Global change will alter the distribution of organisms around the planet. Dustin Marshall and Mariana Álvarez-Noriega found rising ocean temperatures will impact early life stages of marine invertebrates and change the patterns in the distribution of species that we see today. In particular, species in which mothers invest heavily in offspring will be the biggest losers. These species occur predominantly at the poles.
In terrestrial environments seeds often disperse in the wind, with shape and size affecting how far they travel. It turns out much the same happens in the ocean but currents rather than wind carry marine larvae to their new homes. Larvae able to feed spend much longer in the water column meaning these species have far greater dispersal capabilities.
As with plants, dispersal is crucial to the survival of marine populations. Arriving larvae can seed new areas, re-seed vulnerable populations, and provide genetic variation for subsequent generations in far-flung regions. There is a downside: if temperature affects dispersal, it will also shape how species are affected by global warming.
Not all marine species use the same reproductive or life-history strategies which can mean differences in dispersal distances from centimetres to hundreds of kilometres for different species. Interestingly, there is a well-recognised relationship between latitude (or temperature) and reproductive strategy.
Species at higher latitudes (nearer the poles) tend to invest more heavily in their offspring and produce non-feeding larvae or bypass the larval stage altogether. This means these species don’t disperse very far. In contrast, tropical species tend to put little effort into provisioning their offspring and produce larvae that can feed. As a result, these larvae can spend a lot more time in the plankton and can be dispersed vast distances.
Dustin and Mariana wanted to know how these dispersal relationships might change as global temperatures change. To address this question, they revisited the database of marine invertebrates classified into feeding / development types from a previous study. They established relationships between temperature and development mode so they could then explore how predicted temperatures for 2100 would change patterns in distributions.
So, how will global warming affect these relationships? We know species’ in warmer waters are more likely to produce large numbers of feeding larvae able to remain in the water column for weeks at a time. As waters warm, these species are well placed to extend their range.
In contrast, species based in cooler waters tend to invest heavily in individual offspring, meaning that they develop quicker and settle closer to their parents. This reproductive strategy means that such species are more vulnerable to rapid global change as moving to new areas will, of necessity, be step-wise and slow.
Species at the poles will therefore be the biggest losers because not only will their lower dispersal lifestyles mean they will be slow to access cooler waters but also the options are limited; there is nowhere to go.