Mammalian body size is determined by interactions between climate, urbanization, and ecological traits

Anthropogenically-driven climate warming is a hypothesized driver of animal body size reductions. Less understood are effects of other human-caused disturbances on body size, such as urbanization. We compiled 140,499 body size records of over 100 North American mammals to test how climate and human population density, a proxy for urbanization, and their interactions with species traits, impact body size. We tested three hypotheses of body size variation across urbanization gradients: urban heat island effects, habitat fragmentation, and resource availability. Our results demonstrate that both urbanization and temperature influence mammalian body size variation, most often leading to larger individuals, thus supporting the resource availability hypothesis. In addition, life history and other ecological factors play a critical role in mediating the effects of climate and urbanization on body size. Larger mammals and species that utilize thermal buffering are more sensitive to warmer temperatures, while flexibility in activity time appears to be advantageous in urbanized areas. This work highlights the value of using digitized, natural history data to track how human disturbance drives morphological variation.

Resource Allocation Among Cladodes of Opuntia Robusta From East-central Mexico

The Optimal Defense Hypothesis (ODH) predicts that younger, more valuable plant organs should be better defended. We tested this hypothesis in Opuntia robusta Wendl. since its sequential, hierarchical and segmented architecture permits the consideration of a possible reallocation of secondary metabolites among cladodes with different age. We performed a field study taking samples of vegetative tissues from cladodes of different orders during eight months, and a field experiment, by covering either apical or basal cladodes with a fabric. We determined the content of 4-hydroxybenzoic acid (4-HBA), chlorogenic acid (CGA), quercetin (QUE) and salicylic acid (SA). 4-HBA and CGA followed the predictions of ODH. QUE followed an inverse tendency. SA did not show any tendency related either with the position of the cladode or the treatment (light vs. shade) however, its concentration was positively correlated with the concentration of 4-HBA. As we detected SA only in a low proportion of cladodes and 4-HBA in all cladodes, we hypothesize the conversion of the latter metabolite to the former one. Contrary to ODH, CGA presented lower concentrations in apical than in basal cladodes when co-occurred with SAL, and QUE was transferred from apical (younger) to basal (older) cladodes. In this study, we found contradictions in the premises and outcomes of both the Resource Availability Hypothesis (RAH) and the ODH, suggesting that a new hypothesis concerning the plant defense against stress factors should be proposed.

Mammalian body size is determined by interactions between climate, urbanization, and life history traits

Anthropogenically-driven climate warming is a hypothesized driver of animal body size reductions. Less understood are effects of other human-caused disturbances on body size, such as urbanization. We compiled 140,499 body size records of over 100 North American mammals to test how climate and urbanization, and their interactions with species traits, impact body size. We tested three hypotheses of body size change across urbanization gradients; urban heat island effects, fragmentation, and resource availability. Our results unexpectedly demonstrate urbanization is more tightly linked with body size changes than temperature, most often leading to larger individuals, thus supporting the resource availability hypothesis. In addition, life history traits, such as thermal buffering, activity time, and average body size play critical roles in mediating the effects of both climate and urbanization on intraspecific body size trends. This work highlights the value of using digitized, natural history data to track how human disturbance drives morphological change.

Context dependency of biotic interactions and its relation to plant rarity

AimBiotic interactions can determine rarity and commonness of species, however evidence that rare and common species respond differently to biotic stress is scarce. This is because biotic interactions are notoriously context-dependent and traits leading to success in one habitat might be costly or unimportant in another. We aim to identify plant characteristics that are related to biotic interactions and may drive patterns of rarity and commonness, taking environmental context into account.LocationSwitzerlandMethodsIn a multi-species experiment, we compared the response to biotic interactions of 19 rare and 21 widespread congeneric plant species in Switzerland, while also accounting for variation in environmental conditions of the species’ origin.ResultsOur results restrict the long-standing hypothesis that widespread species are superior competitors to rare species to only those species originating from resource rich habitats, in which competition is usually strong. Tolerance to herbivory and ambient herbivore damage on the other hand, did not differ between widespread and rare species. In accordance to the resource-availability hypothesis, widespread species from resource rich habitats where more damaged by herbivores (less defended) than widespread species from resource poor habitats – such a growth-defense tradeoff was lacking in rare species. This indicates that the evolutionary important tradeoff between traits increasing competitive-ability and defence is present in widespread species but may have been lost in rare species.Main conclusionsOur results indicate that biotic interactions, above all competition, might indeed set range limits, and underlines the importance of including context-dependency in studies comparing traits of common and rare or invasive and non-invasive species.

Habitat productivity is a poor predictor of body size in rodents

The “resource availability hypothesis” predicts occurrence of larger rodents in more productive habitats. This prediction was tested in a dataset of 1,301 rodent species. We used adult body mass as a measure of body size and normalized difference vegetation index (NDVI) as a measure of habitat productivity. We utilized a cross-species approach to investigate the association between these variables. This was done at both the order level (Rodentia) and at narrower taxonomic scales. We applied phylogenetic generalized least squares (PGLS) to correct for phylogenetic relationships. The relationship between body mas and NDVI was also investigated across rodent assemblages. We controlled for spatial autocorrelation using generalized least squares (GLS) analysis. The cross-species approach found extremely low support for the resource availability hypothesis. This was reflected by a weak positive association between body mass and NDVI at the order level. We find a positive association in only a minority of rodent subtaxa. The best fit GLS model detected no significant association between body mass and NDVI across assemblages. Thus, our results do not support the view that resource availability plays a major role in explaining geographic variation in rodent body size.

Parental feeding decisions in dunnocks from a resource availability perspective: assessing the role of nestling tongue spots and body size

A recent comparative study found that when resources are limited, parents primarily allocate food to superior nestlings, whereas when resources are abundant, parents evenly allocate food to all nestlings, putting forward what we refer to as the resource availability hypothesis (RAH). This ecological perspective adds valuable insights to the two classical hypotheses, sexual conflict and parent–offspring conflict, that explain the diversity of feeding patterns observed in nature. We investigated the parental allocation decisions between monogamous pairs and polyandrous trios of dunnocks. We first assessed whether the number of spots on the tongues of nestlings signals condition to the breeding parents. We then explored the RAH in an intra-specific context. We found evidence to suggest that the number of nestling tongue spots signals condition to breeding parents. We also found that monogamous and polyandrous females and polyandrous males prioritized nestlings in better condition, yet monogamous males fed nestlings evenly. This finding is consistent with the RAH in our population if monogamous males secure more resources due to their superior quality, thus resulting in the even feeding of nestlings. Another explanation for the observed parental feeding decisions that is indirectly associated with the RAH is sexual conflict (the uncertainty of paternity). Finally, we found that an assumption regarding feeding patterns in dunnocks that was established in another population was not fulfilled here; thus, it is worth noting that testing assumptions is a vital step towards assessing the generality of former findings, paving the way to credible scientific practice.

Tolerance to herbivory and the resource availability hypothesis

The resource availability hypothesis (RAH), the most successful theory explaining plant defence patterns, predicts that defence investment is related to the relative growth rate (RGR) of plant species, which is associated with habitat quality. Thus, fast-growing species should show lower resistance than slow-growing species, which would lead fast growers to sustain higher herbivory rates, but the fitness consequences of herbivory would be greater for slow growers. The latter is often assumed but rarely tested. In a temperate rainforest, we tested the expected pattern of tolerance to herbivory derived from the RAH: that fast-growing species should be more tolerant than slow-growing species. We also evaluated whether other plant features covary with RGR (leaf lifespan, shade tolerance and leaf toughness) and thus could also contribute to the patterns of tolerance to herbivory. As expected, seedlings from tree species with higher RGR showed greater tolerance to herbivory. Among the three plant features included, only leaf lifespan showed a significant association with RGR, but RGR was the best predictor of tolerance. We argue that plant tolerance to herbivory must be evaluated to properly verify the assumptions of the RAH.