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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
Seventy years after Dobzansky suggested that biotic interactions are more important in the tropics, ecologists are still assembling evidence and elucidating potential mechanisms behind such macroecological patterns. My thesis contributes to this field by demonstrating that predation rates and the strength of mutualistic associations decrease with elevation in the New World tropics. I also tease apart possible mechanisms behind these patterns, which are likely ultimately linked to changes in temperature and productivity with elevation.Using manipulative experiments and field observations across 4000-meter elevational gradients in the equatorial Andes, I show that decreasing predation rates on arthropods at higher elevations are driven by decreasing predator abundance and activity. Ants were responsible for 80% of predation in the lowlands, but were replaced by other predators above 1500m, revealing a parallel qualitative gradient of predation. Along the same gradients, I show that the frequency of ant-hemipteran mutualistic associations also decreases with elevation, driven by a decrease in ant and hemipteran abundance. However, ant abundance limits the interaction above 1500m and hemipteran abundance below, which shaped the resource-consumer dynamics occurring in partner aggregations.I provide evidence that, in response to pervasive ant predation in the lowlands, hemipterans may ‘bribe’ ants with honeydew primarily to dissuade them from predating upon them, rather than, as generally assumed, to obtain their defensive services. This is the first documentation of a mutualistic interaction involving prey offering a reward to predators in exchange for their lives, challenging our fundamental understanding of both mutualism and predation.Finally, I show that anti-predator investment in hemipteran communities does not decrease at the same rate as predation rates at higher elevations, suggesting that the fitness effects of predation may decline at slower rate than predation rates. Hemipteran investment in ant-mutualism across elevations mirrored the contribution of ants to predation further supporting the idea of ant-hemipteran mutualism as an anti-predator strategy from ants themselves.
The relative costs and benefits of group living change with group size. In the social spider Anelosimus eximius, as colonies grow, the number of insects captured per capita decreases, but the size of insects increases, causing biomass captured per capita to peak at intermediate colony sizes. One aspect of group living that changes with group size is competition for resources. Whether intraspecific competition occurs via scramble vs. contest competition can affect the stability and survival of the group or population. By feeding large and small prey to artificial colonies of the social spider Anelosimus eximius, we investigated whether prey size could alter the type of competition that takes place and, thus, potentially, influence colony population dynamics. We found that large prey were shared more evenly, and that individuals in poor condition were more likely to feed when prey were large. Next, we investigated whether the condition of individuals vary as a function of colony size by measuring condition of individual spiders in a wide range of nest sizes. We found that, due to reducing per capita food supply as colonies grow, individuals have lower condition in larger colonies. We also found that condition variance decreases with colony size, further suggesting that scramble competition predominates in large colonies. Although dispersing females were larger and in better condition than philopatric ones, nests established by dispersing females had low survival rate, suggesting that dispersal is costly. Dispersers, therefore, likely face multiple constraints. They have to be large enough to stand a chance of survival following dispersal, but, due to dispersal costs and benefits of group living, should not disperse except from large colonies. Diminishing resources in large colonies, however, coupled with scramble competition, should make it hard for individuals to accumulate sufficient resources to disperse. This combination of factors may contribute to the observed sudden extinction of large colonies that fail to disperse representing a paradox of how social spider metapopulations persist. Using an individual-based simulation model, we demonstrate that rare increases in food supply due to environmental stochasticity may precipitate occasional mass dispersal from large colonies, allowing the metapopulation to persist.
Understanding the suite of ecological conditions that favor sociality —the tendency of organisms to form groups and cooperate— is key to understanding the origin, maintenance and contribution of social groups to biodiversity. The ecological dynamics of sociality can in turn have many consequences that feed back to influence the way species use the available resources, interact with other species, and persist in nature. The causes and consequences of sociality thus arise from the interplay of organisms and ecological processes. My thesis includes three studies that provide insight into some of the ecological processes that influence sociality and in turn the consequences that sociality may have in resource use and community structure. In the first study (Chapter 2), I use ecological niche modeling to predict the geographical distribution of social and subsocial New World Anelosimus spiders and explore their ecological correlates across latitude and elevation. Using a comparative approach, I further show that elevational patterns are strongly associated with differences in climatic conditions between social systems. In the next study (Chapter 3), I explore the role of group living and cooperation in resource use in a natural community of Anelosimus spiders of similar body size, but with behaviors ranging from near-solitary to fully social. I conduct surveys of prey capture in four sympatric Anelosimus species in Brazil and find that level of sociality and cooperation greatly shape resource use and act to separate different species into different ecological niches. Finally, I conduct feeding experiments to analyze in more detail the emergent patterns of resource use in two sympatric spiders with similar level of sociality but different body size (Chapter 4). I find that differences in resource use arise through differences in foraging efficiency emerging from the interplay of sociality and individual traits (e.g. body size). My thesis highlights the importance of ecological processes in the broad-scale spatial distribution of sociality and its potential consequences in resource use, community structure and ultimately the maintenance of local diversity. These studies also emphasize the work that remains to be done in such exciting area of research.
No abstract available.
Master's Student Supervision (2010 - 2020)
Our study considers how predation by ants and intense rainfall affect the proportion of three-dimensional (3D) versus two-dimensional (2D) spider webs along a precipitation gradient. We predicted that if predator protection benefits of 3D webs outweighed the costs of rain damage, the proportion of 3D webs would increase with annual rainfall, which is expected to correlate with predation intensity (the predation hypothesis). Alternatively, if the costs of rain damage were more significant, we expected a decrease in the proportion of 3D webs with annual precipitation (the rain intensity hypothesis). To assess how predation and rain intensity affect the proportion of 3D webs, we selected seven sites along a rain gradient in western Ecuador. First, we verified annual rainfall and January to April rain intensity data using geographic information systems (GIS). Second, we surveyed up to 120 webs along six separate transects at each site. In areas adjacent to the transects, we estimated ant predation intensity using tuna baits. Finally, using the same transects, we determined how vegetation lushness changed with rainfall. To do so, we measured leaf area, canopy cover, and the diameter at breast height of adjacent trees. We found that 3D webs increased in proportion with annual rainfall, which correlated positively with predatory ant abundance, consistent with the predation hypothesis, but counter to the rain intensity hypothesis. We found, however, that in areas of greater precipitation, lusher vegetation provided greater shelter under which spiders built their webs. As such, we suggest that greater availability of immediate cover in lusher and wetter habitats would mitigate the destructive power of intense rainfall, allowing the predator protection benefits of 3D webs to be realized despite the simultaneous occurrence of strong rains. Microhabitat factors may thus interact with broader-scale biotic and abiotic factors in structuring web-building spider communities.
Metacommunity theory has advanced understanding of mechanisms shaping community structure. Four main models (neutral, patch-dynamics, species-sorting, and mass-effects) have been recognized to explain these mechanisms, differing in their assumptions about the effects of environmental filtering and species traits on community composition. Here, I focus on complex, three-dimensional spider webs of two social and two solitary species as habitat patches for associated arthropods in a tropical rainforest in Ecuador. I used variance partitioning and various analyses of metacommunity structure to study the role of environmental filtering and dispersal in this system. I found that local patch characteristics, such as patch size and host species, predominantly affected local community composition. Webs of social spider species had higher richness, more variable communities, and proportionally more aggressive (i.e. predatory) web associates. Behavioral characteristics of the host spiders, such as sociality and aggressiveness, seemed to play an important role, as well, in shaping community composition on these patches. In a colonization experiment, there was indication of high dispersal rates at a short temporal scale and some evidence of species dominance at a longer temporal scale. I conclude that environmental filtering is responsible for the patterns of species distribution and that, given the conjunctive high dispersal and species specialization, the metacommunity patterns in this system seem to best be explained by a combination of the species sorting and mass effects models.
Species ranges, which are manifestations of species ecological niches in space, are generally determined by gradients of abiotic and biotic factors. In group-living organisms, not only the properties of individuals, but also those of their groups, should interact with environmental challenges and opportunities to determine a species range. Social and subsocial spiders are well known for having distinct geographical distributions. Intriguingly, subsocial species in the genus Anelosimus are absent from the lowland tropical rainforest where social congeners thrive. Previous studies have attributed this absence to increasing rain intensity and predation, particularly by ants, closer to the rainforest. After confirming that these factors do indeed increase in intensity approaching the lowland tropical rainforest, I test these hypotheses by transplanting nests of the subsocial Anelosimus elegans from its native lower montane cloud forest (1000m) to the lowland tropical rainforest (400m). At both locations I performed a fully factorial ant and rain exclusion experiment and monitored colony survival over time. I found that survival was lower in the lowlands, but improved by the exclusion of rain and ants. At the native higher elevation habitat, in contrast, colony survival did not differ between treatments and controls, confirming that neither intense rains nor predation are factors that negatively impact colony survival in the native habitat. At both locations, large colonies were able to build more webbing, suggesting that larger groups with limited dispersal may benefit from reduced per capita web maintenance in addition to increased predator protection. These findings would explain why subsocial Anelosimus, with small single-family groups and dense webs, have been unable to colonize the lowland tropical rainforest where their social congeners thrive.
Dietary differentiation is an integral component of species coexistence, and among solitary predators, body size differences allow each species to capture a different range of prey sizes. Social predators, however, are able to capture much larger prey than an individual, so prey size use is additionally influenced by group size and behavioural dynamics. To investigate this, we looked at cooperative hunting among three species of sympatric group-living spiders in Brazil that construct colonies of different sizes and are known to capture different sizes of prey. We performed feeding experiments to determine whether differential prey size use is produced by differences in group behaviour and group size. For each species we measured the level of cooperation and examined how colony size influenced group behaviour. We found that two of the species which live in equally large, multi-generational colonies displayed differences in their cooperation and prey size selectivity that are consistent with differences in prey size use previously observed: the species which captures larger prey in natural hunting scenarios showed higher levels of cooperation among hunters during the trials, and had more individuals participate when presented with large prey. The third species, which lives in smaller, temporary colonies, displayed the highest levels of cooperation and prey capture success, despite capturing the smallest prey on average in natural hunting scenarios. This disparity likely reflects the natural size distribution of colonies of this species, which is greatly dominated by solitary individuals that cannot capture the largest prey on their own. This study shows that behavioural differences among group-living predators, in addition to colony size differences, may be responsible for differential prey size use.
Sociality – cooperative group living – is ubiquitous in the natural world, yet our understanding of its evolution is still in its infancy. In this thesis, I explore two poorly understood aspects of the evolutionary origin and consequences of sociality using social cobweb spiders (Anelosimus spp.) as a model system. First, I examine how pre-exisiting traits have contributed to the evolution of alloparental care – the care of non-descendant offspring – in social cobweb spiders. I begin by showing alloparental care is extensive in wild social cobweb spider nests. I then test the hypothesis that alloparental care occurs as a result of a lack of discrimination against foreign egg sacs. In support of this hypothesis, I show that subsocial species from clades sister to the social species freely care for foreign egg sacs. This suggests that a lack of offspring discrimination is ancestral to sociality in cobweb spiders and alloparental care likely emerged spontaneously along with group living. This may have facilitated the evolution of sociality by immediately providing the group-level benefits of alloparental care. Secondly, I examine how social life may have altered natural selection acting on social cobweb spiders. In social cobweb spider nests, the protection offered by a communal nest and the presence of alloparents may have relaxed natural selection on individual maternal care behaviour. Using a comparative approach, I test the hypothesis that sociality is associated with reduced maternal care behavioural phenotypes. I show that social species from independently derived social clades score significantly lower than their subsocial sister taxa on six different assays of maternal care, including the probability of repairing damaged egg sacs and of abandoning egg sacs in the face of simulated predation. Integrating a number of supporting facts, I interpret this result as suggestive of relaxed natural selection on maternal care behaviour as a consequence of sociality. Together, the two comparative studies I present reveal a key role for pre-existing traits in the origin of sociality and that the forces of evolution are likely altered in concert with the onset of social life.