Francesca Porri

Environmental Domains and Range-Limiting Mechanisms: Testing the Abundant Centre Hypothesis Using Southern African Sandhoppers

Predicting shifts of species geographical ranges is a fundamental challenge for conservation ecologists given the great complexity of factors involved in setting range limits. Distributional patterns are frequently modelled to “simplify” species responses to the environment, yet the central mechanisms that drive a particular pattern are rarely understood. We evaluated the distributions of two sandhopper species (Crustacea, Amphipoda, Talitridae), Talorchestia capensis and Africorchestia quadrispinosa along the Namibian and South African coasts, encompassing three biogeographic regions influenced by two different oceanographic systems, the Benguela and Agulhas currents. We aimed to test whether the Abundant Centre Hypothesis (ACH) can explain the distributions of these species’ abundances, sizes and sex ratios and examined which environmental parameters influence/drive these distributions. Animals were collected during a once-off survey at 29 sites over c.3500 km of coastline. The ACH was tested using a non-parametric constraint space analysis of the goodness of fit of five hypothetical models. Distance Based Linear Modelling (DistLM) was performed to evaluate which environmental traits influenced the distribution data. Abundance, size and sex ratio showed different patterns of distribution. A ramped model fitted the abundance (Ramped North) and size (Ramped South) distribution for A. quadrispinosa. The Inverse Quadratic model fitted the size distribution of T. capensis. Beach slope, salinity, sand temperature and percentage of detritus found on the shore at the time of collection played important roles in driving the abundance of A. quadrispinosa. T. capensis was mainly affected by salinity and the morphodynamic state of the beach. Our results provided only some support for the ACH predictions. The DistLM confirmed that the physical state of the beach is an important factor for sandy beach organisms. The effect of salinity and temperature suggest metabolic responses to local conditions and a role in small to mesoscale shifts in the range of these populations.

Testing source-sink theory: the spill-over of mussel recruits beyond marine protected areas

Source-sink theory has contributed to our understanding of the function of protected areas, particularly due to their role as population sources. Marine reserves are a preferred management tool for the conservation of natural populations, creating areas of good quality habitat and thus improving population connectivity by enhancing larval supply and recruitment among shores. Despite recent advances in the study of protected areas in the context of the source-sink theory, rigorous and empirical testing of marine reserves as metapopulation sources for the adjacent areas remain largely unexplored. We investigated the role of marine reserves as population sources, whether there was spill-over beyond the reserve boundaries and if so, whether spill-over was directional. We measured percentage cover and recruitment of mussels (Perna perna) at two reserves and two comparably sized exploited control areas on the south-east coast of South Africa where unprotected populations are severely affected by artisanal exploitation. Adult abundances were enhanced within reserves, but decreased towards their edges. We predicted that recruitment would mirror adult abundances and show directionality, with northern shores having greater recruitment following the prevalent northward flow of near-shore currents. There were, however, no correlations between adult abundances and recruitment for any months or shores, and no clear spatial patterns in recruitment (i.e. similar patterns occurred at reserves and controls). The results emphasise that, while reserves may act as important refuges by protecting adult abundances, their influence on promoting recovery of near-by exploited shores through larval spill-over may be overestimated.

Contrasting environments shape thermal physiology across the spatial range of the sandhopper Talorchestia capensis

Integrating thermal physiology and species range extent can contribute to a better understanding of the likely effects of climate change on natural populations. Generally, broadly distributed species show variation in thermal physiology between populations. Within their distributional ranges, populations at the edges are assumed to experience more challenging environments than central populations (fundamental niche breadth hypothesis). We have investigated differences in thermal tolerance and thermal sensitivity under increasing/decreasing temperatures among geographically separated populations of the sandhopper Talorchestia capensis along the South African coasts. We tested whether the thermal tolerance and thermal sensitivity of T. capensis differ between central and marginal populations using a non-parametric constraint space analysis. We linked thermal sensitivity to environmental history by using historical climatic data to evaluate whether individual responses to temperature could be related to natural long-term fluctuations in air temperatures. Our results demonstrate that there were significant differences in the thermal response of T. capensis populations to both increasing/decreasing temperatures. Thermal sensitivity (for increasing temperatures only) was negatively related to temperature variability and positively related to temperature predictability. Two different models fitted the geographical distribution of thermal sensitivity and thermal tolerance. Our results confirm that widespread species show differences in physiology among populations by providing evidence of contrasting thermal responses in individuals subject to different environmental conditions at the limits of the species’ spatial range. When considering the complex interactions between individual physiology and species ranges, it is not sufficient to consider mean environmental temperatures, or even temperature variability; the predictability of that variability may be critical.

Temperature Effects on Reproductive Allocation in the Sandhopper Talorchestia capensis

In invertebrates, environmental temperature may induce mothers to invest differently in the early development of their offspring. In ectotherms, temperature affects offspring phenotype so that colder mothers produce larger eggs. However, developmental mode and maternal size also contribute to the determination of optimal offspring size. When the maternal experience closely matches the offspring’s probable future conditions (e.g., direct developers), it is expected that mothers will produce eggs of similar size within the same brood. While temperature directly affects the size of the eggs (temperature size rule), with potential indirect links to egg number (trade-off between egg size/number), maternal size can be a limiting factor in determining the optimal number of eggs, especially if eggs are brooded. We evaluated the role of temperature in shaping early ontogeny in the sandhopper Talorchestia capensis (Crustacea: Amphipoda), investigating within-brood and among-female variation in the size of the eggs. To test for causal relationships among temperature, maternal size, egg size and number, we used an information theoretic approach combined with path analysis. Sandhoppers invested in smaller eggs at higher temperatures, with no significant within-brood variation in the size of the eggs. Regardless of temperature, we found significantly different investment in egg size among females. Path analyses showed a simultaneous contribution of temperature and maternal size to the optimal size and number of eggs within a single clutch. Strong inter-individual variability in maternal investment could generate phenotypic variation within a population and promote population fitness.

Epigenetic variation among natural populations of the South African sandhopper Talorchestia capensis

Ecological epigenetics is gaining importance within the field of Molecular Ecology, because of its novel evolutionary implications. Linking population ecology to the variation in epigenetic profiles can help explain the effect of environmental conditions on phenotypic differences among populations. While epigenetic changes have largely been investigated through the examination of DNA methylation under laboratory conditions, there is a limited understanding of the extent of DNA methylation variation in wild populations. Assuming that epigenetic variation is important in nature, the conditions experienced by different conspecific populations should result in levels of DNA methylation that are independent of their genetic differentiation. To test this, we investigated levels of DNA methylation among populations of the sandhopper Talorchestia capensis that show phenotypic (physiological) differences in their response to environmental conditions, at the same time evaluating their genetic relationships. Given the high levels of inter-individual physiological variation observed within populations, we further hypothesised that inter-individual differences in methylation would be high. Levels of genetic and epigenetic variation were assessed within and among populations from five localities using the methylation sensitive amplified polymorphism technique. Population differentiation was higher for epigenetics than genetics, with no clear geographical pattern or any relation to biogeography. Likewise, individuals showed greater variability in their epigenetic than their genetic profiles. Four out of five populations showed significant negative relationships between epigenetic and genetic diversity. These results show uncoupling between epigenetic and genetic variation and suggest that: (1) epigenetics are more responsive to local, site-specific environmental conditions than genetics and (2) individual differences in epigenetic profiles drive phenotypic variation within (and most likely among) natural populations. Within populations, epigenetics could offer a level of phenotypic flexibility beyond genetic constraint that allows rapid responses to variable or unpredictable environments, potentially compensating for low genetic variability.

Thermal sensitivity of the crab Neosarmatium africanum in tropical and temperate mangroves on the east coast of Africa

Abstract Mangrove forests are amongst the tropical marine ecosystems most severely affected by rapid environmental change, and the activities of key associated macrobenthic species contribute to their ecological resilience. Along the east coast of Africa, the amphibious sesarmid crab Neosarmatium africanum (=meinerti) plays a pivotal role in mangrove ecosystem functioning through carbon cycling and sediment bioturbation. In the face of rapid climate change, identifying the sensitivity and vulnerability to global warming of this species is of increasing importance. Based on a latitudinal comparison, we measured the thermal sensitivity of a tropical and a temperate population of N. africanum, testing specimens at the centre and southern limit of its distribution, respectively. We measured metabolic oxygen consumption and haemolymph dissolved oxygen content during air and water breathing within a temperature range that matched the natural environmental conditions. The results indicate different thermal sensitivities in the physiological responses of N. africanum from tropical and temperate populations, especially during air breathing. The differences observed in the thermal physiology between the two populations suggest that the effect of global warming on this important mangrove species may be different under different climate regimes.