Explorations and ideas at the intersection between Evolution and Ecology

Jumping Spiders Take the Leafen Path

[caption id="attachment_351" align="alignleft" width="234" caption="Phidippus princeps, a jumping spider. Click image to see them in motion! Image still by Thomas Shahan."][/caption]

As we carve out our domains for home, business or farm among the landscape, we give little thought to our eco-engineering on the previous denizens we "annexed" the land from. It is no secret that our practices are intentionally destructive. But, still, many of us are compelled to preserve what little shreds of biodiversity remain.

In the midst of the cutting and gutting, we really need to know what the appropriate ways to maintain population connectivity are. Continuous stands of forests are becoming in shorter supply. Why does population connectivity matter? Take for example a sinking cruise ship in a warm, tropical location with 5 families aboard. Each family climbs into their own life raft and floats away to nearby deserted islands. Resigned to their predicament, they settle in for the long haul and make their lives anew in this tropical "paradise".

Some islands are close enough for the families to visit one another, while others are father apart. Of course, these are jellyfish and shark infested waters and swimming across channels comes at your own risk! Since swimming out in the open makes you very vulnerable, so only the islands that are closest together are ever able to make contact with each other.

For colonizes to thrive they must grow. Assuming there is no immigration from outside this groups of islands, they only way to grow is if all the islanders mate and reproduce with each other. Otherwise, their family line and backstories will die out with them and, well, we are biologically programmed to not push the self-destruct button! (To an extent...). Population geneticists hold that populations that outcross (mate outside their population) are much better off than populations that inbreed (mating only within a single population). This is because mating within the same group over several generations tends to fix genetic traits that are harmful. By mating outside your population, you have a better chance of masking or even purging harmful genetic traits from your family line. And, let's be honest, who doesn't want to want to purge grandpa's unibrow and hairy ear canal from subsequent generations of their family!

This, of course, holds true for animals and plants. Connecting populations is a fundamental concept in conservation science. While we fragment the world around us into ever smaller pieces, if our goal is to preserve species then we must make sure they can find each other. But one problem not taken into account for many conservation ecologists is the mere fact that communities evolve together. A stand of forest has a long history of interactions between plants, insects, mammals, birds and all the myriad sorts of microfauna and fungi . These historical interactions are encoded within their genetic make-up as unique events which affected their individual demographies.

[caption id="attachment_354" align="alignright" width="150" caption="Remove enough species and the ecosystem collapses. Public domain via Jorge Barrios."][/caption]

Think of it this way, take an ominously-leaning tower of Jenga. While you may remove several pieces and it still stands strong, the stability of the tower is more threatened with each removal and gets dangerously close to collapse. Eventually, all redundancy in the system is lost and with any further removal all could end in catastrophe.

Adding corridors to fragmented landscapes is not the same as adding pieces (species) to the Jenga tower, but much like placing your palm on the unbalanced edge to hold up the tower, corridors strengthen ecological redundancy. The larger the habitat, the more individuals that can be supported and hence more genetic diversity spread out among the populations or subpopulations. Corridors connect isolated patches and in effect make a specie potential habitat potentially larger. In theory, we should be able to construct corridors to connect isolated populations so there can be movement and gene flow between groups. For large mammals and far-flying birds these problems can be overcome by size and ability. But, most of life is not as such!

Take, for instance, the adorable jumping spider (above), Phidippus princeps (Salticidae). To understand how this spider moves across a fragmented landscape, Baker (2007) manipulated corridors connecting patches of an old growth field of clover and alfalfa. Patches were either all unconnected, all connected, or partly connected by vegetated corridors, as opposed to bare corridors (see schema below). Spiders were set loose upon the land from a source patch and their movements were closely monitored.

[caption id="attachment_356" align="aligncenter" width="400" caption="Experimental design from Baker 2007 studying effects of corridors on jumping spider connectivity."][/caption]

Perhaps unsurprisingly, Baker found that P. princeps always preferred vegetated corridors and was never found on bare strips. Even when vegetated corridors were absent and their population was overcrowded 2-3 times their natural density, the spiders rarely moved across the bare ground. This has pretty fundamental consequences for movement and dispersal ecology, underlying the importance of the types of corridors between habitat patches. Invertebrates in particular, typically being littler, are more subject to small changes and micro-habitat conditions. As Baker noted, "If an animal, as in the case of P. princeps, does not respond to density pressures when habitat patches are surrounded by unfavorable habitat, the persistence of fragmented populations may be severely compromised." To put it lightly.

The fact of the matter is that most creatures won't risk it out in the open. They need cover to help ensure their survival when moving around. This is a nice arguement for conserving natural corridors between habitat patches, as opposed to creating corridors for animals after we've destroyed their habitat. In a recent review on the effects of corridors on habitat connectivity, Gilbert-Norton and colleagues (2010) found that among the 78 experiments published in 35 research articles (including Baker 2007) since 1988 corridors helped increase movement between habitat patches by about 50%. This is a nontrivial number when we are talking about species at the brink of local extinction. Additionally, the effect of these corridors was much stronger for natural corridors (with preexisting vegetation) than manipulated corridors (those created by us), so much so the authors concluded that maintaining existing natural habitat corridors were a worthwhile conservation endeavor.

ResearchBlogging.orgBaker, L. (2007). Effect of corridors on the movement behavior of the jumping spider (Araneae, Salticidae). Canadian Journal of Zoology, 85 (7), 802-808 DOI: 10.1139/Z07-061

Gilbert-Norton, L., Wilson, R., Stevens, J., & Beard, K. (2010). A Meta-Analytic Review of Corridor Effectiveness Conservation Biology, 24 (3), 660-668 DOI: 10.1111/j.1523-1739.2010.01450.x

The views expressed are those of the author and are not necessarily those of Scientific American.

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