This summer was a perfect example of environmental changes. In the northern states, some waterfowl species never migrated south last fall, and this spring’s surveys revealed that some birds migrated north earlier than normal. Conversely, milkweed (Asclepias spp.) emerged one to two weeks early and migrating monarch butterflies arrived two to three weeks late in all the northeastern states. Areas in a few northeastern states were stricken with a historic “100-year or more drought.” On the other hand, many areas in the semi-arid southwest experienced historical devastating floods.
Is this a fluke? Or is this the new norm? Perhaps somewhere in the middle, but more likely these weather and climatic changes are the ‘new normal.’ The events and data support that assessment.*
How do living organisms respond?
Published studies by biologists have been documenting the impact of climate change on the environment, especially species that are adapting and not adapting. We can learn about impacts on organisms by examining changes in cyclic and seasonal natural phenomena of plants and animals in relation to climate. These seasonal changes and cycles are known as phenology. Noting the times of year that specific plants bloom, or when birds migrate are two examples. Comparing the phenology of many species over a period of time can reveal informative clues on how changes in climate may affect them. Many studies along this model of investigation demonstrate that the living environment is indeed impacted.
United Kingdom researcher Stephen Thackeray(1) and his colleagues analyzed the phenology of a wide range of species. They used 10,003 phenological data sets to determine if and how much species’ phenology have changed over a minimum of 20 years. The analysis revealed that phenology has shifted in unequal rates in different species groups. Thus, climate change leads to disruptions of the phenological match between species, which often impacts ecological relationships.
Another question the researchers asked was how sensitive events in their life cycles are to the two most common variables in climatic change: temperature and precipitation. Both variables have changed in an uneven process over the flow of seasons. How does this impact species relationships? Some periods of the year have warmed faster than others, which may affect two interrelated species with equal temperature sensitivities but at different times. This could shift their phenological events at different rates and cause a mismatch in their relationship.
For example, milkweed plants emerging and flowering much earlier than normal resulted in sub-optimal conditions for the late-arriving monarch butterflies to use the plants for breeding. Additionally, the persistent hot and humid weather in the northeast could impact monarch larva (caterpillar) by either accelerating or arresting development.
The study authors also discovered a difference of sensitivity to temperature variations at different positions of the food chain (referred to as trophic levels). Species at different levels did not differ in the time of year at which they were sensitive to annual variations in temperature. But they did vary in how sensitive they were.
Species in higher levels of the food chain (the secondary consumers) are less sensitive to temperature changes than species at the bottom (the producers and primary consumers). These species are twice as sensitive to temperature changes than upper level species. Secondary consumers are also less sensitive to precipitation variations.
The authors then combined the species sensitivities with a future climate scenarios. They forecast that primary consumers -birds, insects, small mammals, etc- will shift the timing of their phenological events by twice as much as will species at other levels of the food chain. One reason their response varies is because species at different tropic levels respond differently to exactly the same temperature cue. Species respond differently to temperature during various times of the year.
The above example of the milkweed and monarch butterfly mismatch could impact the breeding success and thus population numbers of the butterflies. Both species have different physiological mechanisms that determine their phenological events and use different cues to determine their timing. Although these cues will be correlated to some extent, the cue used by the consumer -in this example, the monarch butterfly- is less reliable than that of the the plant they rely on. This cue unreliability in the consumers may mean that they will evolve with less temperature sensitive phenology than those species at the trophic level they rely on.
Ecologist Marcel Visser (at Netherlands Institute of Ecology) calls attention to moving from conventional two-species interaction research to a more holistic approach: investigating the effects of climatic change on the entire food-web. In a review(2) of the Thackeray, et al. study, Visser additionally proposes that impacts by phenological mismatches could be buffered by other mechanisms in their ecosystems.
To help us understand the consequences of phenological mismatches and thereby form predictions, he proposes questions that should be considered in studying changes in climate changes and relationships:
How are the strengths of the links in a food web affected by phenological mismatches? What happens if the phenology of species at one trophic level shifts more than that of species at another? Does this lead to the loss of some links and the formation of others? Does this destabilize the web? Such analyses would be a stepping stone from studying the phenological shifts of species to understanding the effects of
climate change on ecosystem function.(2)
Stink bug preys on larva.
An example for a holistic ecosystem approach is field observations (my own and in the literature) that have suggested that as prolonged temperatures increase, depredation and parasitism of monarch larvae and adults increase. Is this a function of differences in phenology of monarchs and its predators, or changes in all vegetation and species interactions (a complex of one or more phenological overlapping and mismatches) in the habitat? Do temperature mismatches in other members of the monarch habitat increase risk or rates of depredation?
One research team suggested that migration of monarch butterflies may have evolved as an adaptation to decrease depredation and parasitism in their breeding habitats. If monarch adults were to delay or ignore cues to migrate because of changing climate, how would that impact their overall population?
Adding to the complexity, climate sensitivity in species is not fixed. Phenological mismatches can lead to selection on the timing of phenological events. Resilience to environmental challenges can alter phenology, but over time can also result in genetic changes to sensitivity, thereby fixing phenological changes. Conventional theory on temperature range sensitivity of monarch adults and larvae states that it quite narrow. However, some observations(3) of their coping mechanisms with prolonged high temperatures in the Pacific Northwest sub-population questions if this sensitivity range is more flexible than conventional thought, or if this could be a developing adaptation.
Some researchers are already investigating genetic changes accompanying phenological adaptations to climate change (e.g. genetic alterations in melanin associated with plumage and physiology in European owls that have adapted to changing ecosystems). Such complex studies must be conducted to forecast the impacts of climate change and phenological responses and ecosystem function.
Research by Thackeray, Visser, and other colleagues demonstrates that long-time series of data are essential for such investigations. They also applaud and encourage professional and citizen scientists to continue collecting and submitting observations to add to the data pool. As Visser commented, “The additional advantage is that observing phenological shifts in, sometimes literally, your own backyard drives the message of global climate change home.”
(1) Thackeray, SJ, et al. “Phenological sensitivity to climate across taxa and trophic levels”. Nature 535, 241–245 (14 July 2016)
(2) Visser, ME. “Interactions of climate change and species”. Nature 535, 236–237 (14 July 2016)
(3) Anecdotal observations by Dr. David James, Washington State University entomologist, in central Washington and myself at Malheur National Wildlife Refuge, eastern Oregon.
* The main difference between weather and climate is time. Weather is the atmospheric local events over a short period of time. Climate is an average of the weather over much longer time in a region or globally. Sure, we can agree that weather and climate is cyclic, with highs and lows historically up and down. Also, a few episodic variances from the average can be expected. But climate does not vary as greatly as weather. The trends clearly demonstrate that climate is changing. Modern paleoclimate technologies can now add to the 70-year human records of climatic changes, both which confirm that climate change is a reality. Those changes have accelerated, more than any other equal span of time in historical evidence.