Repeated patterns in the evolution of size in island plants

<p>For reasons not fully understood, animals often evolve predictably on islands. For example, radiations of large, flightless birds are a common element of many island biotas. However, our understanding of how plants evolve on islands is comparatively poor. Further, an investigation into the evolution of island plants could help resolve unanswered questions about island animals. This thesis investigates insular size changes in a range of plant functional traits. First (Chapter 2), I explored size changes in 9 species of vines that have colonized islands from the New Zealand and Australian mainland. I asked whether leaf–stem allometry prohibits leaves and stems from evolving independently from one another. Island populations consistently produced larger leaves than did mainland populations. Moreover, changes in leaf size were not associated with concomitant changes in stem size, suggesting that trait allometry does not govern trait evolution on islands. Next (Chapter 3), I asked whether plants obey the infamous island rule, a putative trend in island evolution wherein small animals become large on islands and large animals become small. I demonstrate that plant stature and leaf area obey the island rule, and seed size does not. My findings illustrate that the island rule is more pervasive than previously considered, but that support for its predictions vary among plant functional traits. Third (Chapter 4), I demonstrate that the island rule results from evolutionary drift along bounded trait domains. The island rule has long been hypothesized to result from a suite of selective pressures. Applying my model to island plants, I show that evolutionary drift is the most parsimonious explanation for the island rule pattern. Finally (Chapter 5), to explore insular patterns in leaf size evolution, I conducted a large-scale, macroevolutionary analysis of leaf size on 98 of New Zealand’s offshore islands. Leaf gigantism was emblematic of island populations, and was most prominent in taxa with variable leaf morphologies on the mainland. Further, leaf gigantism was greatest in populations inhabiting old, distant islands, suggesting that time since divergence is a direct predictor of morphological differentiation between mainland and island populations. Overall, this thesis reveals novel patterns, and helps disentangle the distinct roles of natural selection and drift, in the evolution of plant form and function on islands. Finally, this thesis illustrates how investigating the changes in plant traits can help identify the evolutionary mechanisms operating on islands.</p>

Repeated patterns in the evolution of size in island plants

<p>For reasons not fully understood, animals often evolve predictably on islands. For example, radiations of large, flightless birds are a common element of many island biotas. However, our understanding of how plants evolve on islands is comparatively poor. Further, an investigation into the evolution of island plants could help resolve unanswered questions about island animals. This thesis investigates insular size changes in a range of plant functional traits. First (Chapter 2), I explored size changes in 9 species of vines that have colonized islands from the New Zealand and Australian mainland. I asked whether leaf–stem allometry prohibits leaves and stems from evolving independently from one another. Island populations consistently produced larger leaves than did mainland populations. Moreover, changes in leaf size were not associated with concomitant changes in stem size, suggesting that trait allometry does not govern trait evolution on islands. Next (Chapter 3), I asked whether plants obey the infamous island rule, a putative trend in island evolution wherein small animals become large on islands and large animals become small. I demonstrate that plant stature and leaf area obey the island rule, and seed size does not. My findings illustrate that the island rule is more pervasive than previously considered, but that support for its predictions vary among plant functional traits. Third (Chapter 4), I demonstrate that the island rule results from evolutionary drift along bounded trait domains. The island rule has long been hypothesized to result from a suite of selective pressures. Applying my model to island plants, I show that evolutionary drift is the most parsimonious explanation for the island rule pattern. Finally (Chapter 5), to explore insular patterns in leaf size evolution, I conducted a large-scale, macroevolutionary analysis of leaf size on 98 of New Zealand’s offshore islands. Leaf gigantism was emblematic of island populations, and was most prominent in taxa with variable leaf morphologies on the mainland. Further, leaf gigantism was greatest in populations inhabiting old, distant islands, suggesting that time since divergence is a direct predictor of morphological differentiation between mainland and island populations. Overall, this thesis reveals novel patterns, and helps disentangle the distinct roles of natural selection and drift, in the evolution of plant form and function on islands. Finally, this thesis illustrates how investigating the changes in plant traits can help identify the evolutionary mechanisms operating on islands.</p>

The island rule and its application to multiple plant traits

<p>The Island Rule refers to a continuum of body size changes where large mainland species evolve to become smaller and small species evolve to become larger on islands. Previous work focuses almost solely on animals, with virtually no previous tests of its predictions on plants. I tested for (1) reduced floral size diversity on islands, a logical corollary of the island rule and (2) evidence of the Island Rule in plant stature, leaf size and petiole length. Endemic island plants originated from small islands surrounding New Zealand; Antipodes, Auckland, Bounty, Campbell, Chatham, Kermadec, Lord Howe, Macquarie, Norfolk, Snares, Stewart and the Three Kings. I compared the morphology of 65 island endemics and their closest ‘mainland’ relative. Species pairs were identified. Differences between archipelagos located at various latitudes were also assessed. Floral sizes were reduced on islands relative to the ‘mainland’, consistent with predictions of the Island Rule. Plant stature, leaf size and petiole length conformed to the Island Rule, with smaller plants increasing in size, and larger plants decreasing in size. Results indicate that the conceptual umbrella of the Island Rule can be expanded to plants, accelerating understanding of how plant traits evolve on isolated islands.</p>

The island rule and its application to multiple plant traits

<p>The Island Rule refers to a continuum of body size changes where large mainland species evolve to become smaller and small species evolve to become larger on islands. Previous work focuses almost solely on animals, with virtually no previous tests of its predictions on plants. I tested for (1) reduced floral size diversity on islands, a logical corollary of the island rule and (2) evidence of the Island Rule in plant stature, leaf size and petiole length. Endemic island plants originated from small islands surrounding New Zealand; Antipodes, Auckland, Bounty, Campbell, Chatham, Kermadec, Lord Howe, Macquarie, Norfolk, Snares, Stewart and the Three Kings. I compared the morphology of 65 island endemics and their closest ‘mainland’ relative. Species pairs were identified. Differences between archipelagos located at various latitudes were also assessed. Floral sizes were reduced on islands relative to the ‘mainland’, consistent with predictions of the Island Rule. Plant stature, leaf size and petiole length conformed to the Island Rule, with smaller plants increasing in size, and larger plants decreasing in size. Results indicate that the conceptual umbrella of the Island Rule can be expanded to plants, accelerating understanding of how plant traits evolve on isolated islands.</p>

An Italian dinosaur Lagerstätte reveals the tempo and mode of hadrosauriform body size evolution

During the latest Cretaceous, the European Archipelago was characterized by highly fragmented landmasses hosting putative dwarfed, insular dinosaurs, claimed as fossil evidence of the “island rule”. The Villaggio del Pescatore quarry (north-eastern Italy) stands as the most informative locality within the palaeo-Mediterranean region and represents the first, multi-individual Konservat-Lagerstätte type dinosaur-bearing locality in Italy. The site is here critically re-evaluated as early Campanian in age, thus preceding the final fragmentation stages of the European Archipelago, including all other European localities preserving hypothesized dwarfed taxa. New skeletal remains allowed osteohistological analyses on the hadrosauroid Tethyshadros insularis indicating subadult features in the type specimen whereas a second, herein newly described, larger individual is likely somatically mature. A phylogenetic comparative framework places the body-size of T. insularis in range with other non-hadrosaurid Eurasian hadrosauroids, rejecting any significant evolutionary trend towards miniaturisation in this clade, confuting its ‘pygmy’ status, and providing unmatched data to infer environmentally-driven body-size trends in Mesozoic dinosaurs.

Genetic data from the extinct giant rat from Tenerife (Canary Islands) points to a recent divergence from mainland relatives

Evolution of vertebrate endemics in oceanic islands follows a predictable pattern, known as the island rule, according to which gigantism arises in originally small-sized species and dwarfism in large ones. Species of extinct insular giant rodents are known from all over the world. In the Canary Islands, two examples of giant rats, † Canariomys bravoi and † Canariomys tamarani , endemic to Tenerife and Gran Canaria, respectively, disappeared soon after human settlement. The highly derived morphological features of these insular endemic rodents hamper the reconstruction of their evolutionary histories. We have retrieved partial nuclear and mitochondrial data from † C. bravoi and used this information to explore its evolutionary affinities. The resulting dated phylogeny confidently places † C. bravoi within the African grass rat clade ( Arvicanthis niloticus ). The estimated divergence time, 650 000 years ago (95% higher posterior densities: 373 000–944 000), points toward an island colonization during the Günz–Mindel interglacial stage. † Canariomys bravoi ancestors would have reached the island via passive rafting and then underwent a yearly increase of mean body mass calculated between 0.0015 g and 0.0023 g; this corresponds to fast evolutionary rates (in darwins (d), ranging from 7.09 d to 2.78 d) that are well above those observed for non-insular mammals.

Palaeohistology reveals a slow pace of life for the dwarfed Sicilian elephant

The 1-m-tall dwarf elephant Palaeoloxodon falconeri from the Pleistocene of Sicily (Italy) is an extreme example of insular dwarfism and epitomizes the Island Rule. Based on scaling of life-history (LH) traits with body mass, P. falconeri is widely considered to be ‘r-selected’ by truncation of the growth period, associated with an early onset of reproduction and an abbreviated lifespan. These conjectures are, however, at odds with predictions from LH models for adaptive shifts in body size on islands. To settle the LH strategy of P. falconeri, we used bone, molar, and tusk histology to infer growth rates, age at first reproduction, and longevity. Our results from all approaches are congruent and provide evidence that the insular dwarf elephant grew at very slow rates over an extended period; attained maturity at the age of 15 years; and had a minimum lifespan of 68 years. This surpasses not only the values predicted from body mass but even those of both its giant sister taxon (P. antiquus) and its large mainland cousin (L. africana). The suite of LH traits of P. falconeri is consistent with the LH data hitherto inferred for other dwarfed insular mammals. P. falconeri, thus, not only epitomizes the Island Rule but it can also be viewed as a paradigm of evolutionary change towards a slow LH that accompanies the process of dwarfing in insular mammals.

Do Winds Control the Confluence of Subtropical and Subantarctic Surface Waters East of New Zealand?

<p>The confluence region east of New Zealand is one of only a few places in the world where the Antarctic Circumpolar Current meets the strong southwardflowing boundary current of a subtropical gyre. The convergence of subtropical and subantarctic water creates strong fronts. The fronts have clear signatures in height and temperature that make them appropriate places to investigate ocean/climate variability. The location and extent of the New Zealand confluence should respond to changes in large-scale wind patterns, as changes in South Pacific currents have been linked to wind shifts. However, recent studies have shown that highly energetic eddies, local winds, and the bathymetry may be significant controls of currents and associated fronts. This thesis investigates the temporal and spatial variability of the confluence and evaluates its response to variability in South Pacific winds. Analysis of the 18-year time series, from January 1993 to December 2010, of sea surface height mapped from satellite altimetry was used to investigate the location and extent of fronts and the eddy activity and relate these to the wind forcing. Wind stress data were used with the Island Rule to estimate the winddriven transport of the western boundary currents that feed the confluence. In addition, the climate modes Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) were used to examine the influence of the principal modes of atmospheric variability. Time series of the local wind stress curl and local climate indices were calculated and compared to the intensity of the confluence to test any influence of local forcing. In addition, bathymetric effects were investigated by evaluating evidence for preferred front locations near topographic features. Sea level anomalies in the confluence region are increasing at 3.4 cm decade⁻¹. The sea surface height gradients and the eddy kinetic energy are also increasing at a rate of 0.01 cm km⁻¹ and 23 cm² s⁻² per decade respectively, indicating an intensification of the fronts and eddy activity in the confluence. There is a high and significant correlation (r = 0.84) between the front and eddy signals reflecting baroclinic instabilities inherent in the fronts. Difference in transport anomalies across the confluence derived from the Island Rule are also increasing at 8.8 Sv decade⁻¹. SAM and SOI indices showed little or no correspondence with variability in the confluence intensity and eddy kinetic energy, and the same lack of correspondence was observed in local winds and local indices. While these results suggest a connection between the variability in the confluence and South Pacific winds, there is a preferential location of the strongest fronts and eddy activity northeast of Bounty Plateau and Bollons Seamount, indicating some bathymetric control. The correspondence between basin-scale winds and sea surface height gradients in the confluence region indicates that if wind stress continues to increase, as current trends predict, front intensity and eddy activity will also increase, enhancing the transfer of heat and nutrients that, respectively, influence energy transfer and biological productivity.</p>

Do Winds Control the Confluence of Subtropical and Subantarctic Surface Waters East of New Zealand?

<p>The confluence region east of New Zealand is one of only a few places in the world where the Antarctic Circumpolar Current meets the strong southwardflowing boundary current of a subtropical gyre. The convergence of subtropical and subantarctic water creates strong fronts. The fronts have clear signatures in height and temperature that make them appropriate places to investigate ocean/climate variability. The location and extent of the New Zealand confluence should respond to changes in large-scale wind patterns, as changes in South Pacific currents have been linked to wind shifts. However, recent studies have shown that highly energetic eddies, local winds, and the bathymetry may be significant controls of currents and associated fronts. This thesis investigates the temporal and spatial variability of the confluence and evaluates its response to variability in South Pacific winds. Analysis of the 18-year time series, from January 1993 to December 2010, of sea surface height mapped from satellite altimetry was used to investigate the location and extent of fronts and the eddy activity and relate these to the wind forcing. Wind stress data were used with the Island Rule to estimate the winddriven transport of the western boundary currents that feed the confluence. In addition, the climate modes Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) were used to examine the influence of the principal modes of atmospheric variability. Time series of the local wind stress curl and local climate indices were calculated and compared to the intensity of the confluence to test any influence of local forcing. In addition, bathymetric effects were investigated by evaluating evidence for preferred front locations near topographic features. Sea level anomalies in the confluence region are increasing at 3.4 cm decade⁻¹. The sea surface height gradients and the eddy kinetic energy are also increasing at a rate of 0.01 cm km⁻¹ and 23 cm² s⁻² per decade respectively, indicating an intensification of the fronts and eddy activity in the confluence. There is a high and significant correlation (r = 0.84) between the front and eddy signals reflecting baroclinic instabilities inherent in the fronts. Difference in transport anomalies across the confluence derived from the Island Rule are also increasing at 8.8 Sv decade⁻¹. SAM and SOI indices showed little or no correspondence with variability in the confluence intensity and eddy kinetic energy, and the same lack of correspondence was observed in local winds and local indices. While these results suggest a connection between the variability in the confluence and South Pacific winds, there is a preferential location of the strongest fronts and eddy activity northeast of Bounty Plateau and Bollons Seamount, indicating some bathymetric control. The correspondence between basin-scale winds and sea surface height gradients in the confluence region indicates that if wind stress continues to increase, as current trends predict, front intensity and eddy activity will also increase, enhancing the transfer of heat and nutrients that, respectively, influence energy transfer and biological productivity.</p>

Topographic Influences on the Wind-driven Exchange Between Marginal Seas and the Open Ocean

The wind-driven exchange through complex ridges and islands between marginal seas and the open ocean is studied using both numerical and analytical models. The models are forced by a steady, spatially uniform northward wind stress intended to represent the large-scale, low-frequency wind patterns typical of the seasonal monsoons in the western Pacific Ocean. There is an eastward surface Ekman transport out of the marginal sea and westward geostrophic inflows into the marginal sea. The interaction between the Ekman transport and an island chain produces strong baroclinic flows along the island boundaries with a vertical depth that scales with the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. The throughflows in the gaps are characterized by maximum transport in the center gap and decreasing transports towards the southern and northern tips of the island chain. An extended island rule theory demonstrates that throughflows are determined by the collective balance between viscosity on the meridional boundaries and the eastern side boundary of the islands. The outflowing transport is balanced primarily by a shallow current that enters the marginal sea along its equatorward boundary. The islands can block some direct exchange and result in a wind-driven overturning cell within the marginal sea, but this is compensated for by eastward zonal jets around the southern and northern tips of the island chain. Topography in the form of a deep slope, a ridge, or shallow shelves around the islands alters the current pathways but ultimately is unable to limit the total wind-driven exchange between the marginal sea and the open ocean.