Can the native Rusty Fig, Ficus rubiginosa, beat climate change?

Abstract

<p>This study investigated aspects of male and female reproductive success in a widespread, keystone dry-rainforest species, <i>Ficus rubiginosa</i>, the Rusty Fig. The primary aim of the study was to determine if climate change was impacting on reproductive success in <i>F. rubiginosa</i> and/or was likely to impact in the future. Four key questions relevant to plant population ecology in fragmented landscapes are addressed: <b>Question 1</b>. What impacts do climatic variation and fig-population fragmentation have on flowering and fruiting phenology in <i>F. rubiginosa</i> trees growing in a temperate, seasonal environment? <b>Question 2</b>. What impacts do climatic variation and fig-population fragmentation have on pollinator dispersal among <i>F. rubiginosa</i> trees and, if there are impacts, what are the consequences for pollen dispersal and fruit set? <b>Question 3</b>. What impacts do climatic variation and fig-population fragmentation have on frugivore visitation to <i>F. rubiginosa</i> and, if there are impacts, what are the consequences for seed dispersal? <b>Question 4</b>. What have the long-term patterns of dispersal and range extension by <i>F. rubiginosa</i> been in eastern Australia? Field work concentrated in the western half of <i>F. rubiginosa</i>'s range in northern New South Wales, Australia. Four key areas of population ecology relating to phenology, pollination success, avian visitation and genetic variation were addressed. Environmental factors impacting on F<i>F. rubiginosa</i>'s fitness in these four areas were identified and measured over a three-year period between January 2014 and January 2017. Further field work was conducted over the 3,000 km range of the species in July to October 2015 to collect genetic samples for Next Generation Sequencing and Single Nucleotide Polymorphism analysis to determine current and historic patterns of genetic variation across the species' fragmented distribution.</p><p> Major findings from this study were that phenology was impacted by temperature and rainfall. Low winter temperatures caused syconium development to slow down and go into stasis. Low rainfall stimulated the extension of male and female phenophases in trees through staggered syconium development, which increased reproductive assurance but reduced the number of ripe fruit on trees, thereby reducing a tree's attractiveness to frugivore seed-dispersers. Pollination success was reduced in drier parts of <i>F. rubiginosa</i>'s range compared with that in more mesic environments. Extreme fragmentation - single trees - provided pollinators with refuge from parasitoids leading to increased male and female fitness in <i>F. rubiginosa</i>. Retention of unpollinated fruit on trees increased female fitness by increasing apparent crop size and attracting greater visitation by frugivores. <i>Ficus rubiginosa</i> was a pivotal and keystone resource for twice as many insectivorous bird species as frugivore species. Drier climates during the last ice age led to contraction of <i>F. rubiginosa</i> populations to four probable refugia, scattered over 22° (2,500 km) of latitude. Dry coastal zones that functioned as biogeographic barriers to gene flow during the last ice age have functioned as corridors facilitating dispersal more recently.</p><p> Signals of early population decline can be difficult to recognise in long-lived species such as large <i>Ficus</i> trees. However, several factors that impacted on pollination success and seed production in <i>F. rubiginosa</i> were identified during this project. Among these factors was fire, projected to keep increasing in frequency and intensity under the influence of climate change. Flower and fruit production was delayed by up to five years and more in many trees across the study area, as trees recovered (resprouted) after fire. Lower rainfall resulted in slower and reduced fruit production, and fruit that were produced were smaller and had fewer seeds. Smaller crops of ripe fruit in trees attracted lower avian visitation and, hence, reduced seed dispersal. Lower pollination success was recorded during drier conditions. And gene flow was obstructed by dry areas between populations. All of these factors and events point to reductions in fitness and early population decline. They are all consequences of deteriorating rainfall and increasing temperatures, which have been experienced across the study area over the last few decades. Further decline is likely and local population extinctions are possible in those parts of <i>F. rubiginosa</i>'s range where climate change results in further reductions in rainfall and ongoing increases in temperature. This will negatively impact on the communities <i>F. rubiginosa</i> lives in and on ecosystem services the affected plants and animals provide within the local and surrounding environments.</p><p> <i>Ficus rubiginosa</i> would be an important species to use in restoration as it provides a year-round food resource for frugivores and, as found in this study, a year-round and unique food resource for insectivorous woodland birds, many of which are in decline in this region and others globally.</p>