This review summarizes current research on plant-pollinator networks, stressing the need to include more studies in tropical areas. Tropical areas have a richer diversity in plants and animals than non-tropical areas, resulting in the several network patterns such as low connectance and higher modularity. However, these patterns are overlooked when tropical areas are not equally analyzed, limiting accurate understanding of this relationships.

This statewide assessment details the forest conditions of Hawaii in hopes of developing strategies towards conservation. It includes information comparing the major changes in vegetation before and after the arrival of humans on the island.

A recording of a bird's native calls is played to lure the species into an area.

Researcher Sean McDonald placed speakers playing non-native bird calls near native plants in danger of extinction in hopes of attracting these birds to these plants and increasing seed dispersal.

Read more in Inside Science: https://www.insidescience.org/news/playing-birdsongs-save-trees

This study found that phenotypical traits such as the size of a bird's beak and the fruit's diameter were strong factors that determined the likelihood of successful interactions, along with species abundance.

Islands suffer significant losses of large frugivorous birds due to their small size, isolation from mainlands,and limited amount of species. These large birds tend to be flightless (making them an easy target for predation) and make it possible for large seeded plants, particularly trees, to be dispersed throughout the island.

More information about the causes and consequences of large frugivorous bird extinction can be found in the Forbes article: https://www.forbes.com/sites/grrlscientist/2018/10/13/does-it-really-matter-if-just-one-species-goes-extinct/#48e3b358610b

The results from the simulated extinction, along with the measurement of network interaction patterns (modularity, nestedness, and specialization), were all similar between O'hau and other tropical or non-tropical locations.

The authors designed hypothetical scenarios where generalist or specialist species in a network went extinct, then determined the severity of these extinctions by measuring the amount of additional species that would (theoretically) go extinct as a consequence.

The use of a null model ensures that the results found by these simulations are not a coincidence, that is, not due to chance.

A weighted network assigns some form of quantitative value to each connection between two partners (an example is shown below in figure 4). In this case, the value assigned was the frequency of interaction.

The authors surveyed data from seed dispersal networks across a variety of habitats, noting that O'ahu was unique in that the majority of its population was mostly made up of introduced species, with the original bird species of the island going extinct.

Non-living parts of an ecosystem, such as elevation or rainfall

The authors conclude that plants and animals at different sites across the island develop their own unique pattern of interactions, whether the sites are populated by mostly native or novel species.

Referring to figure 3, the majority of dissimilarity across sites was due to linkage turnover, represented as the gray bars. This means that even though the sites shared similar species, the interactions were different.

The authors found that the majority of the sites in O'ahu had different sets of interactions between plant and bird species.

The authors measured the overall dissimilarity between different locations by two factors:

species turnover when — two  locations do not share similar interaction patterns because they are inhabited by different species,

and linkage turnover — when species found in both locations develop different interactions specific to their site

A species' fundamental niche encompasses all of the possible roles it has in its environment, whereas the realized niches are the actual roles that a species plays in its environment, taking into account competition, predation, and other interactions with neighboring species.

The video below further explains this: https://www.youtube.com/watch?v=-6COob_bymw. 

The authors concluded that introduced species do not necessarily have to be evolutionarily related to native species to form similar interaction networks. This conclusion was based on their results demonstrating that introduced species on the island successfully formed modular networks comparable to the networks that were formed between native species. 

The introduced birds and plants integrate successfully into the ecosystem of O'ahu because they follow the same strategies used by the native plants and birds: creating beneficial relationships with specific species and carefully sharing the resources of the island with other organisms. 

The authors concluded that overall, the complex network structures of the island's ecosystem on both the local and regional scale remained the same, even though these networks were now infiltrated with invading plant and animal species.

At individual sites, the authors again saw species mostly interact with only a subset of the total partners available. 

Dalsgaard and colleagues found that tropical areas have a lot of bird species that are obligate frugivores, meaning that they only eat fruited plants. Because fruit is their sole diet, these birds interact with a large variety of plants to ensure that they're consuming enough food to live.

This type of behavior is commonly associated with a low specialized network because the birds are not displaying any preference toward any particular plant(s).

A species that interacts with a wide number of species in ecological network. 

With the original bird species extinct, how well does the introduced bird species spread the seeds of native plants? Does this newer bird species show a preference toward introduced species of plants?

To address these questions, the authors collected poop samples over the course of 3 years from 21 different birds found in 7 different locations across the island of O'ahu. The supplemental figure 1 and table 1 describe the 7 locations' average rainfall, coordinates, and elevation to demonstrate the diversity of these areas. Another set of tables listed the different species and plants (introduced and native) found at each site.

Differences in geography affects the interaction patterns developed between species. Climate change, differences in species richness, and human impact were shown to dictate the types of networks that dominated those areas.

Bascompte, Jordano, and Olsen investigated coevolutionary interactions across a wide range of locations, measuring the levels of dependence between various species of plants and animals. They showed that most of these interactions are asymmetric, meaning that one species depends more heavily on the relationship than the other. This asymmetry supports high biodiversity and coexistence of multiple species in an ecosystem.

To determine if an observation is a consequence of a measured phenomenon, and not by chance, researchers must test (and reject) the null hypothesis. A null hypothesis states that a result or observation is due to chance, and so should be disregarded as insignificant. When researchers can reject the null hypothesis, that means their results from a study were not due by chance.

In this case, the authors test the significance of the identified patterns in the network by comparing these values to a null model, a generated collection of values randomized to produce a pattern based on no ecological mechanism (Gotelli and Graves, 1996). If the observed values differ from the range of null values defined by the null model's confidence interval, they are considered significant.

The authors concluded that the majority of plants and animals involved in the seed dispersal networks of O'ahu are introduced species, and that the majority of seeds being dispersed by these birds are from introduced plant species.

The authors identified 44 different plant species and 15 different birds in O'ahu. Figure 1 depicts each bird (left column) and each plant (right column) as rectangles. The lines connecting a bird to a plant represent a seed dispersal event.

In a 2010 Hawaii statewide assessment of forest conditions and trends, a map illustrated that major vegetation types for multiple islands, especially O'ahu, experienced severe changes in comparison to before the arrival of humans.

The introduction of novel seed dispersers (aka birds) is an important factor regarding in the survival of native Hawaiian plant species. One study showed that the distribution of seeds from native plants is becoming increasingly dependent on not native but foreign birds. C. Chimera and D. Drake also found that these introduced birds tended to spread more seeds from non-native plants rather than native plants.

Native plants on the Hawaiian island of Kauai are threatened by changing climate, invasive species, and destruction by feral pigs. Efforts led by the Plant Extinction Prevention Program (PEPP) are working toward saving these endangered species and preventing the loss of further species.

Read more in the grist: https://grist.org/article/hawaiis-rarest-plants-are-in-crisis-meet-the-people-fighting-to-save-them/

A mutualistic network is a web of beneficial partnerships between organisms. Disturbances to that relationship, like from one of the organisms becoming extinct, or by the intrusion of another species, can be harmful for the original partners of that relationship.

Brodie et al. reviews the concept of secondary extinction, the idea that the extinction of a species, caused by human activity, can lead to the loss of additional species.

Below is a diagram from the review depicting different types of secondary extinctions. Co-extinction is when the direct impact of humans (red arrow) leads to the loss of one species, causing the loss of another species, which can then cascade into a series of extinctions. Human activity (yellow arrows) also affects interactions between species (gray arrows).

By simulating the network patterns between plants and pollinators, the authors demonstrated that plants were more resistant to extinction with the removal of specialized pollinators versus generalized pollinators. Specialized pollinators only interact with a few plant species, whereas generalized pollinators interact with a larger range of plant species. When a generalized pollinator is lost, more plants are affected.

Previously, the majority of network analysis was qualitative. Blüthgen et al introduce two quantitative measurements that uses the frequency of interactions to describe specialization at the species level and across a network.

This study provides evidence that ‘Alalā, an endangered, native bird species in Hawaii, displays seed dispersal behaviors and promotes seed germination of various native plants. Based on these results, the authors suggest that ‘Alalā can contribute towards the restoration of native fruiting plants in Hawaiian forest communities.

Fricke et al investigates how well mutualistic systems between plants and animals protect against coextinction. They find that the diversity in partners protects species in mutualistic interactions because they do not rely solely on one or few partners for survival.

The Anthropocene defines the epoch or geological time when human activity significantly impacted the global environment. Based on the criteria for defining a new epoch and supporting geological evidence, the authors propose two possible dates to mark the beginning of the Anthropocene: 1610 and 1964

Mooney and Cleland describe the evolutionary consequences of invasive species on the invaders themselves and on native species. Modifying behavior to adapt to a new environment, competing for similar resources, predation, or creating hybrid progeny by breeding with natives species are just a few examples.

The authors merged data sets from multiple studies investigating fruit-frugivore interactions and illustrated that most networks across various neotropical areas were significantly nested and modular. The authors also concluded that combining results from different studies can be useful for analyzing the ecological structures of broad regions, and supporting conservation efforts.

Nestedness can contribute towards the robustness of a network. If species that share fewer interactions are the first to go extinct, the remaining mutualistic system have a greater chance of survival. The authors also calculate a coefficient as a parameter of a network’s robustness.

The phylogenetic or evolutionary relationship between species can predict the types and quantity of interaction patterns they exhibit in a network. Using a simulation of extinction events, the authors also demonstrated that the extinction of one species can result in the extinction of others species that are evolutionarily related.

The authors studied the feeding behaviors of birds in the Galapagos island. They discovered that these birds ate a not just a select few, but a wide variety of plants on the island. They called this behavior interaction release, a survival tactic where animals expand their niche.

In contrast to Foster, Chimera found that introduced birds in Hawaii dispersed predominately the seeds of non-native plants. Possible explanations could be that the non-native plants produce larger fruit, that are more abundant and have a smaller seed size. These qualities are more enticing to birds and enhance the plants’ chances of getting eaten and later deposited.

Foster and colleagues investigated how often introduced, fruit-eating birds on the islands of Hawaii consume and disperse seeds from native plants versus seeds from exotic plants. They found that seeds from native plants made up the majority of the introduced bird species’ diets. This supports the potential of introduced bird species playing a role in the conservation efforts of native habitats.

Capable of growing into a plant

Normally depicted as the "tree of life" (shown below), the phylogenetic tree traces the genetic lineage of organisms over time. Two species share a phylogenetic relationship when they share a common ancestor. An example of a phylogenetic tree can be found at https://www.nationalgeographic.org/media/tree-life/

The authors of the cited paper found that phylogenetic relationships can influence the type of networks species build and explain the type of species involved in these interactions.

When studying larger areas of the island, the authors observed that species engaged in only a small portion of the possible interactions available, with groups of species specifically interacting amongst each other.

A niche is an organism's role in its ecosystem, describing how it utilizes the resources and interacts with living and nonliving factors of its environment.

Niche broadening is when a species expands its roles in its habitat to enhance its chances of survival.

The distribution of a seed by being eaten by an animal and later excreted in its feces.

In another study taking place in Hawaii, researchers are investigating invasive predators such as the black rat to implement protective measures for native bird populations of the Hawaii Volcanoes National Park. More on this study can be found here: https://www.usgs.gov/centers/pierc/science/monitoring-bird-and-rat-behavior-improve-invasive-species-management?qt-science_center_objects=0#qt-science_center_objects

Invasive species can maintain interactions and behaviors with their surroundings similar to the native species they replaced, even if both species are not related.

A group of living organisms that has recently moved into a new ecosystem

The authors visited multiple areas in Hawai'i to study how seeds from plants (originally from the area or recently introduced) were being distributed by invasive birds.

The spread of invasive species (living organisms not originally found in a specific ecosystem) causes disruptions to wildlife in a variety of ways. Some examples include outcompeting native plants or animals for resources and food, or carrying diseases harmful to native species. More information on this topic can be found at NWF's "Invasive Species": https://www.nwf.org/Educational-Resources/Wildlife-Guide/Threats-to-Wildlife/Invasive-Species

D. Carstensen and others quantified the variability of interactions between plants and pollinators across different spaces. They found that these interactions vary greatly between different locations due to the abundance of species, which determined the likelihood of partners interacting with each other.

Traveset and colleagues observed that bird-plant interactions on the Galápagos islands are highly generalized, meaning the birds developed relationships with a wide variety of plants, and vice versa. These broad interactions offer more opportunities for species to gain food. However, the authors also warn that these additional interactions promote the reproduction and survival of invasive plant species, at the cost of native species.

In response to lack of food and increased populations, animals broaden the scope of species they interact with beyond their original interactions.

An animal that eats primarily fruit

When the behavior between species in one area differs from the behaviors between species in another location.

A new relationship or pattern of behavior between plants and animals.

The authors concluded that novel species' interactions, developed over a relatively short period of time, still resemble much older interactions between native species.

Binary calculations are a straightforward form of measurement that states the presence or absence of an interaction.

The authors found that neither abiotic (rainfall and elevation) nor biotic (invasive species) factors influenced the variations in plant and animal interactions measured from different sites with similar species populations.

Results in the supplement section show that abiotic factors (rainfall and elevation) affected the composition of species found at a site, thereby creating different network interactions between sites.

Living parts of an ecosystem, in this case, invasive species on the island

The authors compared the similarities and differences between species' interactions across different locations around the island, taking into account the unique environments of each site.

Organized into distinct, independent groups

A deviation or disturbance from the norm.

From an island

A group of islands

The current geological age where human activity is the dominant influence on climate change and the environment.