Ignored Invertebrates

ResearchBlogging.orgMost recreation ecology studies focus their efforts on how recreation may impact vertebrates or, most often, plants. Up until now, invertebrates were pretty much ignored in the field. However, the latest issue of Entomological Review contains an article by two Russian scientists examining how “recreation load” might impact the behavior of hortophilous animals (invertebrates that live in the grass layer).  Specifically, Khabibullin and Khabibullin wanted to know (1) what types of responses do insects show when faced with stressors caused my the mechanical act of recreation? (2) Do different species show different tactics to cope with these stressors? (3) How effective are these strategies?

Beetle on a trail in Joshua Tree National Park

What did they do? The study was conducted along a popular river in South Cis-Ural Region of Russia. The two study species were two types of beetles; the twenty-two spot ladybug (Psyllobora vigintiduopunctata) and the fourteen-spot ladybug (Coccinula quatuordecimpustulata). Both ladybugs are common to the region, do not actively fly in search of food, and are easy to spot in the grass along the river bank. The twenty-two spot ladybug is the more sedentary of the two species as it feeds on mildew while the fourteen-spot ladybug is an active predator hunting aphids.

Twenty-two spot ladybug

The researchers wanted to study the impact of walking through a patch of grass as someone might do while hiking off-trail. Instead of actually trampling the insects, the researchers imitated the impact of walking by using their hands to flatten the grass slightly (such as might happen by a recreationist brushing against the grass while walking by). The hand movement was repeated until the ladybug fell from the grass stalk.  The researchers recorded whether the ladybug flew away at the approach of the human, whether the ladybug stayed put, whether the ladybug tried to fly away when it was falling, and whether the ladybug entered a state of thanatosis (the defensive mechanism of faking death) and how long before it started moving again. Then, to decide how much damage recreationist might cause by actually stepping on the ladybugs, a few specimens were trampled.

Fourteen-spot Ladybug

What did Khabibullin and Khabibullin find out? Well it turns out that ladybugs cannot see very well and thus not a single specimen in the study tried to fly away when a human approached.  The ladybugs were very sensitive to the mechanical impacts and over 90% of each species fell from the grass after the first impact (brush of the researchers hand) – however a few specimens of  the fourteen-spot beetle held on for four or five swipes. Only twice were the beetles observed trying to fly during their fall and these two instances were viewed as outliers and left our of the analysis.  Both species went into a state of “playing dead” at about the same rate (30%) after falling.  Overall, the fourteen-spot ladybug resumed activity more quickly than the twenty-two spot ladybug regardless of whether it went into a state of thanatosis or not. Also an interesting observation, a few copulating couples were observed and even after recreation impacts, the insects did not separate and continued copulating. And, believe it or not, the trampled ladybugs did not show any negative consequences – they resumed normal activity after being stepped on.

So What Does It All Mean? Based on the responses of the two species the researchers categorized two different types of defense strategies exhibited by the ladybugs in response to mechanical disturbance by recreation.  The twenty-two spot ladybug exhibited a “busy” strategy; it fell easily off the plant and then quickly resumed it’s activity. The fourteen-spotted ladybug exhibited an “inert” strategy; it fell reluctantly, often entered a state of thanatosis, and slowly resumed activity. When using a defensive strategy it is in the best interest of the species to maximize safety while minimizing costs associated with energy expenditure and time loss.  And with all defense mechanism there are advantages and drawbacks to each (see Table 1) and the “preferred” defense mechanism will depend on specific conditions (amount of recreation, present of predators, etc).

Table 1 from Khabilullin and Khabilullin (2011)

The authors conclude that in an area where recreation disturbance is moderate the presence of recreation reduces the impact of other limiting factors (such as insectivorous and herbivorous vertebrates). Therefore the efficient use of time and energy becomes more important than maintaining safety as recreation puts less risk on these species of beetles. So overall, the busy strategy is a better use of time and energy when recreation is the main source of disturbance.

Yes, I know this has nothing to do with recreation ecology but cats are cute.

Reference:
V.F. Khabibullin, & A.F. Khabibullin (2011). Mechanical Aspect of Recreation Load and Defensive Behavioral Strategies in Grass-Dwelling Insects Entomological Review, 91 (8)

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2 thoughts on “Ignored Invertebrates

  1. That is an interesting question Ryan – I had not really heard of this beetle or this issue before. I could not find anything about it in the literature (at least as far as recreation impacts are concerned). So I looked at the latest General Management Plan from Coral Pink Sand Dunes State Park (2005) and they address the Tiger Beetle (and an endangered species of milkweed that is found in the state park as well). Interestingly enough they have not done anything since 1994 when they closed certain areas to OHV use to protect the Tiger Beetle. At the end of the paragraph about the Tiger Beetle they also state that this species populations fluctuate and the GMP specifically says “The exact causes of the fluctuations are not known, but the population trends may follow climatic cycles.” The GMP only briefly mentions the potential for beetle loss from OHVs. So it seems as if the state park is trying to say that OHVs may not be a significant threat to the Tiger Beetle – which I do not believe at all.

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