By Ollerton et al.
Sometimes science does not go the way you
plan. This study began as an experiment to test whether the showy outer florets
of Knautia arvensis (Field Scabious) help attract pollinators. These
enlarged “ray florets” sit around the edge of the flower head and make the
inflorescence more visually conspicuous. The original question was simple: if
these ray florets are reduced in size, do pollinators visit the flowers less
often, or spend less time on them?
The answer turned out to be: not really.
But that “failed” result became interesting in a different way.
In 2001, flower heads of Knautia
arvensis growing at Bradlaugh Fields in Northampton (UK) were
experimentally altered. Some were left untouched as controls, some had only the
tips of the ray florets cut off, and others had most of the outer ray-floret
tissue removed. The plants were then observed to see how three main groups of
pollinators responded: bumblebees, butterflies and other day-flying
Lepidoptera, and hoverflies. Nectar production and seed production were also
measured. |
| Experimental manipulation of Knautia arvensis
inflorescences, showing a Control inflorescence (left), an inflorescence with
a Small cut (centre), and an inflorescence with a Large cut (right). |
The cutting treatments did not
significantly change how often pollinators visited the flowers. Bumblebees and
hoverflies were generally more frequent visitors than butterflies, but all
three groups continued to visit and pollinate the manipulated flowers. Nor did
the treatments clearly affect how long insects stayed on the flower heads. In
other words, reducing the ray florets did not make the flowers obviously less
attractive to their pollinators.
However, the treatments did have effects
that were not expected at the time. The most severe cutting treatment reduced
nectar quality. Nectar volume stayed about the same, but nectar concentration
fell substantially in the large-cut flowers. Once the analyses accounted for
missing nectar data and differences in volume, the large-cut flowers also
contained less sugar overall. This suggests that the plant may have responded
to damage by reducing its investment of resources in those flower heads.
The most striking effect was on seeds.
Cutting the ray florets did not significantly reduce the number of seeds
produced, nor the proportion of flowers that set seed. But the seeds from the
small-cut and large-cut treatments were roughly one fifth lighter than seeds
from uncut control flower heads.
This matters because seed number is not the
whole story of plant reproduction. A plant may produce the same number of
seeds, but if those seeds are smaller and less well provisioned, their chances
of germinating, establishing, and surviving may be reduced. Larger seeds often
produce stronger seedlings, especially in competitive grassland environments.
So the damage may have affected reproductive quality rather than reproductive
quantity.
With hindsight, the study is therefore best
understood not as a successful test of whether ray florets attract pollinators,
but as an accidental experiment in florivory - the eating or damaging of
flowers by animals. Florivory is common in nature but is less studied than leaf
herbivory. Many studies show that damage to flowers can reduce pollinator
visits and lower seed production. This experiment suggests a subtler
possibility: floral damage may leave pollination apparently unaffected, while
still changing how the plant allocates resources to nectar and seeds.
The broader lesson is that archived or
apparently inconclusive experiments can still be valuable. Data that once
seemed difficult to interpret can become meaningful when viewed through a
different ecological framework. Here, an experiment that did not answer its
original question nevertheless revealed something important: damage to floral
tissues can alter plant resource allocation in ways that are not obvious from
pollinator behaviour or seed number alone.
A remaining mystery is what the ray florets
of Knautia arvensis actually do. In this experiment, they did not have a
strong effect on attracting or retaining pollinators. They may still contribute
to the overall visual display of the flower head, or they may have a more
subtle role that was not detected here. Alternatively, they may not be strongly
adaptive in this species. Further experiments would be needed to resolve that
question.
Overall, the study shows that plant
responses to floral damage can be complex and unpredictable. Pollinators may
continue to visit damaged flowers, and seed numbers may remain stable, but the
quality of the plant’s reproductive output can still decline. That makes
florivory an important but underappreciated influence on plant reproduction.
Read the scientific publication in JPE here!
