Asialoglycophorin-containing liposomes and their contents (¹²⁵I-labeled bovine serum albumin) were taken up by a perfused rat liver with subsequent digestion of their protein components. The uptake of these liposomes required Ca²⁺ as well as desialylation. The process was inhibited partially by asialofetuin and completely by further addition of asialoglycophorin to the perfusate.     

Uptake of asialo-glycophorin by the perfused rat liver and isolated hepatocytes

Perfused rat livers took up asialo-glycophorin, a glycoprotein derived from human erythrocyte membraneds, with a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for the clearance of 7 min. As a comparison, asialo-orosomucoid was taken up by this system with a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 3.5 min. Both proteins were digested and their ¹²⁵I labels were released to the perfusate as free ¹²⁵I^?. EGTA completely inhibited uptake of these glycoproteins, but not uptake of denatured bovine serum albumin. Addition of Ca²⁺ reversed the inhibition nearly completely. Isolated hepatocytes had an uptake rate of approximately 3 ng/min per 10⁶ cells for the asialo forms of glycophorin, orosomucoid and fetuin. Cellular uptake of each of these asialoglycoproteins could be inhibited by one of the other proteins. Asialo-fetuin caused a 95% inhibition of the uptake rate of asialo-orosomucoid by the perfused liver. This fetal calf glycoprotein had a similar inhibitory effect on asialo-glycophorin, but only after an initial 40% of the asialo-glycophorin had been taken up by the liver at an almost normal rate during the first 30 min of perfusion. The possiblity of an alternative hepatic removal system for asialo-glycophorin is suggested.     

Uptake of asialo-glycophorin by the perfused rat liver and isolated hepatocytes.

Perfused rat livers took up asialo-glycophorin, a glycoprotein derived from human erythrocyte membranes, with a t1/2 for clearance of 7 min. As a comparison, asialo-orosomucoid was taken up by this system with a t1/2 of 3.5 min. Both proiteins were digested and their 125I labels were released to the perfusate as free 125I-. EGTA completely inhibited uptake of these glycoproteins, but not uptake of denatured bovine serum albumin. Addition of Ca2+ reversed the inhibition nearly completely. Isolated hepatocytes had an uptake rate of approximately 3 ng/min per 10(6) cells for the asialo forms of glycophorin, orosomucoid and fetuin. Cellular uptake of each of these asialoglycoproteins could be inhibited by one of the other proteins. Asialo-fetuin caused a 95% inhibition of the uptake rate of asialo-orosomucoid by the perfused liver. This fetal calf glycoprotein had a similar inhibitory effect on asialo-glycophorin, but only after an initial 40% of the asialo-glycophorin had been taken up by the liver at an almost normal rate during the first 30 min of perfsuion. The possibility of an alternative hepatic removal system for asialo-glycophorin is suggested.     

Observations on the (Ca2+ plus Mg2+)-ATPase activator found in various mammalian erythrocytes.

1. A soluble activator of membrane (Ca2+ plus Mg2)-ATPase is present in hemolysates of the newborn calf and cow, the new born and adult pig as well as human erythrocytes. 2. The activator is also found in reticulocytes of the adult pig. 3. The activator obtained from any of the above species is capable of stimulating the membrane (Ca2+ plus Mg2+)-ATPases of the other species, regardless of the age of the animals. 4. The results obtained from density fractionation of human erythrocytes revealed that the soluble factor has little simulatory effect on membranes of young erythrocytes from which it is derived but caused a marked stimulation on (Ca2+ plus Mg2+)-ATPase activity of the intermediate aged and old erythrocyte membranes. 5. The above observations support the following conclusions: (a) the extremely low levels of (Ca2+ plus Mg2+)-ATPase in cow erythrocytes is not due to the lack of a (Ca2+ plus Mg2+)-ATPase activator; (b) the distribution of (Ca2+ plus Mg2+)-atpase activator is not species specific and the differences in the level of membrane (Ca2+ plus Mg2+)-ATPase activity in various species of cells is an inherent property of that particular membrane (c) the (Ca2+ plus Mg2+)-ATPase activator is present at least from the time of reticulocyte formation and remain during tthe life span of the erythrocyte.     

Self-organization of collective escape in pigeon flocks

Bird flocks under predation demonstrate complex patterns of collective escape. These patterns may emerge by self-organization from local interactions among group-members. Computational models have been shown to be valuable for identifying what behavioral rules may govern such interactions among individuals during collective motion. However, our knowledge of such rules for collective escape is limited by the lack of quantitative data on bird flocks under predation in the field. In the present study, we analyze the first GPS trajectories of pigeons in airborne flocks attacked by a robotic falcon in order to build a species-specific model of collective escape. We use our model to examine a recently identified distance-dependent pattern of collective behavior: the closer the prey is to the predator, the higher the frequency with which flock members turn away from it. We first extract from the empirical data of pigeon flocks the characteristics of their shape and internal structure (bearing angle and distance to nearest neighbors). Combining these with information on their coordination from the literature, we build an agent-based model adjusted to pigeons’ collective escape. We show that the pattern of turning away from the predator with increased frequency when the predator is closer arises without prey prioritizing escape when the predator is near. Instead, it emerges through self-organization from a behavioral rule to avoid the predator independently of their distance to it. During this self-organization process, we show how flock members increase their consensus over which direction to escape and turn collectively as the predator gets closer. Our results suggest that coordination among flock members, combined with simple escape rules, reduces the cognitive costs of tracking the predator while flocking. Such escape rules that are independent of the distance to the predator can now be investigated in other species. Our study showcases the important role of computational models in the interpretation of empirical findings of collective behavior.     

Some causes of the variable shape of flocks of birds

Flocks of birds are highly variable in shape in all contexts (while travelling, avoiding predation, wheeling above the roost). Particularly amazing in this respect are the aerial displays of huge flocks of starlings (Sturnus vulgaris) above the sleeping site at dawn. The causes of this variability are hardly known, however. Here we hypothesise that variability of shape increases when there are larger local differences in movement behaviour in the flock. We investigate this hypothesis with the help of a model of the self-organisation of travelling groups, called StarDisplay, since such a model has also increased our understanding of what causes the oblong shape of schools of fish. The flocking patterns in the model prove to resemble those of real birds, in particular of starlings and rock doves. As to shape, we measure the relative proportions of the flock in several ways, which either depend on the direction of movement or do not. We confirm that flock shape is usually more variable when local differences in movement in the flock are larger. This happens when a) flock size is larger, b) interacting partners are fewer, c) the flock turnings are stronger, and d) individuals roll into the turn. In contrast to our expectations, when variability of speed in the flock is higher, flock shape and the positions of members in the flock are more static. We explain this and indicate the adaptive value of low variability of speed and spatial restriction of interaction and develop testable hypotheses.