Why Species Counts Fail for Seed Dispersal in Brazil’s Atlantic Forest

In tropical forest ecology, seeing many fruit-eating species around a tree is generally taken as a positive sign.

The usual reasoning is that more visitors will transport more seeds to fresh sites.

Conservation schemes therefore often use animal diversity as a dependable indicator of woodland health.

Research in Brazil tested that idea directly and showed that it breaks down in important areas.

Only a small number of species performed nearly all of the work, while a straightforward animal count gave no indication of which species they were.

Counting the wrong thing

The research was led by Eduardo D. B. Rigacci, a biologist at the University of Campinas (UNICAMP) in Brazil.

Alongside colleagues, he observed animals feeding over two fruiting seasons in Atlantic Forest fragments. The trees involved cannot reproduce by themselves.

Instead, they rely on animals to consume their fruit and move the seeds to other locations. Ecologists refer to this as seed dispersal, a process on which up to 90% of woody tropical plants rely.

For decades, researchers assessed these relationships using a single measure: the number of frugivores, or fruit-eating species, that shared a forest with a particular tree.

A higher number of species was presumed to provide more dependable seed dispersal. Rigacci and his colleagues examined whether that was true.

A small number of seed-dispersal heavy lifters

The study focused on two trees: the jussara palm and the silver cecropia.

Both are keystone species that fruit during the dry season, when few other food sources are available. For birds and monkeys, they provide a vital resource.

Extended observation revealed a clear pattern: a limited group of species carries out most seed dispersal.

The remaining visitors make comparatively minor contributions. Although more than 20 fruit-eating species were recorded at the trees, nearly all were birds. Most larger animals have already disappeared from these fragments.

Large fruit-eaters, including toucans, have declined through hunting and habitat loss. One study also found that the palm’s seeds became smaller after its largest dispersers vanished.

What observation revealed

Visit totals tell only part of the story. During more than 350 hours of observation, the team recorded which animals visited each tree and the number of seeds removed by every visitor.

Yet a seed that is moved does not necessarily reach somewhere suitable for growth. The researchers therefore tested germination, with markedly uneven results.

Seeds that had passed through a bird’s digestive system germinated much more frequently than those left to drop naturally.

For the jussara palm, about 70% of gut-passed seeds germinated, compared with roughly 18% of untouched seeds.

The silver cecropia showed the same pattern less strongly, at around 40% versus 9%. A small number of birds raised germination to more than 90%. Whatever occurs during digestion, the seed plainly gains an advantage.

As the climate warms

The researchers then modelled the future. Drawing on climate projections for the coming decades, they mapped where the two trees and their fruit-eaters might still survive by 2070.

Both tree species are expected to experience reduced ranges. The silver cecropia could lose between a fifth and a third of its range, while the palm could lose somewhat less, in an area forecast to warm by as much as 3.3°C (6°F).

Fruit-eaters are expected to fare no better. Their distributions could shrink by a quarter to well over a third, with monkeys and other canopy specialists facing the greatest exposure.

As ranges contract, many species may be unable to cross open farmland to find cooler habitat. Fewer dispersers mean smaller forests.

When species numbers mislead

This was the point at which species counts failed. Compared with a more comprehensive measure of how effectively seeds were moved and germinated, the simple total disagreed across roughly 60% of each tree’s range.

In some places, the species total gave an unduly hopeful impression. The most effective carriers were often those most likely to disappear, making conditions appear safer than they actually were.

In other areas, it understated the system’s resilience because it overlooked hardy species that persisted. The discrepancy was greatest for the jussara palm.

Lessons from the forest

For the jussara palm, species numbers accounted for less than 13% of the change in seed dispersal under warming.

The silver cecropia performed better: the count matched germination reasonably closely, although it still did not forecast how many seeds were actually transported.

Until now, projections of warming’s impact on these forests have relied solely on species totals.

Incorporating the role performed by each animal had not previously been attempted across entire landscapes. That approach revealed the shortfall.

Two trees, two different stories

The different responses of the two trees are explained by the animals that disperse their seeds.

Silver cecropia produces very small seeds that almost any fruit-eater can swallow, distributing the work across many species. If one or two resilient dispersers are lost, the system could still buckle.

The jussara palm does not have that buffer. Its larger fruit can be handled only by certain birds, leaving its future dependent on a short list. The pale-breasted thrush, a common songbird, is particularly important.

That thrush also withstands warming better than most species, indicating that the palm may persist in areas where a raw species count would dismiss its prospects. Lose one or two resilient workers, however, and the system could buckle.

Implications for conservation

Species totals are an unstable tool for predicting whether a forest will continue to regenerate.

An identical number of fruit-eaters can represent a robust system in one location and a vulnerable one elsewhere, depending on which animals are still present.

For conservation, this changes the priorities. Protecting a set target number of species is insufficient if the animals carrying out most seed dispersal disappear without notice.

The study supports monitoring what animals do rather than simply how many are present. Its significance extends beyond two Brazilian trees.

Seed-dispersing animals are declining globally, and a recent review found that forests lose the capacity to establish their next generation when the appropriate movers disappear.