Critical (ecological) care in the land of the dodo: invasive species removal

By Eva Colberg, postdoctoral fellow at Cornell University. Nearly all of Mauritius’s contemporary conservation plights are rooted in or exacerbated by the effects of invasive, non-native species. To see what restoration can do for the island’s few remaining forests, Dr. Eva Colberg joined members of the Tropical Island Biodiversity, Ecology & Conservation research group to visit (and weed) one of the island’s forest restoration sites.

Two of Mauritius’s prominent ecological invaders, a macaque and strawberry guava. Photo: Eva Colberg.

Red stems of strawberry guava (Psidium cattleyanum) form a wall dense enough to prevent walking through most of Mauritius’s remaining forests. Beyond impeding movement, the thick guava understory also reduces overstory tree fitness and disrupts native forest growth and succession. Originally from South America, strawberry guava is a classic case of a non-native, invasive species outcompeting and reducing habitat quality outside its native range (and islands are particularly vulnerable to invasion).

Litter basket ferns (Asplenium nidus) and other native species grow in the understory space freed up in a 20-ha area since the UNDP-funded removal of strawberry guava and other invasives in 1996. Photo: Eva Colberg.

Strawberry guava is far from the only invader threatening Mauritius’s flora and fauna. Alien ants disrupt pollination of native plants, an effect compounded by invasive plant presence. Conflicts between fruit farmers and a keystone seed disperser, the Mauritian flying fox (Pteropus niger), could be due to poor habitat quality and low native fruit production in invaded forests. Invasive macaques (Macaca fascicularis) further disrupt plant reproduction by breaking branches and eating fruits before they’re ripe, and eating and stealing nectar from native flowers without pollination.

Vincent Florens and an undergraduate student discuss the diversity of epiphytes found in a weeded section of forest at Black River Gorges National Park. Photo: Eva Colberg.

The ongoing onslaught of invasion means there’s no time to waste for restoration ecologists like F.B. Vincent Florens, Associate Professor at the University of Mauritius. “We have so many rare species on the brink of extinction [over 80% of the island’s endemic flowering plants are threatened], and have to work at the same time and learn as we go.” His life experience and ecological studies point to invasive species management as the island’s best hope for restoration and conservation, which he likens to healthcare. “First you save the person from dying and then you can treat the other issues.”

Although the views from Black River Gorges National Park are stunning, they also show the sparseness of the park’s forest overstory, with fewer and farther-between survivors.

Although avian re-introductions and rewilding small islets with tortoises are sexier solutions than mere weeding, the best way to keep Mauritius’s mainland forests from dying is through invasive plant removal. After weeding, native trees in all forest strata produce more flowers and fruit, woody plants increase in species richness and seedling density, and butterfly diversity and abundance also increase. These many benefits can be furthered and maintained by follow-up weeding and other subsequent measures (including the promise of biochar to suppress weed regeneration).

Recently described and known to only a few locations, the orchid (Polystachya jubaultii) grows in a weeded forest remnant at Black River Gorges National Park. Photo: Eva Colberg.

Despite decades’ worth of evidence pointing to the efficacy of invasive plant removal in Mauritius, it still isn’t widely implemented. Less than 5% of the island’s few remaining forests have been weeded of invasive plants, and even the best-protected forests are already dominated by invasive undergrowth. Frustratingly, some of the resources that could be used for invasive removal have instead hindered restoration via removal of native pioneer and nurse tree species. “We can do a lot of science, can come up with a lot of facts, but how do we get people to do what they don’t want to do?” Indeed, it’s far easier to uproot a small plant than to change someone’s mind, and Prof. Florens has an entire country to convince that saving their native forests is not only possible, but worth the effort.

Hard times for hemiepiphytes: Aroids have trouble making a comeback in second-growth forests

Estefania Fernandez Barrancos is a PhD student and Christensen Fellow at the University of Missouri St. Louis, where she is affiliated with the Harris World Ecology Center and the Center for Conservation and Sustainable Development at the Missouri Botanical Gardens. Estefania has previously written about how to restore bromeliad populations. Here she describes a recent study asking how well hemiepiphytic aroids recover in secondary forests in Panama.

Most people know aroids as the familiar swiss cheese plants found growing in hotels and shopping malls. But few people realize that the aroid family (Araceae) is the fifth most diverse plant family on Earth. These plants provide essential food and refuge for birds, bats, insects, and primates in tropical forests throughout the world.

Like many other plants, aroid populations are dropping because the rainforests where they live are being converted into farms. My new research shows that aroids are also slow to recolonize new forests that become available.

City aroid (left, Monstera deliciosa in a building), country aroid (right, Monstera sp. in a Colombian forest). Photo sources: Left – Maja Dumat CCBY 2.0; Right – Thomas Croat via Tropicos.

Before I describe that research though, here is some botanical jargon for the uninitiated. Epiphytes (a.k.a. air plants) are plants that grow on other plants (but not as parasites). Hemiepiphytes are plants that grow on other plants but only for part of their lives. Many aroids are hemiepiphtyes because they start life in the soil of the forest understory and grow until they find a tree. Then they climb up the tree and live above the ground, but they always keep a connection to solid earth.

To study their recovery, I surveyed hemiepiphytic aroids in native tree plantations (9-years old), natural secondary forests (8-14-years old), and mature forests (>100-years old) near the Panama Canal. These forests are part of Agua Salud – a tropical forest restoration experiment led by the Smithsonian Tropical Research Institute. In the dense forest, I found aroids by looking for their stems coming down from the trees, then I followed the stem with binoculars until I found their leaves, which helped me identify the species. In all, I surveyed 1479 trees this way.

Estefania Fernandez (below) and field assistant Carlos Diaz (above) look for aroids in a mature forest tree in Panama.

I found out that there were virtually no aroids in secondary forests or plantations. I recorded more than 2000 aroids from at least ten species growing on trees in mature forest, but in secondary forests and plantations I found less than 1% as many aroids and only three species.

Why do aroids have recovery troubles?

One reason for the lack of aroids could be that seeds from adult aroids in mature forests can’t reach the new forests. This seems unlikely because all of the secondary forests and plantations in my study were close to mature forests full of aroids, less than one kilometer away. Also, birds that are present in secondary forests are known to eat aroid fruits and disperse their seeds.

Another reason could be that the young forest canopy is too open for aroid seeds to germinate and grow. Unlike most plants, some aroids start out life growing away from light and towards darkness. (This has another great word: skototropism). It seems counterintuitive since most plants need light. But it is actually a good strategy. By growing away from light, aroid seedlings are more likely to run into a tree, which they need to climb up into the canopy and get to the light that they need to photosynthesize. So it is possible that there is too much light in the young forests and it keeps the aroid seedlings from finding a host tree.

Whether dispersal or establishment limits aroids in secondary forests, it is likely that more time will help. As forests become older and darker and birds bring in more seeds, aroid populations should eventually begin to recover. My research suggests that there is a considerable lag time required for aroids to recolonize disturbed habitats such as secondary forests and plantations.

More importantly, my study highlights how important it is to hold onto old forests. Forest restoration is a poor substitute for mature forest conservation. To the extent that we can prevent older forests from being cut down, it will help preserve many species of aroids as well as other plant and animal species that are threatened by habitat loss.

Aroid being pollinated by scarab beetles at Barro Colorado Island, Panama. Source: www.aroid.org.

You can read more about Estefania’s research in her new open-access paper in Tropical Conservation Science, or on other posts from Natural History of Ecological Restoration (here and here).

Drivers of epiphyte recovery in secondary forests in southeastern Brazil

Alex Fernando Mendes is an undergraduate researcher in the Tropical Silviculture Lab at the University of São Paulo, Brazil. He describes his thesis project, undertaken in dozens of forest fragments in the endangered Atlantic Forest biome. Currently, Alex is analyzing his data as a visiting researcher in the Center for Conservation and Sustainable Development.

Historically, intensive agriculture in the Brazilian Atlantic Forest has caused large-scale deforestation of this biome. However, new legal requirements, land exhaustion, and the shifting priorities of farmers have recently allowed forests to regenerate on some formerly farmed lands. Given the unique nature of the Atlantic Forest, its high species endemism, and its potential for providing ecosystem services, the Tropical Silviculture Lab (LASTROP) at the University of São Paulo, coordinated by Prof. Pedro Brancalion and its partners, initiated a project in 2014 to better understand the structure and composition of these new forests.

However, forests aren’t made solely of trees. Among the plant components of a forest, there are others life forms such as epiphytes, lianas, and herbs that contribute to biodiversity and provide food, water, and shelter for many animal species. Epiphytes are plants that use other plants as support. Due to their sensitivity to environmental changes, epiphytes can be used as bioindicators. Therefore, we asked how these plants are doing in these young regenerating forests. And what landscape and local attributes facilitate or hinder their recolonization?

Epiphyte species found in second-growth forests of the Atlantic Forest – A) Philodendron bipinnatifidum Schott; B) Lepismium houlletianum (Lem.) Barthlott.; C) Ionopsis utricularioides (Sw.) Lindl.; D) Catasetum fimbriatum (E. Morren) Lindl. & Paxton; E) Aechmea bromeliifolia (Rudge) Baker; F) Billbergia sp.

 

To try to answer these questions, we are studying the epiphyte communities in 40 second-growth forests (i.e., forests that were once completely cut down). We are considering three landscape drivers (distance from watercourses, distance from forest edge and forest cover in a 1-km buffer around the remnant) and four local drivers (previous land use, forest age, liana abundance, and tree basal area). We expect that forests close to watercourses would provide the moisture required by epiphytes. We expect to find more epiphytes further from the forest edge since forest cover in more conserved forests may limit their establishment. Since our forests regenerated from abandoned eucalyptus plantation and pastures, we want to check if the non-native eucalyptus could act as a filter preventing epiphytes recolonization. We also expect that older forests and forests with more basal area could house more epiphytes than young forests. Finally, field observations made us wonder if lianas could compete with epiphytes by occupying the same niche.

Of the more than 6,000 phorophytes (trees that could support epiphytes) sampled in these 40 forests we found 398 epiphytes belonging to 21 morphospecies distributed in 4 families (Araceae – 1 species, Bromeliaceae – 14 species, Cactaceae – 5 species, Orchidaceae – 5 species). Only three species, Tillandsia pohliana, Tillandsia tricholepis, and Ionopsis utricularioides, represented more than half (59.5%) of all epiphytes found in second-growth forests. The genus Tillandsia was expected to be abundant in these young forests, since these are disturbance-adapted species that can even be found growing on power lines in cities.

Epiphytes of the genus Tillandsia (Bromeliaceae) are often found in extreme microenvironments in urban areas (Photos by A. Mendes, 2017).

Our analysis is in progress, but our preliminary observations suggest that forests closer to watercourses and closer to forest edge are more likely to have epiphyte recolonization than forests far from edge and watercourses. Forests regenerated on pastures have more epiphytes than those on abandoned eucalyptus plantation. Our dataset will soon be upgraded with new forest types: conserved and disturbed old-growth forests, and mixed tree plantings for forest restoration, totaling approximately 70 forests with epiphyte samples.

With this research, we hope to find out the local and landscape factors that contribute to epiphyte recolonization in second-growth forests. In practice, this will allow us to locate sites with limited potential for spontaneous colonization of this life form to take actions that promote colonization and establishment, such as introducing individuals. Finally, by identifying epiphytes species that are more sensitive to disturbance, we can focus our reintroduction interventions.

Small, remnant forests are surrounded by cattle pastures in southern Brazil.

Note: The image of Philodendron bipinnatifidum featured at the top of this post was taken by David Stang. 

Vascular epiphyte restoration using bromeliad transplants in Southern Costa Rica

Estefania Fernandez is a Bascom Fellow who recently finished her master’s thesis at the University of Montpelier, France. Last year, Estefania wrote about her preliminary results on tropical forest restoration and vascular epiphyte reintroductions in Costa Rica. Here, she describes the final results, recently published in Restoration Ecology.

A transplanted bromeliad, Aechmea dactylina flowering in a 10-year old tree plantation.

Vascular epiphytes are plants that germinate and root on other plants without taking their nourishment from their host plant, and they represent 50% of the flora in some tropical forests and 9% of all vascular plants worldwide. If you are a plant lover, then you most likely have one or several vascular epiphytes in your house. Some of the most appreciated horticultural families include orchids (Orchidaceae), aroids (Araceae), and bromeliads (Bromeliaceae).

Vascular epiphytes also play key roles in our ecosystems. They are crucial to forest water and mineral recycling as they intercept rainfall and prevent rapid run-off and nutrient leaching. Vascular epiphytes are also exceptional microhabitats where invertebrate communities find refugia and birds and arboreal mammals forage.

Transplanted individual of Werauhia gladioliflora

Despite their importance in forest ecosystems, vascular epiphytes are rarely taken into account in forest restoration. This is problematic because vascular epiphytes are often among the slowest plants to recolonize regenerating forests.

In 2015-2016, I tested whether transplanting epiphytes into young restoration sites could be a viable strategy to accelerate their reestablishment. I used a bromeliad for my experiment, Werauhia gladioliflora (H. Wendl.) J.R. Grant, which was common in remnant forest but had not been found during epiphyte surveys in nearby restoration areas. In March-June 2015, I transplanted 60 bromeliads into three restoration plantations near Las Cruces Biological Station in southern Costa Rica. I revisited the sites in January-February 2016, nine months after transplantation, to monitor survival and arthropod recolonization.

Happily, over 75% bromeliads survived and the number of arthropods on branches with bromeliads was seven times greater than in branches without bromeliads. Additionally, I observed that bromeliads buffered the local microclimate; during the driest and hottest times of the day, the interior of the bromeliads was moister and cooler than ambient air.

Transplanted individuals of Werauhia gladioliflora (left) hosted considerably more arthropods in their rosettes than could be found on the stems of trees that had not received a transplant. GN, JG, and MM are three study sites near Las Cruces Biological Station in southern Costa Rica. Photo by Dave Janas.

Restoring arboreal refugia

My research suggests that transplanting fallen epiphytes onto trees in restored sites contributes to the recovery of vascular epiphyte diversity in these ecosystems and has the additional benefits of bringing back arthropod diversity to these sites. Epiphytes, and specifically “tank” epiphytes that retain water in their rosettes, help stabilize microclimatic conditions, a critical function in light of climate change, which may put arboreal communities at special risk. Indeed, the body temperature of many animals such as invertebrates entirely depends on ambient temperatures but rising temperatures could push arboreal animal communities to the ground. Epiphytes offer ideal refugia from high temperatures and drought and their presence in tree canopies and understory is critical to preserve arboreal animal communities. Transplanting other epiphyte families or even entire epiphyte communities found on fallen branches could be tested in the future to broaden this strategy.

Estefania inspects a flowering individual of an Aechmea dactylina transplant

This work was supported by a grant from the National Science Foundation.

Transplanted bromeliads improve microclimate and facilitate arthropods in restored forests

Estefania Fernandez is a masters student at the University of Montpellier, France. She spent the past six months working with scientists in the Center for Conservation and Sustainable Development on a tropical forest restoration experiment in southern Costa Rica.

Costa Rica is one of the world’s most biodiverse countries, hosting 4% of flowering plant species in an area representing only 0.03% of the Earth’s terrestrial surface. With a large diversity of ecosystems, ranging from mangroves to cloud forests, Costa Rica hosts a unique family of (almost exclusively) Neotropical plants: the Bromeliaceae, commonly called bromeliads. With their colorful inflorescences and strikingly patterned leaves, numerous bromeliads are cultivated around the world for their ornamental value. Less is known, however, about their ecology in tropical ecosystems, particularly in regenerating forests.

Werauhia gladioliflora rosette, showing its overlapping leaves.

Many of the so-called “tank bromeliads” are epiphytes, meaning that they grow non-parasitically on other plants. These bromeliads have ample rosettes of overlapping leaves, capable of holding considerable amounts of water. These water tanks keep them hydrated, and plant detritus that accumulates in these structures also provides bromeliads with nutrients. Arthropods take refuge in bromeliad rosettes, and consequently these plants attract mammals and birds seeking prey. Mutualistic ants build their nests in bromeliad rhizospheres, or root zones, and frogs lay eggs in the tanks. When sufficiently numerous in tree canopies, bromeliads can stabilize local temperature and humidity.

Water stored inside a W. gladioliflora tank. (Photo courtesy of Dave Janas)

Despite these important ecological roles, vascular epiphytes like bromeliads are often scarce in regenerating tropical forests. Their recovery could be slowed by limited seed dispersal or by a lack of suitable recruitment sites. One way to overcome dispersal limitation is to transplant individuals. In our study area in southern Costa Rica, transplanting bromeliads is relatively simple because they are easily found on fallen tree branches in the old growth forest reserve at Las Cruces Biological Station. We hypothesized that transplanting bromeliads from the old growth forest into 10-year old forest restoration sites would buffer local temperatures and increase arthropod abundance and diversity compared to bare, control branches.

Measuring local temperature in a transplanted Aechmea dactylina.

To test our hypothesis, we transplanted 120 bromeliads into three restoration sites in southern Costa Rica. The restoration sites are part of the Islas Project, an NSF-funded restoration experiment led by Drs. Karen Holl and Rakan Zahawi. Bromeliads were sterilized and attached to tree branches in the restoration sites with twine. Each day, we measured the microsite temperature on branches with and without transplanted bromeliads, as well as ambient temperature in the nearby air. To characterize arthropod colonization, we extracted and identified arthropods (to order) from transplanted bromeliads after two and three weeks.

We found that transplanted bromeliads decreased local temperatures on tree branches, creating a less stressful microclimate for other organisms. Bromeliads also facilitated arthropods; transplanted bromeliads were quickly colonized, especially by ants. We also observed small frogs inside of some bromeliad tanks, but none on the bare branches where we did not transplant bromeliads.

We found this frog (Craugastor stejnegerianus) in a small Catopsis sessiliflora tank. (Photo courtesy of Dave Janas)

Our observations suggest that bromeliad transplantation can buffer microclimates and create useful structures for invertebrates. If so, this method could improve restoration outcomes for canopy flora and fauna. Given that this experiment was conducted over a single field season, it is still an open question whether transplanted bromeliads will survive over longer time periods. It will also be important to learn whether transplanted bromeliads will facilitate colonization by other epiphytic plants. We did find some evidence of this as ferns were already growing in several bromeliads’ rhizospheres after two months.