Frontiers of Biogeography

In this issue Minchin and Quigley review the disseminules known to be carried by ocean currents to western Europe. The image is of a stranded fruit of the coconut (Cocos nucifera) on a Florida shore, the largest to make a North Atlantic crossing and rarely viable after floating 110 days. Upon stranding on a tropical or sub-tropical shore germination may take place. Photo by Dan Minchin.

We connect evidence that 20 oC is the most stable temperature for cellular processes with macroecological observations. Examples show that temperatures warmer than ~20 oC result in decreases in: aquatic species’ tolerance to low oxygen; marine pelagic and benthic algal productivity; pelagic and benthic predation rates; global species richness in pelagic fishes, plankton and benthic invertebrates; and genetic diversity; but increased extinctions in the fossil record. The realised thermal niche of reef fishes and invertebrates globally is narrowest among species with distributions centred on 20 oC, as also seen in microbes. While many species have evolved to live at warmer and colder temperatures, most species live at, and extinctions in the fossil record across seven phyla were lower at, 20 oC. The mathematical “Corkrey” model, which predicts that thermal breadth should be minimized and species richness maximised at 20 oC across all Domains of life, provides an explanation for this “20 oC effect”. A literature search found highest species richness at ~20 oC across life in air and water, including animals, plants and microbes. That life seems centred around ~20 oC implies fundamental constraints that compromise the ability of extant tropical species to adapt to higher temperatures.

The purpose of this article is to exemplify how certain types of historic toponyms (placenames) can be employed as an aide to biogeographers in revealing past distributions of species and ecosystems, but also the need for additional interrogation of their likely veracity. Some of the toponyms bestowed by the Dutch explorer, Maerten van Delft, who surveyed the northern coasts of Australia’s Melville Island and the Cobourg Peninsula in 1705, serve as examples for further examination. The expedition conferred 61 toponyms and topographic descriptors, some of which are enigmatic given what we know of the ostensive distribution of Australian fauna in the region at the time. Presumably, the names referred to animals seen on the expedition. Cartographic, documentary, linguistic, and natural science sources were consulted to analyse the meanings of the toponyms. It shows that some the toponyms were based on misidentification due to unfamiliarity of the endemic fauna, whilst one did not refer to an animal at all. Another toponym raises the tantalising prospect that thylacines were present on Melville and Greenhill Islands at the time.

The “more individuals hypothesis” suggests that increases in the total number of individuals per species leads to increases in community richness. Abundance, body size distributions and richness do vary with latitude in several taxonomic groups. However, support for this hypothesis has otherwise been mixed. In this paper, we investigate latitudinal changes in all three variables for marine bivalves along the eastern coastline of Australia. We utilise a large, uniformly sampled field dataset of 5670 shells representing 157 species that spans 20º of latitude and crosses a major biogeographic transition. For each of 15 field sites, 10 quadrats were randomly placed and completely sampled, making it possible to quantify total abundance. Species richness was calculated using a new estimator based on the geometric series distribution. Body size was computed as the geometric mean of length and width. Despite uncovering a strong latitudinal gradient in species richness, we found no significant gradient in total abundance and body size at any taxonomic level. Although previous work found family-level trends in bivalve size, it was done at a larger spatial scale and therefore did not pertain to individual communities. Environmental variables do correlate with both abundance and richness, but they are not directly related. Because we find no gradient in abundance and no relationship between abundance and richness, we reject the “more individuals hypothesis” for our system. Instead, latitudinal richness trends in coastal Australia may result from an environmental gradient in dispersal constraint.

The marula tree, Sclerocarya birrea (S. birrea) (A. Rich.) Horchst, is native to Africa, used to restore drylands, and introduced outside Africa as a pilot towards commercial cultivation due to its economic potential. However, there is a global paucity of information regarding where subspecies can survive beyond Africa. We aimed to predict and quantify global-scale suitable areas for S. birrea and its subspecies beyond their native ranges under the current environmental conditions and future warming climates. The areas were predicted by using MaxEnt algorithm using occurrence data from Africa and, climatic and topographical environmental variables and, the Max Planck Institute for Meteorology and Hadley Climate Center’s global Earth Systems Models under shared socio-economic pathways (SSPs) greenhouse gas concentrations, SSP3-7.0, for the year 2050 and 2080. The results show that the models’ predictive power was robust, with the Receiver Operating Characteristic’s Area under the Curve (AUCs) ranging from 0.90-0.98. Currently, suitable areas exist in all continents except Europe and Antarctica and, occupy 3,751,057 km2 to 24,632,452 km2 of Earth’s terrestrial area scattered in 54 to 107 countries predominantly in global biomes with climatic conditions ranging from desert tropical to temperate humid. Under future climates, the areas will retract by 64-100%, shifting to high latitudes and being limited to global biomes with tropical desert-to-desert temperate, Mediterranean warm conditions, and some regions of Eastern Europe will become suitable areas. Suitable areas for S. birrea and its subspecies exist beyond Africa, and they will retract and migrate to high latitudes under future warming climates.

Disseminules have drifted in long distance dispersal from the Americas to the coasts of Europe with records extending to the Arctic Ocean and southwards to the Macaronesian islands. The parent plants originate from tropical wetland forests to boreal conditions. Their disseminules undergo different ocean crossing times according to buoyancy duration. We use plastic-drift as a surrogate for the likely risks that disseminules endure during long-distance oceanic spread relating to their size, behaviour and losses from sinking. While origins of disseminules have a wide latitudinal biogeography we have used the records of the extensive strandings on Floridian, Bermudian and the east coast of North American shores as the region with which to compare the comparatively sparse arrivals in Europe. Studies on plastic drift show that the Atlantic current flow is mainly directed towards the northern coast of Ireland, that also covers the west coast of Scotland and south-west Britain. We have concentrated our records to this region, in particular to the Atlantic coasts of Ireland. Few disseminules arrive on European shores in a viable state, and for those that do, the temperate conditions may not suit germination. Some stranding events are likely to have anthropogenic involvement; but the great majority are most probably naturally distributed. Historically, the ethnographic significance suggests stranding events have taken place over centuries and most probably regularly took place during the Holocene. The increase of records in recent decades is likely to have been due to interested observers with access to taxonomic guides, the popular literature and on-line information.

Species may respond to climate change by redistributing their distribution areas, but because they do not share the same climatic affinities, they should not respond in the same way. Consequently, distribution shifts of species that are currently found in sympatry may change the extent of the area of sympatry and therefore interspecific interactions at the local scale. In Western Europe, the green lizards Lacerta agilis and Lacerta bilineata live in partial sympatry, share morphological similarities, and can locally compete for resources. In this study, we used a correlative species distribution model (SDM), Maxent, to explore the effects of climate change on the distribution of suitable areas for each species and also within their sympatric area under future scenarios. Our simulations showed that all L. agilis subspecies are more likely to lose suitable habitats throughout their distribution areas whatever the scenario. Conversely, even if L. bilineata should lose less suitable areas, gains may be more restricted. In addition, when looking within the areas both reachable for L. a. agilis and L. bilineata to identify where sympatric areas will occur, we find that they should be spatially restricted. Climatic refugia potentially shared for both lizards may therefore be limited. Consequently, competitive exclusion expected in areas suitable for both species (i.e., the potential area of sympatry) could decrease in the future as climate changes likely lead L. agilis subspecies and L. bilineata to follow different species range shifts trajectories over the 21st century.

Climate change is driving a rapid but highly variable redistribution of life on Earth, comparable in scale and magnitude to changes historically only seen over tens of thousands of years. Despite increased research effort, the complex mechanisms driving these changes in geographical distribution of species, or ‘range shifts’, remain only superficially understood. Attempts to understand the processes underpinning species responses are hampered by the paucity of comprehensive, longterm datasets, few theoretical frameworks, and lack of strategic direction and cross-fertilisation with related ecological fields. As an emerging, dynamic field, range shift ecology would benefit from integrating concepts and approaches from other related, more established areas of research, such as invasion ecology. Here, we use a systematic literature review and bibliographic analysis to assess the level of knowledge exchange between range shift ecology and invasion ecology. We found that while the two fields are inherently strongly related, the level of exchange and integration of ideas via citation networks does not reflect the closeness of the fields in terms of concepts, theories, and practice. Although range shift papers cite invasion papers more often than vice versa, the citation rate is generally quite low for both. These findings are evidence of the increasing need to move away from discipline-focused interpretation and communication of scientific results, towards greater research integration and connection between related ecological fields. Increased knowledge and data exchange between range shift and invasion fields could improve mechanistic understanding of range shifts and species invasions under climate change, enhance the predictive capacity of models and better inform management and conservation efforts.

Biogeography attempts to find explanations for the distributions of species, based on their past histories and present environmental conditions. Historically, biogeographic studies were modelled on intuitive and expert knowledge, whereas recent studies have advanced with the aid of digitised natural history collections, computational power and repeatable methods. In South Africa, biogeographical studies on insects are greatly lacking and very little is known about the zoogeographic patterns for many insect groups in the region. South Africa has a high level of diversity and endemism of orthopterans (> 800 species), making them an ideal group to investigate zoogeographic patterns. The aim of this study was to identify and describe the zoogeographical patterns of South Africa’s orthopteran species, based on the distributions and levels of diversity of all major families. Point locality data was used to conduct a hierarchical cluster analysis based on the shared presence of species to delimit zoogeographical centres. In addition, delimited centres were compared to plant-based biomes and phytogeographic regions. Results showed that orthopteran species richness was evenly distributed across the region and clustered into six zoogeographical centres. There was a primary split, separating species into a western winter-rainfall and an eastern summer-rainfall group. The western and eastern regions contained three centres each, with the east being less diverse and taxonomically distinctive than the west. Strong consensus was seen between orthopterans and the Greater Cape Floristic Region, and between orthopterans and the Cape zoogeographic region for butterflies, reinforcing the notion that this region is representative of a biochorion. In addition, this region had the greatest numbers of orthopteran families, highlighting its importance for orthopteran diversification.