Macroevolutionary Changes of Plants on Islands

<p>Insularity is known to produce predictable evolutionary changes in plants. For example, herbaceous plants often evolve woodiness and seeds tend to have reduced dispersal capabilities on islands. However, our understanding of how other plant traits may evolve on islands is lacking. Furthermore, plants are modular organisms and by investigating evolutionary changes in specific plant traits we may better understand macroevolutionary processes on islands. In this thesis, I investigate evolutionary changes in a range of plant traits on islands. First, I tested for evolutionary changes in seed size on islands (Chapter 2). Island plants consistently produced larger seeds than mainland relatives. Furthermore, this result was consistent regardless of differences in dispersal mode, growth form and evolutionary history. Selection may favour increased seed size to reduce dispersal distances. Additionally, selection may favour larger seeds due to the competitive advantage conferred to developing seedlings. Many animal taxa exhibit increased sexual size dimorphism (SSD) on islands, as predicted by the niche variation hypothesis. However, patterns of SSD among dioecious plants on islands are unknown. In Chapter 3 I tested for differences in SSD of dioecious plants that colonized four island groups from New Zealand (mainland). The degree of SSD did not vary predictable between island and mainland plants, contrary to predictions of the niche variation hypothesis. However, SSD was consistently female biased on the mainland and results suggest selection is acting to increase the size of both sexes on islands. Evolutionary changes in island plants may be a response to herbivory by unique large browsers. For example, the divaricate growth form is common in the New Zealand flora and may have deterred browsing moa. In Chapter 4 I tested for differences in traits associated with the divaricate growth form between plants from mainland New Zealand and Chatham Island. Results suggest that an absence of moa on Chatham Island has relaxed selection for traits associated with the divaricate growth form. An emerging body of research suggests aposematism (warning signals to herbivores) may be common in plants. However, previous investigations have not appreciated the fact that the perspective of terrestrial herbivores changes as plants grown vertically. Furthermore, ontogenetic changes in the capacity of plants to defend themselves may influence the reliability of warning signals. In Chapter 5 I tested for ontogenetic changes in two potentially aposematic signals produced by Pseudopanax crassifolius. Aposematism on upper leaf surfaces peaked early in ontogeny, providing a dishonest signal of defense. Conversely, signaling on the underside of leaves peaked later in ontogeny and scaled positively with structural defenses. The results of this thesis suggest selection is acting on specific plant traits on islands. Evolutionary pathways, such as the evolution of woodiness, may be better explained by considering selection acting on other plant traits. For example, selection acting on seed size may facilitate evolutionary size changes evident at later life-history stages. A lack of consensus exists regarding the role of insular herbivores in the evolution of island plants. The results of Chapters 4 and 5 suggest herbivory has played an important role in the evolution of novel morphology of island plants. Considering trait specific changes of plants on islands may further our understanding of prominent evolutionary pathways by pinpointing the action of selection.</p>

How to Become a Generalist Species? Individual Niche Variation Across Habitat Transformation Gradients

Species in transformed habitats, frequently labeled as environmental generalists, tend to show broader niches than species in natural habitats. However, how population niche expansion translates into changes in the niches of individual organisms remains unclear, particularly in the context of habitat transformation. Niche expansion could be a product of individuals having broader niches, greater distances among individuals’ niches, or a combination of both processes. This would challenge the traditional conceptions on niche dynamics, which emphasize the role played by individual specialization (IS). Here, using stable isotopes, we computed total niche width (TNW), its within- and between-individual components (WIC and BIC), and IS (the ratio WIC/TNW), in 13 populations of 6 bird species and 8 populations of 3 frog species in natural and transformed habitats. We confirmed that species had broader niche width in transformed than in natural habitats, yet population niche expansion across habitats was mainly a product of increased distance between individuals. Within each habitat type, increases in TNW were linked to increases in WIC for all habitat types, while relationships between TNW and BIC were found in transformed but not in natural habitats. Hence, both increased individual niche width and increased distance among individuals were apparent within habitats, particularly in transformed ones, where increases in WIC dominated. Neither across or within habitats was niche expansion associated with increasing IS. Therefore, our results overturn traditional conceptions associated with the niche variation hypothesis and illustrate that niche expansion is not invariably associated with increased IS, because the distance between individual’s niches (BIC) can increase, as well as the breadth of those niches (WIC).

Feeding Strategies of Co-occurring Newt Species across Different Conditions of Syntopy: A Test of the “Within-Population Niche Variation” Hypothesis

Intraspecific trait variation in generalist animals is widespread in nature, yet its effects on community ecology are not well understood. Newts are considered opportunistic feeders that may co-occur in different syntopic conditions and represent an excellent model for studying the role of individual feeding specialization in shaping the population dietary strategy. Here, we investigated the diet of three newt species from central Italy occurring in artificial habitats in different coexistence conditions to test the predictions of the niche width (NW) variation hypotheses. Population NW did not vary among species and between presence and absence of coexisting species. An overall positive relationship between individual specialization and population NW was observed. However, this pattern was disrupted by the condition of syntopy with newt populations showing an individual NW variation invariant with population NW in presence of coexisting species, whereas it was larger in populations occurring alone. The observed pattern of newt behavior was not consistent with any of the proposed scenarios. We found a consistent pattern with the degree of individual specialization being (1) size-dependent (specialized individuals increasing within larger sized species) and (2) assemblage-complexity-dependent (specialized individuals increasing in syntopic populations in comparison to singly populations).

Principles of niche expansion

Niche expansion is attained by adaptations in two generalized phenotypical traits—niche position and niche width. This gives room for a wide range of conceptual ways of niche filling. The niche variation hypothesis reduces the range by predicting that expansion occurs by increasing variation in niche position, which has been debated on empirical and theoretical grounds as also other options seem possible. Here, we propose a general theory of niche expansion. We review empirical data and show with an eco-evolutionary model how resource diversity and a trade-off in resource acquisition steer niche evolution consistent with observations. We show that the range can be reduced to a discrete set of two orthogonal ways of niche filling, through (1) strict phenotypical differentiation in niche position or (2) strict individual generalization. When individual generalization is costly, niche expansion undergoes a shift from (2) to (1) at a point where the resource diversity becomes sufficiently large. Otherwise, niche expansion always follows (2), consistent with earlier results. We show that this either–or response can operate at both evolutionary and short-term time scales. This reduces the principles of niche expansion under environmental change to a notion of orthogonality, dictated by resource diversity and a resource-acquisition trade-off.

Feeding limitations in temperate anurans and the niche variation hypothesis

The niche variation hypothesis (NVH) states that populations with wider niches are more phenotypically variable. The NVH has important ecological and evolutionary implications but has been controversial since its inception. Recent interpretations have supported the NVH by directly comparing among-individual diet variation with population dietary niche breadth. Traditional studies of the NVH focused on morphological traits as proxies of niche variation, with contradictory results. Gape-limited predators may be relatively likely to show effects of morphological variation on diet breadth because gape size can strongly limit diet. We used five anurans to test NVH predictions, including three true frogs, Rana catesbeiana, R. clamitans, and R. sphenocephala, and two toads, Anaxyrus americanus and A. fowleri. We combined recent and traditional approaches by comparing both individual variation in diet and variation in gape width with dietary niche breadth. We found support for the NVH within two species of the three true frogs but not for either toad species, a difference likely driven by greater strength of the feeding limitation caused by gape width in the frogs. Toads had higher gape width to snout-vent length ratios, reducing the strength of the feeding limitation imposed by gape width. We found strong support for the NVH among species; species with more among-individual variation in diet and species with more variation in gape width had broader niches. Our results highlight the circumstances under which the NVH is applicable and demonstrate an example in which the NVH is supported through both traditional and recent interpretations.

Trophic specialization at the individual level in a terrestrial generalist salamander

Information on individual trophic specialization may be relevant to better understand the ecological adaptation of populations to their environment and the evolution of their realized trophic niche. In this study, we analysed the trophic specialization at the individual level in a population of the plethodontid Northwest Italian Cave Salamander (Speleomantes strinatii (Aellen, 1958)), a terrestrial generalist predator. Salamanders were sampled in northwestern Italy on the forest floor in autumn (n = 49) and spring (n = 47) along with their available prey. In autumn, when trophic resources showed a twofold reduction in abundance, the population trophic niche width (TNW = 2.58) was significantly broader than during spring (TNW = 2.25), and in both seasons, individual specialization (IS) was significantly higher than expected by chance (P = 0.001). There were no sexual or ontogenetic differences in IS within each season, but IS in autumn was significantly higher than in spring (IS = 0.34 and IS = 0.41, respectively; P = 0.01). These findings are in accordance with the niche variation hypothesis, which predicts a positive relationship between TNW and IS. Therefore, while the population became more generalist, individual salamanders shifted towards a more specialized diet by adapting their feeding behaviour to changes in prey availability.