Protected areas have been implemented worldwide to preserve diversity and maintain ecosystem function. Marine Protected Areas (MPAs), which often restrict commercial and recreational fishing and foreshore development, have been shown to increase fish abundance and biomass, compared with unprotected, or partially protected areas.
However, an overarching goal of MPAs is to maintain community stability – so are they actually doing this?
We used a 10-year snorkel survey in Batemans Marine Park in southern NSW to test whether changes in the identity of fish species (turnover) varies between sites that are protected (MPAs), unprotected (OPAs = outside park areas), and partially protected (PPAs – where recreational fishing is still allowed).
We found that MPAs have a greater capacity to retain species through time, as shown by more shallow zeta diversity decline (i.e., turnover), compared with unprotected (OPAs) and partially-protected (PPAs) areas. This was particularly important for large-bodied harvested species, including red morwong and bream.
Our decadal study provides evidence that MPAs are working – in Batemans Marine Park protection produces stabilising effects on fish diversity. Whether these results hold more generally across other marine parks, and whether MPAs can also confer resilience to fish communities in the face of environmental change still needs to be determined.
This work was funded by the RAAP grant awarded to SIMS and IMOS by the NSW Governmment, and survey work done by Nature Coast Marine Group, and is now published in Conservation Biology. See ‘publications’ or contact me for a PDF copy.
Ever since reading classic papers by Conover and Schultz 1995 and Levins 1969, I have been fascinated by patterns of countergradient variation in traits, whereby genetic variation opposes environmental variation across ecological gradients. I have even observed these patterns in our wall lizard experiments – under a common temperature, embryos from high altitude (cold) populations develop faster than those from low altitude (warm) environments.
Is this a general pattern across all reptiles? How and why might this happen?
For egg laying species, temperature experienced during embryonic development can have important fitness consequences. Incubation temperature can affect hatching success (survival), alter size at hatching, growth rate and the quantity and quality of offspring. Temperature poses a strong influence on physiological rates underlying energy acquisition and use. For example, relative to warm environments, low nest temperatures often increase development time (time from fertilization to hatching) and decrease metabolic rate (rate of energy expenditure), yet in cold climates it is crucial that embryos complete development and commence feeding and growth before the onset of winter. Countergradient variation can enable populations to compensate for the direct effects of temperature on physiological rates, to ensure persistence of populations under extreme climatic regimes.
Another reason why reptiles may show countergradient variation in development is to increase the efficiency use of their finite energy reserves. The thermal sensitivities of developmental and metabolic rates determine how energy use during development (fertilization until nutritional independence) scales with temperature (see article here and blog post here). Increasing either development time, or metabolic rate will increase the costs of development, and therefore reduce the amount of residual energy at hatching. The recently proposed Development Cost Theory explains how the relative temperature sensitivities of development time and metabolic rate determine the amount of energy expended at any given temperature. At cooler developmental temperatures, development time is often extended more than metabolic rate decreases, so cold environments generally increase total energy use, which reduces energy available upon hatching. Developmental Cost Theory can provide a useful framework for detecting local adaptation by providing a potentially general mechanism to explain the temperature sensitivity of development time and metabolic rate and fitness across environmental thermal gradients.
I compiled data from the literature on common garden and reciprocal transplant studies to test for evidence of countergradient variation in development time and metabolic rate across cold- and warm-adapted populations of reptiles. I found that most studies (17/22) show evidence for countergradient variation between development time and environmental temperature, supporting the generality of countergradient variation in reptile development. Development under cool conditions necessitates a countergradient adaptive response for faster development and earlier hatching time, enabling embryos to hatch before winter while resources are still available and ensuring residual energy at hatching. However, I found little support to suggest that countergradient variation is common for metabolic rate – overall, reptile embryos from locally-adapted cooler climates did not maintain higher metabolic rates compared with populations from warmer climates.
Another selection analysis has just been accepted for publication in Evolution Letters! This work was in collaboration with Dustin Marshall, Matt Hall and Craig White at Monash University.
Metabolic rate, or the rate at which organisms uptake, transform, and expend energy varies substantially across individuals of the same species, even after accounting for differences in body size and temperature. What drives this variation? Metabolic rates set the pace-of-life – higher metabolic rates are linked to faster growth, earlier onset of reproduction, and shorter lifespan, while low metabolic rates are associated with a slow pace-of-life (slow growth, late onset of reproduction and long lifespan).
Variation in metabolic rates are likely to have fitness consequences, and be under strong selection. Evolutionary theory predicts that over time, selection should deplete variation in traits, yet variation in metabolic rates is ubiquitous. Variable selection regimes may maintain trait variation, by selecting for different metabolic rates across different environments, where a high or low pace-of-life is advantageous. While this theory is well established, field estimates of selection on metabolism across environments are historically rare.
To investigate the role of environmental variation in maintaining trait variation, we measured the metabolic rates of individual marine bryozoans, experimentally manipulated their competitive environment, and monitored their survival, reproduction, and pace-of-life in the subtidal.
We found that selection on metabolic rate varies among competition environments separated by only a few centimetres – competition selects for a faster pace-of-life, relative to competition-free environments. High-metabolism individuals are better able to withstand intense competition, however low-metabolism individuals live longer, and are likely to have higher fitness under competition-free conditions. Hence, the environment-dependent nature of selection on metabolism and the pace-of-life is likely to maintain variation in metabolic rates.
Being in (semi) lockdown in Sweden has made me nostalgic for field work. Last April in sunny Italy our field team (Mara, Lu, Tobias, Geoff & Nathalie) collected over 100 wall lizards (Podarcis muralis) from sites near Rome. We sampled brown and green phenotypes across low to high altitude ranges and brought them back to the lab in Lund, Sweden. The field work was fantastic – and not just because of the post-work gelato in the evenings 🙂
I am now back in the lizard room, collecting clutches from females and planning the next set of experiments. It is not quite as scenic as last year but the lizards are doing a fantastic job, providing lots of eggs for us to test for convergence in maternal effects.
Last year I was able to measure metabolic and developmental rates in hundreds of eggs that had been incubated across warm and cool temperature regimes. I also analysed egg yolk for thyroid hormone analysis in collaboration with Suvi Ruuskanen in Turku, Finland.
I am still continuing with the analysis but so far the results look promising – we are seeing differences in maternal investment and developmental thermal physiology across populations from low – mid – high altitude and between green and brown phenotypes! We think these population-level differences may have allowed Podarcis muralis to expand its range into high altitude and non-native high latitude regions.
This year we continue with thermal physiology measures, manipulating the variance in developmental temperatures using programmable incubators to see how embryos from different altitudes and parental phenotype respond in their thermal physiology.
My final PhD thesis chapter is now out in Ecology Letters: Linking life‐history theory and metabolic theory explains the offspring size‐temperature relationship. This work was in collaboration with my two PhD supervisors, Dustin Marshall and Craig White, along with Robert Bryson-Richardson at Monash University. We combined experimental work on a bryozoan (Bugula neritina) and zebra fish (Danio rerio) with two meta-analyses on over 70 ectotherm species to determine how the costs of development scale with temperature.
One common pattern in life-history theory is that among and within species, mothers in cold environments produce larger offspring. This pattern was first observed by Gunnar Thorson, a marine larval biologist in the 1950’s, and since then, tens of studies have reported that when mothers are reared under cooler temperatures, they produce larger offspring:
Relationship between the magnitude of change in offspring size with temperature (Hedges’ g) and change in experimental temperature. For the majority of 34 ectotherm species, an increase in maternal brooding temperature results in a decrease in offspring size.
One potentially general explanation for this pattern is how the costs of development scale with offspring size. If we assume that mother’s must provision their offspring with enough energy to sustain them throughout development (i.e fertilisation until onset of feeding), then offspring size must be proportional to the costs of development. Here, we measured the costs of development as development time x metabolic rate. We expect that as temperatures increase, development time decreases and metabolic rate increases:
We reared individual larvae (Bugula neritina) and embryos (Danio rerio) across 4 temperatures and measured development time and metabolic rate. We found that while metabolic rate does increase with temperature, development time decreases at a much faster rate, such that overall, the costs of development decrease with temperature. We think that mother’s may be offsetting these costs under cooler environments by provisioning their offspring with more energy, thus making larger offspring under cooler temperatures.
To see whether this pattern holds more generally, we conducted a meta-analysis on over 80 species of ectotherms and found the same pattern for the vast majority of species:
Interestingly, we also found that at temperature beyond those that we tested, the costs of development begin to increase. This is because the temperature sensitivity of metabolic rate becomes higher than that of development time. Thus, our results have implications for global changes in temperature – under extreme temperature increases, we expect developmental costs to increase, and mothers may need to make larger offspring. Such effects would reduce the productivity of ectotherms across a wide range of species. An important next step is to determine how the temperature sensitivity of development time and metabolic rates can evolve over short and long time scales.