Monday, March 19, 2012

An Early Spring Isn't Always a Good Thing

It is pretty well agreed upon within the scientific community that climate change is happening. It has become increasingly urgent that we nail down all of the individual facets of this very large change and the operation of environmental drivers. One of these is the operation of weather as a driver for population dynamics.

It is important at this stage to define weather versus climate. Weather is the state of the atmosphere at a particular time and place such as heat, cloudiness, dryness, sunshine, wind, rain, etc. Climate is the weather conditions prevailing in a place over a long period of time. So when we talk about weather, it is as a single climate driver that may have detectable effects on populations of organisms. These effects can be direct density-independent, have indirect effects on populations (act of food sources, predators, etc.), or act of population dynamics (age structure, life stages, etc.).

A new study, published in Ecology Letters, takes a look at how a single climate parameter can determine population dynamics in a butterfly species. The focal butterfly species they chose to work with was the Mormon Fritillary (Speyeria mormonia) because it is a species that is distributed throughout the North American Rocky Mountains and has non-overlapping generations. The adults of this species feed on nectar (the preferred floral nectar host is Erigeron speciosus) and young males feed from mud, dung and carrion. The females will mate once and lay their eggs singly in some leaf litter located near a host plant.The larvae will over-winter as unfed first instars and will develop into adults in about 6 weeks. Previous studies have shown that the fecundity of adult females declines linearly with their food intake with their eggs deriving up to 80% of their carbon composition from the sugars taken up by the adults. The host plants are useful in that they can be counted for flour availability which can be a broad indicator of nectar (food) availability for these females. Then add in the weather component. Frosts occurring early in the growing season will kill developing flower buds, reducing the food availability in the system. Keeping this in mind, the researchers hypothesized that snow melt time in the first year would affect butterfly fecundity through flower abundance (a delayed density-dependent indirect effect). In the year following this they predicted that snow melt time would directly affect the developing larvae. Remember, the larvae overwinter and mortality could occur due to exposure (a density-independent direct effect).

To test this, the researchers set up study sites in the Rocky Mountains of Colorado. They determined the floral hosts preferences and distribution of the hosts plants of the butterflies. They counted the blooms every other day in teach of their plots every year from 1975 to 2009 (except 1990). They also caught butterflies, took their demographics (size, sex, etc.), numbered their wings, and observed their feeding behaviors. Finally they recorded the snow melt timing within their sites.

After all kinds of population growth analysis that I'm not going to go into (you're welcome), they found that the timing of the snow melt affects the population dynamics of these butterflies both directly and through the density-dependent indirect effects on flower availability. The early snow melt reduced the flower (and therefore the food) supply, adversely affecting the butterfly population growth rate. In the second, consecutive year, the combined effects explained more than four-fifths of the variation in the population growth rate. Just a single weather parameter (in this case snow melt) can have multiple effects on population growth. This study is among the first to demonstrate these indirect effects as well as documenting the multiple effects that a single weather parameter can have on population dynamics. These researchers were able to take long-term data and apply it to data models to understand both the effects on a single species as well as understanding species interactions in the context of climate change. These types of experiments and models can help to predict changes in populations in the future and even across populations and species.

Here's the paper:

Boggs, Carol L. and David W. Inouye. (2012) A single climate driver has direct and indirect effects on insect population dynamics. Ecology Letters: published online March 14, 2012 (DOI: 10.1111/j.1461-0248.2012.01766.x)

Science Daily article about this paper:  Early Spring Drives Butterfly Population Declines: 'Ahead-of-Time' Snowmelt Triggers Chains of Events in the Mormon Fritillary Butterfly

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