Search

Granivore ant foraging should respond to seed density through changes in individual and collective behaviour. Overall seed availability and the presence of multiple types of seeds in a patch may affect these behaviours. We presented ants with three species of seeds in different mixtures and in two seasons contrasting in seed availability to assess differences in probabilities of seed collection. Removal rates were recorded every 2 h. We fitted a model that uses data on seed removal to disentangle the individual (probability that an ant removes a seed of species i, pi) and collective (number of active ants h) components of foraging. We found that removal increased with seed density due to increased pi values and recruitment of new ants. This suggests that ants evaluated resource density, collecting seeds only from dense patches. This was confirmed by observations. The presence of additional seeds of other species in the seed mixture elicited recruitment but had nearly no effect on pi. This may be observed if different ants prefer a seed species and ignore the rest. In the low seed-availability season, h increased, ants travelled longer distances, and foraging was directed to the most profitable seed-patches. This selectivity for some seed-patches during times of scarcity may appear surprising. It is true that, by abandoning low-density patches, individuals may reduce their seed acquisition rates, but also increase collective gains by forming trails only to dense patches. This may compensate for the large collective investments in the form of more scouts that explore longer distances.

Plant-soil feedback (PSF) occurs when plants change the biota and physicochemical properties of the soil, and these changes affect future survival or growth of plants. PSF depends on several factors such as plant functional attributes (e.g., life cycle or photosynthetic metabolism) and the environment. PSF often turn positive under dry conditions because soil biota confers drought tolerance. Conspecifics and close relatives share pathogens and consume similar resources, exerting negative PSF on each other. These ideas have mostly been tested under controlled conditions, while field studies remain scarce. To reevaluate these findings in nature, we analyzed plant-soil feedbacks over a drought-stress gradient in a phosphorus-limited semiarid grassland. We planted seedlings of 17 species in plots where community composition had been monitored for six years. To determine PSF intensity, we measured how seedling longevity was affected by previous occupancy of conspecifics and heterospecifics. The previous occupancy-survival relationship (OSR) was used as a proxy for PSF. Evidence for OSRs was found in one-third of the species pairs, with inconclusive evidence for the rest suggesting weak feedbacks. This is in line with the expectation that PSFs in the field are weaker than under controlled conditions. As expected, positive PSFs were more frequent as drought stress increased. The strongest OSRs were caused in dry plots by C4 perennial grasses, which had very positive OSRs on several C3 annual forbs, but negative effects on each other. Well-documented differences between these two functional groups may explain this result: C3 plants are more sensitive to drought, and thus may be favored by tolerance-conferring microbiota; in contrast, water-efficient C4 perennial grasses compete for phosphorus strongly, perhaps driving strong negative PSFs between them. Finally, close relatives had more negative OSRs on each other than on distant relatives as expected, although only in dry plots. This pattern was mostly due to the negative effects of closely related C4 grasses under dry conditions, and their positive effects on distantly related dicots. Our results highlight the importance of plant traits and of the environmental context in determining the direction and strength of PSFs under field conditions.

Facilitation by tussocks is common in high-altitude tropical environments. It is thought that facilitation results from stress amelioration, but it is unclear which of the many stressors acting in these environments is ameliorated. We aimed at determining the relative importance of different stressors as drivers of facilitation by the tussock Festuca tolucensis in Mexico. We manipulated five stressors in the field: minimum temperatures by using electric radiators that kept plants warm; maximum temperatures by means of reflective sand that precluded temperature build-up during the day; UV radiation by using screens opaque to UV; poor soil properties by comparing soils from beneath tussocks and from bare ground; and low water availability by adding vermiculite to the soil. The performance (survival and growth) of Mexerion sarmentosum (a plant usually associated with Festuca) in these treatments was compared to that recorded under tussocks and in bare ground. Amelioration of extreme temperatures had the largest positive effects on Mexerion survival. UV radiation and increased soil humidity did not affect survival, although humidity increased growth rates. Nevertheless, tussocks reduced the growth of Mexerion, which is consistent with observations of competition between plants and soil microorganisms favoured by tussocks. Our results highlight the importance of the extreme daily fluctuations in temperature that characterise tropical mountains as fundamental drivers of their dynamics.

Microsite limitation (ML) occurs when the number of suitable sites for development determines population size. Seed limitation (SL) arises when seeds cannot occupy all microsites. Both limitations have been measured on relative scales, but absolute values would allow analyzing the relationship between them and their contributions to population regulation. A tradeoff is expected because small-seeded species are prolific and thus could have low SL, whereas large seeds perform better under different conditions, lowering ML. We assessed whether there is such tradeoff, and if it is mediated by seed size. We measured the absolute magnitudes of limitations through seed addition experiments, and tested whether they determine population density in 14 herbs with different seed sizes in a semiarid grassland. All species were limited by microsites and seeds. The tradeoff occurred only during germination, but was seemingly not mediated by seed size. Annual species were more limited by microsites than perennials, which suffered more SL. Population densities were smaller for species with greater ML measured over the life cycle, but marginally so when ML was measured during germination. SL only reduced annual-plant density. These negative correlations indicate that our measurements correspond to actual limitations. Microsites limited populations depending on the process analyzed (germination, survival and whole life-cycle) and the life history of species, suggesting that an integral approach to ML is required rather than focusing only on germination. Comparing annuals and perennials, we found that the strongest limitation is the main determinant of population size. The tradeoff was consistent with seed size and competition–colonization tradeoff, but our results were inconclusive. The SL–ML tradeoff seems to arise from still-unidentified evolutionary constraints, and may contribute to the maintenance of species diversity, preventing any single species from becoming dominant and excluding its neighbors.